Transcript
Hig h Reli abi li ty Power System Design Buenos Air es, Arg enti na June 25 & 26, 2009
Keene M. Mats P.E. Regio nal Elect ri caluda, Manager Seni or Memb er IEEE IEEE/PES Dist in gu is hed L ectu rer
[email protected]
Agenda
3 case stu dies for hi gh relia bi li ty po wer systems
Desig n co ncepts Start wi th basics for simpl e circ uit d esign
Con si dera ti on s fo r tempera tu re, safe ty , etc .
Buil d s ystem wit h t ransfor mers, swit chgea r, etc.
Overa ll pow er s yst em design
2008 Nation al Elect rical Cod e (NEC)
“ Bible ” for de signing e lectrica l syste ms in U SA
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U.S. Typi cal Sys tem Vol tages
120 V, for m ost small l oads li ke la pto ps
120/240 V, 1-phase di st ri bu ti on 208Y/120 V, 3-phase distribution
480Y/277 V, 3-phase distribution
4.16Y/2.4 kV, 3-phase distribution
12.47Y/7.2 kV, 3-phase distribution
Uti li ty Dis tr ib ut io n: 12 kV, 23 kV, 34.5 kV, etc .
Uti li ty Trans mi ss io n: 46 kV, 60 kV, 115 kV, etc.
All at 60 Hz
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p 480 V, 3-Phas e Pow er Cables & Cond ui ts Combination Motor Starter
Cir cui t Brea ker (Over Curr ent Pro tecti ve Devi ce) Motor Starter
Moto r Cont act or Mot or Overlo ad Cables & Cond ui ts M 100 Hp Mot or
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
BASIC ELEMENTS
Loa d: 100 Hp pu mp f or movin g l iqui d Cables & Cond uit : Conv eys pow er, sa fely, from motor sta rter to pump
Motor Overload: P rovi des pr ote cti on t o mot or f rom overload cond it ion s (e.g., bim etallic str ip, e lectron ic)
Motor Contactor: A llow s passa ge of power to mo tor
from source Cir cui t Brea ker ( OCPD): Prov ides ove rl oad and sho rt circuit prote ction
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
Cables & Cond uit : Conv eys pow er, sa fely, from power source to mo tor sta rter
Pow er Sou rce: 480 V, 3-phase, 60 Hz
Contro l: Not s hown in s ingle line dia gra m
Cont rol Metho ds: Level sw it ch, flow s enso r, pressur e senso r, ma nual st art /sto p, auto mated cont rol sys tem, PLC, DCS, SCADA, etc.
PLC = Pro gr amm abl e Lo gi c Contr ol ler DCS = Dis tr ib ut ed Cont ro l Syst em
SCADA = Supervis ory Cont rol and Data Acqui si ti on
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
DESIGN CAL CULA TIONS
A. Determine full-load current, IFL B. Size mo to r s tarter
C. Size ov erc ur rent p ro tecti on , brea ker
D. Size con duc tor s f or cables
E. Size gro und ing con duc tor
F. Size co nd ui t f or cables
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
A. Determine Full-Load Current, IFL
Thr ee meth od s 1) Calc ulate fr om pow er s our ce
2) Dir ectl y f rom mot or name plate
3) From NEC Tabl e 430.250
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
1) Calc ul ate IFL f ro m p ow er s ou rc e: kVA IFL = -------------------------------------Sq Rt (Phases) x Vol tage
Where, Phases = 3
Where, Vol tage = 480 V, or 0.48 kV
Where, k VA = k W/PF
Where, PF = Pow er fact or , ass um e typ ic al 0.85 Wher e, kW = Hp x 0.746 kW/Hp
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
Thu s, kW = 100 Hp x 0.746 kW/Hp = 74.6 kW
kVA = 74.6 kW/0.85 PF = 87.8 kVA And,
87.8 kVA IFL = ----------------------------- = 105.6 A Sq Rt (3) x 0.48 kV
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
2) IFL d ir ectl y f rom mot or name plate :
Depends ontor whethe r m te oto r h as b een p urc hased to ins pect mo n amepla
Many di ff erent moto r designs
Resu lt s i n di ff erent IF Ls f or exact same Hp
High effi ci ency m oto rs w il l have low er IFL
Low efficiency a nd lo wer cos t mot ors w ill have hi gh er IFLs
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
3) IFL f rom NEC Table 430.250
NEC Table 430.250 = Full-Load Current, Three-Phase Alternating-Current Motors
Most com mon m oto r ty pe = Indu cti on-T ype S qui rrel Cage and Woun d Rotor mot ors
NEC Tabl e 430.250 inc lu des IF Ls fo r vari ou s induc tion motor Hp size s ve rsus motor volt age
Mot or voltages = 115 V, 200 V, 208 V, 230 V, 460 V, and 575 V.
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NEC Tabl e 430.250, Motor Ful l-Lo ad Cur rent s
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IFL fo r 100 Hp, 460 V, Ind uc tio n Typ e Mot or
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
Thr ee metho ds , su mm ary
1) Calc ul ate fr om po wer s ou rc e = 105.6 A 2) Dir ect ly fr om mo to r n ameplate = Depends o n motor design and efficiency
3) From NEC Table 430.250 = 124 A
Why is th ere a di ff erence?
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
Thr ee metho ds , su mm ary
1) Calc ul ate fr om po wer s ou rc e >>> a) Does no t acco unt for mot or effic iency b) Had t o assume som e typ ic al p ower factor c) Small er Hp mot ors wil l h ave ve ry low PF
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
Thr ee metho ds , su mm ary
2) Dir ect ly fr om m ot or name pl ate >>> a) Mos t acc ur ate b) Actu al mo to r may no t be available to see nameplate c) Usu all y t he case when de si gn i s exe cu ted befor e equi pment pur chase and i nst all ati on d) Even aft er i ns tallati on , mo to r may have to b e replaced e) New m ot or may be le ss eff ic ient, or hi gh er IFL
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
Thr ee metho ds , su mm ary
3) Fro m NEC Tabl e 430.250 >>> a) Mos t co ns erv ati ve, si nc e IFL is u su all y hi gh er b) Avoid s installing condu ctors fo r high e ffi cie ncy mot or (low er IFL), but m ay b e too small f or a repla ceme nt low eff ici ency mot or (hig her I FL) c) This is s afety cons idera ti on t o pr event a fi re d) Use of I FL fr om t able is require d by NE C for sizing conductors e) For 100 Hp, 460 V mo tor, IFL = 124 A
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
B. Siz e Mot or Start er
U.S. us es st andard NE MA cl ass st arter s izes Main d if fere nce is in s ize of m oto r co ntactor
Moto r con tactor mu st be siz ed to carry f ull -load cur rent a nd starti ng in-rush cur rent (a bou t 5.5 x IFL)
Allows motor starter manufacturers to build starters wit h fewe r di ffere nt si ze con tactors
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
For 460 V, 3-phase mo to rs : NEMA Starter Size
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Max Hp
1
10
2
25
3
50
4
100
5
200
6
400
7
600
Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
For 208 V, 3-phase mo to rs : NEMA Starter Size
Max Hp
1
5
2
10
3
25
4
40
5
75
For s ame mo to r Hp, IFL is hi gh er f or 208 V vs . 460 V; th us , max Hp f or 208 V is lo wer
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
Size Mot or Start er Sum mary
For 100 Hp, 460 V, 3-phas e mo tor : Mot or st arter si ze = NEMA Size 4
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
C. Size Overc ur rent Pro tect io n, Brea ker
Circ uit bre aker c ome s w ith combi nation motor sta rter Size is base d o n t he mot or IFL
Min im um br eaker s ize = IFL x 125%
For 100 Hp, 460 V, 3-phas e mo tor ,
Min imum br eaker si ze = 124 A x 1.25 = 155 A
Next h ig her s tand ard avail able si ze = 175 A
Maxi mu m br eaker si ze >>> per N EC
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
NEC Tabl e 430.52 = Maxim um Rati ng or Setti ng of Moto r B ranch- Cir cui t Short -Cir cui t and Ground -Fault Pro tecti ve Devi ces
Depends on t ype of mo tor
Depend s o n t yp e of OCPD
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NEC Table 430.52, Maxi mum OCPD for Mot ors
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
Per NEC Table 430.52,
Maximu m OCPD fo r 100 Hp, 460 V mo tor = IFL x 250%
Maximu m br eaker si ze = 124 A x 2.5 = 310 A
Next h ig her s tand ard avail able si ze = 350 A
Why t he di ff erence?
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
Recall,
Min imu m b reaker s ize = 175 A Maxi mu m b reaker s ize = 350 A
To all ow f or m oto r st arti ng i n-rush = I FL x 5. 5
In-ru sh cu rrent = IFL x 5.5 = 124 A x 5.5 = 682 A
682 A exceeds 175 A and 350 A b reaker , bu t b reaker won’t trip duri ng norm al starting of about 5 se conds
Br eaker is i nv ers e ti me, no t in st ant aneous, a nd allows short- time ove rcurre nt conditions
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
D. Size Con du ct or s f or Cabl es
Cond uct ors m ust b e si zed to c arry ful l-loa d cur rent, continuously
Sizin g c ri teri a is based on IFL x 125%, again
For 100 Hp, 460 V, 3-phas e mo tor ,
Min im um co nd uc to r amp aci ty = 124 A x 1.25 = 155 A
NEC Tabl e 310.16 go vern s c on du ct or amp aci ty
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
NEC Tabl e 310.16 = Al lo wabl e Amp aci ti es of Ins ul ated Condu ct or s Rated 0 T hr ou gh 2000 Vol ts , 60°C (140°F Thr ough 194°F), Not Mor e Than Th ree Cur rent-C arr yi ng Con du ct or s i n Racewa y, Cabl e, or Eart h (Dir ect ly B ur ied) , Base d on Amb ient Temperat ure of 30°C (86°F)
incl ude s ampa citi es for copper a nd alumi num conductors
Standa rd engi neeri ng practic e = use C u c ond uct ors Incl ud es temp eratu re r ati ng s of 60°C, 75°C, and 90 °C
Use 75°C beca us e of ratin g o f d evi ce te rm in ati on s
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NEC Tabl e 310.16, Cond uc tor Am pacity
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NEC Tabl e 310.16, Cond uc tor Am pacity
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
The U.S. use s a non -uni versa l sys tem for identi fyi ng conduc tor siz es
AWG = American Wire Gage (higher the number, the small the con duc tor d iame ter)
kcmil = Thousand circ ula r m ils (based on cross secti on al area)
A more universal method is to identify conductor size s by the cros s-se cti onal a rea of the cond uct or, using square mill ime ters, or m m 2
NEC Chapt er 9, Tabl e 8, Con du ct or Pro pert ies, has a transla tion table
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NEC Chapt er 9, Tabl e 8, Con du ctor Pro pert ies
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NEC Chapt er 9, Tabl e 8, Con du ctor Pro pert ies
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
For 100 Hp, 460 V, 3-phas e mo tor ,
Min im um co nd uc to r amp aci ty = 124 A x 1.25 = 155 A Min im um co nd uc to r si ze = 2/0 AWG (67.43 mm 2)
Ampacity of 2/0 AWG (67.43 mm 2) = 175 A
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
Cabl es f or 480 V po wer ci rc ui ts are available wi th st and ard 60 0 V cl ass c abl es
Cables mus t b e sui tably r ated for dry , damp, or wet conditions
For above grou nd appli cations , dry and d amp r ated cables are a cc ept abl e
For und ergr oun d duct bank appl ic ati ons , dry and we t cabl es are essent ial
Many dif fere nt kin ds of 600 V ins ulatio n/ja cket t ype cabl es are avail able
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
The four mos t com mon 600 V cable s are a s f oll ows :
RHW = Flame -reta rd ant , mo is tu re-resis tant th erm os et THHN = Flame-retardant, heat-resistant, thermoplastic
THWN = Flame -reta rd ant , moi st ur e- and heatresistant, thermoplastic
XHHW = Flame -retard ant , moi st ur e-resi st ant , thermoset
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
Standa rd engi neeri ng practic e is to use he avy dut y cabl es fo r r eli abil it y and f ewer cha nces for failur es
For all po wer c ir cu it s, us e XHHW-2, 90°C wet and d ry (cross -link ed the rmosetting polyethyle ne insul ation )
For s mall li ght ing and r ecepta cle circ uit s, use THHN/THWN, 90°C dr y, 75°C wet
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
E. Size Gro und in g Condu ct or
Ground ing co nduct or is very, ve ry im port ant Requi red for gro und f ault return path to up str eam ci rc ui t br eaker (or OCPD)
Brea ker m ust sense the fa ult and t rip in o rder to c lear the fault
Or, if a fuse , the fus e eleme nt mus t m elt thr oug h
NEC Tabl e 250.122 go vern s th e min im um si ze of grounding conductors
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
NEC Tabl e 250.122 = Mini mu m Size Equi pm ent Grou ndi ng Cond uct ors f or Ground ing Ra ceway a nd Equipment
Standa rd engi neeri ng practic e is t o u se Cu conductors for both powe r and grounding
Size of gr oundin g cond uctor s is based on r ating of up st ream b reaker, fu se (or OCPD)
Why?
If gr oundi ng c onduct or i s to o sm all (and t herefore hi gh er im peda nc e), the O CPD may no t dete ct th e ground fault return
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NEC Tabl e 250.122, Gro un di ng Con du ctor s
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
For 100 Hp, 460 V, 3-phas e mo tor :
Mini mum si ze brea ker in starter = 175 A Next hi gh er s ize br eaker in NEC 250.122 = 200 A
Then, gro un di ng co nd uc to r = 6 AWG (13.30 mm 2)
Maxim um si ze brea ker in s tarte r = 350 A
Next hi gh er s ize br eaker in NEC 250.122 = 400 A
Then, gro un di ng co nd uc to r = 3 AWG (26.67 mm 2)
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
Min Max
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
For mo st mo tor applications, the mini mum s izing calcu latio n i s adequa te (us in g IFL x 125%)
Conc ern wo uld o nly b e wit h mot or st art ers t hat take an excessiv e amou nt o f ti me to start
Thu s, gro un di ng co nd uc to r = 6 AWG (13.30 mm 2)
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
F. Size Cond ui t f or Cabl es
Size of con dui t d epends on q uanti ty and s ize of cables insi de
Fir st, calcul ate cros s-se cti onal are a of all c ables i n the condui t
Dif fere nt cable manufa ctu rers pr odu ce ca bles wi th slig htly diff erent dia meters
If act ual c abl e data shee t is available, th en t ho se cable di amete rs can be us ed
If n ot , su ch as d ur in g d esi gn , th e NEC Tabl e is us ed
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
NEC Chapt er 9, Tabl e 5 = Dim ensi on s o f Insu lated Con du ct or s and Fix tu re Wir es, Typ e XHHW
Tabl e in cl ud es cable diame ter a nd c abl e cr os ssecti on al are a
Select cable cro ss -secti on al area si nc e we have to calcu late ba sed on cable a reas and c on du it areas
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NEC Chapt er 9, Table 5, Cabl e Dim ens ions
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
For 100 Hp, 460 V, 3-phas e mo tor , 2
Cir cuit = 3-2/0 AWG (67.43 mm ), 1-6 AWG (13.30 mm 2) GND
In one condui t
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
Per NEC Table: 2
Area of 2/0 AWG (67.43 mm ) cabl e = 141.3 mm Area of 6 AWG (13.30 mm 2) cab le = 38.06 mm 2 Tot al c ro ss -secti on al are a of all cables = 3 x 141.3 mm 2 + 1 x 38.06 mm 2 = 462.0 mm 2
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2
Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p 2
Next , sele ct mi ni mu m c on du it si ze fo r 462.0 mm tot al cable cross -section al area
Cri teria of mi nim um co ndu it i s gov erned by N EC Chapt er 9, Tabl e 1 = Perc ent of Cros s Sect io n o f Condui t and Tubing for Conduc tors
Very rare ly do es a ci rc ui t h ave onl y 1 or 2 cabl es (DC circuits)
Majo rit y o f c ir cui ts are ove r 2 ca bles
Thus, ma ximum cross sectio n of cable s to condui t is 40%, also kno wn as “ Fil l Facto r”
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of
NEC Chapt er 9, Table 1, Maximum Fi ll Factor
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
Why d oes the N EC li mi t t he fil l f act or to 40%?
Two major factors : 1) Cable D amage Duri ng Inst all ati on – If t he condu it has too m any cable s in t he con dui t, then the pul li ng tensi on i nc reases and t he cable coul d be da mage d with bro ken insulation
2) Therm al Heat Management – Heat emana tes f ro m cables whe n cu rrent flo ws throu gh t hem (I 2xR), and eleva ted t emp eratur es i nc reases resi st anc e and reduce s ampa city of condu ctor
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
Simi lar to ca bles, dif fere nt co ndu it m anuf actu rers produc e condui ts wi th sl ight ly di ffere nt di ameters
If actu al c on du it data sh eet i s ava il abl e, th en t ho se con dui t d iame ters can be used
If n ot , su ch as d ur in g d esi gn , th e NEC Tabl e is us ed
NEC Chapt er 9, Tabl e 4 = Dimens io ns and Perc ent Area of Conduit and Tubing, Article 344 – Rigid Metal Con du it (RMC) or Ar ti cl e 352 and 353 – Rig id PVC Cond ui t (PVC), Sched ul e 40
Standard engi neeri ng pr act ic e = 21 mm di amete r minimum condui t siz e
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NEC Chapt er 9, Tabl e 4, RMC Con du it Dim ensi on s
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NEC Chapter 9, Tabl e 4, PVC Con du it Dim ens ion s
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
RMC is us ually u sed abo ve gr ou nd and wh ere mecha nical prot ectio n i s r equir ed to prot ect the cabl es f rom damage
PVC = Pol y-Vin yl -Chl or ide
PVC is us uall y used in und ergr oun d duc tbanks
PVC Sch edu le 40 is th in ner w all th an Schedu le 80
Con cr ete enc asement aro un d PVC Sch edu le 40 pro vi de the me chanical prot ecti on, pa rt icu larly wh en tre nchi ng or diggi ng i s being pe rfor med late r
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
For the 100 Hp, 460 V, 3-ph ase mo to r, 2
Tot al c abl e area = 462.0 mm For RM C, a co nd ui t d iameter of 41 mm has an a rea of 1333 mm 2
Fill Fact or = Total Ca bl e Ar ea/Con du it Ar ea
Fill Fact or = 462 mm 2/1333 mm 2 = 34.7%
FF < 40%, and is co mp li ant wi th th e NEC
A larger conduit could be used: 53 mm = 2198 mm 2
Fill Fact or = 462 mm 2/2198 mm 2 = 21.0% >>> OK
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Sim pl e Cir cui t Design for 480 V, 100 Hp Pum p
For PVC, a co nd ui t d iameter of 41 mm has an a rea of 1282 mm 2 Not e th e area of 1282 mm 2 for PVC is sli ghtl y l ess than t he area of 1333 mm 2 fo r RMC
Fill Fact or = 462 mm 2/1282 mm 2 = 36.0%
FF < 40%, and is co mp li ant wi th th e NEC
A larger conduit could be used: 53 mm = 2124 mm 2
Fill Fact or = 462 mm 2/2124 mm 2 = 21.7% >>> Sti ll OK
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Vol tage Dro p Cons id eration s
For sh ort c irc uit l engths , volt age drop consi deration s will not a pply
But for lon ger le ngt hs, the in crea sed re sis tance in cables wil l affe ct v olt age dro p
If so , the condu cto rs s hou ld b e inc reased in size to min imi ze vol tage dro p
Con si der p revi ou s exampl e wit h t he 100 Hp, 460 V, 3pha se motor c ircu it
Cons ider tw o c ir cui t l engt hs: 25 mete rs, or 500 meters fo r ill ustr ation
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Vol tage Dro p Cons id eration s
Very basic fo rm ul a fo r Vdro p = (1.732 or 2) x I x L x Z/L
There are mor e exact f or mu las to u se, bu t t he goal is to calc ulate the a ppr oxi mate Vdro p t o t hen de termine if or ho w to com pensa te
For 3- ph ase ci rc ui ts : us e 1.732, Sq Rt (3)
For 1- phase ci rcu its : us e 2, for rou nd t rip length
Where, I = lo ad c ur rent (124 A f or 100 Hp pu mp ) Where , L = cir cu it lengt h (25 m or 500 m)
Where Z /L = im peda nc e per uni t l engt h
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Vol tage Dro p Cons id eration s
For Z/L d ata, us e NEC Chapt er 9, Table 9 = Alternating-Current Resistance and Reactance for 600-Vol t Cables, 3-Phase, 60 Hz, 75°C (167°F) – Thr ee Singl e Conduct ors in Condui t
For mos t appl icatio ns, a ssu me a pow er factor of 0.85
Then, th e co lu mn headi ng of “ Eff ect iv e Z at 0.85 PF for Uncoa ted Coppe r Wires” can be easil y used
Sub-colum ns in cl ude opt ion s for PV C con dui t, Aluminum conduit, and Steel conduit
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NEC Chapt er 9, Tabl e 9, Z fo r Con du ctor s
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Vol tage Dro p Cons id eration s
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Vol tage Dro p Cons id eration s
For st eel co nd ui t, Z/L = 0.36 ohms /ki lo mete r
For PV C con du it , Z/L = 0.36 oh ms /kil om eter Happens to be same Z /L
Oth er t abl e ent ri es are dif fere nt betwee n s teel and PVC fo r exa ct same si ze of co nd uc to r
The dif fere nce is due pri marily t o ind uct ance fr om intera ctio n wi th t he ste el c ondui t
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Vol tage Dro p Cons id eration s
For 100 Hp, 460 V, 3-ph ase mo tor , wi th L = 25 m:
Vdro p = 1.732 x I x L x Z/L Vdro p = 1.732 x 124 A x .025 km x 0.36 oh ms /km
= 1.94 V
Vdr op (%) = Vdr op /Sys tem Volt age
Vdrop (%) = 1.94 V/480 V = 0.4%
What i s c ri teria for excessiv e Vdro p?
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Vol tage Dro p Cons id eration s
The NEC doe s not dic tate Vdro p l imi tations
A lower than normal voltage at device is not a safety consi deration ; onl y ope ration al fun ctio nalit y of device
How ever, N EC has a Fine Pri nt Not e (FPN) that recommends a ma xi mum Vdro p of 5%
An FPN is optional, and not binding per the NEC
Thu s, Vdr op of 0.4% is acceptable NEC 210.19(A)(1) = Cond uc to rs -Min im um Am paci ty and Size, Gener al, FPN No. 4
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NEC 210.19(A)(1), FPN No. 4, Vo ltage Dr op, 3%
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Vol tage Dro p Cons id eration s
For 100 Hp, 460 V, 3-ph ase mo to r, wi th L = 500 m:
Vdro p = 1.732 x I x L x Z/L Vdro p = 1.732 x 124 A x .5 km x 0.36 oh ms /km
= 38.66 V
Vdr op (%) = Vdr op /Sys tem Volt age
Vdrop (%) = 38.66 V/480 V = 8.1%
Thi s Vdrop far e xc eeds th e 5% li mi t
How do we comp ensate for excessiv e Vdro p?
Page - 74
Vol tage Dro p Cons id eration s
To co mpensa te for exce ssi ve Vdro p, most c omm on method is to i ncrea se siz e of conduc tors Mus t inc rease si ze of pr evio us 2/0 AWG (67.43 mm 2) conduc tors , or lo wer imp edance of con ducto rs
Per NEC Chapt er 9, Tabl e 9, fo r 300 kc mi l (152 mm 2):
For st eel c on du it, Z/L = 0.213 oh ms /kil om eter
For PV C con du it , Z/L = 0.194 oh ms /kil om eter
Recalcu late V dr op wi th 300 kcm il (152 mm 2) conductors
Page - 75
Vol tage Dro p Cons id eration s
For 100 Hp, 460 V, 3-ph ase mo to r, wi th L = 500 m,
and wi th s teel co nduit : Vdro p = 1.732 x 124 A x .5 km x 0.213 ohm s/km = 22.87 V
Vdr op (%) = Vdr op /Sys tem Volt age
Vdrop (%) = 22.87 V/480 V = 4.7%
This Vdro p is now below the 5 % li mi t
Page - 76
Vol tage Dro p Cons id eration s
For 100 Hp, 460 V, 3-ph ase mo to r, wi th L = 500 m,
and w it h PVC con dui t: Vdro p = 1.732 x 124 A x .5 km x 0.194 ohm s/km = 20.83. V
Vdr op (%) = Vdr op /Sys tem Volt age
Vdrop (%) = 20.83 V/480 V = 4.3%
This Vdro p i s also b elow the 5 % li mit
Page - 77
Vol tage Dro p Cons id eration s
Wit h in cr eased c on du ct or s fr om 2/0 AWG (67.43 mm 2) to 300 kcm il (152 mm 2), the cond uit may no w b e to o s mall, result in g i n a FF exc eedi ng 40%
Per NEC Chapt er 9, Table 5:
Area of 300 kcmil (152 mm 2) cabl e = 292.6 mm 2
What about the pre vious groun ding conduc tor of 6 AWG (13.30 mm 2) cabl e?
Page - 78
Vol tage Dro p Cons id eration s
NEC requi res th at w hen in cr easi ng s ize of conduc tors to compensa te for vo ltage drop, the ground ing conduc tor must be incre ased in siz e by the same pro por ti on
NEC 250.122(B) = Size of Equi pm ent Gro un di ng Con du ct or s, Inc reased in Size
Page - 79
NEC 250.122(B), Increase Ground f or Vdrop
Page - 80
Vol tage Dro p Cons id eration s
Mus t c alc ul ate % in cr ease in cr os s-se ct io nal a rea of pha se conductor s
Then u se that same % incr ease for the gro und ing conductor
Inc rease fr om 2/0 AWG (67.43 mm 2) to 300 kc mi l (152 mm 2) = 152 mm 2/ 67.43 mm 2 = 225%
Inc rease of g ro und in g c on du ct or of 6 AWG (13.30 mm 2) by 225% = 13.30 mm 2 x 225% = 30.0 mm 2
Use NEC Chapt er 9, Tabl e 8, to select a con du ct or cl os e to 30.0 mm 2
Page - 81
NEC Chapt er 9, Tabl e 8, Con du ctor Pro pert ies
Page - 82
Vol tage Dro p Cons id eration s
NEC Chapt er 9, Table 8 sh ows t hat 2 AWG (33.62 mm 2) is cl os e to and excee ds t he ca lc ul ated value of 30.0 mm 2
In s ome case s, the inc rease in phase conduc tor may result in a very large %, especia ll y w hen s tarting wi th sma ll conductors
May b e pos si ble that a ppl yi ng that % incr ease re sul ts in a gro und ing c ond uct or l arger than the phase
conductors That d oesn’ t s oun d very r eason able
Page - 83
NEC 250.122(A), Li mi t Inc rease Grou nd for Vdr op
Page - 84
Vol tage Dro p Cons id eration s
Thus, fina l c ircu it adjus ted fo r v oltage drop = 2
2
3-300 kc mi l (152 mm ), 1-2 AWG (33.62 mm ) GND Now, ve ry u nli kely the pre vio us c ond uit size of 41 mm i n di amete r, or eve n t he ne xt si ze of 53 mm w il l be adequ ate t o keep FF less th an 40% Need t o r e-calc ul ate th e to tal c able area
Page - 85
Vol tage Dro p Cons id eration s
Page - 86
Vol tage Dro p Cons id eration s
Per NEC Chapt er 9, Tabl e 5: 2
2
Area of 300 kcmil (152 mm ) cabl e = 292.6 mm Area of 2 AWG (33.62 mm 2) cab le = 73.94 mm 2
Tot al c ro ss -secti on al are a of all cables = 3 x 292.6 mm 2 + 1 x 73.94 mm 2 = 951.7 mm 2
Need to re-calc ulate mini mum con dui t di ameter
Page - 87
NEC Chapt er 9, Tabl e 4, RMC Con du it Dim ensi on s
Page - 88
Vol tage Dro p Cons id eration s
Per NEC Chapt er 9, Table 4:
For RM C,2a co nd ui t d iameter of 53 mm has an a rea of 2198 mm
Fil l Fact or = 951.7 mm 2/2198 mm 2 = 43.3%
FF > 40%, and i s i n v io lati on of th e NEC
For RM C, a co nd ui t d iameter of 63 mm has an a rea of 3137 mm 2
Fil l Fact or = 951.7 mm 2/3137 mm 2 = 30.3% >> OK
Page - 89
NEC Chapter 9, Tabl e 4, PVC Con du it Dim ens ion s
Page - 90
Vol tage Dro p Cons id eration s
Per NEC Chapt er 9, Table 4:
For PVC, 2a co nd ui t d iameter of 53 mm has an a rea of 2124 mm
Fil l Fact or = 951.7 mm 2/2124 mm 2 = 44.8%
FF > 40%, and i s i n v io lati on of th e NEC
For PVC, a co nd ui t d iameter of 63 mm has an a rea of 3029 mm 2
Fil l Fact or = 951.7 mm 2/3029 mm 2 = 31.4% >> OK
Page - 91
Page - 92
Vol tage Rati ng s o f Motor /Starter & U ti li ty Sup pl y
Recall,
Uti li ty su pp ly = 480 V, no mi nal Moto rs and mot or starters r ati ng = 460 V
Why 20 V di ff erenc e?
Page - 93
Vol tage Rati ng s o f Motor /Starter & U ti li ty Sup pl y
To gi ve the mot or a chance to start u nder less th an nomina l conditions
Uti li ty can’t gu arantee 480 V at all ti mes
Heavily loa d uti lit y ci rcui ts reduce util ity volt age
Sometimes ha ve ca pacitor banks to b oos t vo lt age or auto t ap changin g tr ansf orm ers or vol tage regulators
Unless util ity has a history of po or v oltage deliv ery pr of il es, ass um e 480 V, or 1. 0 per u ni t (pu )
Page - 94
Vol tage Rati ng s o f Motor /Starter & U ti li ty Sup pl y
Assuming utility is 480 V, you have built-in 20 V mar gi n, or 460 V/480 V = 4.3% of vol tage mar gi n
Generally, mot ors require 9 0% volt age mini mum to start
Wit h r espect to mo to r: 460 V x 0.90 = 414 V is mini mum volt age at mo tor termin als to s tart
Wit h r espect t o u ti li ty sup ply : 480 V – 414 V = 66 V, or 414 V/480 V = 15.9% of voltage mar gin
Page - 95
Vol tage Rati ng s o f Motor /Starter & U ti li ty Sup pl y
Prefe r t o avoi d g ett in g n ear 414 V, ot herwi se ris k motor not sta rting
Account for lower utility voltage by design con sid eration beyon d 20 V margin
Hence, the 5 % vol tage dro p l imi t i s i mpo rt ant
Can’t c ontro l uti lit y suppl y vol tage, but can control design consi deration s
Page - 96
Page - 97
Let’ s Ad d a Seco nd 100 Hp Pum p
Ident ical 100 Hp, 460 V, 3-ph ase mo tor
Same ca bl es and co nd ui t, inc reased in s ize fo r Vdrop 3-300 kc mi l (152 mm 2), 1-2 AWG (33.62 mm 2) GND
But run in para llel to first ci rcui t
Why n ot com bin e all 7 cables int o on e large r conduit?
Note the gro und ing con duc tor can be share d
Pos si bl e, bu t t here are con sequences
Page - 98
Let’ s Ad d a Seco nd 100 Hp Pum p
The ma jor con sequence is coi nci dent hea ti ng effects on e ach ind ivid ual ci rcui t
Recall , heati ng effects of cur rent th rou gh a con duc tor gene rate s hea t i n th e for m of los ses = I 2xR
The NEC dict ates a mpacity derating for mul ti pl e circuits in one conduit
NEC Tabl e 310.15(B)(2)(a) = Adj us tment Fact or s f or Mor e Than T hr ee Cur rent-C arr yi ng Con du ct or s i n a Raceway o r Cabl e
Page - 99
Let’ s Ad d a Seco nd 100 Hp Pum p
Page - 100
Let’ s Ad d a Seco nd 100 Hp Pum p
Thu s, for 6 ca bl es i n on e co nd ui t, the de ratin g of 4-6 cables requi res a n amp aci ty d eratin g of 80%
The previ ou s ampaci ty of 285 A fo r 300 kcm il (152 mm 2) mus t b e derate d as fol low s:
4-6 cabl e derat ing = 285 A x 0.80 = 228 A
Previous l oad current has not ch anged: 124 A x 125% = 155 A
Derated amp aci ty of 228 A i s g reater t han 15 5 A If t here are 7 cable s i n t he condu it, why don ’t we use th e 2 nd li ne for 7-9 cabl es w it h a dera ti ng of 70%?
Page - 101
Let’ s Ad d a Seco nd 100 Hp Pum p
Because th e 7 th cable is a groundi ng c onduct or, a nd is there fore not a “ curr ent-ca rryi ng cond uctor ” New du al ci rc ui t = 3-300 kc mi l (152 mm 2), 1-2 AWG (33.62 mm 2) GND Previous condui t si ze of 63 mm i s no w pr oba bly too sm all and w il l r esul t i n a FF < 40% per NE C
Page - 102
Let’ s Ad d a Seco nd 100 Hp Pum p
Per NEC Chapt er 9, Tabl e 5: 2
2
Area of 300 kcmil (152 mm ) cabl e = 292.6 mm Area of 2 AWG (33.62 mm 2) cab le = 73.94 mm 2
Tot al c ro ss -secti on al are a of all cables = 6 x 292.6 mm 2 + 1 x 73.94 mm 2 = 1829.5 mm 2
Need to re-calc ulate mini mum con dui t di ameter
Page - 103
NEC Chapt er 9, Tabl e 4, RMC Con du it Dim ensi on s
Page - 104
Let’ s Ad d a Seco nd 100 Hp Pum p
Per NEC Chapt er 9, Table 4:
For RMC, th e pr evi ou s c on du it di amete r of 63 mm has an area of 3137 mm 2
Fil l Facto r = 1829.5 mm 2/3137 mm 2 = 58.3%
FF > 40%, and i s i n v io lati on of th e NEC
For RM C, a co nd ui t d iameter of 78 mm has an a rea of 4840 mm 2
Fil l Facto r = 1829.5 mm 2/4840 mm 2 = 37.8% >> OK
Page - 105
NEC Chapter 9, Tabl e 4, PVC Con du it Dim ens ion s
Page - 106
Let’ s Ad d a Seco nd 100 Hp Pum p
Per NEC Chapt er 9, Table 4:
For PVC, th e pr evi ou2 s c on du it di amete r of 63 mm h as an ar ea of 3029 mm
Fil l Facto r = 1829.5 mm 2/3029 mm 2 = 60.4%
FF > 40%, and i s i n v io lati on of th e NEC
For PVC, a co nd ui t d iameter of 78 mm has an a rea of 4693 mm 2
Fil l Facto r = 1829.5 mm 2/4693 mm 2 = 39.0% >> OK
Page - 107
Page - 108
Cabl e Temp eratu re Con si dera ti on s
Why?
As temperature of copper increases, the resistance increases
Common w hen condu it i s locate d in bo iler room or on roof in di rect sunlight
Volt age at l oad = Volt age at s our ce – Volt age drop i n cir cui t betwe en
Recall , E = I x R, wh ere I is co ns tant fo r l oad R in cr eases wit h tempera tu re, th ereby in cr easi ng Vdrop
Page - 109
Cabl e Temp eratu re Con si dera ti on s
Hig her amb ient t emp eratur e may dic tate larger conductor
NEC Tabl e 310.16 gov ern s dera ti ng of co nd uc to r amp aci ty du e to eleva ted tempera tu re
NEC Tabl e 310.16 = Al lo wabl e Amp aci ti es of Ins ul ated Condu ct or s Rated 0 T hr ou gh 2000 Vol ts , 60°C (140°F Thr ough 194°F), Not Mor e Than Th ree Cur rent-C arr yi ng Con du ct or s i n Racewa y, Cabl e, or Eart h (Dir ure ect lyofB30°C ur ied)(86°F) , Base d on Amb ient Temperat
This i s bot tom h alf o f previous ampa city table
Page - 110
NEC Tabl e 310.16, Conductor Temp Derati ng
Nominal
Page - 111
Cabl e Temp eratu re Con si dera ti on s
For ambient temperature between 36°C and 40°C, pr evi ou s ampacit y mu st b e dera ted to 0. 88 of nom inal a mpacity
The previ ou s ampaci ty of 285 A fo r 300 kcm il (152 mm 2) mus t b e derate d as fol low s:
Temperat ure der ati ng @ 36-40°C = 285 A x 0.88 = 250.8 A
Previous l oad current has not ch anged: 124 A x 125% = 155 A
Derated amp aci ty of 250.8 A i s gr eater t han 15 5 A
Page - 112
Cabl e Temp eratu re Con si dera ti on s
For ambient temperature between 46°C and 50°C, pr evi ou s ampacit y mu st b e dera ted to 0. 75 of nom inal a mpacity
The previ ou s ampaci ty of 285 A fo r 300 kcm il (152 mm 2) mus t b e derate d as fol low s:
Temperat ure der ati ng @ 46-50°C = 285 A x 0.75 = 213.8 A
Previous l oad current has not ch anged: 124 A x 125% = 155 A
Derated amp aci ty of 213.8 A i s gr eater t han 15 5 A
Page - 113
Cabl e Temp eratu re Con si dera ti on s
The two der ated amp aci ties of 250.8 A and 213.8 A, were bo th gr eater th an t he ta rg et ampacit y o f 155 A
We alr eady c om pensate d fo r Vdr op w it h larger conductors
If we ha d th e fi rst Vdro p exa mpl e wit h 25 m ci rcu it length, the con duc tor s mi ght have to b e inc reased du e to eleva ted temperature
Page - 114
Cabl e Temp eratu re Con si dera ti on s
Recall , target amp acit y = 155 A
Recall , no n-Vdr op co nd uc to r was 3-2/0 AWG (67.43 mm 2), 1-6 AWG (13.30 mm 2) GND
Recal l, amp aci ty of 2/0 AWG (67.43 mm 2) = 175 A
For der ati ng at 36°C to 40°C = 175 A x 0.88 = 154 A
Clo se eno ug h t o t arg et ampacit y o f 155 A, OK
But for second t empera tur e range :
For der ati ng at 46°C to 50°C = 175 A x 0.75 = 131 A
Ampacity is too low; must go to next size larger
Page - 115
Page - 116
What if Feeder is Part UG and Part AG?
Undergro und duc tbank has cool er t empera tur es
Aboveground can vary but will be worst case What if con duit r un is th rough bo th typ es?
NEC all ow s s electi ng lo wer U G amp aci ty
But ve ry re stri ctiv e
NEC 310.15(A)(2), Am paci ti es fo r Cond uc to rs Rated 0-2000 Vol ts , General, S elect ion of Am paci ty , Exception
NEC 10 ft o r 10%, wh ich ever i s l ess
Page - 117
What if Feeder is Part UG and Part AG?
Conduit Above Ground Page - 118
What if Feeder is Part UG and Part AG?
Conduit From Underground Page - 119
NEC 310.15(A)(2), Am paci ty in Mixed Con du it
Page - 120
What if Feeder is Part UG and Part AG?
NEC 310.15(A)(2), Except ion , says t o u se l ow er ampacity when dif fere nt ampaciti es apply
Howeve r, ca n u se high er ampa ci ty if secon d l engt h of con dui t after transit ion is less th an 3 me ters ( 10 ft) or t he length of the higher a mpacity c ond uit is 10 % of enti re ci rcu it , whi cheve r is l ess Higher Ampacity, 3 m (10 ft)
Lower Ampacity, 24 m (80 ft)
Page - 121
Page - 122
Sim pl e Cir cui t Desi gn fo r a 120 V, 1-Phase Lo ad
Dup lex receptacles a re gene rally co nv eni enc e recept acles f or mo st any 120 V, 1-phase load
Sing le loa ds l ik e a cop y machin e or refrig erator c an be pl ug ged int o a rece pt acl e
Est im ate refr ig erato r l oad demand = 1 000 VA
IFL = VA/V = 1000 VA/120 V = 8.33 A
IFL x 125% = 8.33 A x 1.25 = 10.4 A
Use NEC Tabl e 310.16 to select co nd uc to r si ze gr eater t han 10.4 A
Page - 123
NEC Tabl e 310.16, Cond uc tor Am pacity
Page - 124
Sim pl e Cir cui t Desi gn fo r a 120 V, 1-Phase Lo ad
Per NEC Table 310.16, 2
14 AWG (2.08 mm ) has an ampacit y o f 20 A 12 AWG (3.31 mm 2) has an ampacit y o f 25 A
Both would work
But st and ard engi neeri ng p racti ce is to u se 12 AWG (3.31 mm 2) mi nim um fo r all p ower- rela ted circu it s
Why?
To neglect a mb ient t emp eratur e by b ein g conserva tive for s impl icit y wi th b uilt -in 25 % margin
Page - 125
Sim pl e Cir cui t Desi gn fo r a 120 V, 1-Phase Lo ad
Select ci rc ui t br eaker b ased on IFL x 125% = 10.4 A
Br eaker mus t always b e equ al t o or gr eater th an l oad curr ent to pr ote ct the conductor
At 120 V, smallest panelboard breaker is 15 A
Next avail able larg er s ize is 20 A
For sm all mo ld ed case br eakers, mus t d erate maxim um all ow abl e amp eres to 80% of br eaker rati ng
Br eaker d erati ng : 15 A x 0.80 = 12 A max all ow abl e
Br eaker d erati ng : 20 A x 0.80 = 16 A max all ow abl e
Page - 126
Sim pl e Cir cui t Desi gn fo r a 120 V, 1-Phase Lo ad
Why?
Biggest re ason i s t hat a continu ous l oad 2tends to bui ld up heat i n t he bre aker, ca used by I R
The overlo ad eleme nt in a sm all mo ld ed c ase brea ker is a bimeta ll ic s tri p of dis sim il ar meta ls t hat sepa rate whe n t he curre nt flow ing thru the m exce eds i ts rating
The eleva ted t emp eratur e over t im e can ch ang e the resistance of th e metals a nd m ove clos er t o t he actual trip po int
At 15 A or 20 A, the manufacturing tolerances on the trip po int is no t accurate
Page - 127
Sim pl e Cir cui t Desi gn fo r a 120 V, 1-Phase Lo ad
Need t o be con servativ e and pr event nu is anc e tripping
Select 20 A b reaker
Standard e ng in eeri ng p racti ce is to us e 20 A br eakers r egardl ess of th e lo ad demand
That i ncl udes a loa d t hat r equi res onl y 1 A
Why?
Page - 128
Sim pl e Cir cui t Desi gn fo r a 120 V, 1-Phase Lo ad
Overcurr ent pro tection ind eed m ay b e 5 A extra in select in g a 20 A b reaker
This r eall y onl y aff ects o verload condi ti ons w hen the dema nd cu rr ent exc eeds 15 A or 20 A
Under shor t ci rcu it c ond it ion s, sa y 2000 A of f ault cur rent, bot h brea kers wil l vi rt uall y tr ip at the same time
Refri gera tor is very un li kely t o d raw say, 12 A, because it s m ax d emand is 8.33 A
Page - 129
Sim pl e Cir cui t Desi gn fo r a 120 V, 1-Phase Lo ad
If t he compr esso r m oto r w ere to lo ck up and fr eeze, tha t wou ld no t rea lly be a short circ uit
But the curr ent flo w to the compr essor mot or woul d be abo ut 5.5 ti mes t he IFL (or th e same when t he motor s tarts on in -rush)
Mot or lo ck ed r ot or cu rr ent is th en 5.5 x 8.33 A = 45.8 A
This excee ds bot h 15 A or 20 A, with or w it hou t t he 80% derati ng
Page - 130
Sim pl e Cir cui t Desi gn fo r a 120 V, 1-Phase Lo ad
If all br eakers i n a pane lb oard w ere 20 A, then it wou ld be easy to swap out if brea ker f ail s
Or u se a 20 A sp are brea ker in st ead o f w or ry in g abou t a 15 A br eaker be ing too small i n t he futur e
Cos t d if fere nt ial is tr iv ial betwee n 15 A and 2 0 A breakers
Use NEC Tabl e 250.122 to select gr ou nd in g conductor
Page - 131
NEC Tabl e 250.122, Gro un di ng Con du ctor s
Page - 132
Sim pl e Cir cui t Desi gn fo r a 120 V, 1-Phase Lo ad
Gro un di ng co nd uc to r i s 12 AWG (3.31 mm 2) based on br eaker r ati ng of 20 A Cir cuit = 2-12 AWG (3.31 mm 2), 1-12 AWG (3.31 mm 2) GND
Recall , for small l igh ti ng and rece ptacle ci rcu it s, use THHN/THWN, 90°C dr y, 75°C wet
This ti me we us e NEC Chapt er 9, Tabl e 5, fo r Type THHN/THWN cabl e
Page - 133
Sim pl e Cir cui t Desi gn fo r a 120 V, 1-Phase Lo ad
Page - 134
Sim pl e Cir cui t Desi gn fo r a 120 V, 1-Phase Lo ad
Per NEC Table: 2
2
Area of 12 AWG (3.31 mm ) cab le = 8.581 mm Tot al c ro ss -secti on al are a of all cables = 2 x 8.581 mm 2 + 1 x 8.581 mm 2 = 25.7 mm 2
Use NEC Chapt er 9, Tabl e 4 to select co nd ui t s ize
Page - 135
NEC Chapt er 9, Tabl e 4, RMC Con du it Dim ensi on s
Page - 136
Sim pl e Cir cui t Desi gn fo r a 120 V, 1-Phase Lo ad
Per NEC Chapt er 9, Table 4:
For RM C, a co nd ui t d iameter of 16 mm has an a rea of 204 mm 2
Fill Fact or = 25.7 mm 2/204 mm 2 = 12.6%
FF < 40%, OK
For RM C, a co nd ui t d iameter of 21 mm has an a rea of 353 mm 2
Fill Fact or = 25.7 mm 2/353 mm 2 = 7.3%, OK
Page - 137
NEC Chapter 9, Tabl e 4, PVC Con du it Dim ens ion s
Page - 138
Sim pl e Cir cui t Desi gn fo r a 120 V, 1-Phase Lo ad
Per NEC Chapt er 9, Table 4:
For PVC,2 a co nd ui t d iameter of 16 mm has an a rea of 184 mm
Fill Fact or = 25.7 mm 2/184 mm 2 = 14.0%
FF < 40%, OK
For PVC, a co nd ui t d iameter of 21 mm has an a rea of 327 mm 2
Fill Fact or = 25.7 mm 2/327 mm 2 = 7.9%, OK
Page - 139
Sim pl e Cir cui t Desi gn fo r a 120 V, 1-Phase Lo ad
Bot h c ond uit diame ters of 16 mm and 21 mm, for bo th RMC and PVC woul d w or k
Standard e ng in eeri ng p racti ce is to us e 21 mm conduits for a ll circuit s
Why?
Allows future addition of cables
Cos t di ff erenti al i s tr iv ial betwee n 16 mm and 21 mm conduits
Page - 140
Sim pl e Cir cui t Desi gn fo r a 120 V, 1-Phase Lo ad
Also prevents poor workmanship by installer when bending condu it
Need a co nd ui t bender th at pr od uc es ni ce even ang led s weep aro un d 90 degr ees
Small diame ter con dui t c an easi ly be be nt too sharply and pinc h the condu it, the reby re duc ing t he availa ble cr oss -section al area of the con dui t
Page - 141
Panelbo ard Desi gn
The 20 A br eakers f or th e du pl ex r eceptacles wo ul d be cont ain ed i n a pane lb oard
There a re 3-ph ase panelb oar ds : 208Y/120 V fed f rom 3-ph ase tr ans fo rm ers
Where, 208 V is t he ph ase-to-p hase vo ltage, or 120 V x 1.732 = 208 V
Page - 142
Panelbo ard Desi gn
Page - 143
Panelbo ard Desi gn
There a re 1-ph ase panelb oar ds : 120/240 V fed fr om 1phase tr ansf orm ers
Where, 240 V is th e phase- to -phase vol tage wi th a center- tapped neutr al
Phase A to neut ral i s 120 V
Phase B to neut ral i s 120 V
Phase A to Phase B is 240 V
Selection of panelbo ard d epends on typ e of loads to be po were d
Page - 144
Panelbo ard Desi gn
If all lo ads are 120 V, then e it her pane lb oard w oul d suffice
If so me lo ads are 240 V, 1-phase, li ke a small air co nd it io ner, th en yo u need th e 120/240 V, 1-phase panelboard
If so me lo ads are 208 V, 3-phase, li ke a fa n o r pu mp , then y ou need t he 208Y/120 V, 3-ph ase panelb oar d
Giv en a ch oi ce on lo ad vo lt age requi reme nt s, the 208Y/120 V, 3-ph ase panelb oar d al low s m or e flexibilit y with a sma ller continuo us bus rating in amperes
Page - 145
Panelbo ard Sch edu le Calc ul ati on
1 of 3
2 of 3
3 of 3 Page - 146
Panelbo ard Desi gn
View 1 of 3:
Each lo ad i s entered in th e sp reads heet Each l oad’s dema nd VA is ent ered in to th e spreadsheet
Each load’s brea ker is entere d w it h t ri p r ati ng and 1, 2, or 3 po les (120 V or 208 V)
Page - 147
Panelbo ard Sch edu le Calc ul ati on
Page - 148
Panelbo ard Desi gn
View 2 of 3:
Tot al L1, L2, and L3 VA l oads at b ot to m Total both s ides of V A load subt otals a t bot tom
Page - 149
Panelbo ard Sch edu le Calc ul ati on
Page - 150
Panelbo ard Desi gn
View 3 of 3:
Add all VA loads for entire panelboard Calcu late con ti nuo us c urr ent d emand
Mult ipl y by 12 5% to c alc ulate min im um cu rrent bu s rating
Select n ext available bus ratin g si ze
Page - 151
Panelbo ard Sch edu le Calc ul ati on
Page - 152
Page - 153
TVSS Desi gn
TVSS = Trans ient Vol tage Sur ge Sup pr ess io n
A TVSS unit is designed to protect downstream equi pment fr om t he dama gin g eff ects of a high vol tage spi ke or t ransient
The TVSS uni t esse nti all y c li ps t he higher por tio ns of the volt age spi ke and sh unt s th at ene rgy to g rou nd
Thu s, the T VSS un it sh ou ld be size d t o acco mm od ate hig her l evels of energy
The sma ll m ulti ple outle t st rip for y our ho me televisi on or comput er is simi lar but not t he same
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TVSS Desi gn
Energy leve l d epends on where in the power sy stem yo u pl ace th ese TVSS uni ts
The low er i n t he power sy st em t he TVSS un it is loc ated, the le ss l ikely t he vol tage spi ke will be hig h
Some of t he energy is di ssi pated throu gh variou s transfo rmers a nd l engt hs of cable s, or imp edance
Howeve r, it wou ld be prudent enginee rin g t o alwa ys pl ace a TVSS un it in fr on t o f each pane lb oard f or addit ional protection f or all loads fe d fr om t he panelboard
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TVSS Desi gn
Cost is not grea t f or TVSS uni ts
Prud ent in vestme nt fo r ins urance to pr otect loads More impo rt ant i s pl acin g TVSS uni ts f urt her upstr eam i n po wer s ystem to prot ect all loa ds
480 V sw it ch gear, 480 V mo to r c on tr ol cent er, 480 V panelbo ard , 208 V panelb oard , etc .
Import ant to have LE D ligh ts indi cating funct ionality of TVSS uni t
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Shor t Circ uit Impa ct on Conduc tor s
The availa ble sho rt cir cui t c an h ave an i mpact o n t he siz e of t he conduc tors in ea ch ci rcui t
The upst ream brea ker or fus e must clea r the fault before the conducto r burns up
The “ time to burn” depends on the siz e of the conduc tor and t he available short circ uit
Most impor tant: the highe r th e short circ uit, the qui cker the fa ult mus t b e cl eared
Okon it e has an e xcellent table that sho ws thi s relationship
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Shor t Circ uit Impa ct on Conduc tor s
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4000 A Sho rt Cir cu it
Must clea r f ault wit hin 100 cycles or 1.667 sec
1 AWG (42.41 mm 2) Page - 161
10000 A Shor t Cir cu it
Must clea r fault with in 16 cycl es or 0.267 sec
1 AWG (42.41 mm 2) Page - 162
10000 A Short Circui t
Must clea r fault wit hi n 100 cy cles or 1.67 sec
4/0 AWG (107.2 mm 2) Page - 163
Shor t Circ uit Impa ct on Conduc tor s
For s ame short ci rcui t, la rger conduct or allows more ti me to cl ear f aul t
Must sele ct pro per brea ker size , or adju st tri p s ett ing if adjust able bre aker to cle ar f ault wit hin the “ burn through” tim e
Same for fu ses wh en f us es are us ed
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