Cablesizer | Cable Sizing to IEC and NEC Standards
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Principles of Cable Sizing
1. Introduction
Cable ( or conduct or) sizing is t he process of select ing appropriat e sizes for elect rical power cable
conduct ors. Cable sizes are t ypically decribed in t erm s of cross- sect ional area, Am erican Wire Gauge
( AWG) or kcm il, depending on geographic region.
The proper sizing of cables is im port ant t o ensure t hat t he cable can:
Operat e cont inuously under full load wit hout being dam aged
Provide t he load wit h a suit able volt age ( and avoid excessive volt age drops)
Wit hst and t he worst short circuit s current s flowing t hrough t he cable
Cable sizing m et hods do differ across int ernat ional st andards ( e.g. I EC, NEC, BS, et c) and som e st andards
em phasise cert ain t hings over ot hers. However t he general principles t hat underpin all cable sizing
calculat ion do not change. When sizing a cable, t he following general process is t ypically followed:
Gat her dat a about t he cable, it s inst allat ion condit ions, t he load t hat it will carry, et c
Det erm ine t he m inim um cable size based on am pacit y ( cont inuous current carrying capacit y)
Det erm ine t he m inim um cable size based on volt age drop considerat ions
Det erm ine t he m inim um cable size based on short circuit t em perat ure rise
Select t he cable based on t he highest of t he sizes calculat ed in t he st eps above
2. Data Gathering
The first st ep is t o collat e t he relevant inform at ion t hat is required t o perform t he sizing calculat ion.
Typically, you will need t o obt ain t he following dat a:
( 1 ) Ba sic ca ble da t a - t he basic charact erist ics of t he cable's physical const ruct ion, which includes:
Conduct or m at erial - e.g. copper or alum inium
I nsulat ion or cable t ype - e.g. PVC, XLPE, EPR ( for I EC cables) , TW, THHW, XHH, et c ( for NEC cables)
Num ber of cores - single core or m ult icore ( e.g. 2C, 3C or 4C)
( 2 ) Loa d da t a - t he charact erist ics of t he load t hat t he cable will supply, which includes:
Num ber of phases, e.g. t hree phase or single phase
Syst em / source volt age
Full load current ( A) - or calculat e t his if t he load is defined in t erm s of power ( kW)
Full load power fact or ( pu)
Dist ance / lengt h of cable run from source t o load - t his lengt h should be as close as possible t o t he
act ual rout e of t he cable and include enough cont ingency for vert ical drops / rises and t erm inat ion of
t he cable t ails
( 3 ) Ca ble in st a lla t ion - how t he cable will be inst alled, which includes:
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Cablesizer | Cable Sizing to IEC and NEC Standards
I nst allat ion m et hod - e.g. cable t ray / ladder, in conduit / raceways, against a wall, in air, direct ly
buried, et c
Am bient or soil t em perat ure at t he inst allat ion sit e
Cable grouping, i.e. t he num ber of ot her cables t hat are bunched t oget her or inst alled in t he sam e
area
Cable spacing, i.e. whet her cables are inst alled t ouching or spaced
Soil t herm al resist ivit y ( for underground cables)
For single core t hree- phase cables, are t he cables inst alled in t refoil or laid flat ?
3. Cable Selection Based on Ampacity
Current flowing t hrough a cable generat es heat t hrough t he resist ive losses in t he conduct ors, dielect ric
losses t hrough t he insulat ion and resist ive losses from current flowing t hrough any cable screens / shields
and arm ouring.
A cable's const it uent part s ( part icularly t he insulat ion) m ust be capable of wit hst anding t he t em perat ure
rise and heat em anat ing from t he cable. The am pacit y of a cable is t he m axim um current t hat can flow
cont inuously t hrough a cable wit hout dam aging t he insulat ion. I t is som et im es also referred t o as t he
cont inuous current rat ing or current carrying capacit y of a cable.
Cables wit h larger conduct or cross- sect ional areas ( i.e. m ore copper or alum inium ) have lower resist ive
losses and are able t o dissipat e t he heat bet t er t han sm aller cables. Therefore a 16 m m 2 ( or 6 AWG) cable
will have a higher am pacit y t han a 4 m m 2 ( or 12 AWG) cable.
3.1 Base Ampacities
I nt ernat ional st andards and m anufact urers of cables will quot e base am pacit ies for specific t ypes of cable
const ruct ions ( e.g. copper conduct or, PVC insulat ed, 0.6/ 1kV volt age grade, et c) and a base set of
inst allat ion condit ions ( e.g. am bient t em perat ure of 40°C, inst allat ion in conduit / raceways, et c) . I t is
im port ant t o not e t hat t hese am pacit ies are only valid for t he quot ed t ypes of cables and base inst allat ion
condit ions.
3.2 Installed / Derated Ampacities
When t he proposed inst allat ion condit ions differ from t he base condit ions, derat ing ( or correct ion) fact ors
can be applied t o t he base am pacit ies t o obt ain t he act ual inst alled current rat ings.
I nt ernat ional st andards and cable m anufact urers will provide derat ing fact ors for a range of inst allat ion
condit ions, for exam ple am bient / soil t em perat ure, grouping or bunching of cables, soil t herm al resist ivit y,
et c. The inst alled current rat ing is calculat ed by m ult iplying t he base current rat ing wit h each of t he
derat ing fact ors, i.e.
I c = I b . kd
where I c is t he inst alled / derat ed am pacit y of t he cable ( A)
I b is t he base cable am pacit y ( A)
k d are t he product of all t he derat ing fact ors
For exam ple, suppose a cable had an am bient t em perat ure derat ing fact or of k am b = 0.94 and a grouping
derat ing fact or of k g = 0.85, t hen t he overall derat ing fact or k d = 0.94x0.85 = 0.799. For a cable wit h a
base am pacit y of 42A, t he inst alled / derat ed am pacit y would be I c = 0.799x42 = 33.6A.
4. Cable Selection Based on Voltage Drop
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Cablesizer | Cable Sizing to IEC and NEC Standards
A cable's conduct or can be seen as an im pedance and as a result , whenever current flows t hrough a cable,
t here will be a volt age drop across it , derived by Ohm 's Law ( i.e. V = I Z) . The volt age drop will depend on
t wo t hings:
Current flow t hrough t he cable - t he higher t he current flow, t he higher t he volt age drop
I m pedance of t he conduct or - t he larger t he im pedance, t he higher t he volt age drop
The im pedance of t he cable is a funct ion of t he cable size ( cross- sect ional area) and t he lengt h of t he
cable. Most cable m anufact urers will quot e a cable's resist ance and react ance in Ohm s/ km or Ohm s/ ft .
For AC syst em s, t he m et hod of calculat ing volt age drops based on load power fact or is com m only used.
Full load current s are norm ally used, but if t he load has high st art up current s ( e.g. m ot ors) , t hen volt age
drops based on st art ing current ( and power fact or if applicable) should also be calculat ed.
For a t hree phase syst em :
For a single phase syst em :
Where V is t he t hree phase or single phase volt age drop ( V)
I is t he nom inal full load or st art ing current as applicable ( A)
R c is t he ac resist ance of t he cable ( Ohm s/ km or Ohm s/ ft )
Xc is t he ac react ance of t he cable ( Ohm s/ km or Ohm s/ ft )
\ cos\ phi is t he load power fact or ( pu)
L is t he lengt h of t he cable ( m or ft )
When sizing cables for volt age drop, a m axim um volt age drop is specified, and t hen t he sm allest cable size
t hat m eet s t he volt age drop const raint is select ed. For exam ple, suppose a 5% m axim um volt age drop is
specified. 16m m 2 , 25m m 2 and 35m m 2 cables have calculat ed volt age drops of 6.4% , 4.6% and 3.2%
respect ively. The 25m m 2 cable is select ed as it is t he sm allest cable t hat fulfils t he m axim um volt age drop
crit eria of 5% .
Maxim um volt age drops are t ypically specified because load consum ers ( e.g. appliances) will have an input
volt age t olerance range. This m eans t hat if t he volt age at t he appliance is lower t han it s rat ed m inim um
volt age, t hen t he appliance m ay not operat e correct ly.
I n general, m ost elect rical equipm ent will operat e norm ally at a volt age as low as 80% nom inal volt age.
For exam ple, if t he nom inal volt age is 230VAC, t hen m ost appliances will run at > 184VAC. Cables are
t ypically sized for a m ore conservat ive m axim um volt age drop, in t he range of 5 t o 10% at full load.
5. Cable Selection Based on Short Circuit Temperature Rise
N ot e t hat short circuit t em perat ure rise is not required for cable sizing t o NEC st andards.
During a short circuit , a high am ount of current can flow t hrough a cable for a short t im e. This surge in
current flow causes a t em perat ure rise wit hin t he cable. High t em perat ures can t rigger unwant ed react ions
in t he cable insulat ion, sheat h m at erials and ot her com ponent s, which can prem at urely degrade t he
condit ion of t he cable. As t he cross- sect ional area of t he cable increases, it can dissipat e higher fault
current s for a given t em perat ure rise. Therefore, cables should be sized t o wit hst and t he largest short
circuit t hat it is expect ed t o see.
The m inim um cable size due t o short circuit t em perat ure rise is t ypically calculat ed wit h an equat ion of t he
form :
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Cablesizer | Cable Sizing to IEC and NEC Standards
Where A is t he m inim um cross- sect ional area of t he cable ( m m 2 )
i is t he prospect ive short circuit current ( A)
t is t he durat ion of t he short circuit ( s)
k is a short circuit t em perat ure rise const ant
The t em perat ure rise const ant is calculat ed based on t he m at erial propert ies of t he conduct or and t he
init ial and final conduct or t em perat ures. I EC 60364 - 5 - 54 calculat es it as follows:
For copper cables:
For alum inium cables:
Where \ t het a i and \ t het a f are t he init ial and final conduct or t em perat ures respect ively.
As a rough guide, t he following t em perat ures are com m on for t he different insulat ion m at erials:
M a t e r ia l
M a x Ope r a t in g
Te m pe r a t u r e
oC
Lim it in g
Te m pe r a t u r e
oC
PVC
75
160
EPR
90
250
XLPE
90
250
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