Cablesizer | Cable Sizing to IEC and NEC Standards H e lp N e w Ca ble 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: http://www.cablesizer.com/help/principles/[3/13/2013 11:24:25 AM] 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 http://www.cablesizer.com/help/principles/[3/13/2013 11:24:25 AM] 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 : http://www.cablesizer.com/help/principles/[3/13/2013 11:24:25 AM] 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 Copyright © 2012 http://www.cablesizer.com/help/principles/[3/13/2013 11:24:25 AM] About Disclaimer Contact Us
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