Zone4info.com Two basic factors that influence the sizing of conductors feeding a transformer Two basic factors that influence the sizing of conductors feeding a transformer Posted by Zone 4info April 28 Two basic factors that influence the sizing of conductors feeding a transformer follow: Size the conductors adequately to carry the expected primary current of the transformer. Determine allowable ampacity of conductors in accordance with 310.15 Protect conductors in accordance with NEC 240.4 or 240.100. For circuits operating at or below 600 V, the conductors will generally be larger than the transformer FLA to be protected by the transformer primary OCP in accordance with Section 240.4. In the case of circuits operating above 600 V, the rules in Section 240.100 permit the conductor to be protected by a fuse not greater than three times the conductor rating or a circuit breaker not greater than six times the

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5/14/2015 Zone4info.comTwobasicfactorsthatinfluencethesizingofconductorsfeedingatransformer

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Zone4info.comTwobasicfactorsthatinfluencethesizingofconductorsfeedingatransformer

Twobasicfactorsthatinfluencethesizingofconductorsfeedingatransformer

PostedbyZone4infoApril28

Twobasicfactorsthatinfluencethesizingofconductorsfeedingatransformerfollow:

Sizetheconductorsadequatelytocarrytheexpectedprimarycurrentofthetransformer.Determineallowableampacityofconductorsinaccordancewith310.15

ProtectconductorsinaccordancewithNEC240.4or240.100.

Forcircuitsoperatingatorbelow600V,theconductorswillgenerallybelargerthanthetransformerFLAtobeprotectedbythetransformerprimaryOCPinaccordancewithSection240.4.Inthecaseofcircuitsoperatingabove600V,therulesinSection240.100permittheconductortobeprotectedbyafusenotgreaterthanthreetimestheconductorratingoracircuitbreakernotgreaterthansixtimesthe
5/14/2015 Zone4info.comTwobasicfactorsthatinfluencethesizingofconductorsfeedingatransformer

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conductorrating.Thismatcheswellwiththepermittedtransformerprotectionin450.3(A).

IntheexampleshowninFigure5,theconductorssupplyingthetransformerneedtobeatleast4/0Cu(basedonTable310.16),ifequipmentwith75Cterminationsisassumed.

Twobasicfactorsinfluencetransformersecondaryconductors:

Adequateampacitytocarrytheloadsuppliedbythesecondaryofthetransformer.Determinedinaccordancewith310.15.

Inapplicationsof600Vorless,overcurrentprotectionistypicallylocatedattheloadendofthefieldinstalledconductors,NEC240.21(C)(orPartVIIIofArticle240forsupervisedindustrialinstallations)hasasignificantinfluenceonthesizeoftheseconductors

Forapplicationsover600V,rulessimilartothosein240.21(C)arenotavailable.OCPmustbeprovidedwheretheseconductorsoriginateorasensing/relayingschemewillhavetobeusedtoopenadeviceaheadoftheseconductors.Inthecaseofconductorsconnecteddirectlytothetransformersecondary,itmaybenecessarytoopentheovercurrentdeviceontheprimaryofthetransformerwhenthesecondaryovercurrentsensingmeansdetectanovercurrentcondition.NEC240.100indicatesthatthelocationoftheovercurrentprotectioncanbedeterminedunderengineeringsupervision

Therulesin240.21(C)areapplicableinsituationswherethesecondaryvoltageisatorbelow600V.Theserulesintendtodealwiththelocationoftheovercurrentprotection,butconductorsizeisinfluencedwhentherulesareapplied.Therulesare,inapracticalsense,asetoftradeoffs.Thedesigner/installermustmeetsomeadditionalcriteriaiftheovercurrentprotectionisnotgoingtobelocatedatthepointwheretheconductorsreceivetheirsupply.Giventhatsomelengthofconductorisnecessarytoconnectfrommosttransformersecondaries,therulesin240.21(C)recognizeacceptablemeanstoprovidethatconductorlength

240.21(C)doesnotlimitthenumberoftapsthatarepermittedtobemadetoatransformersecondary.Itdoesspecifytherulesnecessaryforeachtap

Toseehoweachoftheruleswillbeappliedwewillusethefollowingparameters:

Transformerprimary480V,90ATransformersecondary240V,180AAllequipmentterminationsareassumedtobe75C

Reference:Schneider
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