Derating adjustment factors

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rybrama

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on a 4 wire 208/120 v 3 phase wye, if pulling 2 wyes of # 12 thhn stranded for receptacles and flourescent lighting, is the 90degree C rating of 12 thhn to be used or the 75 degree rating of the receptacle terminal
 

Dean83169

Senior Member
Its based on the weakest link. If the outlet or lug is 75 Degree's then the wire has to be based on that. I have had problems with inspectors because they have seen the breaker rated at 90 but the lugs were rated at 75, He had me size according to the 75, but some inspectors do not inforce it.
 

stew

Senior Member
i am assuming by your terminology of 2 ys you mean 2 full boats in which case you have 6 conductors. the conductors then must be derated per 310.15(B)(2)a. to derate when using thhn start with the 90 degree rating of the conductor which is 30 amps, derate at 80% to 24 amps and there you are. 20 amp breaker is just fine.
 
stew said:
i am assuming by your terminology of 2 ys you mean 2 full boats in which case you have 6 conductors. the conductors then must be derated per 310.15(B)(2)a. to derate when using thhn start with the 90 degree rating of the conductor which is 30 amps, derate at 80% to 24 amps and there you are. 20 amp breaker is just fine.

you got that part right but i dont know why not shift the table from 90degree rating to 75 degress rating because the OP did mention 75 C at the repectale terminal location therefore you squeak by at 20 amp with 12 gauge THHN's

note here if i did make a mistake please corret me i did use the metric verison and converted to the standard size


Merci, Marc
 

infinity

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rybrama said:
on a 4 wire 208/120 v 3 phase wye, if pulling 2 wyes of # 12 thhn stranded for receptacles and flourescent lighting, is the 90degree C rating of 12 thhn to be used or the 75 degree rating of the receptacle terminal


Neither. You would need to use a maximum of 20 amps for #12 THHN since 240.4(D) limits it's ampacity to 20 amps for your application. For derating purposes you may use the 90 degree rating but it's adjusted ampacity can not exceed 20 amps.
 
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ray cyr

Senior Member
Location
Yakima, Wash.
I disagree, the wires ampacity, after applying the proper derating factors, is 24. The wire can safely disapate the heat of 24 amperes of current. The fact that the ocpd can not be rated more than 20 amps for a #12 simply means that the wire will be very well protected against overheating due to too much current since the wire can safely carry more current than the ocpd will allow for the circuit.
 
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