transformer primary feeder OCPD

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don_resqcapt19

don_resqcapt19

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In a recent thread there was a brief discussion of the code rules as far as the sizing of the OCPD that protects the conductors. The example was a 112.5kva 480/208Y/120 transformer with a primay FLA of 135 amps, 1/0 primary conductors and a 300 amp OCPD.

It is my postition that there are no code provisions that would permit you to protect the 1/0 primary conductors above their ampacity. If you want to use a 300 amp OCPD to protect the primary side of the transformer as permitted in 450.3(B) you would have to increase the size of the primary conductors to 300 kcmil.

It is my opinion that the overcurrent protection rules in Article 450 apply only to the protection of the transfomer itself and you have to look to the rules in Article 240 for the protection of the primay and secondary conductors.
 
I agree. I would add that FPN #1, immeciately following paragraph 450.3, appears to confirm this viewpoint.
 
I agree completely. That discussion ended prematurely. The 450.3 discusses Overcurrent Protection for Transformers. The FPN #1 in 450.3 says "See 240.4, 240.21, 240.100 & 240.101 for overcurrent protection of conductors."
 
I agree as well.

I had a job in the past where the EE specified 3 AWG to supply a 75 kVa transformer which was enough to carry the load of the transformer but he had us land those 3 AWGs on a 225 amp breaker.

When I asked about it he went on about it being OK because of the turns ratio. :confused:
 
iwire said:
When I asked about it he went on about it being OK because of the turns ratio. :confused:
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Perhaps he didn't notice the fact that the conductor in question is in between the breaker and the first of the turns. :roll:
 
FWIW I agree as well.

I have argued many times that the rules in 450.3 only apply to the transformer itself not to the primary or secondary conductors.

Chris
 
I also am in total agreement.

My question is whether the conductors between the OCPD and the transformer are classified as a "feeder" as the thread title indicates. Aren't they really a "branch circuit"?
 
kevin said:
My question is whether the conductors between the OCPD and the transformer are classified as a "feeder" as the thread title indicates. Aren't they really a "branch circuit"?
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I think they would be a "feeder." Feeders are all circuit conductors between the service equipment, SDS or other power supply and final branch-circuit overcurrent device.
 
There will be overcurrent devices on the other side of the transformer. Until you get to the last one of those, you are still dealing with "feeders."
 
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