Feeder Sizing for K-Rated Transformer

Status
Not open for further replies.
J

jjengineer

Member
Location
Houston
Occupation
Engineer
Hello - Is there any requirement in the NEC that would include upsizing the hot conductors (not just the neutral conductor) on the secondary feeder, when you go from a standard 277/480V - 120/208V 3-phase, dry-type, delta-wye configuration transformer to K-rated (for instance, K-13 rated) transformer? I've ran across a transformer feeder schedule, for example, that states for 112.5 KVA transformer, use 2 sets of 4#3/0, 1#1/0G, 2" conduit for standard transformer, but 2 sets of 3#4/0, 1#350N, 1#1/0G, 2-1/2" conduit for K-rated transformer. Hot conductor upsized #3/0 --> #4/0, and I've been unable to determine why this is the case.

Thank you for any assistance.
 
There are no special NEC requirements for K-rated transformers. Secondary conductors are sized/chosen based on the load.
 
jjengineer said:
Hello - Is there any requirement in the NEC that would include upsizing the hot conductors (not just the neutral conductor) on the secondary feeder, when you go from a standard 277/480V - 120/208V 3-phase, dry-type, delta-wye configuration transformer to K-rated (for instance, K-13 rated) transformer? I've ran across a transformer feeder schedule, for example, that states for 112.5 KVA transformer, use 2 sets of 4#3/0, 1#1/0G, 2" conduit for standard transformer, but 2 sets of 3#4/0, 1#350N, 1#1/0G, 2-1/2" conduit for K-rated transformer. Hot conductor upsized #3/0 --> #4/0, and I've been unable to determine why this is the case.

Thank you for any assistance.
Click to expand...
The transformer is still at 112.5 KVA even when it has a K rating. Not sure why someone would increase the ungrounded conductor size. Is the OCPD after the transformer secondary the same size?
 
infinity said:
The transformer is still at 112.5 KVA even when it has a K rating. Not sure why someone would increase the ungrounded conductor size. Is the OCPD after the transformer secondary the same size?
Click to expand...
In this scenario, there would be a panelboard with a main circuit breaker, sized for 400A (and consequently the bus rating would be 400A).

At this point, I'm tempted to say the schedule is incorrect. The engineer who made it allegedly was very in tune with the code, but senior engineers I've asked do not see why.
 
jjengineer said:
Hello - Is there any requirement in the NEC that would include upsizing the hot conductors (not just the neutral conductor) on the secondary feeder,... and I've been unable to determine why this is the case.
Click to expand...
It's not an NEC requirement, but I believe IEEE 519 has some derating recommendations when harmonics are involved. That may be why they have upsized.
 
I think they are counting the neutral as a current carrying conductor and applying the derating factor for more than three current carrying conductors in a raceway.
 
david luchini said:
It's not an NEC requirement, but I believe IEEE 519 has some derating recommendations when harmonics are involved. That may be why they have upsized.
Click to expand...
I'll need to look into that. Thanks for the pointer.
d0nut said:
I think they are counting the neutral as a current carrying conductor and applying the derating factor for more than three current carrying conductors in a raceway.
Click to expand...
That's what I thought originally, but the calculations didn't add up to the schedule I was referring to. 4-6 current carrying conductors I believe is 80%, when you apply to the 112.5 KVA transformer, that comes out to 500A, which is a larger ampacity than parallel #4/0 in the example. The definition of the NEC says the neutral is considered currrent carrying if a "major portion" of the load consists of nonlinear loads, not sure how to interpret that.

Edit - Correction on my part, adjustment factor is to the wire, not the OCPD. So if you are matching the ampacity of the derated wire to the 400A OCPD, it would be (2) sets of 250 KCMIl (408A ampacity), still not the #4/0 in the example.
infinity said:
With a 400 amp OCPD the two sets of #3/0 in separate raceways is all you need.
Click to expand...
That's what I'm thinking.
 
jjengineer said:
That's what I thought originally, but the calculations didn't add up to the schedule I was referring to. 4-6 current carrying conductors I believe is 80%, when you apply to the 112.5 KVA transformer, that comes out to 500A, which is a larger ampacity than parallel #4/0 in the example. The definition of the NEC says the neutral is considered currrent carrying if a "major portion" of the load consists of nonlinear loads, not sure how to interpret that.
Click to expand...
I don't understand where your 500A is coming from. The 208Y/120V side of your 112.5kVA transformer is 312A. At 125% you would have 390A, therefore a 400A circuit breaker is specified. The #4/0 derated 80% from the 90deg column would be 208A. Paralleled sets would provide your 400A capacity to match your circuit breaker.

The design is providing a K-rated transformer because of the harmonic content of the loads. In my opinion, if your design needs a K-rated transformer with an oversized neutral, that neutral need to be counted as a current carrying conductor.
 
d0nut said:
I think they are counting the neutral as a current carrying conductor and applying the derating factor for more than three current carrying conductors in a raceway.
Click to expand...
I think that you're correct. For more than 50% harmonic loading the nuetral would count as a CCC. (260*80%=208*2=416 amps) Not sure that's very likely in the real world.
 
Status
Not open for further replies.
Top