Wire Gauge

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rswolff001

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Client has a 5kva 480/120 transformer supplying 120vac to two separate circuits inside a control panel. I told them to use 8g wire for the secondary to the circuit breakers for the two circuits. They asked if they needed 8g. And I wasn't sure.. can they land the secondary on a distribution block and then split into the two circuits using smaller gage wire based on the circuit they are supplying? The transformer is on the top of the enclosure, so the distribution block would be no more than maybe 1ft from the transformer. The circuit breakers would also be 1~2ft from the distribution block. Can they do the same for the neutral? I know its acceptable for the mains into the enclosure, not sure if its acceptable for the transformer. Thanks, Bob
 
I agree.. but I'm simply double checking. When one is not sure, one should ask. Sometimes something that's sounds correct isn't for some other reason that may not be obvious.
 
You could leave the transformer with two sets of conductors that each land on their own OCPD that has a rating not exceeding the ampacity of the conductors to avoid tapping a tap.
If you only run a single set of conductors from the transformer, those conductors will have to terminate at an OCPD and the smaller conductors from the OCPD to the loads will also have to terminate at OCPDs.
 
the way I've done it in past is a single set of conductors terminating in an OCPD.. that feeds a distribution block that feeds the other circuits. The circuits being fed from the distribution block could be smaller gage wire. That's normal for incoming supply to the enclosure though this is a different application. I believe that can be done.. correct? or again, am I missing something.
 
It's probably simpler to go from the transformer to each OCPD. But Transformer -> #8 ->50 amp fuse -> #8 -> Distribution block would work also.
 
If you're planning to run #8 to a distribution block and then run smaller conductors to each OCPD instead just run the smaller conductors from the transformer to each OCPD.
 
I wasn't sure that was legal.. how is that much different than coming out of the transformer with 8g into a distribution block, and then continuing with the smaller conductors? Electrically it should be nearly identical. Again, the 8g is only going thru the enclosure top (conduit) approximately 1ft. The two conductor method will work, and that is what I will tell client. Only interested why the distribution block would not be allowed. Lets say I had 8 smaller circuits to feed.. you wouldn't want to have 8 separate feeds coming directly off the transformer would you? I certainly wouldn't design anything that way.
 
rswolff001 said:
Lets say I had 8 smaller circuits to feed.. you wouldn't want to have 8 separate feeds coming directly off the transformer would you? I certainly wouldn't design anything that way.
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The design has to account for not tapping a secondary tap conductor. How that is accomplished is up to the designer. If you had 8 circuits to feed then you should install a panel.
 
rswolff001 said:
the way I've done it in past is a single set of conductors terminating in an OCPD.. that feeds a distribution block that feeds the other circuits. The circuits being fed from the distribution block could be smaller gage wire. That's normal for incoming supply to the enclosure though this is a different application. I believe that can be done.. correct? or again, am I missing something.
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That not what your original post says. I read that post as saying the conductors would run from the transformer to a power distribution bock, and then smaller conductors from the distribution block to OCPDs. That is not compliant. See 240.21
240.21 Location in Circuit.
Overcurrent protection shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductors receive their supply except as specified in 240.21(A) through (H). Conductors supplied under 240.21(A) through (H) shall not supply another conductor except through an overcurrent protective device meeting the requirements of 240.4.
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There are two compliant methods. A conductor from the transformer to an OCPD and smaller conductors from the load side of that OCPD to smaller OCPDs. or two sets of conductors from the transformer to two OCPDs, than to the loads.
 
240.4 B cover feeder taps however, since you are talking about transformer secondary, I think 240.21 C may apply. which states : 240.4 B shall not be permitted for transformer secondary conductors.
Sometimes these things are a confusing rabbit hole and I just make a judgement call and go with it.
 
milmat1 said:
240.4 B cover feeder taps however, since you are talking about transformer secondary, I think 240.21 C may apply. which states : 240.4 B shall not be permitted for transformer secondary conductors.
Sometimes these things are a confusing rabbit hole and I just make a judgement call and go with it.
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If he uses a single set of conductors from the transformer to an OCPD, 240.21(C) would apply to those conductors, and 240.21(B) to the conductors on the load side of that OCPD to the next OCPD.
If he uses two sets of conductors from the transformer to OCPDs, then only 240.21(C) would apply.
 
don_resqcapt19 said:
That not what your original post says. I read that post as saying the conductors would run from the transformer to a power distribution bock, and then smaller conductors from the distribution block to OCPDs. That is not compliant. See 240.21

There are two compliant methods. A conductor from the transformer to an OCPD and smaller conductors from the load side of that OCPD to smaller OCPDs. or two sets of conductors from the transformer to two OCPDs, than to the loads.
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I have always done a single set of conductors out of any of my transformers.. these are all machine mounted and for the control side of the applications. Client was asking if there was some method of not using 8g only because previously they hadn't done that. I don't think previously they met code either but that was a different vendor. It probably would not have been something I would have recommended but they asked if they could use a distribution block. I simply wasn't sure since as far as I can tell, its a kind of blurred line. I asked two licensed electricians and one said he didn't see any problem, and one said he didn't think it would be acceptable. None of us had actually ever been asked or done exactly this. So before I made any judgement, I figured I'd ask and see what the answers were.
 
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