Mismatched parallel feeders - 500 kCM and 600 kCM

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jchristopherslice

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First of all, I am aware that NEC 310.10 prohibits this. However, our client finds them self in a situation where the contractor has installed this contrary to the design. As best we can tell, the design originally (or at other locations) called for 1 set of 4 600 kCM conductors. When changes required 2 sets of 4 500 kCM conductors, apparently the contractor just added one set of 500 kCM, figuring that the other set of 600 kCM would suffice since it was larger than 500 kCM. Our client is looking for justification for temporary code enforcement relief until they can work with the utility to get another set of 500 kCM conductors landed on the utility transformer.

Given that the code says this, why must this be the case? All else being equal, if the relative ampacities of each conductor set created a current divider that didn't exceed the ratings of either feeder, then why shouldn't that be acceptable?
 
jchristopherslice said:
First of all, I am aware that NEC 310.10 prohibits this. However, our client finds them self in a situation where the contractor has installed this contrary to the design. As best we can tell, the design originally (or at other locations) called for 1 set of 4 600 kCM conductors. When changes required 2 sets of 4 500 kCM conductors, apparently the contractor just added one set of 500 kCM, figuring that the other set of 600 kCM would suffice since it was larger than 500 kCM. Our client is looking for justification for temporary code enforcement relief until they can work with the utility to get another set of 500 kCM conductors landed on the utility transformer.

Given that the code says this, why must this be the case? All else being equal, if the relative ampacities of each conductor set created a current divider that didn't exceed the ratings of either feeder, then why shouldn't that be acceptable?
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because NEC 310.10 prohibits this. :)

I wonder why he didn't just add a set of 600s?
 
If the protection for the feeder is 800A, there is no technical reason it wouldn't be safe for a short term situation until it is rectified to be code compliant, but most AHJ's would rather you have it fixed than give some type of temporary approval.
 
ron said:
If the protection for the feeder is 800A, there is no technical reason it wouldn't be safe for a short term situation until it is rectified to be code compliant, but most AHJ's would rather you have it fixed than give some type of temporary approval.
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potentially you could terminate one set of conductors on one panel and the other set on another panel and leave the conductors alone.
 
The problem, in general, is that the current will not divide evenly between the two wires when of different size. If 2 500s will work, then there will probably not be enough extra current on the 600 to overload it but the calculations need to be done to be sure. Two 600s would, of course, be no problem.
 
Using the DC resistance of 500KCM and 600KCM, and the current divider formula, I get about 55% of the current on the 600KCM wire, and about 45% on the 500KCM.

Assuming that you needed 760 amps of capacity (2 sets of 500KCM), I get about 418 amps flowing on the 600KCM, and about 342 amps on the 500KCM.

So the 600 KCM is actually closer to being overloaded than the 500KCM.

These numbers might change a little if you account for inductive reactance, metallic conduit, temp. variations and such.

But if you were conservative with the load calcs, and the actual load is considerably less than 760 amps, I think one could reasonably expect it to be OK for a temp installation.
 
steve66 said:
Using the DC resistance of 500KCM and 600KCM, and the current divider formula, I get about 55% of the current on the 600KCM wire, and about 45% on the 500KCM.

Assuming that you needed 760 amps of capacity (2 sets of 500KCM), I get about 418 amps flowing on the 600KCM, and about 342 amps on the 500KCM.

So the 600 KCM is actually closer to being overloaded than the 500KCM.

These numbers might change a little if you account for inductive reactance, metallic conduit, temp. variations and such.

But if you were conservative with the load calcs, and the actual load is considerably less than 760 amps, I think one could reasonably expect it to be OK for a temp installation.
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Thank you. There's only about 500A demand load. So that might work.
 
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