Parallel Conductors in Single Conduits

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hfd419

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Ok, here is my dilema. I currently have 2 runs of 3.5" EMT going to a 800 Amp gear in an existing school. I need to demo the gear and install a 1200 Amp. Can I use parallel 400 kcmil THHN Cu in each of my two conduits (8 wires per conduit) to achieve this. In essence if I have 2 A, B, C, and N phase in each pipe do I have to derate based on fill or are they viewed as being single conductors since they are joined at each end?????
 
You can parallel conductors in this fashion. The essential requirements are that the conduits contain the same set of conductors (e.g., you couldn't run three parallel sets with two sets in one conduit and one set in a second conduit), and that a given conduit have the same number of A's and B's and C's and N's.

But yes, you would have to derate for having more than three current-carrying conductors in a single conduit. See the last sentence (before the exceptions) of 310.15(B)(2)(a). So you need another solution. 380 amps (the 90C value for 1 400 MCM THHN) times four sets times 70% derating factor gives you a total ampacity of only 1,064 amps.

Welcome to the forum.
 
Soooo, If my gear is straight 480 only with no neutral then I would be in the clear as only having 6 conductors in each conduit and the 80% value instaed of 70% ???
 
Yep. That would give you a total ampacity of 1,216 amps. In fact, looking again at your original description, I should not have taken your statement of "8 conductors per conduit" at face value, without asking a follow-up question. If you did pull a neutral, and if it only carries the unbalanced current from the phase conductors, it doesn't count as being a current-carrying conductor. You could still use the 80% derating factor. However, if there is a significant amount of harmonics present (no idea what "significant" means), then the neutral would carry more than just the unbalanced current from the phase conductors, so it would count, and you would be back to the 70% derating factor.
 
Would an a dedicated equipment ground conductor (if provided, not conduit only) be counted for max raceway fill? (8) 400kCMIL THHN are the max in a 3-1/2" EMT, would an EGC if specified throw him over the top of 40% fill?
 
electroman00 said:
Exactly how would that help him here with his problem??
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He would be able to refer the question back to the engineer whose seal and signature appear on the plans and specs. Thus, if there are any changes, he would have a clear basis for a change in his contract price.
 
ron said:
Would an a dedicated equipment ground conductor (if provided, not conduit only) be counted for max raceway fill?
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I would say definitely yes. The basis of the conduit fill percentage is the ability to pull the conductors without risking jamming or damaging them in the process. That has nothing to do with how much current, if any, is expected to be run through any given conductor. For the same reason, spare conductors and "redundant" conductors (by which I mean, for example, there may be four wires, but only two will be used at a time because of the way loads are switched on and off) would also count against the 40% limit.
 
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