Multiple Conductor Ampacity vs. Conduit Fill

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jsinclair

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I have a question about using the derating factors for more than 3 current carrying conductors in a raceway. I have a situation where I need to run 11, 2 pole 30A circuits to power a piece of equipment. #10 is good for 30A based on 310.16. However, I now have 22 current carrying conductors in a single conduit, which requires me to derate the ampacity of the wire based on 310.15(B)(2)a. The #10 is no good any more and I have to drop to #8's. Which will still fit in my conduit. The question is, if the intent of 310.15(B)(2) is to constantly derate the ampacity of the conductors in a multiwire circuit as the number of conductors increase, then what good are the conduit fill tables?

For instance, take 1" EMT. Its maximum fill per Table C.1 is 16 #10's. A #10 per Table 310.16 is capable of 40A using the 90 degree column. If you apply the adjustment factor of 310.15(2)(a) assuming that 15 of the #10's are current carrying and 1 of them is the ground, the #10's ampacity is reduced to 20A. 20A is too low so the wire size must be increased.

A #8 has an ampacity of 55A using the 90 degree column. Half of 55A is 27.5A which is allowable. I now have to use 16, #8 conductors per the calculations. 16, #8's bumps me up to 1.25" conduit.

Am I missing something here? :roll:
 
Re: Multiple Conductor Ampacity vs. Conduit Fill

Conduit fill table are just that, fill tables. They tell you how many of the same size and type of conductor that will fit in a particular size and type of conduit. I'm not sure I understand your question. If derating factors require you to increase the size of your conductors than you may or may not need to increase the size of your conduit to accommodate the larger conductors. You should approach it this way:

-Find your conductor insulation type and size

-Find # of current carrying conductors

-Apply your derating factors

-See if your derated conductors can supply the load at their derated ampacity

-If not find the size that will

-Find conduit size of derated and possibly resized conductors from the appropriate table.

Maybe you're asking what good are the fill tables if you can't really fill the conduit due to derating. The answer is that they may not help you in sizing the conduit before derating but they will help you to size it after derating.
 
Re: Multiple Conductor Ampacity vs. Conduit Fill

The conduit fill tables of Annex C are strictly used to size the conduit.


Ampacity adjustment as per 310.15(B)(2)(a) is for sizing the conductor.

If your 10 AWG is sized too small, than no matter what size conduit, you will need to increase the size of the conductor. When increasing the size of the conductor, you may need to also increase the size of the conduit.


There are many people who think if they increase the size of the conduit, then they can keep the same size conductor... that is not how it works.

You do not have to agree with the requirements, you just have to follow them ;)
 
Re: Multiple Conductor Ampacity vs. Conduit Fill

Always agree with the requirements. Can't see installing #8's on 30A circuit just because I have 4 current carrying wires in a conduit. This job is going to require a lot of splicing in #10 taps to land them on the provided terminals.
 
Re: Multiple Conductor Ampacity vs. Conduit Fill

J, I suggest an alternative: run a single feeder to a sub-panel near enough to the load(s) to use individual load cables/conduits, which won't require derating.
 
Re: Multiple Conductor Ampacity vs. Conduit Fill

Would it help you understand if I say that you would have to derate for 4 current carrying #12 conductors in a 1 inch conduit, and that the same derating factor would apply if the same 4 wires were in a 5 inch conduit?
 
Re: Multiple Conductor Ampacity vs. Conduit Fill

Or another way, yes, size the conduit last.
 
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