Derating of Conductors: allowable # 10 wires in conduit EMT

[kwired]

I didn't realize that neutrals weren't considered CCW if they were part of a MWBC. I was only made aware recently on one of the other threads. Just so I understand, any time the neutral is shared with two or more other phases on a MWBC it isn't considered a CCW, at least in terms of fill calculations.

Bob

Don't confuse ampacity adjustments with fill calculations. With fill calculations you count everything in the raceway - it is going to take up space. With ampacity adjustments you don't count non current carrying conductors - they do not contribute heat to the raceway.

Example - say you have 3 three phase multiwire branch circuits in a raceway. That would be 9 current carrying ungrounded conductors and three non current carrying (for adjustment purposes) grounded neutral conductors. Total of 12 conductors plus any equipment grounding conductor, but adjustments are based on only 9 of them being current carrying - a 70% adjustment.

Now put 9 two wire circuits (9 ungrounded and 9 grounded neutrals) in place to supply the same loads and you now have 18 current carrying conductors - and must derate based on 18 which puts you in the 50% deration level. That usually means for your average 20 amp circuit, the first example would allow 12 AWG but the second example would require 10 AWG.

Either way is acceptable install, each has advantages and disadvantages that are design issues - not code issues.

 

[Smart $]

Thanks Rob,
So pretty much the neutral should always be considered a CCW unless it is connected to task specific equipment where you will know what load types will be connected.

I don't look at it that way, as do many others. I'll take advantage of not counting a neutral as a CCC every instance I can.

 

[infinity]

I don't look at it that way, as do many others. I'll take advantage of not counting a neutral as a CCC every instance I can.

I agree.