Undersize neutrals? And conduit fill?

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fastline

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Can someone point out or share the NEC part on reduced size neutral on a 3P/1P branch? At the moment I am looking at either looking at a branch with 750mcm 3P conductors, or the higher voltage with 4/0 3P conductors. I thought someone in the tables they mention the smaller neutral? Do I literally have to go a full step up in size to account for "another conductor"? As example table indicates 3 750s in a 3" and need 3.5" for another conductor.

I am also looking for the part of code on reduced size neutrals. Not sure what will be needed on the 1P side yet but code might make that decision for me.
 
Feeder neutrals should be sized per 220.61 with 215.2(A)(2) in mind also (Minimum size).
Sizing raceways with reduced neutrals will require you us Tables 4 & 5, Chapter 9 (or a conduit fill app such as Southwire's)
 
I might be dense but the way I read it is confusing. "unbalanced load in excess of 200A"..... In the case of say a 300A feeder circuit, would that be reduced to 200A on the neutral or 100A? Or am I reading that totally wrong? There is language about the 70% factor but that seems to only apply to household appliances.
 
How much of the load is line to line and how much is line to neutral? In general the maximum neutral load is based on the phase with the most line to neutral load.
 
I don't have a copy of the code with me so trying to use the online page, which they make less than fun.... Basically 90% of the loads on this feeder will be served to 3P CNC machines. Not a single one has a neutral. Intent is to run feeder to a load center and branch out of that to serve individual machines. They are usually connected with EMT plus a #6 ground is taken in for redundancy.

I almost never reduce neutral size because there are a lot of factors that get confusing. But running 750 cable, yeah we don't need near that much 1P
Actually I will serve lighting loads with P-P so really the only reason is for general 120V receptacle use and usually good practice to drag a neutral along. I know I have done it on some and never even connected it.
 
fastline said:
I don't have a copy of the code with me so trying to use the online page, which they make less than fun.... Basically 90% of the loads on this feeder will be served to 3P CNC machines. Not a single one has a neutral. Intent is to run feeder to a load center and branch out of that to serve individual machines. They are usually connected with EMT plus a #6 ground is taken in for redundancy.

I almost never reduce neutral size because there are a lot of factors that get confusing. But running 750 cable, yeah we don't need near that much 1P
Actually I will serve lighting loads with P-P so really the only reason is for general 120V receptacle use and usually good practice to drag a neutral along. I know I have done it on some and never even connected it.
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If there is very little line to neutral load, then the minimum size specified by 215.2(A)(2) will be compliant.
 
The feeder neutral must be the calculated neutral load but not smaller than required in 250.122. Whichever is larger. See 215.2(A)(2).
The conduit fill is another matter. That will have to be calculated as all the conductors in the raceway are not the same size.
 
I might add that if the neutral load is as limited as the OP indicates, I would never run a full size neutral on a circuit of this size as it is big money and wasteful. Just comply with the minimum as it is perfectly safe and compliant.
 
Thanks guys. Applying the 250.122 as the "bare minimum" is very helpful! I was further trying to get clarity on 220.61. I simply don't understand how to apply it in this case!

As for the conduit fill, Augie was quite helpful and Southwire's calc made short work of that showing 3" has plenty of room for the 750s and a reduced neutral to stay under 40%.
 
texie said:
I might add that if the neutral load is as limited as the OP indicates, I would never run a full size neutral on a circuit of this size as it is big money and wasteful. Just comply with the minimum as it is perfectly safe and compliant.
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You got it! I would not even run a neutral if I had to another 750. Stuff is expensive!
 
texie said:
Yes, but if you need a neutral, you need a neutral. There is no compliant or safe way around this. Just keep it to the minimum required by code.
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I only mean that we have many other ways to serve 120V loads. This feeder could technically be dedicated 3P. It might actually happen that way too. Just not sure yet.
 
If the neutral is available at the source I normally pulled at least the 215.2 minimum to subpanels. Seems some always wants some lighting or receptacle.
 
augie47 said:
If the neutral is available at the source I normally pulled at least the 215.2 minimum to subpanels. Seems some always wants some lighting or receptacle.
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Here is an issue I am looking at. The derate of 20% for # of cccs in the raceway. Even applying the 90c ampacity, that knocks 750 Al down from 385 to 348A. That would be a no win for us unless we can wiggle back to the 75c ampacity?
 
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