Sizing the Neutral

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erickench

erickench

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According to NEC 250.24(C) the grounded conductor(Service Neutral) must be sized in accordance with NEC Table 250.66. But according to NEC 220.61 the neutral must be sized to handle the maximum unbalanced load. My question is this: If the service neutral is sized according to NEC Table 250.66 then how is the feeder neutral sized? Is it sized according to NEC 220.61? What I mean by feeder neutral is downstream from the service. We would be connecting two different wire sizes together.
 
Gec 250.66

Gec 250.66

Hi erickench,
I believe 250.66 is used for GEC sizing only, whereas 220.61 calculations encompass sizing for both the SE and Feeder grounded conductors as stated;
"220.61 Feeder or Service Neutral Load
(A) Basic Calculation. The feeder or service neutral load shall be the maximum unbalance of the load determined by this article."
250.24(C)(1) primarily states that the SE (service neutral) shall not be smaller than the required GEC specified in T 250.66.

I hope this helps. rbj
 
You must size your neutral conductor in accordance with 220.61 for the calculated load. Then for the service neutral you must also make sure that the service grounded conductor be no smaller than the conductor specified in table 250.66 based on the ungrounded service entrance conductors.

250.24(C) requires the grounded conductor brought to the service to be no smaller than what is specified in Table 250.66 based on the size of the service entrance conductors.

Chris
 
erickench said:
According to NEC 250.24(C) the grounded conductor(Service Neutral) must be sized in accordance with NEC Table 250.66. But according to NEC 220.61 the neutral must be sized to handle the maximum unbalanced load. My question is this: If the service neutral is sized according to NEC Table 250.66 then how is the feeder neutral sized? Is it sized according to NEC 220.61? What I mean by feeder neutral is downstream from the service. We would be connecting two different wire sizes together.
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I think NEC handbook says to pick larger neutral wire between 250.24(C) and 250.66
 
I went for many years saying, for other than services, size the feeder grounded conductor by the load. That was my complete statement.

The one fine Forum day I was introduced to 215.(A)(1)
"The size of the feeder circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel."


Does this not mean the minimum grounded conductor as well as grounding condcutor is determined by 250.122 ??

Either I am incorrect, or that was a real eye-opener.
 
augie47 said:
I went for many years saying, for other than services, size the feeder grounded conductor by the load. That was my complete statement.

The one fine Forum day I was introduced to 215.(A)(1)
"The size of the feeder circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel."


Does this not mean the minimum grounded conductor as well as grounding condcutor is determined by 250.122 ??

Either I am incorrect, or that was a real eye-opener.
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Great catch Gus!

This makes no sense to me, why would a feeder grounded conductor need to be sized no smaller than the equipment grounding conductor?

For a feeder, the grounded conductor is not used to carry fault current so why would it have to be sized not smaller than the EGC?

Chris
 
After thinking about it, the neutral must be sized no smaller than the EGC in case there was a hot to neutral fault. The neutral must be capable of carrying the fault current to trip the overcurrent device.

Sometimes I need to sit down and think things through so they make sense.:)

Chris
 
yep! after 30 years that startled me too. I don't want to recall how many #10 or #8 neutrals I installed for minor grounded loads on bigger feeders.
And the logic you gave is all I could come up with also.
Betcha that one sneaks by a lot. I leanened it here (Thanks Mike & Guys) and even then I have to hunt every time it's a question.
 
personally i would not run a smaller neutral i would size it the same as the ungrounded conductors. thats just how i feel. theres nothing wrong with running a smaller neutral if you have few L - N loads but thats just something i wouldnt do
 
electricalperson said:
personally i would not run a smaller neutral i would size it the same as the ungrounded conductors. thats just how i feel. theres nothing wrong with running a smaller neutral if you have few L - N loads but thats just something i wouldnt do
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The code allows it, therefore I will do it. Furthermore it is highly unlikely that you could ever overload a neutral conductor when you factor in load balancing and load diversity.
 
peter d said:
The code allows it, therefore I will do it. Furthermore it is highly unlikely that you could ever overload a neutral conductor when you factor in load balancing and load diversity.
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if your feeding a bunch of 3 phase motors and run a smaller neutral, what happens when they decide to add VFD or other nonliner loads? wont the harmonics add in the neutral and burn it up if its undersized?
 
electricalperson said:
if your feeding a bunch of 3 phase motors and run a smaller neutral, what happens when they decide to add VFD or other nonliner loads? wont the harmonics add in the neutral and burn it up if its undersized?
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I don't know...I never deal with that stuff. I know you can certainly use the code allowance to run a smaller neutral for the vast majority of residential and commercial applications.
 
peter d said:
I don't know...I never deal with that stuff. I know you can certainly use the code allowance to run a smaller neutral for the vast majority of residential and commercial applications.
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where in residential would you run a smaller neutral? i never ran conduit in a house before besides underground services and i just use triplex underground or SER for feeders in the house. all that stuff comes with a full size white conductor (black with a yellow stripe for the triplex for the nitpickers :) )
 
electricalperson said:
if your feeding a bunch of 3 phase motors and run a smaller neutral, what happens when they decide to add VFD or other nonliner loads? wont the harmonics add in the neutral and burn it up if its undersized?
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No, if they are 3 phase motors the service neutral wouldn't come into play except for clearing a fault which is the reason for 250.24(C)

Roger
 
roger said:
No, if they are 3 phase motors the service neutral wouldn't come into play except for clearing a fault which is the reason for 250.24(C)

Roger
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i agree with this i guess 3 phase motors wasnt a good example. i stll wouldnt undersize a neutral in a commercial building because of the nonlinear loads like computers. i know the harmonics from flourescant lighting doesnt matter much i would be more concerned with computers. i never undersized a neutral and probably never will
 
roger said:
No, if they are 3 phase motors the service neutral wouldn't come into play except for clearing a fault which is the reason for 250.24(C)
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Good point, the whole 3-phase motor not needing a neutral flew over my head about mach 5.

I figured a VFD might need a neutral, but I've never wired one so I don't know. :)
 
electricalperson said:
i never undersized a neutral and probably never will
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Different strokes.....

Just for an example, I can't see running a full sized neutral across a plant to hit a 480/277 panel with the majority being 3 phase motor loads and a few 277v lights. Full size neutral would be a waste IMO. I know some guys might not like that because it's less wire and possibly a smaller pipe(less markup), but I can live with that. Probably because I'm only an employee.:wink:
 
I don't know much about NEC rules, but the reason to have a neutral wire the same size as the lines is primarily for 3 phase systems where because of faults or load distribution there could be a situation where loads are not balance between the phases. In an ideal 3 phase system there is no current flowing thru the neutral, but if that system became very unbalance the current flowing thru the neutral could be greater than the phase current thus requiring a full sized neutral.
 
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