Downsizing Neutral

M

mossbayelectric

New User
Location
Kirkland WA
Occupation
Electrical Contractor
I am struggling to determine whether 250.102 (C) 1 refers to sizing of the neutral conductor, I have a 200 amp service, and I have used 2/0 for my feeders by using the residential 315.16 table, and according to the AHJ I need to use a #1, and they can't give me a code reference. My belief, and what I have been told is that by using 250.102 C 1, I can use a #4 for my neutral. There is apparently common knowledge that you can size down two sizes, which would be a number one. I even have the Mike holt grounding and bonding book, and it says sizing of the neutral conductor. I just don't know how to communicate this, if this is the case- to the Inspector. I will switch it out this time, but if I can confidently say this table is used to size the neutral then that would be great.
 
mossbayelectric said:
I am struggling to determine whether 250.102 (C) 1 refers to sizing of the neutral conductor, I have a 200 amp service, and I have used 2/0 for my feeders by using the residential 315.16 table, and according to the AHJ I need to use a #1, and they can't give me a code reference. My belief, and what I have been told is that by using 250.102 C 1, I can use a #4 for my neutral. There is apparently common knowledge that you can size down two sizes, which would be a number one. I even have the Mike holt grounding and bonding book, and it says sizing of the neutral conductor. I just don't know how to communicate this, if this is the case- to the Inspector. I will switch it out this time, but if I can confidently say this table is used to size the neutral then that would be great.
Click to expand...
The two size down neutral thing is extremely common but has no basis whatsoever in the NEC
 
Depending on the application, I would start by showing them a calculation with 220.61 showing that you wouldn't need a neutral larger than #4. Then move to section 250.24(C). That is the rule that points you to the grounded conductor must not be smaller than that which would be required by table 250.102(C)(2).
 
When at common sizes I don't do smaller than 2 sizes but when doing sub panels I often do a #4 copper as my go do size for hardware reasons to avoid neutral lug kits. Sometimes dropping from a #2 AL with a inline splice reducer.

Even a 200 amp subpanel full of 120v loads won't need the 2/0 but I've never done as small as a #4 since I like buying mobile home feeder or cable assembly at that size.

Bigger panels I parallel the hots and use a single nuetral that's sized for the load and then some more. Up to a point obviously.
 
mossbayelectric said:
I have a 200 amp service, and I have used 2/0 for my feeders
Click to expand...
Hope that is CU and not AL.

Question
What is you neutral load calculation?
If the system is fully balanced you have next to none. But Any imbalance and any 120V only load will be on the neutrals adding to the size needed.
All this is why the neutral size is based on calculations if you want to "down size" the Neutral. There are times a full size neutral will be required as well. And Code says that max size is no larger that the current carrying conductor.

So you want to convince the inspector, Do the math and show your work and resulting code minimum conductor size. The typical size reduction is 2 sizes down but as already mentioned not directly code referenced. Only way to really get a proper size again is do the math.
 
I don't have the book with me but isn't there an example in the back of the NEC that tells you how to calculate the service neutral size?
 
If you are talking feeders and not service conductors then your neutral can be as small as the equipment grounding conductor. That is assuming the calculated load will handle it.

For a service you cannot be smaller than the grounding electrode conductor from Table 250.102(C)(1)
 
Fred B said:
Hope that is CU and not AL.

Question
What is you neutral load calculation?
If the system is fully balanced you have next to none. But Any imbalance and any 120V only load will be on the neutrals adding to the size needed.
All this is why the neutral size is based on calculations if you want to "down size" the Neutral. There are times a full size neutral will be required as well. And Code says that max size is no larger that the current carrying conductor.

So you want to convince the inspector, Do the math and show your work and resulting code minimum conductor size. The typical size reduction is 2 sizes down but as already mentioned not directly code referenced. Only way to really get a proper size again is do the math.
Click to expand...
He's talking specifically nuetral so I believe this is all in relation to the standard 4/0 4/0 2/0 service wire.
 
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