Neutral Conductor Sizing & 310.15(B)(7)

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George Stolz

George Stolz

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Hospital Master Electrician
I was cruising along, minding my own business, and got broadsided by this graphic:

31015b7neutral.jpg

I don't see how this can be accurate. If I have a calculated neutral load of 100A, it is still protected by a 200A OCPD at the service, so the feeder rating is still 200A, is it not? How can the neutral use the 310.15(B)(7) 100A rating column for sizing?

Looking at 310.15(B)(7)
The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of 215.2, 220.61, and 230.42 are met.
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215.2 would apply to the feeder neutral, and would require a #3 CU THHN.
220.61 would have figured into finding my 100A in the first place.
230.42 would apply to the service neutral, and would require a #3 THHN.

As I see it, my steps in solving this problem for the service conductors are
1. Determine the minimum conductor size for 100A per 310.15(B)(16). (#3 CU THHN)
2. Verify that the neutral is big enough for short circuit current (250.24(C)(1) ---> 250.66 (#4, which is smaller than the #3 required by 230.42)).

So I'm left with a #3, not a #4, right? :huh:
 
215.2 would apply to the feeder neutral, and would require a #3 CU THHN.
220.61 would have figured into finding my 100A in the first place.
230.42 would apply to the service neutral, and would require a #3 THHN.

As I see it, my steps in solving this problem for the service conductors are
1. Determine the minimum conductor size for 100A per 310.15(B)(16). (#3 CU THHN)
2. Verify that the neutral is big enough for short circuit current (250.24(C)(1) ---> 250.66 (#4, which is smaller than the #3 required by 230.42)).

So I'm left with a #3, not a #4, right? :huh:

Geo - 215.2 says the grounded conductor shall not be smaller than req'd by 250.122 => which is a #6.
230.42 says to size per T250.66 => which is a #4.

I don't know how you're coming up with a #3. ??
 
My whole point is that you can't size the grounded conductor per Table 310.15(B)(7), unless you want to run a neutral conductor the same size as your line conductors, and then say you're using 310.15(B)(7).

It is not an ampacity table.
 
George Stolz said:
My whole point is that you can't size the grounded conductor per Table 310.15(B)(7), unless you want to run a neutral conductor the same size as your line conductors, and then say you're using 310.15(B)(7).

It is not an ampacity table.
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Where does it state you cannot use this table unless all the wires are the same size. If the calculated load of the services is 100 amps and #4 is allowed why then wouldn't a #4 neutral be allowed. You are always allowed to downsize the neutral if you can so I don't see why 310.15(B)(7) can't be used for a residence.
 
Let me try it this way:

310.15(B)(7) says that if you have a service rating of "X", then you can use the conductor size listed next to "X".

What is a rating of a service? 230.79 leads me to believe that the rating of the service is the rating of the service disconnecting means. 230.80 tells me that when it consists of more than one disconnect, the combined rating of the handles is the "service rating." (Edit to add: 210.3 tells me that the rating of a branch circuit is the rating of the OCPD as well, but I'm still hunting for the definition of the rating of a feeder. I would call it the rating of the OCPD protecting the feeder.)

So, if I want a 400A 120/240V residential service, then that is displayed by the presence of a 400A handle or a pair of 200A handles comprising the service disconnect. If I have this, then I can use the conductor size listed next to "400" on Table 310.15(B)(7). That is a "400A rated service."

It is not an ampacity table, it is a table to accompany a rule. It is not telling me that a 400 kcmil CU conductor's ampacity changes when I connect it to a house; it is saying that I can blatantly ignore the ampacity if I want to use that table.

The rating of the service didn't change when I went to install the neutral conductor, it is still a 400A service - and that is the row I have to use if I want to use that table for anything. There is not a 200A handle to be found in the picture, so the 200A row of Table (B)(7) is irrelevant to the picture. I can use 400 kcmil, or do some work to find a smaller conductor compliant with 310.15(B)(16).

(Edit x 2 to add the picture that I was looking at as I wrote this post)
 

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Okay take this a step further. Table 310.15(B)(7) allows se cable into the equation. Does it state that se cable must have all the same wire sizes. With your thought process if I use this table then I cannot undersize a neutral conductor at all simply because the table states 200 amp-- 2/0 copper conductors.

Explain to me why the neutral needs to follow 310.16(B)(16) if the load is 100 amps on the neutral for a 200 amp service, yet if the load is 100 amps for the entire service I can use #4 for that neutral.

I understand what you are saying and agree in principal but I don't agree with the interpretation as the intent of the Table.
 
George, why would you use one table for the ungrounded conductors and another for the neutral?:happysad:

That makes no sense.:slaphead:

If the table is good for the ungrounded conductors in this app, why would it be wrong to use it for the neutral/grounded conductor?:?

IMO, you are over thinking this.:)
 
Dennis Alwon said:
Okay take this a step further. Table 310.15(B)(7) allows se cable into the equation. Does it state that se cable must have all the same wire sizes. With your thought process if I use this table then I cannot undersize a neutral conductor at all simply because the table states 200 amp-- 2/0 copper conductors.
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I'm not saying it cannot be reduced; I'm saying it's no longer trying to use Table 310.15(B)(7) when it is reduced. Paragraph 310.15(B)(7) clearly allows a reduced neutral.

Explain to me why the neutral needs to follow 310.16(B)(15) if the load is 100 amps on the neutral for a 200 amp service, yet if the load is 100 amps for the entire service I can use #4 for that neutral.
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Technically, no reason. It's just how the thing is built.

I understand what you are saying and agree in principal but I don't agree with the interpretation as the intent of the Table.
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Keep reading. ;)
jumper said:
George, why would you use one table for the ungrounded conductors and another for the neutral?:happysad:

That makes no sense.:slaphead:

If the table is good for the ungrounded conductors in this app, why would it be wrong to use it for the neutral/grounded conductor?:?

IMO, you are over thinking this.:)
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Behold:
2008 ROP said:
6-64 Log #1341 NEC-P06 Final Action: Reject
(Table 310.15(B)(6))
____________________________________________________________
Submitter: Mike Holt, Mike Holt Enterprises
Recommendation: Add reference to table 310.15(B)(6)
(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For individual dwelling units of one family, two-family, and multifamily dwellings conductors as listed in Table 310.15(B)(6), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service lateral conductors, and feeder conductors that serve as the main power feeder to each dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboards(s). The feeder conductors to a dwelling unit shall not be required to have an allowable ampacity rating greater than their service-entrance conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors and sized in accordance with Table 310.15(B)(6) provided the requirements of 215.2, 220.61, and 230.42 are met.
Substantiation: This section should be re-written to clarify how the grounded conductor is to be sized.
Panel Meeting Action: Reject
Panel Statement: The grounded conductor is sized in accordance with 230.42(C), not Table 310.15(B)(6). 230.42(C) is presently referenced in 310.15(B)(6). The panel assumes that the submitter’s proposal relates to 310.15(B)(6) rather than the Table.
Number Eligible to Vote: 11
Ballot Results: Affirmative: 11
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You know, I think I'm going to make a proposal to make this more clear, and at the same time, get 120/208 into the mix at the same time. The halls are lined with the corpses of proposals to try to get 120/208 into (B)(7), and this may just be the ticket.

6-63 Log #1174 NEC-P06 Final Action: Reject
(310.15(B)(6))
____________________________________________________________
Submitter: Daniel Leaf, Seneca, SC
Recommendation: Add:
“or 120/208-volt” after “120/240-volt”.
Substantiation: Edit. This section should be applicable where dwelling units in apartment buildings are supplied with 120/208-volt systems.
Panel Meeting Action: Reject
Panel Statement: The neutral conductor of a 120/208 volt, 3 wire system does carry significant load. In a 120/240 volt system the load on the neutral conductor is reduced. No substantiation was provided to justify applying the Table to 120/208-volt service conductors.
Number Eligible to Vote: 11
Ballot Results: Affirmative: 11
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jumper said:
Well then, please explain it to me in dummy terms-'cause I am confused.
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I think George did a pretty good job of explaining it, because my original thought was that the graphic was correct. But...

310.15(B) says that the ampacity for conductors shall be as specified in the Allowable Ampacity Tables 310.15(B)(16) thru 310.15(B)(21) as modified in (B)(1) thru (B)(7).

310.15(B)(7) says that for 120/240V Single Phase Dwelling Services and Feeders conductors of a certain type and size shall be permitted to be used for specific size "Service Ratings" and "Feeder Ratings."

In the original graphic, there is one service and one feeder shown. The "rating" of the service/feeder is 200A. There is not two separate services and feeders, one rated 200A and one rated 100A.

Since the neutral conductor with a calculated load of 195A is not itself a "100A rated" feeder or service, Table 310.15(B)(7) does not apply to the neutral conductor. The reduced neutral conductor should have an ampacity based on the Ampacity Tables as noted in 310.15(B).
 
jumper said:
So the whole "load diversity" thing is okay for the ungrounded conductors: but I got to throw it out the window for the neutral?
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I don't understand your question. I would think the neutral calculation already includes "load diversity."
 
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David, he's referring to the inherent load diversity plugged into (B)(7), that we lose if we use (B)(16).

The calculated neutral load does not include diversity, although it had been demanded down.
 
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