310.15(b)(7) and derating

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electrofelon said:
For certain things, like motor full load currents, we are specifically directed to use the tables provided. I see nothing directing us to specifically use the tables for conduit fill.
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What is the actual diameter of a trade size 2 inch conduit? I want to try and verify the calculation
 
electrofelon said:
So would that common neutral be sized for the max theoretical imbalance on the same phase for all three units together? It seems like that is wildly overly conservative but I think it's what the code would require. What size do you typically see used for the situation then?
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Yes, but don't ask me how they actually calculate it. Typically 3-100 amp feeders would get a #3/0 neutral.
Tainted said:
What is the actual diameter of a trade size 2 inch conduit? I want to try and verify the calculation
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Look in Chapter 9 tables.
 
infinity said:
Yes, but don't ask me how they actually calculate it. Typically 3-100 amp feeders would get a #3/0 neutral.
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Okay, so just for the heck of it let's take that 3/0 as gospel, that is 1.7 times the area of a single number one so that definitely buys you some fill.

I think I would definitely go the common neutral route. It's going to make pulling conductors easier having 7 instead of 9 and you're going to save wire. I bet you could even go smaller than 3/0.
 
I am not sure if you're interested in value engineering this at all, but how about using an aluminum common neutral? You could even go with a (probably ridiculously oversized) 250 aluminum as a common neutral along with your six number ones and still be at 36% fill👍🏼
 
electrofelon said:
2.197 OD, .065 wall. Apparently the size is not required in UL 797 so theoretically it could vary from manufacturer to manufacturer if you are being really picky, but every manufacturer I have found specs that dimension.
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I just found out in the NYC electrical code chapter 9 note 6, they tell you to use tables only. If you compare it in the 2014 NEC note number 6 clarified you can use actual numbers or tables. :cry:
 
Tainted said:
I just found out in the NYC electrical code chapter 9 note 6, they tell you to use tables only. If you compare it in the 2014 NEC note number 6 clarified you can use actual numbers or tables. :cry:
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Yet another reason to use a common neutral.

Cheers, Wayne
 
infinity said:
For these installations were typically run a common neutral for every three panels.
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So when the 3 runs separate you set a box and use a 4-way Polaris connector to split the neutral 3 ways? Even if just one feeder splits off from the other two, you could split the neutral 3-ways there and just run separate neutrals with those other two.

Cheers, Wayne
 
wwhitney said:
So when the 3 runs separate you set a box and use a 4-way Polaris connector to split the neutral 3 ways? Even if just one feeder splits off from the other two, you could split the neutral 3-ways there and just run separate neutrals with those other two.

Cheers, Wayne
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It just occurred to me that the only potential problem I can see with the common neutral method is from a metering standpoint. If this was 120/240 single phase we could still meter the three separately but not if it's 120/208 3 phase. Submetering may be a possibility.
 
electrofelon said:
It just occurred to me that the only potential problem I can see with the common neutral method is from a metering standpoint. If this was 120/240 single phase we could still meter the three separately but not if it's 120/208 3 phase. Submetering may be a possibility.
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If it’s 120/208 single phase and not 3 phase, could you meter separately?
 
Tainted said:
If it’s 120/208 single phase and not 3 phase, could you meter separately?
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Sorry, that was a typo I meant to say 120/208 single phase in post #32. Someone will correct me if I am wrong, but I do not believe you could meter these separately where there is only a common neutral available - you would need to meter at the point where there is an individual neutral for each metered circuit.
 
Tainted said:
Hard to find a devils advocate for this answer... But is there anything in the code that tells you to only use values directed by the code rather than actual dimensions?
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Yes, see Note 5 to Chapter 9, Table 1.
5) For conductors not included in Chapter 9, such as multiconductor cables and optical fiber cables, the actual dimensions shall be used.
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However if you have conductors that are not of the same size, Note 6 says you can use either the actual dimensions or the dimensions in Tables 5 and 5A.

That does not make much sense....I will put it on my 2026 PI list.
 
Also note note #10:

"if the actual values of the conductor diameter
and area are known, they shall be permitted to be used."

I suppose there is some ambiguity on whether or not the value on a spec sheet is "actual" or not...🤔
 
Yes we use a sub-metering system in NYC. The shared neutral has no affect on the metering. Basically run a riser up through the first panel up through the next panel on the floor above and then end in the panel on the floor above that. Each panel has two ungrounded conductor and the common neutral is tapped with an insulation piercing connector (IPC tap).
 
According to my favorite calculator, the ninth #1 takes you to 2.5" EMT, unless you use bare conductors.

Isn't there a rounding allowance for conductor fill?
 
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