Mike's Video Neutral Calculation

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augie47

augie47

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In one of Mike's videos when calculating the service load on a bank, he includes rhe 125% continuous load for lighting in the phase conductor calculation which I understand, but not in the neutral sizing. In my trying to explain this I can not referecne the Code Section that adresses it.
What Section covers that ?
 
I am guessing 220.61

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.
The maximum unbalanced load shall be the maximum net
calculated load between the neutral conductor and any one
ungrounded conductor.
Exception: For 3-wire, 2-phase or 5-wire, 2-phase systems, the maximum
unbalanced load shall be the maximum net calculated load between the
neutral conductor and any one ungrounded conductor multiplied by
140 percent.
Click to expand...
 
See also 215.2(A)(1) Exception No. 3: "Grounded conductors that are not connected to an overcurrent device shall be permitted to be sized at 100
percent of the continuous and noncontinuous load."

However, how does the practice of a reduced size neutral in a feeder comply with 240.4?

Cheers, Wayne
 
david luchini said:
You have to look at the entire Article 240.
Click to expand...
If you have a specific section in mind, that would be helpful.

Otherwise, I scanned all of Article 240 and don't see the answer to my question. There are several citations for providing OCPD in the ungrounded conductors. As the current in the grounded conductor is going to be no greater than the maximum current in the ungrounded conductors in a feeder or MWBC (save for 5-wire 2 phase), the OCPD in the ungrounded conductors protect the grounded conductor as well, at the OCPD rating.

But when the grounded conductor has an ampacity below the rating of the OCPD, I still don't see how that complies with 240.4.

Cheers, Wayne
 
david luchini said:
Yes, this is it.
Click to expand...
But 240.4 says "Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G)."

So for say a 100A feeder with a calculated maximum neutral load of 60A, consisting of #3 Cu ungrounded conductors and a #6 Cu neutral conductor, how is the #6 Cu neutral conductor protected against overcurrent in compliance with 240.4?

Or getting back to the OP, if the calculated load is 100A continuous, and so a 125A OCPD, #1 Cu ungrounded conductors, and a #3 Cu grounded conductor are used, how is the #3 Cu grounded conductor protected against overcurrent in compliance with 240.4?

I don't see anything in 240.4(A) through 240.4(G) that would cover these situations, yet other code sections indicate they are allowed. Seems like a conflict.

Cheers, Wayne
 
wwhitney said:
But 240.4 says "Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G)."
Click to expand...
Would it make more sense to you if the "Overcurrent Shall Be Provided in Each Ungrounded Conductor" came before the "Conductors Shall be Protected Against Overcurrent in Accordance With Their Ampacities"?
 
david luchini said:
Would it make more sense to you if the "Overcurrent Shall Be Provided in Each Ungrounded Conductor" came before the "Conductors Shall be Protected Against Overcurrent in Accordance With Their Ampacities"?
Click to expand...
Are you suggesting that 240.15(A) or 240.21 are modifying 240.4? I certainly don't see anything suggesting that.

As written, 240.4 requires the grounded conductor to be protected against overcurrent in accordance with its ampacity and the rules in 240.4(A) through (G). So a rule like 215.A(1) Exception 3 means that, say, for a 100A continuous load, you could use a 125A OCPD, #1 Cu (130A 75C) ungrounded conductors and a #2 Cu (115A 75C) grounded conductor, as 240.4(B) allows the #2 to be protected at 125A. But it would seem to me that using a #3 Cu (100A 75C) grounded conductor would be a violation of 240.4, as the #3 can't be protected at 125A.

Now that doesn't seem to be the intention or common practice, but unless there's something in 240.4 I'm missing, that's the literal meaning. In which case 240.4 needs a section such as 240.4(H) "Where specified elsewhere in the NEC, a grounded conductor may have an ampacity less than the rating of the OCPD protecting the associated ungrounded conductors."

Cheers, Wayne
 
Try this:

If the Code said:
1. Overcurrent protection shall be provided in ungrounded conductors.
2. Overcurrent protection for conductors shall be in accordance with their ampacities specified in 310.15, unless otherwise required or permitted.

Would that make sense to you?

If I'm not required to provide overcurrent protection for the grounded conductor, and/or not permitted to provide overcurrent protection for the grounded conductor, then how would I protect the grounded conductor at it's ampacity?
 
david luchini said:
If the Code said:
1. Overcurrent protection shall be provided in ungrounded conductors.
2. Overcurrent protection for conductors shall be in accordance with their ampacities specified in 310.15, unless otherwise required or permitted.

Would that make sense to you?
Click to expand...
I'd go for that as a set of rules that better match current practice, since (2) is just telling you how to size it, without actually requiring it.

But that's not what we currently have. 240.4 says "Conductors . . . shall be protected against overcurrent in accordance with their ampacities . . ." with no exclusion for the grounded conductor. 240.4 already requires OCPD for the ungrounded (and grounded) conductors, 240.15(A) and 240.21 are redundant in that regard (but impose other requirements).

This also bring me to the question of why the grounded conductor can be sized smaller. For case of 220.61, I can see that it can be protected by calculation. But for 215(A)(1) Exception 3 on a 2-wire circuit, it makes no sense to me. If a 125A breaker sufficiently protects a #3 grounded conductor (if that's the intent of the exception), why doesn't it sufficiently protect a #3 ungrounded conductor? What's difference does the potential relative to earth make?

Cheers, Wayne
 
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