PI: 240.4 (Second)

[wwhitney]

Another. Cheers, Wayne

New paragraph 240.4(I) (with the opening of 240.4 updated to reference the new paragraph):

(I) Neutral Conductors. Neutral conductors sized according to their load calculated per 220.61 shall be permitted to be protected against overcurrent by the overcurrent protective devices for their associated ungrounded conductors.

Statement of Problem and Substantiation for Public Input

220.61 permits a neutral conductor's size to be reduced relative to its corresponding ungrounded circuit conductors, and this is common industry practice. However, 240.4 requires that circuit conductors be protected against overcurrent in accordance with their ampacity, and it currently provides no allowance for a reduced size neutral. As such this common industry practice is in violation of 240.4 as currently written. This change removes this presumably unintended oversight.

 

[letgomywago]

I thought this was settled in the 1920s when fused nuetrals stopped

 

[infinity]

Judging this PI from a voting perspective of a CMP I would say it is reject based on the fact that it is not necessary. As letgo stated it does not have an OCPD and it's size has already been determined to be adequate as part of the calculation.

 

[wwhitney]

This question is very simple, and it's easy not to see it because everyone is used to how things are done.

Say you have a 100A feeder with a neutral whose calculated load under 220.61 is 70A, so you run a 70A neutral with your feeder.

What's the ampacity of the neutral conductor? 70A
What's the rating of the OCPD that protects it? 100A
Is the neutral conductor "protected against overcurrent in accordance with its ampacity"? No

Cheers, Wayne

 

[wwhitney]

it does not have an OCPD and it's size has already been determined to be adequate as part of the calculation.

Neutrals don't have direct OCPD, but they are protected by the OCPD in the associated ungrounded conductors.

For a 2-wire circuit that's obvious. For a multiwire circuit on a 3-phase system, a full size neutral is still protected by the OCPD, because the way the vector math works out, the unbalanced current is never more than the maximum current on any of the ungrounded conductors. And for the very rare 2-phase system (e.g. 3-wire or 5-wire), the neutral current can be up to 141% of the maximum current on the ungrounded conductors, and the neutral is required to be accordingly oversized.

So when we downsize the neutral per 220.61, we have a conductor that is not protected against overcurrent in accordance with its ampacity. That's a conflict with the plain language of 240.4, so 240.4 needs to have an allowance to do this.

Cheers, Wayne

 

[letgomywago]

Neutrals don't have direct OCPD, but they are protected by the OCPD in the associated ungrounded conductors.

For a 2-wire circuit that's obvious. For a multiwire circuit on a 3-phase system, a full size neutral is still protected by the OCPD, because the way the vector math works out, the unbalanced current is never more than the maximum current on any of the ungrounded conductors. And for the very rare 2-phase system (e.g. 3-wire or 5-wire), the neutral current can be up to 141% of the maximum current on the ungrounded conductors, and the neutral is required to be accordingly oversized.

So when we downsize the neutral per 220.61, we have a conductor that is not protected against overcurrent in accordance with its ampacity. That's a conflict with the plain language of 240.4, so 240.4 needs to have an allowance to do this.

Cheers, Wayne

The neutral is protected from fault already since it's sized atleast as big as the equipment ground.

 

[wwhitney]

The neutral is protected from fault already since it's sized atleast as big as the equipment ground.

OK, it's protected against short circuit and ground fault. But when it is reduced in size, it isn't protected against overcurrent (overload) in accordance with its ampacity. So it does not comply with the general rule at the beginning 240.4.

Cheers, Wayne

 

[wwhitney]

220.22 says you size for unbalanced load. It doesn't need more than that.

Presumably you mean 220.61?

It does, in fact, require more than that. Article 220 tells you how to calculate the loads. But you need to look at other articles on how to use those loads to size the wires (210, 215, 230, and 310) and to size the OCPD (240). So if we want to be able to use undersized wires (relative to OCPD), because 220.61 tells us we have a reduced load, then we need an exemption from the general rule in 240.4.

Cheers, Wayne

 

[letgomywago]

OK, it's protected against short circuit and ground fault. But when it is reduced in size, it isn't protected against overcurrent (overload) in accordance with its ampacity. So it does not comply with the general rule at the beginning 240.4.

Cheers, Wayne

220.61 covered this

 

[letgomywago]

Presumably you mean 220.61?

It does, in fact, require more than that. Article 220 tells you how to calculate the loads. But you need to look at other articles on how to use those loads to size the wires (210, 215, 230, and 310) and to size the OCPD (240). So if we want to be able to use undersized wires (relative to OCPD), because 220.61 tells us we have a reduced load, then we need an exemption from the general rule in 240.4.

Cheers, Wayne

At least you sending this in will cement it for people like you who have issues with this.

 

[wwhitney]

220.61 covered this

It just tells you the load. It doesn't tell you how to size the wires, or what OCPD to use, or anything else. The sections that do tell you those things need exemptions for the reduced neutral load allowed by 220.61, otherwise the fact that the load is less is useless. [Well, the sections on conductor sizing refer to the load, which is presumably on a per conductor basis, so that part works without any explicit language.]

Cheers, Wayne

 

[wwhitney]

At least you sending this in will cement it for people like you who have issues with this.

All I want is for the code language to be clear and exact. None of this "it doesn't really mean what it literally says, because we all know you do it a certain way."

Cheers, Wayne