PI: 705.12(B)

[wwhitney]

I'm listing only the new item I propose, rather than reiterate the whole section.

Cheers, Wayne


(7) Where a busbar has only three connections to it, counting the primary power source, each connection shall be protected by an overcurrent device whose rating does exceed the busbar ampere rating. Where the distribution equipment has provisions for a fourth connection, a permanent warning label shall be applied that displays the following or equivalent wording:

WARNING:

EQUIPMENT FED BY MULTIPLE SOURCES. DO NOT MAKE ADDITIONAL CONNECTIONS TO BUSBAR.


Statement of Problem and Substantiation for Public Input

Under the conditions specified in the proposed text, it is impossible to overload the busbar: By Kirkoff's Law, current in must equal current out. So if two of the three connections are acting as power sources (current in), the other must be acting as the sole load (current out), and the current out is suitably limited by its overcurrent device. While if only one of the connections is acting as a power source, the current in is limited as usual.

Also, please note that the second sentence of 2020 NEC 705.12 began "Where distribution equipment or feeders are fed simultaneously by a primary source of electricity and one or more other power source and are capable of supplying multiple branch circuits or feeders, or both." This wording limits the scope of the rest of 705.12 to equipment capable of at least 4 connections: at least two for the power sources, and at least two for the multiple branch circuits/feeders.

However, in the 2023 NEC and the corresponding sentence in 705.12, the qualifier "and are capable of supplying multiple branch circuits or feeders, or both" was omitted. This removed the exemption for equipment only capable of 3 connections.

Therefore, this proposed addition restores the longstanding allowance for equipment capable of only 3 connections. It also extends the allowance to equipment capable of more than 3 connections, so long as a suitable warning is posted, in accordance with the other items in this section.

 

[jaggedben]

I like the general idea.

However I don't agree with your reasoning that this restores an allowance that used to be there.

Say I have a meter main with a 200A main breaker and 200A busbar. In it the only breakers I have (although there are other spaces) are a 125A breaker feeding an ordinary subpanel with only loads, and another 125A breaker feeding a microgrid (or just another subpanel) that has both sources and loads. Say also that the sources connected to the micro-grid feeder can output more than 32A so I can't use the 120% rule.

Under the old 'capable of supplying multiple branch circuits or feeders, or both' there was no allowance for this setup, not only because the panel is 'capable' of feeding more feeders or branch circuits, but also because it does supply two feeders. But under your new proposed rule the setup would be allowed.

Also Kirchhoff's Law. (spelling)

 

[wwhitney]

I like the general idea.

However I don't agree with your reasoning that this restores an allowance that used to be there.

Say I have a meter main with a 200A main breaker and 200A busbar. In it the only breakers I have (although there are other spaces) are a 125A breaker feeding an ordinary subpanel with only loads, and another 125A breaker feeding a microgrid (or just another subpanel) that has both sources and loads. Say also that the sources connected to the micro-grid feeder can output more than 32A so I can't use the 120% rule.

Under the old 'capable of supplying multiple branch circuits or feeders, or both' there was no allowance for this setup, not only because the panel is 'capable' of feeding more feeders or branch circuits, but also because it does supply two feeders.

You didn't quote the full text, it was "fed simultaneously by a primary source of electricity and one or more other power source and are capable of supplying multiple branch circuits or feeders, or both."

While the arrangement you describe can be fed by two sources of supplies, it only supplies one feeder when doing that. And while it can supply two feeders, it only has one source of supply while doing that.

So I'd say that yes, if the equipment couldn't accept any more breakers, it was exempt under the old language. I mean, maybe the "fed by" and "capable of" language are slightly different, and so the implication is that it doesn't have to be simultaneous. But I don't think the simultaneous interpretation is obviously wrong or indefensible.

Thanks for the spelling correction, I will adjust.

Cheers, Wayne

 

[jaggedben]

'Capable of' to me has the broad connotation of 'it could happen, regardless of whether it is happening.' So how the sources or loads are connected or what they are doing at any given moment is irrelevant. I read your 'full quote' language as exempting only equipment such as disconnects or breaker enclosures that are only designed for use with a single circuit. So I don't follow you at all. And I think the CMP will follow you even less.

That said, the actual rule of your proposal has technical merit in my opinion. So my advice is to drop the weak 'restoration' argument and focus on the technical merit.

 

[wwhitney]

That said, the actual rule of your proposal has technical merit in my opinion. So my advice is to drop the weak 'restoration' argument and focus on the technical merit.

OK, I don't find the argument as weak as you do, since to me the "capable of" is implicitly simultaneous with the "two sources of supply". So I'll leave it in. If you like the proposal but think you have a better argument for it, I encourage you to submit your own PI.

Do you think it is worth adding a sentence or two about how with the limitations described in the first paragraph, the resulting worst case I²R heating of the busbar will be no more than the single source of supply case? Or does that go without saying?

Also, as someone with more experience, if you can provide some info on situations where the new section would be useful in the field, I could add a sentence or two about that.

Cheers, Wayne

 

[tortuga]

I'm listing only the new item I propose, rather than reiterate the whole section.

Cheers, Wayne


(7) Where a busbar has only three connections to it, counting the primary power source, each connection shall be protected by an overcurrent device whose rating does exceed the busbar ampere rating. Where the distribution equipment has provisions for a fourth connection, a permanent warning label shall be applied that displays the following or equivalent wording:

WARNING:

EQUIPMENT FED BY MULTIPLE SOURCES. DO NOT MAKE ADDITIONAL CONNECTIONS TO BUSBAR.

I think its a good proposal also but you can distill it down to:
(7) Where a busbar has only three connections to it each connection shall be protected by an overcurrent device whose rating does exceed the busbar ampere rating.

The article 705 warning label madness is out of hand.
I'd delete all warning labels in 705 and just have one rule that says if article 705 is used in a panel a label with the code year and code section used shall be applied.

 

[jaggedben]

I think its a good proposal also but you can distill it down to:
(7) Where a busbar has only three connections to it each connection shall be protected by an overcurrent device whose rating does exceed the busbar ampere rating.

The article 705 warning label madness is out of hand.
I'd delete all warning labels in 705 and just have one rule that says if article 705 is used in a panel a label with the code year and code section used shall be applied.

I agree with code citations on labeling. But there's something to be said for basic instructions, too. It's not like electricians who don't read labels are going to look up code either. 'DO NOT RELOCATE THIS OVERCURRENT DEVICE' is simple enough to follow. The other one for the sum of breakers rule, not so much.

If there's a general criticism to be made of Wayne's proposal here, it's that 705.12 is complicated enough already.