Using feed thru lugs to feed a panel?

[jaggedben]

usually only when there is ambiguity.

Which is the case most of the time when you're talking about the NEC.

 

[gene6]

OK I just looked up 705.12(B)(3)(6) while I am waiting in my truck, this is not vague at all first sentence is declarative and not conditional on anything following. "shall be permitted" is the only qualifier in that first sentence.

 

[wwhitney]

OK I just looked up 705.12(B)(3)(6) while I am waiting in my truck, this is not vague at all first sentence is declarative and not conditional on anything following. "shall be permitted" is the only qualifier in that first sentence.

Yes, read literally the first sentence says that to comply with (2020) 705.12(B)(3), all you need to do is add some feed-thru lugs and feed-thru conductors, and then you can load up the bus all you like. But everyone knows that is not what is meant and would not be safe. The CMPs need to learn how to write logically.

Cheers, Wayne

 

[gene6]

Yes, read literally the first sentence says that to comply with (2020) 705.12(B)(3)

Incorrect it says 705.12(B)(1)

all you need to do is add some feed-thru lugs and feed-thru conductors, and then you can load up the bus all you like.

Incorrect 705.12(B)(1) (a) or (b) would not permit that.

 

[wwhitney]

Incorrect

My statement was correct. The unreasonable literal reading goes like this:

- 705.12(B)(3) says "One of the following methods shall be used to determine the ratings of busbars:" OK we get to pick any subsection; if it complies with the subsection, then the busbar complies with 705.12(B)(3).

-705.12(B)(3)(6) says in the first sentence: "Connections shall be permitted on busbars of panelboards that supply lugs connected to feed-through
conductors." End of sentence, no qualification. So if we have lugs and feed-through conductors, then the busbar qualifies. The next sentence says something about how to size the feed-through conductors, that's fine, you have to comply with that. The last sentence says "Where . . . shall
be permitted . . ." and just offers you additional allowances you already had, it doesn't impose any requirement. It's meaningless.

Basically 705.12(B)(3)(6) is written wrong. The first sentence of the other 5 subsections of 705.12(B)(3) all have a qualifier, "where" something is true. So 705.12(B)(3)(6) should be rewritten along the following lines:

Connections shall be permitted on busbars of panelboards that supply lugs connected to feed-through conductors, where the following requirements are met:
- The bit about sizing the conductors per 705.12(B)(1)
- The bit about "an overcurrent device . . . installed at the supply end of the feed-through conductors,"
- The bit about complying with 705.12(B)(3)(1), (2), or (3) (presumably notwithstanding the feed-thru lugs)

But even that rewrite is not good enough, as 705.12(B)(3)(1), (2), or (3) have no language limiting applicability to a busbar without feed-through lugs (although in the case of (2), if the feed-through lugs are at the opposite end of the busbar, that would rule out using (2)). So if you can comply with one of those, ignoring the feed-through lugs, you're already done with 705.12(B)(3), you don't need to look at 705.12(B)(3)(6).

Thus in addition the first sentence of 705.12(B)(3) would need to be changed to something like: "Busbars that supply lugs connected to feed-through conductors shall comply with 705.12(B)(3)(6). All other busbars shall use of one of the following methods to determine the ratings of the busbars."

Cheers, Wayne

 

[gene6]

Yes, read literally the first sentence says that to comply with (2020) 705.12(B)(3), all you need to do is add some feed-thru lugs and feed-thru conductors, and then you can load up the bus all you like.

Please provide some examples of how one could "load up the bus all you like" and comply with 705.12(B)(1)?

 

[wwhitney]

Please provide some examples of how one could "load up the bus all you like" and comply with 705.12(B)(1)?

OK, 200A service, meter/main/distribution with 200A main breaker and feed-thru lugs, 200A feed-thru conductors to a 200A main breaker subpanel. Now let's put another 400A of load breakers on the service panel bus, and 200A of PV breakers. The feed-thru conductors still comply with 705.12(B)(1)(b) because of the main breaker on the subpanel.

Cheers, Wayne

 

[gene6]

OK, 200A service, meter/main/distribution with 200A main breaker and feed-thru lugs, 200A feed-thru conductors to a 200A main breaker subpanel. Now let's put another 400A of load breakers on the service panel bus, and 200A of PV breakers. The feed-thru conductors still comply with 705.12(B)(1)(b) because of the main breaker on the subpanel.

Cheers, Wayne

Nope that's a violation of 705.12(B)(1)(b)

 

[wwhitney]

Nope that's a violation of 705.12(B)(1)(b)

Care to expand on that? I don't see it.

Cheers, Wayne

 

[gene6]

So in your example you have typical meter main
Load side of main is bolted to busbars.

Are these busbars a feeder?
Are these busbars conductors?
Does the busbar feed-thru?

 

[wwhitney]

Ah, I was wondering if you were thinking about applying 705.12(B)(1) to busbars. To answer your first question, yes, busbars are feeders per the definition, but the usage of the word feeder in the NEC often, and in 705.12(B)(1) in particular, excludes busbars in panelboards.

First, note that 705.12(B)(1) refers to "the opposite end of the feeder from the primary source overcurrent device," If busbars are feeders under 705.12(B)(1), where does a feeder end? The only place it can end is a branch circuit breaker. I.e. the premises wiring system ends up as one large feeder graph starting at the service point and extending to each branch circuit breaker. There would be no internal feeder ends within that graph. That is certainly contrary to the usual understanding of 705.12(B)(1).

Also, let's see what happens to 705.12(B)(3) if busbars also have to satisfy 705.12(B)(1):

- 705.12(B)(3)(1) is the equivalent of 705.12(B)(1)(a), no problem.

- 705.12(B)(3)(2) becomes useless. If the busbar has loads on it between the primary source connection and the other source connection at the opposite end of the busbar, then 705.12(B)(1)(a) would require the busbar to effectively comply with 705.12(B)(3)(1) instead.

[If there is only one load breaker between the two source connections (the busbar has just 3 connections), then 705.12(B)(1)(b) could be used, and so there's no problem. In fact, the 705.12(B) would be improved if 705.12(B)(3) were limited to busbars with 4 or more connections, and busbars with only 3 connections were subject to 705.12(B)(1) instead.]

- 705.12(B)(3)(3) similarly becomes useless.

I conclude that "feeder" in 705.12(B)(3)(1) means "non-busbar feeder." But again, the NEC is illogically written, so we are forced to do this type of interpretation.

Cheers, Wayne

 

[gene6]

Ah, I was wondering if you were thinking about applying 705.12(B)(1) to busbars. To answer your first question, yes, busbars are feeders per the definition,

We agree

the usage of the word feeder in the NEC often, and in 705.12(B)(1) in particular, excludes busbars in panelboards.

706.12(B)(3)(6) carves out the single case of a panelboard with feed thru lugs, these particular types of panels are considered part of a service or feeder depending if they have a main at the supply end.

In this special case of 706.12(B)(3)(6) the busbar in the supplying panelboard together with the feeder wires to the second panel are all conductors of one feeder that "feeds-though", only then can one use 705.12(B)(1).

[If there is only one load breaker between the two source connections (the busbar has just 3 connections), then 705.12(B)(1)(b) could be used, and so there's no problem.

Agreed

In fact, the 705.12(B) would be improved if 705.12(B)(3) were limited to busbars with 4 or more connections, and busbars with only 3 connections were subject to 705.12(B)(1) instead.

Agreed, that would make it clear.

 

[tortuga]

Thanks all for your further comments, been a busy week i'll have to read through all this later with my codebook and a beverage.

 

[wwhitney]

In this special case of 706.12(B)(3)(6) the busbar in the supplying panelboard together with the feeder wires to the second panel are all conductors of one feeder that "feeds-though", only then can one use 705.12(B)(1).

I agree that is a plausible logical way to read 705.12(B)(3)(6) based on the definitions. However, I'm confident that is not what was intended, and that in 705.12(B), "feeder" is used to mean a non-busbar feeder, and that "feed through conductors" is meant to exclude the busbar.

Cheers, Wayne

 

[tortuga]

OK looking into this deeper, as I'd like to understand this and the intent of CMP4.
it appears the CMP wrote their own proposal
On the nfpa website there is a document:
70_A2020_NEC_P04_FD_FRStatements.pdf
On page 163 there is the first occurance of the new paragraph 705.12(B)(3)(6) it was 705.12(B)(3)(f) at that revision
Submitter Full Name: NEC-CMP Panel 04
Submittal Date:
Thu Jan 18 16:55:22 EST 2018
First Revision No. 8902-NFPA 70-2018 [ Section No. 705.12 ]

(f) Connections shall be permitted on busbars that supply feed-through lugs and conductors
connected to the lugs opposite the main source of supply. The ampacity of the busbar and connected
feeders shall not be less than the sum of the primary source overcurrent device and 125 percent of the
power-source output circuit current.

The intent is for 705.12(B)(3)(6) to treat the busbars and the attached feeders as one and have them be sized
as what would become 705.12(B)(1)(a) in the final 2020 NEC.


Then there were 5 other proposals from the public (and one of their names sounds familiar).

I can't find where they voted on these proposals and what the outcome was -yet.
They usually give 'panel statements' that are enlightening as to their intent.
But one of the proposals was to remove the "opposite the main source of supply" so that passed.

Then later in the document: 70_A2019_NEC_AAC_SD_ballotfinal.pdf
Page 134
Submitter Information Verification
Committee:
NEC-AAC
Submittal Date: Tue Feb 19 10:07:05 EST 2019
Committee Statement:
The Correlating Committee has taken this action on SR 8148 to correct a circular reference.

They make these changes:

Connections shall be permitted on busbars of panelboards that supply feed-through lugs and

conductors connected to the lugs opposite the main source of supply. feed-through conductors. The

feed-through conductors shall be sized in accordance with 705.12(B)(1) . Where an overcurrent

device is installed at the supply end of the feed-through conductors, the busbar in the supplying

panelboard shall be permitted to be sized in accordance with 705.12(B)(3) (1) through 705.12(B)

(3) (3). The ampacity of the busbar and connected feeders shall not be less than the sum of the

primary source overcurrent device and 125 percent of the power-source output circuit current.

Thats a pretty big change to correct a circular reference.

 

[tortuga]

the other file I was looking for is 70_A2019_NEC_P04_PCResponses.pdf
looks like they considered all the public inputs / code change proposals and labeled them all as

Rejected but see related SR

All they say in the Resolution Statements regarding705.12(B)(3)(6) is

705.12(B)(3)(6) is further edited to clarify how panelboards with feed-through conductors shall be
sized and how the feed-through conductors are sized.

 

[gene6]

Wayne I am surrendering to all your interpretations so far other than this one:

I would say the diagram does not comply with 2020 NEC 705.12(B)(3)(6). That requires "an overcurrent device . . . installed at the supply end of the feed-through conductors," to allow the use of 705.12(B)(3)(3) for the panel busbar.

The main breaker in the supplying panel protects the feed-thru, a MLO would not.

 

[wwhitney]

Wayne I am surrendering to all your interpretations so far other than this one:

Sorry if this was combat by text, I hope you mean you find my arguments convincing. Totally agree that these sections were not written by anyone with adequate logical training.

The main breaker in the supplying panel protects the feed-thru, a MLO would not.

I can buy that if there are no secondary sources attached to breakers on the bus. In which case the secondary source (the only reason to be looking at 705.12) would be attached via the feed-thru lugs, i.e. in the panel supplied by the feed-through conductors, or a feeder interconneciton to the feed-through conductors. If there's a secondary source on a breaker on the panelboard bus, then there's no one breaker protecting the feed-through conductors, and since 705.12(B)(3)(6) uses the singular, I would say that doesn't comply.

Cheers, Wayne

 

[tortuga]

We seem to be honing in on this mysterious "supply end" breaker wording that was added in the revision.
Where would this breaker be if not the main in downstream panel 'B" then?
If the feeder conductors have lugs at the "supply end" from the perspective of the conductors then you dont need this section.
I think"supply end" is from the perspective of the panel "B"

 

[LarryFine]

Is my thought in post #37 a viable suggestion?