120% rule and end-of-busbar

[prof.dmiranda]

I'm started working at a PV design company. One of our clients instructed us to always include a main breaker in the sub-panel if said sub-panel, is fed through lugs and is not protected, whenever we design with a backfeed breaker inside the 120% rule. The reasoning is that the new MCB in the sub-panel will "break" the bus bar from the main panel, thus making the end of the bus bar the bottom of the main panel. If the new 200A breaker is not there, then the end of the bus bar would be at the bottom of the sub-panel. They want to be inside the NEC 705.12 (3)(2) Where two sources, one a primary power source and the other another power source, are located at opposite ends of a busbar that contains loads...I apologize if the terms I used are not correct. I hope I explained myself in a clear manner.

https://imgur.com/avO2Qp5

 

[prof.dmiranda]

I think I made the post without an actual question or doubt. So, I would like to know if the need for a breaker to "protect" the loads is correct or if the pv breaker, as it is in the drawing, can be installed at the bottom of the main breaker without any further changes.

 

[jaggedben]

In my opinion the request is more or less correct but the reasoning is wrong. You put a main breaker in the sub because otherwise the sub with 200A busbar could be fed by up to 221 amps. The code section is 705.12(B)(1) [2020 NEC reference], which is not the same as the 120% rule.

In my opinion the compliance of the design shown is still questionable because if the feed through lugs are at the opposite end of the busbar then the 30A breaker is not at the opposite end, and the code section you are quoting (120% rule, opposite end) is not complied with. This can be gotten around by doing a load side tap from the feeder to a fused disconnect, instead us using a breaker, although arguably the two designs are equally safe.

 

[prof.dmiranda]

In my opinion the request is more or less correct but the reasoning is wrong. You put a main breaker in the sub because otherwise the sub with 200A busbar could be fed by up to 221 amps. The code section is 705.12(B)(1) [2020 NEC reference], which is not the same as the 120% rule.

In my opinion the compliance of the design shown is still questionable because if the feed through lugs are at the opposite end of the busbar then the 30A breaker is not at the opposite end, and the code section you are quoting (120% rule, opposite end) is not complied with. This can be gotten around by doing a load side tap from the feeder to a fused disconnect, instead us using a breaker, although arguably the two designs are equally safe.

Thanks for pointing that out. What would be the practical definition of end-of-the-bus-bar, then?

 

[jaggedben]

How about: the farthest away point at which you can make an electrical connection according to the manufacturers instructions

 

[wwhitney]

What version of the NEC?

2017 NEC 705.12(B)(2)(3) does not directly address the question of feed-thru lugs on the bus. So the most stringent interpretation of 705.12(B)(2)(3)(b) would not allow its use when there are feed-thru lugs in use at the opposite end of the bus from the utility supply, as the interconnected source would not be at the opposite end.

2020 NEC 705.12(B)(3)(6) does address feed-thru lugs. It uses the language "Where an overcurrent device is installed at the supply encl of the feed-through conductors, the busbar in the supplying panel board shall be permitted to be sized in accordance with 705.12(B)(3)(1) through 705.12(B)(3)(3)." That language about "supply end" to me suggests that the 200A breaker would have to be in an enclosure next to the meter main, rather than a main breaker in the subpanel. But it's pretty unclear and I may have that wrong.

2023 NEC 705.12(B)(5) changes that language to "Where an overcurrent device is installed at either end of the feed-through conductors, panelboard busbars on either side of the feed-through conductors shall be permitted to be sized in accordance with 705.12(B)(1) through (B)(3)." Which clarifies the location language that was confusing in the 2020 NEC, and it tacitly suggests when applying 2023 705.12(B)(2) to either busbar, the feed-through lugs may be ignored when determining what is the "opposite end" of the busbar.

Cheers, Wayne

 

[jaggedben]

....
2023 NEC 705.12(B)(5) changes that language to "Where an overcurrent device is installed at either end of the feed-through conductors, panelboard busbars on either side of the feed-through conductors shall be permitted to be sized in accordance with 705.12(B)(1) through (B)(3)." Which clarifies the location language that was confusing in the 2020 NEC, and it tacitly suggests when applying 2023 705.12(B)(2) to either busbar, the feed-through lugs may be ignored when determining what is the "opposite end" of the busbar.

Cheers, Wayne

Tacitly? Feed through lugs may or may not be at the opposite end. If they are not, then the permission you quote let's you ignore them when they have an overcurrent device. If they are at the opposite end, then the permission doesn't let you ignore them if you still want to qualify the installation by the 'opposite end' rule. (You could ignore them if you use (1) or (3) instead of (2).) Since it depends on details I wouldn't say there's any 'tacit' permission to ignore in all situations.

 

[tortuga]

2020 NEC 705.12(B)(3)(6) does address feed-thru lugs. It uses the language "Where an overcurrent device is installed at the supply encl of the feed-through conductors, the busbar in the supplying panel board shall be permitted to be sized in accordance with 705.12(B)(3)(1) through 705.12(B)(3)(3)." That language about "supply end" to me suggests that the 200A breaker would have to be in an enclosure next to the meter main, rather than a main breaker in the subpanel. But it's pretty unclear and I may have that wrong.

I think we have discussed this before under the 2020 'supply end' has the same meaning as 'either end '. As they meant 'supply end' of the end panel, and as evidenced by the 2033 clarification.

 

[wwhitney]

Tacitly? Feed through lugs may or may not be at the opposite end.

If we are going to call those plug-on devices with lugs only, no OCPD, "feed through lugs," then I agree. IIRC UL67 defines "feed through lugs" as being at the opposite end of the bus. But perhaps just because it was written before those plug-on devices were available. Or maybe I'm misremembering.

If they are not, then the permission you quote let's you ignore them when they have an overcurrent device. If they are at the opposite end, then the permission doesn't let you ignore them if you still want to qualify the installation by the 'opposite end' rule.

My thinking is this:

- 2023 NEC 705.12(B) just says "one of the following methods shall be used". It doesn't say "all of the following shall apply as applicable."
- Since 705.12(B)(1) through (4) don't have any language limiting them to cases without feed through lugs, they may always be applied even if there are feed through lugs; we don't even have to read 705.12(B)(5).
- So the only reason for 705.12(B)(5) to exist is to provide an extra allowance not otherwise available. And that allowance is to ignore the feed-through lugs when applying 705.12(B)(2) if the provisions of 705.12(B)(5) are satisfied.

Note that if I have a busbar with the connection pattern "source 1 - loads - source 2 - one single load", where each connection to the bus is limited by a breaker to at most the busbar ampacity, it's not possible to overload the bus. [At least for the case that there are no other connections to the feed-through conductors between the feed-through lugs and the referenced OCPD; this requirement should probably be added to 705.12(B)(5).]

This is unlike the explicit allowance we are given in 705.12(B)(4), which obviously allows overloading the bus. 705.12(B)(4) really should have some language to the effect "the portion of the busbar that excludes the secondary source connected at the end, and all load breakers between that secondary source and the center feed, shall comply with 705.12(B)(3)."

Cheers, Wayne

 

[jaggedben]

If we are going to call those plug-on devices with lugs only, no OCPD, "feed through lugs," then I agree. IIRC UL67 defines "feed through lugs" as being at the opposite end of the bus. But perhaps just because it was written before those plug-on devices were available. Or maybe I'm misremembering.
...

There are at three ways you could have conductors connected to lugs instead of breaker:
-lugs that match your UL quote
-plug on 'subfeed' lugs like you mention
-main lug panel converted to main breaker with a breaker to the regular bus and a hold down kit.

I see no reason why this rule should apply to the first one but not all three.

 

[jaggedben]

...


My thinking is this:

- 2023 NEC 705.12(B) just says "one of the following methods shall be used". It doesn't say "all of the following shall apply as applicable."
- Since 705.12(B)(1) through (4) don't have any language limiting them to cases without feed through lugs, they may always be applied even if there are feed through lugs; we don't even have to read 705.12(B)(5).
- So the only reason for 705.12(B)(5) to exist is to provide an extra allowance not otherwise available. And that allowance is to ignore the feed-through lugs when applying 705.12(B)(2) if the provisions of 705.12(B)(5) are satisfied.
...

I think that's a mostly a very good way to make sense of a rule that I'm not convinced was actually as thoroughly thought through when written and adopted. But the last sentence is still not logical. You have an allowance to follow other rules that could be ambiguous without the allowance, not an allowance to ignore one part of one of those rules at your discretion. Connecting above feed through lugs that are at the opposite end clearly violates the intent of the opposite end rule; it's not electrically different than if the lugs were a breaker on the busbar instead.

705.12(B)(4) is different because it's limited to dwellings.

 

[tortuga]

This is unlike the explicit allowance we are given in 705.12(B)(4), which obviously allows overloading the bus. 705.12(B)(4) really should have some language to the effect "the portion of the busbar that excludes the secondary source connected at the end, and all load breakers between that secondary source and the center feed, shall comply with 705.12(B)(3)."

You mean this scenario?

 

[wwhitney]

You have an allowance to follow other rules that could be ambiguous without the allowance

I don't see the other rules as being ambiguous in the presence of feed-thru lugs. (1) ignores them, they are immaterial. For (2), if they are at the end of the bus opposite the primary supply, then you just can't use (2). And (3) ignores them, which is a mistake but how it's written. (3) needs an internal limitation with respect to feed-thru lugs.

So we have a few imperfect interpretations of (5):

(A) It's attempting to modify (1), (2) and (3); if you have subfeed lugs, you must satisfy (5) before you can use (1), (2), or (3). But the text at the beginning of 705.12(B) is not written in a way that allows (5) to modify (1), (2), and (3).
(B) It's providing an additional allowance not present in (1), (2), or (3), but does so only implicitly.
(C) It's useless text that does nothing.

I'm arguing for (B); I guess you're arguing for (A) or (C)?

Note that if in (2) the subfeed lugs aren't at the end of the bus, they are immaterial to compliance with (2). So the only additional allowance that (5) can offer with respect to (2) is to allow the subfeed lugs at the end of the bus and allow connecting of the secondary source at the penultimate connection.

Note further that (2) could be modified in general to permit one load (OCPD limited) connection between the secondary source connection and the end of the bus, without any risk of overloading the bus. Or maybe with most bus layouts, as there are two bottom most positions, each such position will connect the very bottom of the bus for one pole, and the second pole (and third pole) would already be not quite farthest?

Cheers, Wayne

 

[wwhitney]

You mean this scenario?

Could be, but we don't even need feed thru lugs. Just take a 200A center fed panel, with a sum of breakers count of 250A per pole, and arrange the breakers as "40A secondary source - center fed main -- all load breakers". Complies with 2023 NEC 705.12(B)(4) but obviously allows up to 240A of current on the half of the bus that has the load breakers.

Cheers, Wayne

 

[tortuga]

(3) needs an internal limitation with respect to feed-thru lugs.

Do you think this arangement would be allowed with 705.12(B)(3) and 705.12(B)(5)?

 

[wwhitney]

Do you think this arangement would be allowed with 705.12(B)(3) and 705.12(B)(5)?

I think it should not be allowed by a properly written 705.12(B), as it allows 220A of current on the red bus between the 100A inverter breaker and the 20A breaker below that.

As to whether it complies with 2023 NEC 705.12(B)(3) and 705.12(B)(5) as written, it obviously does comply with 705.12(B)(5). And I think it complies with 705.12(B)(3) as written, as 705.12(B)(3) is missing the required language to count the breaker protecting the feed thru conductors as if it were on the red bus.

Cheers, Wayne

 

[jaggedben]

I don't see the other rules as being ambiguous in the presence of feed-thru lugs. (1) ignores them, they are immaterial. For (2), if they are at the end of the bus opposite the primary supply, then you just can't use (2).

This bit about (2) is incorrect. You can backfeed the lugs by putting a breaker in the panel fed by them or by tapping that feeder, for example. As I suggested in my initial response to this thread.

So we have a few imperfect interpretations of (5):

(A) It's attempting to modify (1), (2) and (3); if you have subfeed lugs, you must satisfy (5) before you can use (1), (2), or (3). But the text at the beginning of 705.12(B) is not written in a way that allows (5) to modify (1), (2), and (3).
(B) It's providing an additional allowance not present in (1), (2), or (3), but does so only implicitly.
(C) It's useless text that does nothing.

I'm arguing for (B); I guess you're arguing for (A) or (C)?

I'm arguing for (C). It's the most perfect of the imperfect interpretations. I believe the intention was mainly to address the following...

And (3) ignores them, which is a mistake but how it's written. (3) needs an internal limitation with respect to feed-thru lugs.

...and they went about it in the wrong way.

Note that if in (2) the subfeed lugs aren't at the end of the bus, they are immaterial to compliance with (2).

Correct, although they probably still need an overcurrent device to comply with 705.12(B)(1). But said section takes care of that. It's the same for (1).

So the only additional allowance that (5) can offer with respect to (2) is to allow the subfeed lugs at the end of the bus and allow connecting of the secondary source at the penultimate connection.

I follow you here, but remember I'm arguing the whole thing is useless text that does nothing on the face of its meaning. And you're looking for a possible meaning between the lines to justify the section's existence. Also, you are arguing the section exists to provide this allowance for (2), but you can't say what allowance is needed for (1), as you more or less pointed out yourself at top of the reply I'm quoting. The whole section just really doesn't make consistent sense.

Note further that (2) could be modified in general to permit one load (OCPD limited) connection between the secondary source connection and the end of the bus, without any risk of overloading the bus. Or maybe with most bus layouts, as there are two bottom most positions, each such position will connect the very bottom of the bus for one pole, and the second pole (and third pole) would already be not quite farthest?

Each busbar has an opposite end position. A side by side panel could have two breakers in that position, and with tandem breakers you could have 4 poles connected to the opposite end. But all this is a little too much for a code revision.

 

[tortuga]

This bit about (2) is incorrect. You can backfeed the lugs by putting a breaker in the panel fed by them or by tapping that feeder, for example. As I suggested in my initial response to this thread.

Like so:

 

[wwhitney]

This bit about (2) is incorrect. You can backfeed the lugs by putting a breaker in the panel fed by them or by tapping that feeder, for example. As I suggested in my initial response to this thread.

OK, yes of course, I was limiting my consideration to the case that the secondary source is connecting via a separate breaker in the panel in question.

Cheers, Wayne

 

[wwhitney]

Like so:

OK, but 2023 NEC 705.12(B)(5) is immaterial for that install. You could do that with the 2017 NEC, it just complies with the 120% rule directly.

Cheers, Wayne