Backfeed breaker in meter-main with feed through lugs to MLO?

ryangittens

Member
Location
usa
Situation: Meter-main combo with 150A main breaker and available breaker space in the meter-main panel. However there are feed-through lugs at the end of the bus going to a MLO panel.

Question: Seems obvious to me that you can't add a backfeed breaker to the meter-main panel unless the feed through conductors and panel are rated for the main plus the PV OR the main is downsized. Is there any specific reference in the NEC?
 

wwhitney

Senior Member
Location
Berkeley, CA
705.12(B)(3)(6) is new to the 2020 NEC, I believe previous versions were silent on this issue.

I don't understand the point of the last sentence of 705.12(B)(3)(6), as if an overcurrent device is installed at the supply end of the feed-through conductors, those "feed through" conductors are just a normal feeder and are obviously covered by any of the preceding parts of 705.12(B)(3).

When the meter-main feed through lugs are on the opposite end of the busbar as the main breaker, and the feed through conductors are properly protected by the meter-main main breaker, and they land on in MLO panel, a logical interpretation would be to treat the two busbars and feed-through conductors like a single busbar. Which would allow the the 120% rule to apply to a connection in the MLO panel opposite its main lugs. But I don't think there's any language in the NEC to allow this.

Cheers, Wayne
 

wwhitney

Senior Member
Location
Berkeley, CA
But I don't think there's any language in the NEC to allow this.
I take that back, it looks like all the necessary language is there and has been for a while. 705.12(B)(3)(2) simply applies to each panel separately. For the MLO panel, the two sources are connected at opposite ends, and the main breaker in the meter main protects the busbar of the MLO panel. For the meter-main, again the two sources are connected at opposite ends.

So I guess 705.12(B)(3)(6) is just for the case of a panel with a utility supply, another power source connection, and a set of feed through lugs that is a different, third connection to the busbar. Still unclear on what the last sentence of 705.12(B)(3)(6) is trying to do.

Cheers, Wayne
 

jaggedben

Senior Member
Situation: Meter-main combo with 150A main breaker and available breaker space in the meter-main panel. However there are feed-through lugs at the end of the bus going to a MLO panel.

Question: Seems obvious to me that you can't add a backfeed breaker to the meter-main panel unless the feed through conductors and panel are rated for the main plus the PV OR the main is downsized. Is there any specific reference in the NEC?
You are essentially correct. As Wayne explained, this is implicit in the 2017 and 2014 NEC, and explicit in the 2020 NEC. You would be allowed to connect in the MLO panel at the opposite end. (Technically you possibly could connect in the main panel if the feeder and MLO panel are sized for the current from both sources, but that's not likely an existing setup you'll encouter.)
 

jaggedben

Senior Member
....

So I guess 705.12(B)(3)(6) is just for the case of a panel with a utility supply, another power source connection, and a set of feed through lugs that is a different, third connection to the busbar. Still unclear on what the last sentence of 705.12(B)(3)(6) is trying to do.

Cheers, Wayne
The last sentence essentially is saying: If you put an overcurrent device just downstream of the feed-through lugs, you can treat them like another breaker in the panel... And if you don't, you must treat the installation as if you're doing a feeder tap. It is making explicit what has been common sense for those of us who understand current-flow in these systems.
 

ryangittens

Member
Location
usa
705.12(B)(3)(6) is new to the 2020 NEC, I believe previous versions were silent on this issue.

I don't understand the point of the last sentence of 705.12(B)(3)(6), as if an overcurrent device is installed at the supply end of the feed-through conductors, those "feed through" conductors are just a normal feeder and are obviously covered by any of the preceding parts of 705.12(B)(3).

When the meter-main feed through lugs are on the opposite end of the busbar as the main breaker, and the feed through conductors are properly protected by the meter-main main breaker, and they land on in MLO panel, a logical interpretation would be to treat the two busbars and feed-through conductors like a single busbar. Which would allow the the 120% rule to apply to a connection in the MLO panel opposite its main lugs. But I don't think there's any language in the NEC to allow this.

Cheers, Wayne
Thanks, that makes perfect sense to think of it as one busbar and is an easy way to explain it.
 

ryangittens

Member
Location
usa
You are essentially correct. As Wayne explained, this is implicit in the 2017 and 2014 NEC, and explicit in the 2020 NEC. You would be allowed to connect in the MLO panel at the opposite end. (Technically you possibly could connect in the main panel if the feeder and MLO panel are sized for the current from both sources, but that's not likely an existing setup you'll encouter.)
Very good. Thanks for confirming.
 

wwhitney

Senior Member
Location
Berkeley, CA
The last sentence essentially is saying: If you put an overcurrent device just downstream of the feed-through lugs, you can treat them like another breaker in the panel... And if you don't, you must treat the installation as if you're doing a feeder tap. It is making explicit what has been common sense for those of us who understand current-flow in these systems.
That may be the intent, but the language they've used doesn't achieve that for any logical reading:

First, 705.12(B)(3) says "One of the following methods shall be used to determine the rating of the busbars". So that means you can determine compliance by just reading one of the subsections. Text in a subsections can't restrict the applicability of the other subsections, the restrictions would go in 705.12(B)(3) itself.

Moreover, 705.12(B)(3)(6) simply uses the phase "shall be permitted" That's inclusive language and does not indicate a restriction.

Lastly, 705.12(B)(3)(1) should not be limited by the presence of subfeed lugs.

Cheers, Wayne
 

jaggedben

Senior Member
That may be the intent, but the language they've used doesn't achieve that for any logical reading:

First, 705.12(B)(3) says "One of the following methods shall be used to determine the rating of the busbars". So that means you can determine compliance by just reading one of the subsections. Text in a subsections can't restrict the applicability of the other subsections, the restrictions would go in 705.12(B)(3) itself.

Moreover, 705.12(B)(3)(6) simply uses the phase "shall be permitted" That's inclusive language and does not indicate a restriction.
I see your points in favor of tweaking the way it is constructed, although I think the intent still comes through.
Lastly, 705.12(B)(3)(1) should not be limited by the presence of subfeed lugs.
Yes it should, and I'll back that up with an example I've run across in real life several times.

Let's say twenty years ago someone replacd a 100amp service panel. For the new equipment they used a 125A MLO panel but installed a backfed 100A breaker as a main. They did this because it was a cheap and widely available solution with no code problems. Perhaps at the same time or later someone also wanted to add a subpanel. They see the unused main lugs in the panel, and say to themselves "well, I can use those as feed through lugs as long as my feeder is rated 100A, and I can save space in the panel and the cost of a breaker." And they were correct about that per code and nothing was unsafe after they did that.

Now I come along and want to add a 40A solar breaker. And everything would be fine if the sub weren't connected to the feed-through lugs. But that 100A feeder can now see 100+40 amps combined, so the feeder is no longer protected from overload. If there's no space left to put it on a breaker instead of the lugs then I'll have to find some other solution. 705.12(B)(3)(6) addresses my other options explicitly.
 

wwhitney

Senior Member
Location
Berkeley, CA
For your example to speak to 705.12(B)(3)(1), the solar breaker would have to be 25A. Either way, though, your example is about protecting the wire-type feeder. So the rules to handle it don't belong in 705.12(B)(3), which is about protecting busbars.

Cheers, Wayne
 

jaggedben

Senior Member
For your example to speak to 705.12(B)(3)(1), the solar breaker would have to be 25A.
Correct, 25A or anything less. Lost track of the math there. But the point still stands. In previous codes, 705.12(B)(3)(1) was no less immune to the feed-through lug loophole than (2) or (3).

Either way, though, your example is about protecting the wire-type feeder. So the rules to handle it don't belong in 705.12(B)(3), which is about protecting busbars.
I kind of see your point, but given that most people don't realize that busbars are technically feeders, I think something had to be said about feed-through lugs. And the lugs are on the busbar, so I think it makes sense to say it in the section on busbars. Otherwise I think the implicit applicability of 705.12(B)(1) would be missed by most people. I agree that the construction and organization is slightly illogical and could be tweaked to better match the conventions that the code supposedly follows.
 

ryangittens

Member
Location
usa
So in essence, 705.12(B)(3)(6) is saying that you can connect a backfeed breaker on a panel with feed through lugs using 705.12(B)(3)(1) but if there is an ocpd protecting the feed through conductors you can use 705.12(B)(3)(1) - 705.12(B)(3)(3) (treat the panel like the lugs aren't there)?
 

jaggedben

Senior Member
So in essence, 705.12(B)(3)(6) is saying that you can connect a backfeed breaker on a panel with feed through lugs using 705.12(B)(3)(1) but if there is an ocpd protecting the feed through conductors you can use 705.12(B)(3)(1) - 705.12(B)(3)(3) (treat the panel like the lugs aren't there)?
In essence it is saying that you must protect conductors that are connected to feed through lugs.

I don't think ...(B)(3)(1) is special compared to the others.
 
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