Busbar connection at meter-main panel with feedthrough

[ryangittens]

Reviewer is saying that the OCPD for the feedthrough conductors needs to be at the supply end (in the meter-main combo). Is there any code reference that says that? Won't adding a main breaker to the sub panel suffice per NEC 2017 705.12(B)(2) or NEC 2020 705.12(B)(3)(6)?

 

[don_resqcapt19]

In that drawing the sum of the main breaker and the PV breaker is not permitted to exceed 120% of the main panel bus rating. 705.12(B)(2)(3)(b) applies.

 

[jaggedben]

Reviewer is saying that the OCPD for the feedthrough conductors needs to be at the supply end (in the meter-main combo). Is there any code reference that says that? Won't adding a main breaker to the sub panel suffice per NEC 2017 705.12(B)(2) or NEC 2020 705.12(B)(3)(6)?

View attachment 2570015

Unfortunately 705.12(B)(3)(6) does refer to the supply end. It's a very unnecessary rule IMHO since the 125A breaker provides overload protection for the circuit and panel.
If the feeder is 25ft or less you could try asking if they will let you treat it as a 240.21(B) tap. But that by no means shuts the door on their reading of 705. You might just have to try to move the feeder to a 125A breaker on the busbar, or insert a 125A overcurrent device near the main.

 

[jaggedben]

In that drawing the sum of the main breaker and the PV breaker is not permitted to exceed 120% of the main panel bus rating. 705.12(B)(2)(3)(b) applies.

You can't state that definitively without knowing the busbar rating and the other loaf breakers in the panel.

 

[don_resqcapt19]

You can't state that definitively without knowing the busbar rating and the other loaf breakers in the panel.

If it wasn't a meter main, I might agree with you.

 

[ryangittens]

Unfortunately 705.12(B)(3)(6) does refer to the supply end. It's a very unnecessary rule IMHO since the 125A breaker provides overload protection for the circuit and panel.
If the feeder is 25ft or less you could try asking if they will let you treat it as a 240.21(B) tap. But that by no means shuts the door on their reading of 705. You might just have to try to move the feeder to a 125A breaker on the busbar, or insert a 125A overcurrent device near the main.

Okay I came to similar conclusion. It depends on their interpretation of 705.12(B)(3)(6). Even that code points to the feeders which doesn't mention the location and just says "load side". But as you said it does mention supply end unfortuantely which cound be intrepreted as it needs to be there. This scenario comes up a lot so I wish it was clearer.

 

[ryangittens]

So in the 2023 codes it was clarified to say "either end" lol.

 

[wwhitney]

What is the rating of the busbar in the meter main?

Cheers, Wayne

 

[ryangittens]

What is the rating of the busbar in the meter main?

Cheers, Wayne

Busbar is 200A with a 150A main

 

[wwhitney]

Busbar is 200A with a 150A main

Then the bus complies with 2020 NEC 705.12(B)(3)(1) and you may ignore 705.12(B)(3)(6).

Cheers, Wayne

 

[ryangittens]

Then the bus complies with 2020 NEC 705.12(B)(3)(1) and you may ignore 705.12(B)(3)(6).

Cheers, Wayne

Are you sure about that? That's probably the case only if the feedthrough conductors and sub panel were sized for 200A which is usually the case. But in this instance they are only sized for 125A.

 

[ryangittens]

Are you sure about that? That's probably the case only if the feedthrough conductors and sub panel were sized for 200A which is usually the case. But in this instance they are only sized for 125A.

Sorry I didn't clarify what the ratings for the subpanel and feedthrough wire were. Here I was just questioning the location of the subpanel OCPD.

 

[jaggedben]

If it wasn't a meter main, I might agree with you.

It makes no difference whether the panel has a meter attached.

 

[wwhitney]

Are you sure about that?

I'm sure on the physics--if the sum of the ratings of the sources is less than the bus rating, then the bus is protected from overload. The issue of feed-thru conductors only affects the "120% rule" and the "sum of all breakers rule".

As for code language, 705.12(B)(3)(6) is clearly misworded as to the intention. The only affirmative requirement it has is "the feed-through conductors shall be sized in accordance with 705.12(B)(1)," which is already true. The other two sentences are of the "shall be permitted" variety and thus can't take away permissions already granted by 705.12(B)(3)(1)-(5).

To achieve what is really intended by 705.12(B)(3)(6), 705.12(B)(3)(3) really needs to start with "where the busbar does not supply feed-through conductors, . . . " Such a limit isn't necessary on 705.12(B)(3)(2), as the feed-thru conductors at the end of a bus typically make it impossible to comply with 705.12(B)(3)(2) (and if the feed-thru conductors are from the middle of the bus, so you can comply with 705.12(B)(3)(2), the feed-thru conductors again don't matter as far as the physics of protecting the busbar.) And such a limit isn't necessary on 705.12(B)(3)(1), again on the physics.

That's probably the case only if the feedthrough conductors and sub panel were sized for 200A which is usually the case. But in this instance they are only sized for 125A.

That makes the feeder supplied by the feed-thru conductors a feeder tap, and you need to comply with the tap rules, along with 2020 NEC 705.12(B)(1) and (2). The 125A main breaker in the panel supplied satisfies 705.12(B)(1)(b).

Cheers, Wayne

 

[jaggedben]

Then the bus complies with 2020 NEC 705.12(B)(3)(1) and you may ignore 705.12(B)(3)(6).

Cheers, Wayne

705.12(B)(3)(6) is pretty weird, but it seems to come into play whenever feed through lugs are present on a panel regardless of how and whether the panel is qualified under other sections. So I don't agree that your statement is logical.

 

[wwhitney]

705.12(B)(3)(6) is pretty weird, but it seems to come into play whenever feed through lugs are present on a panel regardless of how and whether the panel is qualified under other sections. So I don't agree that your statement is logical.

I expanded on that in my post simultaneous with your post above.

Let me also add that 705.12(B)(3) starts off "One of the following methods shall be used to determine the ratings of busbars:" which means that as soon as you satisfy one of the methods listed, you may ignore the content of all the other items. Any intention to require one to use the 705.12(B)(3)(6) method necessitates sufficiently limiting all the other allowances, which was not done.

Cheers, Wayne

 

[jaggedben]

...
As for code language, 705.12(B)(3)(6) is clearly misworded as to the intention. ...

Let's just say I fully agree with this. If I were an AHJ, I would enforce overcurrent protection for the feeder based on the obvious intent. But yours is a good argument that the overcurrent device at the supply end is not strictly speaking a *requirement*.

 

[ryangittens]

I'm sure on the physics--if the sum of the ratings of the sources is less than the bus rating, then the bus is protected from overload. The issue of feed-thru conductors only affects the "120% rule" and the "sum of all breakers rule".

As for code language, 705.12(B)(3)(6) is clearly misworded as to the intention. The only affirmative requirement it has is "the feed-through conductors shall be sized in accordance with 705.12(B)(1)," which is already true. The other two sentences are of the "shall be permitted" variety and thus can't take away permissions already granted by 705.12(B)(3)(1)-(5).

To achieve what is really intended by 705.12(B)(3)(6), 705.12(B)(3)(3) really needs to start with "where the busbar does not supply feed-through conductors, . . . " Such a limit isn't necessary on 705.12(B)(3)(2), as the feed-thru conductors at the end of a bus typically make it impossible to comply with 705.12(B)(3)(2) (and if the feed-thru conductors are from the middle of the bus, so you can comply with 705.12(B)(3)(2), the feed-thru conductors again don't matter as far as the physics of protecting the busbar.) And such a limit isn't necessary on 705.12(B)(3)(1), again on the physics.


That makes the feeder supplied by the feed-thru conductors a feeder tap, and you need to comply with the tap rules, along with 2020 NEC 705.12(B)(1) and (2). The 125A main breaker in the panel supplied satisfies 705.12(B)(1)(b).

Cheers, Wayne

Okay I see what you're getting at

 

[tortuga]

This is very similar to my thread today, here is how this would comply under the 2023 NEC.
As long as the feed-through conductors are fully rated 125A:

 

[ggunn]

Are you sure about that? That's probably the case only if the feedthrough conductors and sub panel were sized for 200A which is usually the case. But in this instance they are only sized for 125A.

Why would they need to be 200A if the main breaker is 150A?