Sum Of All Breakers Rule

[BackCountry]

I’m a little confused on 704.12B(3)(3)’s intent when we factor in a BESS.

Scenario: 400A MSP with 400A main breaker.

Battery backup IQ system controller 2 (Enphase in this case) is fed by a 200A breaker off of the 400A bus.

Existing loads (280A) remain on the 400A bus and backup loads are moved to a new backup loads panel.

The BESS and PV are 83.5A after multiplying the inverter output of both by 125%.

The sum of all breakers rule says not compliant because the existing loads remaining on the 400A bus (280A) plus the 200A backup system breaker are > 400A. But are they really? The inverter max continuous output is 83.5A. There is no scenario where more than 400A could ever pass through the bus bar.

If the 280A loads were at max, and the backup system consumed more than 120A — the main breaker trips.

If the PV system and ESS are outputting max, there isn’t more than 280A of load on the bus bars, so the bus bar rating isn’t exceeded.

If the backup loads panel was at max, and the existing loads were at max, the main breaker would trip.

I don’t understand why the sum of all breakers rule wouldn’t be clarified to say that it’s the sum of all load breakers excluding the battery backup system breaker itself, there’s never a scenario where the backup system breaker can supply and consume simultaneously — it’s either consuming from the bus or supplying up to its max output, which in both scenarios never exceeds the bus capacity.

 

[ggunn]

We all have times when we wish the NEC would say something different than it does, but it says what it says, and it's pretty easy to tell if a panel is compliant with the sum of all breakers rule. There is no provision in the rule for breakers that won't be drawing or supplying current to the bus simultaneously. The only wiggle room is when there are breakers that do not connect to all the busbars; it's the sum of breakers on a busbar that must not exceed the rating of the busbar.

 

[BackCountry]

We all have times when we wish the NEC would say something different than it does, but it says what it says, and it's pretty easy to tell if a panel is compliant with the sum of all breakers rule. There is no provision in the rule for breakers that won't be drawing or supplying current to the bus simultaneously. The only wiggle room is when there are breakers that do not connect to all the busbars; it's the sum of breakers on a busbar that must not exceed the rating of the busbar.

There are ways around it — like using the PCS rule, I just think the sum of all breakers rule is a common problem we encounter as we transition California’s panels which are mostly all combination meter main and loads panels over to backup. If we were like the rest of the country with a meter socket that feeds a load center in the basement, it’d be no problem — intercept the feeder and call it good. Unfortunately we have to move every circuit over to a backup loads panel, and we usually want to leave a few circuits on the main bus to avoid draining the battery during an outage, hence why I run into this so often.

 

[ggunn]

There are ways around it — like using the PCS rule, I just think the sum of all breakers rule is a common problem we encounter as we transition California’s panels which are mostly all combination meter main and loads panels over to backup. If we were like the rest of the country with a meter socket that feeds a load center in the basement, it’d be no problem — intercept the feeder and call it good. Unfortunately we have to move every circuit over to a backup loads panel, and we usually want to leave a few circuits on the main bus to avoid draining the battery during an outage, hence why I run into this so often.

I don't know what PCS stands for, but of course one only needs to comply with one of the provisions under 705.12(B).

 

[BackCountry]

I don't know what PCS stands for, but of course one only needs to comply with one of the provisions under 705.12(B).

705.13

Power control system

 

[wwhitney]

I’m a little confused on 704.12B(3)(3)’s intent when we factor in a BESS.

The intent is to provide a very simple to verify rule that provides a small amount of needed flexibility. As such, the rule is written to not require thinking about what is connected to each breaker.

If the goal were to provide more flexibility at the expense of greater complexity for field verifying compliance, then it could require each breaker, including the primary breaker protecting the bus, to be labeled as "source only," "load only" or "load/source". Then the requisite check would be, for each collection of breakers labeled "source" for which the sum of their ratings exceeds the bus rating, is the sum of the ratings of the other breakers labeled "load" no more than the bus rating?

But that's never going to be incorporated into the NEC, I gather. : - )

Cheers, Wayne

 

[jaggedben]

I’m a little confused on 704.12B(3)(3)’s intent when we factor in a BESS.

Scenario: 400A MSP with 400A main breaker.

Battery backup IQ system controller 2 (Enphase in this case) is fed by a 200A breaker off of the 400A bus.

Existing loads (280A) remain on the 400A bus and backup loads are moved to a new backup loads panel.

The BESS and PV are 83.5A after multiplying the inverter output of both by 125%.

The sum of all breakers rule says not compliant because the existing loads remaining on the 400A bus (280A) plus the 200A backup system breaker are > 400A. But are they really? The inverter max continuous output is 83.5A. There is no scenario where more than 400A could ever pass through the bus bar.

If the 280A loads were at max, and the backup system consumed more than 120A — the main breaker trips.

If the PV system and ESS are outputting max, there isn’t more than 280A of load on the bus bars, so the bus bar rating isn’t exceeded.

If the backup loads panel was at max, and the existing loads were at max, the main breaker would trip.

I don’t understand why the sum of all breakers rule wouldn’t be clarified to say that it’s the sum of all load breakers excluding the battery backup system breaker itself, there’s never a scenario where the backup system breaker can supply and consume simultaneously — it’s either consuming from the bus or supplying up to its max output, which in both scenarios never exceeds the bus capacity.

To speak to the original intent of this rule: To my understanding, it was primarily to exempt solar AC combiner panels from the 120% rule. We got other benefits as installers, but it wasn't the intent to make a rule that covers all the possible permutations of Kirchoff's law in a panel, as you're more or less wanting to do.

Of course you make a good case for exempting a particular breaker in a particular scenario, but I don't know how hopeful we can be of convincing the code making panel to 'complicate' this rule. I believe Wayne has tried and failed at least once already, with a simpler formulation than in the previous post.

But let me point out a couple other places where we might get more flexibility down the road...

First, the wording of 705.13 in both the 2020 NEC and the 2023 NEC, strictly speaking, allows us to ignore 705.12 completely if the PCS monitors the busbar. In other words, as long as a system can monitor the current coming from both the service and the backup micro-grid and ensure the sum doesn't exceed the busbar rating, we can ignore 705.12. With such a system we shouldn't even have to do things use the PCS to limit the output current to the panel to comply with the 120% rule. I'm not sure if/when we'll get manufacturers providing such features and documenting them sufficiently to pass AHJ muster, and it will take some educating and convincing of AHJs if we do. But it's potentially something to pay attention to. Simply: "My system monitors the busbar to ensure that total current is less than the busbar rating. See 705.13."

Second, take note of the new language in the 2023 NEC in 705.12(B) that says "for ... distribution equipment with no specific listing and instructions for combining multiple sources" the familiar rules will apply. Note how this allows for manufacturers to provide equipment that comes with its own rules. Maybe manufacturers will start taking advantage of this language, or maybe not, and who knows how quickly. But again potentially worth paying attention to.

 

[jaggedben]

Also keep in mind that any rule change that complicates this rule makes the label longer and more incomprehensible.

(In some thread I posted a pic where someone was making fun of this label, but I can't find it right now.)

 

[BackCountry]

Also keep in mind that any rule change that complicates this rule makes the label longer and more incomprehensible.

(In some thread I posted a pic where someone was making fun of this label, but I can't find it right now.)

I had a new inspector today say “where are all of the rest of the labels.” I flipped to the label sheet in the plan set and said: “they’re all here.” He seemed to mull that over and then moved on to asking if E curbs actually seal to the roof. Didn’t care that the original panel multi wires weren’t handle tied, or that you couldn’t inspect the ground rod, or anything else. What is the deal with the label police???

 

[BackCountry]

To speak to the original intent of this rule: To my understanding, it was primarily to exempt solar AC combiner panels from the 120% rule. We got other benefits as installers, but it wasn't the intent to make a rule that covers all the possible permutations of Kirchoff's law in a panel, as you're more or less wanting to do.

Of course you make a good case for exempting a particular breaker in a particular scenario, but I don't know how hopeful we can be of convincing the code making panel to 'complicate' this rule. I believe Wayne has tried and failed at least once already, with a simpler formulation than in the previous post.

But let me point out a couple other places where we might get more flexibility down the road...

First, the wording of 705.13 in both the 2020 NEC and the 2023 NEC, strictly speaking, allows us to ignore 705.12 completely if the PCS monitors the busbar. In other words, as long as a system can monitor the current coming from both the service and the backup micro-grid and ensure the sum doesn't exceed the busbar rating, we can ignore 705.12. With such a system we shouldn't even have to do things use the PCS to limit the output current to the panel to comply with the 120% rule. I'm not sure if/when we'll get manufacturers providing such features and documenting them sufficiently to pass AHJ muster, and it will take some educating and convincing of AHJs if we do. But it's potentially something to pay attention to. Simply: "My system monitors the busbar to ensure that total current is less than the busbar rating. See 705.13."

Second, take note of the new language in the 2023 NEC in 705.12(B) that says "for ... distribution equipment with no specific listing and instructions for combining multiple sources" the familiar rules will apply. Note how this allows for manufacturers to provide equipment that comes with its own rules. Maybe manufacturers will start taking advantage of this language, or maybe not, and who knows how quickly. But again potentially worth paying attention to.

I agree that a PCS solves everything… I need to get our EE to start using that as the interconnection method. I just wish a little logic would apply

I agree that all of the methods are complex both for an inspector and for an installer. I just find this whole backup loads scenario and moving everything over to be incredibly time consuming and difficult. We were out till 8pm last night moving loads over and getting our customer back online. Then 4 hours with Enphase support today because of another battery software error. I’m burnt

 

[BackCountry]

The intent is to provide a very simple to verify rule that provides a small amount of needed flexibility. As such, the rule is written to not require thinking about what is connected to each breaker.

If the goal were to provide more flexibility at the expense of greater complexity for field verifying compliance, then it could require each breaker, including the primary breaker protecting the bus, to be labeled as "source only," "load only" or "load/source". Then the requisite check would be, for each collection of breakers labeled "source" for which the sum of their ratings exceeds the bus rating, is the sum of the ratings of the other breakers labeled "load" no more than the bus rating?

But that's never going to be incorporated into the NEC, I gather. : - )

Cheers, Wayne

You never know! Maybe we can band together for some code influence.