120% panelboard question

[photonboy]

I have been asked if the backfeed breaker must always in all circumstances be placed at the furthest point on the bus from a main breaker (or lugs in case of a sub). This comes up because some of the jobs we're seeing have ample electrical space available, like say a 225A bus with a 175A main breaker which leaves 95A available and the backfeed is only 20A. Installers are asking if with that much available ampacity does the PV backfeed still need to be at furthest point from MB.

Personally, my thinking is that when backfeeding a bus the 120% rule is always in effect and the breaker should always be at the furthest point, even if we are not up against the ampacity limits. Others have said it's only important when we are close to a maxed out bus.

I'm looking for a definitive answer as this will affect quite a few installers we work with and I get the question quite often, we need a clear policy and my personal opinion isn't cutting it.

Thanks

 

[wwhitney]

See (2020) NEC 705.12(B)(3)(1); contrast that with 705.12(B)(3)(2).

Cheers, Wayne

 

[ggunn]

I have been asked if the backfeed breaker must always in all circumstances be placed at the furthest point on the bus from a main breaker (or lugs in case of a sub). This comes up because some of the jobs we're seeing have ample electrical space available, like say a 225A bus with a 175A main breaker which leaves 95A available and the backfeed is only 20A. Installers are asking if with that much available ampacity does the PV backfeed still need to be at furthest point from MB.

Personally, my thinking is that when backfeeding a bus the 120% rule is always in effect and the breaker should always be at the furthest point, even if we are not up against the ampacity limits. Others have said it's only important when we are close to a maxed out bus.

I'm looking for a definitive answer as this will affect quite a few installers we work with and I get the question quite often, we need a clear policy and my personal opinion isn't cutting it.

Thanks

If the sum of the main breaker rating and 125% of the max inverter current is equal to or less than 100% of the busbar rating, the PV breaker can be anywhere in the panel. As Wayne says, (2020) NEC 705.12(B)(3)(1).

 

[jaggedben]

See (2020) NEC 705.12(B)(3)(1); contrast that with 705.12(B)(3)(2).

Cheers, Wayne

Or 705.12(B)(3)(3).

 

[jaggedben]

I have been asked if the backfeed breaker must always in all circumstances be placed at the furthest point on the bus from a main breaker (or lugs in case of a sub). ...

The answer is no, not always.

 

[photonboy]

Ok, so what I'm getting from the comments and trying to decipher the code sections is that if the PV backfeed + Main Breaker ampacity is anywhere between 101% and 120% of the bus rating then the PV breaker has to be at the furthest point from the MB and have a "do not move" warning label. If the PV backfeed + Main Breaker ampacity is anywhere less than OR at 100% of the bus rating then the backfeed can go anywhere, even right next to the MB.

Would this be a correct interpretation?

 

[Carultch]

Ok, so what I'm getting from the comments and trying to decipher the code sections is that if the PV backfeed + Main Breaker ampacity is anywhere between 101% and 120% of the bus rating then the PV breaker has to be at the furthest point from the MB and have a "do not move" warning label. If the PV backfeed + Main Breaker ampacity is anywhere less than OR at 100% of the bus rating then the backfeed can go anywhere, even right next to the MB.

Would this be a correct interpretation?

That's correct. Essentially, if you don't put the interconnection breaker opposite the main supply, the 120% rule gets demoted to a 100% rule.

As a fine print note, it isn't the PV backfeed breaker that matters, but it is the calculation that sizes the backfeed breaker. I.e. 1.25*I_inv, where I_inv is the total inverter output current, as of NEC2014. We don't have a word for that Ampere figure, so the NEC is a bit verbose to specify it. An example where it makes a difference: 600A bus & 600A main, with 120A of headroom provided by the 120% rule. The 2014+ rule 96A of inverter current on a 125A breaker. The pre-2014 rule would mean a max 110A breaker for 88A of inverter current. Essentially, rounding errors are no longer a show stopper.

The underlying idea is that the 120% rule allows you to take partial credit for Kirchhoff's current law. If current comes from opposite sides of the bus, it is subtractive, rather than additive. If it comes from the same side, it is additive, and the solar assisting the main supply would create a blindspot for the main breaker. A slight overload exceeding the busbar rating would go undetected. But, due to other factors at play, such as mutual breaker heating, the NEC doesn't allow opposite feeding to be a 200% rule as full credit for KCL would imply, so there is an industry compromise to allow it to be up to 120%.

Also due to the limitations of the NEC rules, by strict reading of the code's wording, just one single breaker beyond the PV breaker, is enough to demote the 120% rule. Even if the objectionable breakers aren't even close to adding up to exceed the busbar rating, the NEC simply doesn't consider this nuance.

 

[jaggedben]

Ok, so what I'm getting from the comments and trying to decipher the code sections is that if the PV backfeed + Main Breaker ampacity is anywhere between 101% and 120% of the bus rating then the PV breaker has to be at the furthest point from the MB and have a "do not move" warning label. If the PV backfeed + Main Breaker ampacity is anywhere less than OR at 100% of the bus rating then the backfeed can go anywhere, even right next to the MB.

Would this be a correct interpretation?

No. Not quite. Sorry but the others are wrong. You need to look again at 705.12(B)(3)(3), what I call in short hand the 'sum of all breakers' rule. It allows another option where what you say above does not need to be true. The PV+Main Breaker ratings can even exceed 120% of the busbar ratings by a lot.

 

[Carultch]

Essentially, there are multiple options in 705.12 to qualify an interconnection to a busbar, and they are all options that are independent of each other. You are only required to comply with one of the options at any given panelboard, and the choice is independent of the rule you may be using at a different panelboard.

One issue that isn't clarified, is for interconnections at a subpanel, whether you are required to apply interconnection rules to every panelboard rank upstream of the POI, all the way to the service point. This would consider the worst case scenario that local loads in the subpanel are zero, and loads in the main panel are possibly-maximum. Not an unlikely case either, since a detached building may use its loads a lot less often than loads in a main building.

 

[ggunn]

Ok, so what I'm getting from the comments and trying to decipher the code sections is that if the PV backfeed + Main Breaker ampacity is anywhere between 101% and 120% of the bus rating then the PV breaker has to be at the furthest point from the MB and have a "do not move" warning label. If the PV backfeed + Main Breaker ampacity is anywhere less than OR at 100% of the bus rating then the backfeed can go anywhere, even right next to the MB.

Would this be a correct interpretation?

Almost. It isn't the rating of the PV backfed breaker you use in that math, but 125% of your total inverter maximum output current.

Also, as jaggedben says there is also the alternative bus qualification method where the total of all breaker ratings landing on the busbars (in this math it IS the rating of the backfed PV breaker(s) that is used), both load and supply, does not exceed the rating of the busbars. This article is what allows the use of PV AC combiner panels, but it is very rarely applicable to an MDP.

 

[ggunn]

One issue that isn't clarified, is for interconnections at a subpanel, whether you are required to apply interconnection rules to every panelboard rank upstream of the POI, all the way to the service point.

I dunno; it seems pretty clear to me. If current from a PV system enters a subpanel, depending on which loads are active it can also enter every panel in series all the way back to the service. Those panels do not all need to be qualified by the same part of 705.12(B)(3), but they do all need to be qualified.

 

[Carultch]

I dunno; it seems pretty clear to me. If current from a PV system enters a subpanel, it can also enter every panel in series back to the service. Those panels do not all need to be qualified by the same part of 705.12(B)(3), but they do all need to be qualified.

It's a rule that you have to read between the lines to know you have to apply. I agree that it should be applied, since local load diminishing to zero is a realistic possibility, especially in a detached building. I also agree that each panel in the ranks, can qualify under whichever rule is most convenient to use, independently of all the others.

 

[wwhitney]

One issue that isn't clarified, is for interconnections at a subpanel, whether you are required to apply interconnection rules to every panelboard rank upstream of the POI, all the way to the service point.

"Where distribution equipment . . . is fed simultaneously by a primary source(s) of electricity and one or more other power source(s) . . ." seems to me to clearly cover all distribution equipment between the primary source and the other power source(s).

Cheers, Wayne

 

[ggunn]

It's a rule that you have to read between the lines to know you have to apply.

So, what else is new? The NEC is a convoluted piece of literature.

 

[SKSolar]

Question on solar backfeed breaker:

As I learnt from ggunn that If the sum of the main breaker rating and 125% of the max inverter current is equal to or less than 100% of the busbar rating, the PV breaker can be anywhere in the panel.

Is there any specific difference between a solar backfeed breaker and the other circuit breakers other than the amperage differences in the Main Service panel? What I mean is a solar breaker must have a specific feature that a circuit did not have to.

Let say I have a 40A solar backfeed breaker and I also have a 40A circuit breaker, say, connected to an air conditioner. Can I interchange their connections?

 

[jaggedben]

Yes?

Put this this way...
Other than in the 705.12 "120% rule" , I know of no other restrictions in the NEC regarding where an overcurrent devices for any circuit may be located in a panelboard or switchboard. You can put loads anywhere you want. The only restriction is for power sources.

 

[ggunn]

Yes?

Put this this way...
Other than in the 705.12 "120% rule" , I know of no other restrictions in the NEC regarding where an overcurrent devices for any circuit may be located in a panelboard or switchboard. You can put loads anywhere you want. The only restriction is for power sources.

On a bus qualified under NEC2023 705.12(B)(2), which is the 120% rule and has been moved around a bit but not changed, all loads must be between the utility feed and the PV feed.

 

[jaggedben]

On a bus qualified under NEC2023 705.12(B)(2), which is the 120% rule and has been moved around a bit but not changed, all loads must be between the utility feed and the PV feed.

But that's really just a restatement of the same rule. My point was that I know of no similar rules in any other section of the code. And none that apply to loads when no interconnected power sources are connected.

 

[Dennis Alwon]

But remember if you use that method it is likely someone will come along and add more load to the panel. Is it really just as easy to install the breaker at the bottom of the buss

 

[jaggedben]

But remember if you use that method it is likely someone will come along and add more load to the panel. Is it really just as easy to install the breaker at the bottom of the buss

Not sure which method you're referring to. But something like that only makes sense where a panel would presently comply with both the "120% rule" and the "sum of all breakers rule". Unless you can only use the latter rule, and the sum would exceed the rating, adding a load doesn't matter.