Solar Combiner Boxes - Bus Bar Sizing

[unless]

Is there code specific wording on how to handle bus bar sizing for AC Combiner Panels?
I have a 800A MLO panel (with a 800A 100% rated breaker upstream) that collects the outputs of ten inverters (each rated for 72.2A continous current).
There are no loads on this panel, just power sources.
The output of this panel is what ties into the main switchboard (load side connection) and follows 705.12(B) for bus bar sizing etc.
Each inverter has a 100A breaker in this panelboard (total of 10).

My question is...does my 800A panelboard (AC Combiner Box) need to have an upsized bus bar akin to 705.12(B)? There are no loads and the max it will ever see is 722A of continuous current.
If I follow 705.12(B)(2)(3)(c) my bus bar is required to be 1000A which seems crazy to me since there are no loads. Am I missing something? Do I need to upsize? If not, where does the code support this approach?

Thanks! Long time follower, first time posting.

 

[jaggedben]

I don't think you'll find what you need in the code.

Your best bet would probably be the exception to 705.60(B), which allows an 'assembly' to be utilized at 100% of its rating if it's listed for that. But your panelboard and all your breakers in it would have to be listed for such. You'd have to have 80A breakers listed for operation at 100%, and the busbar would have to be so listed, and perhaps it would all have to be listed together as an 'assembly.' Not sure such a thing exists, and I'd gather that getting something field listed, if even possible, is not easier than getting a 1000A panelboard.

Keep in mind that throughout the code, for most applications and not just solar, continuous current requires a 125% multiplier. Thus 722A of continuous current requires 902A rated conductors and such. That's why it's not 'crazy'. This also likely applies to your upstream equipment with the 100% rated 800A breaker, regardless of the breaker rating, although as a 'switchboard' there may be some wiggle room as those are not explicitly mentioned in the 2017 NEC.

 

[ggunn]

Is there code specific wording on how to handle bus bar sizing for AC Combiner Panels?
I have a 800A MLO panel (with a 800A 100% rated breaker upstream) that collects the outputs of ten inverters (each rated for 72.2A continous current).
There are no loads on this panel, just power sources.
The output of this panel is what ties into the main switchboard (load side connection) and follows 705.12(B) for bus bar sizing etc.
Each inverter has a 100A breaker in this panelboard (total of 10).

My question is...does my 800A panelboard (AC Combiner Box) need to have an upsized bus bar akin to 705.12(B)? There are no loads and the max it will ever see is 722A of continuous current.
If I follow 705.12(B)(2)(3)(c) my bus bar is required to be 1000A which seems crazy to me since there are no loads. Am I missing something? Do I need to upsize? If not, where does the code support this approach?

Thanks! Long time follower, first time posting.

The word on the street is that in the 2020 NEC the 1.25 multiplier on inverter nameplate current is going away in some places, this one in particular, but for the time being you may be stuck with the 1000A requirement. I feel your pain.

 

[electrofelon]

The word on the street is that in the 2020 NEC the 1.25 multiplier on inverter nameplate current is going away in some places, this one in particular, but for the time being you may be stuck with the 1000A requirement. I feel your pain.

What is the justification for getting rid of the continuous multiplier?

 

[ggunn]

What is the justification for getting rid of the continuous multiplier?

The 1.25 multiplier for establishing minimum OCPD on inverters isn't because of continuous inverter current. It doesn't have anything to do with limiting inverter current at all. All it does is establish a window for the inverter to operate in and it doesn't make sense for it to be 25% wider than the the maximum current that the inverter can deliver, especially in the OP's situation where he's qualifying his bus under 705.12(B)(2)(3)(c)* where it's the breaker ratings that the calculation is based on.

*I think that's right; I don't have my code book handy.

 

[electrofelon]

The 1.25 multiplier for establishing minimum OCPD on inverters isn't because of continuous inverter current. It doesn't have anything to do with limiting inverter current at all. All it does is establish a window for the inverter to operate in and it doesn't make sense for it to be 25% wider than the the maximum current that the inverter can deliver , especially in the OP's situation where he's qualifying his bus under 705.12(B)(2)(3)(c)* where it's the breaker ratings that the calculation is based on.

*I think that's right; I don't have my code book handy.

I am confused. Since an inverters output is considered continuous, both intuitively and as stated in 690.8(B), shouldn't the OCPD be rated at 125% of the output current just like every other OCPD that is subject to continuous loads?

 

[jaggedben]

ggunn I was looking through the 2nd Draft and the only thing I see that resembles what you're describing is something that doesn't apply to overcurrent devices or busbars, or the OP's situation. 705.12 doesn't have a change like that. (The change that does somewhat resemble your remark is that they are changing the conductor ampacity requirements to be like 690, where it's either 125% or conditions-of-use, but not both at the same time. But that only applies to conductor sizes. It's the new/renumbered 705.28, btw.)

I guess I have always just assumed I understood the reasoning behind the 125% rule for continuous current. (In a nutshell, I've assumed more heat requires higher rated components, or else you get nuisance tripping.) I've never seen the logic or the empirical science spelled out. That said, I don't really see how it's different for inverters than for loads.

 

[jaggedben]

[MENTION=160161]unless[/MENTION]

It looks like there will be a section in 2020 NEC that will allow 'other configurations' when done under 'engineering supervision'. Unfortunately, the similar section in the 2017 NEC only allows this for 'multiple ampacity busbars.

You might be able to get your 800A panelboard approved if you can get an engineer to declare it safe and get the AHJ to permit alternative methods. 90.4, second paragraph. Always worth a mention.

 

[electrofelon]

I guess I have always just assumed I understood the reasoning behind the 125% rule for continuous current. (In a nutshell, I've assumed more heat requires higher rated components, or else you get nuisance tripping.) I've never seen the logic or the empirical science spelled out. That said, I don't really see how it's different for inverters than for loads.

My understanding of it is that circuit breakers are tested at 100% of their rating but the conditions for that test is in free air. A breaker in the wild can be packed in with others that are also under load, generating heat, etc....But the 125% adder also applies to fuses which doesnt seem to follow from that reasoning.....

 

[ggunn]

ggunn I was looking through the 2nd Draft and the only thing I see that resembles what you're describing is something that doesn't apply to overcurrent devices or busbars, or the OP's situation. 705.12 doesn't have a change like that. (The change that does somewhat resemble your remark is that they are changing the conductor ampacity requirements to be like 690, where it's either 125% or conditions-of-use, but not both at the same time. But that only applies to conductor sizes. It's the new/renumbered 705.28, btw.)

I guess I have always just assumed I understood the reasoning behind the 125% rule for continuous current. (In a nutshell, I've assumed more heat requires higher rated components, or else you get nuisance tripping.) I've never seen the logic or the empirical science spelled out. That said, I don't really see how it's different for inverters than for loads.

Apparently I was misinformed or I misunderstood what I was told. Carry on.