Using different 705.12(B)(2)(3) methods for multiple panels in a PV system

[ggunn]

Is a line side interconnection feasible? If it is, it would render all of this moot.

 

[jaggedben]

I'm gonna assume that 43A is inverter nameplate and that you need to multiply by 125% to get 54A as Wayne described.

If you can do a supply side connection as ggun suggested, then this might be one where you should just downsize the PV system to 40A nameplate inverter output. (Microinverters?) Then all your 200A components will be rated for the sum of the MCB and 125% of inverter (150A+50A). And all of the applicable rules would be met.

 

[ohmti787]

Can't do a line side tap. Would have to either derate the MCB and/or work around the different methods in 705.12(B)(2).

But I spoke to the inspector and he's fine with a breaker at the indoor panel.

Thanks y'all. I really appreciate all your input

 

[ohmti787]

Yes. See 705.12(B)(2)(2).

The easiest way I've found around that situation is to add a main breaker to the subpanel, but it isn't possible in all situations.

I know there's wording on the 2020 code about these scenarios, but the 2017 doesn't touch on the feed through lugs situation. Now I have this inspector saying that the load center inside should be consider "part" of the MSP outside. So, according to him, whatever MCB I install at the load center inside should be included in the "....sum of overcurrent devices..." for the busbar outside where I'm installing my PV breaker. Am I missing something here?

 

[ggunn]

I know there's wording on the 2020 code about these scenarios, but the 2017 doesn't touch on the feed through lugs situation. Now I have this inspector saying that the load center inside should be consider "part" of the MSP outside. So, according to him, whatever MCB I install at the load center inside should be included in the "....sum of overcurrent devices..." for the busbar outside where I'm installing my PV breaker. Am I missing something here?

Are you qualifying the outside MSP busbars under 705.12(B)(3)(3)? Can't you use 705.12(B)(3)(2), aka the 120% rule?

 

[jaggedben]

Are the feed through lugs in the MSP at the opposite end of the busbar from the main?

 

[ohmti787]

It's a 50A PV breaker, so I can't use the 120% rule. Option 705.12(B)(2)(3)(c) is the method that I'm trying to push. Either that, derate the main, or do a MSP upgrade. Currently it's a 200A MSP w/ a 200A MCB, and yes, the feed through lugs are in the opposite end of the MCB. If I follow this guy's logic, even if I tap the feed through conductors and slap a MCB at the load center inside, I would still be overloading the busbar outside.

 

[wwhitney]

Seems like the only good option is to downsize the MCB in the MSP to 175A, interconnect via splicing to the feeder supplied by the feed-thru lugs, and ensure that the downstream panel on that feeder has a main breaker.

Cheers, Wayne

 

[jaggedben]

I think you still have options. The inspector is being logical as far as I see and may be reasonable if presented with a strong explanation. I would probably derate the main, tap the feeder (one option), and put the MCB in the sub.

The inspector has a point about the sum of all breakers rule, even if he perhaps isn't explaining it well. The feed through lugs really are a loophole to that rule, (which has been at least partially closed in the 2020 NEC). So throw out that option

The rest of my logic goes like this...

You cannot use the 120% option with your PV breaker in the MSP, because your breaker cannot be at the opposite end, because the feed through lugs are at tue opposite end. That leaves you two options...

Option 1) Backfeed the feed-through lugs, because they are at the opposite end. This requires downsizing the MCB to 175A for you 50A PV breaker. That deals with qualifying the MSP. If you tap the feeder, you still have to deal with the potential overload of the sub downstream, which the inspector may let you deal via a main breaker in the sub? Alternatively, you can just put your PV breaker at the the opposite end of the sub, although I'm gathering that the location may be infeasible.

Option 2) Downsize the MSP main breaker to 150A. This lets you meet the first rule in the busbar section of the code, where your MCB and PV don't exceed the busbar and therefore the location of the PV breaker doesn't matter. You still need the MCB in the sub for the same reason as above.

BTW, in rare cases it might be feasible to put the sub on a breaker in the MSP.

Edit: Wayne gave the short version while I was typing the legal case for it. ;-)

 

[ggunn]

It's a 50A PV breaker, so I can't use the 120% rule. Option 705.12(B)(2)(3)(c) is the method that I'm trying to push. Either that, derate the main, or do a MSP upgrade. Currently it's a 200A MSP w/ a 200A MCB, and yes, the feed through lugs are in the opposite end of the MCB. If I follow this guy's logic, even if I tap the feed through conductors and slap a MCB at the load center inside, I would still be overloading the busbar outside.

His logic is correct, IMO.

Here's something: If the MSP happens to be a 200A Cutler Hammer (Eaton CH not BR) panel, the busbar is rated at 225A. We have it in writing and we use it. BR may be the same but we only have the documentation for CH. We use this even when it's a feedthough panel and no one yet (knocking vigorously on wood) has called us on the "other end of the bus" thing.