705.12(B)(2)(3)(c) question

[jaggedben]

What is the difference, electrically speaking, between a 90A breaker in a panel and 90A of OCP on the other end of a conductor set connected to feedthrough lugs on that same panel? There is none that I can see; the NEC's silence on this point is, as you say, a loophole.

That's a fair point. Another fair point is that the rule is overly conservative and the loophole doesn't result in an unsafe installation. The output from the solar/battery would have to exceed 200A for the busbar to exceed 200A, even if the sum of the load breakers were also 200A (in compliance with the rule).

 

[ggunn]

That's a fair point. Another fair point is that the rule is overly conservative and the loophole doesn't result in an unsafe installation. The output from the solar/battery would have to exceed 200A for the busbar to exceed 200A, even if the sum of the load breakers were also 200A (in compliance with the rule).

I can only tell you how I would interpret the NEC on this point if I were to be asked to affix my PE seal to the design, which I have already stated. Theoretical points on current flow or declarations that the NEC is overconservative are never part of a compliance discussion for me, but YMMV.

 

[wwhitney]

So if the grid sense was moved to the shop panel he is okay?

Not necessarily, because of this:

We added the Lug kit to move back up loads to the back up loads panel. The solar is also downstream from the Lug kit.

We don't know the configuration of the panel(s) supplied by the lug kit. The one-line diagram in the OP is incomplete, so all bets are off.

The way to do an interconnection like this, in my opinion, is to use the feeder interconnection rules with an "H"-topology. The H is a 200A feeder, with the MID in the middle of the central segment. Then the upper left is the 200A grid connection, the lower left is a non-backup load panel, the upper right is all the inverters, and the lower right is the backup load panel. If the MID has double line side and double load side lugs, that facilitates such an installation.

Cheers, Wayne

 

[electro7]

What is the grid sense for, exactly? Does the solar/battery system have a backup function? Does it really need a 20A breaker or would 10A suffice? Are there loads connected downstream and what is the load calc for them?

It does have a back-up function with a transfer. It is similar to an ATS in that is operates a relay coil to allow the transfer. I'll have to check with the manufacturer on whether I can do a 10A breaker, but I believe I should be able to.

What is the difference, electrically speaking, between a 90A breaker in a panel and 90A of OCP on the other end of a conductor set connected to feedthrough lugs on that same panel? There is none that I can see; the NEC's silence on this point is, as you say, a loophole. If that battery sensor, whatever it is, were on a 10A instead of a 20A breaker, I'd stamp it.

I believe the difference (not related to this code section we are speaking of) would be of the conductors connected to the lug kit since it is a MLO panel. Right now they are sized for 200A, same as the main breaker. If the 90A of solar were landed on the bus shown in the drawing then the feed through conductors could be overloaded. We have to have the solar downstream anyways to function in a back-up scenerio.

I can see the math with the 100A shop breaker, 20A grid sense and 90A of solar backfeed being an issue with the code section in the subject title. Even though I am 99% sure the grid sense isn't a load I am going to try and get that on a 10A breaker. I believe that would answer all questions and sounds like ggunn would put his stamp on it

Thanks guys!

 

[wwhitney]

I believe the difference (not related to this code section we are speaking of) would be of the conductors connected to the lug kit since it is a MLO panel.

How about a complete one line diagram, including the location of the MID?

If you have a single panel connected to the lug kit, and that panel is supplied by the grid via the feeder from the lug kit, and has breakers for the solar and the battery, then that panel also needs to meet one of the options of 2017 NEC 705.12(B)(2)(3)(a), (b), or (c).

If the solar is instead interconnected to a feeder between the lug kit and the panel, that panel needs to be a main breaker panel, not an MLO panel.

Cheers, Wayne

 

[electro7]

How about a complete one line diagram, including the location of the MID?

If you have a single panel connected to the lug kit, and that panel is supplied by the grid via the feeder from the lug kit, and has breakers for the solar and the battery, then that panel also needs to meet one of the options of 2017 NEC 705.12(B)(2)(3)(a), (b), or (c).

If the solar is interconnected to a feeder between the lug kit and the panel, that panel needs to be a main breaker panel, not an MLO panel.

Wayne, its a Sunpower Hub+ that is downstream of the lug kit. It has basically 3 panels in one enclosure. All of that is taken into consideration within that unit.

My main thing was that I didn't want to downsize the homeowners main 200A breaker if I met the 705.12(B)(2)(3)(c) code section.

 

[ggunn]

I can see the math with the 100A shop breaker, 20A grid sense and 90A of solar backfeed being an issue with the code section in the subject title. Even though I am 99% sure the grid sense isn't a load I am going to try and get that on a 10A breaker. I believe that would answer all questions and sounds like ggunn would put his stamp on it

You could also reduce the shop breaker from 100A to 90A and get it down to 100% that way.

You changed your avatar and it faked me out.

 

[electro7]

You could also reduce the shop breaker from 100A to 90A and get it down to 100% that way.

Ah, yes. That will probably be the thing to do since the manufacturer said 15A minimum for the grid sense breaker.

Thanks again!

 

[ggunn]

Ah, yes. That will probably be the thing to do since the manufacturer said 15A minimum for the grid sense breaker.

Thanks again!

You might also ask your AHJ if they maybe don't consider the grid sense breaker to be a load or source. I had some success with that approach for surge arrestor breakers when I had a similar situation.

 

[wwhitney]

Wayne, its a Sunpower Hub+ that is downstream of the lug kit. It has basically 3 panels in one enclosure. All of that is taken into consideration within that unit.

Do you have a sufficiently detailed spec sheet or installation manual for the Sunpower Hub+ that you could post or direct me to? I couldn't find the details on Sunpower's web site.

Thanks,
Wayne

 

[Designer101]

cant you use
USE 705.13 if you are in NEC 2020 OR NEC 2023
most of the equipment that has batteries comes with Pcs features.
if the equipment dont have pcs feature, how about relcoating that 100A sub panel to back up section of mid Device then you can use

705.12(B)(2)(3)(c)​

with no problem at all