120% rule on subpanels and feeder passthrough lugs?

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hitehm

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We are installing a dual inverter system w/ battery BU. We were unsure how this should be wired to the SE BU interface (transfer switch) so SE sent me this diagram attached. The flow from inverters back to the MSP seems straight forward but it raised several ?s about the infamous 120% rule and having the "other power source" (aka solar) breaker on a subpanel and not in the MSP. Any help clearing this up would be appreciated. My questions are:

1 - Since they are using passthrough lugs on the feeder to the BU subpanel "C", which passes through the transfer switch to the 200A main on the subpanel at "B", does the 120% rule apply to the MSP? If so how, since there is no solar breaker attached to the MSP to calculate?
2 - Is subpanel "C" subject to the 120% rule itself? It's not Service Equipment but it is on the load side of the Service Disconnect, and per 705.12B.2.3.b has both a "primary source" and "another power source at the opposite end". So what's the rule for that?
3 - In the inverter combiner panel on the right, is the breaker at "D" necessary? Can't this panel be MLO or does the production meter in between make a difference? The meter is not a "load" correct?combined inverters w notes.jpg
 
Up.codes says NV is using the 2017 NEC:


1) Every panel and feeder between the inverters and the service has to comply with 705.12(B). The 120% rule, 705.12(B)(2)(3)(b) is just one option for busbars, along with (a) and (c). All of the rules state to use 125% of the "power source output current" rather than breaker size. For inverters that would 125% of the maximum continuous output current.

So the MSP has to comply with one of the 3 options in 705.12(B)(2)(3), and you use 125% of the ratings of the inverters.

2) Yes, subpanel (C) has to comply with one of the 3 options in 705.12(B)(2)(3). The SE7600 has a maximum output current of 32A, so with two of them, 125% of the power source output current would be 80A. But 120% * 225 = 270A, and 270A < 200A + 80A. So subpanel (C) as drawn doesn't comply with 705.12(B)(2)(3)(b). (It could if the main breaker is downsized.) It would comply with (c) if the sum of the BU load breakers is less than 125A per pole, and you could change the 100A breaker to 80A to increase that limit to 145A. Otherwise, it does not comply with 705.12(B)(2)(3).

3) As far as I can see it could be MLO.

BTW, I'm not familiar with the SolarEdge Backup Interface (I need to look that up), but it shows spaces for 40A breakers. I would have thought it was designed for the inverters to be interconnected there, is that not the case?

Cheers, Wayne
 
Thanks for the help Wayne. As the for BU interface spaces for breakers, I also questioned that but I think the reason they don't use those in this situation is because the Production Meter needs to take in the combined inverter output BEFORE any loads so it only calculates solar production regardless of where that power is directed and also so it does not "sense" any grid power. I believe the production meters are not bi-directional and will sum internally in one direction regardless of the direction of net power. I don't know if there's a way to use the BU interface as a combiner and also connect a Production Only meter. Do you see a way? It also begs the question - Then what are the BU interface breakers used for?
 
hitehm said:
As the for BU interface spaces for breakers, I also questioned that but I think the reason they don't use those in this situation is because the Production Meter needs to take in the combined inverter output BEFORE any loads so it only calculates solar production regardless of where that power is directed and also so it does not "sense" any grid power.
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Right, the production meter, that makes sense. The largest breaker the Backup Interface supports for inverters is 63A, which is not enough for (2) SE7600. So if the utility requires a single production meter, you can't use the Backup Interface to connect the inverters, and you end up with the topology shown in the OP.

With the production meter in that location, how does that work when operating off grid (in backup mode)? If it the billing is NEM, that's fine, but if it's "buy all / sell all" with different rates for buy and sell, you'd still be paying the spread for the privilege of running your own microgrid.

hitehm said:
It also begs the question - Then what are the BU interface breakers used for?
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Installations that don't require a Production Meter, or where the combined inverters can go to a single breaker in the Backup Interface. Presumably the majority of installations.

Cheers, Wayne
 
wwhitney said:
With the production meter in that location, how does that work when operating off grid (in backup mode)? If it the billing is NEM, that's fine, but if it's "buy all / sell all" with different rates for buy and sell, you'd still be paying the spread for the privilege of running your own microgrid.

Cheers, Wayne
Click to expand...
Ah, very good point! The POCO might argue that all grids need occasional repair/maintenance and as a "partner" with or "supplier" for them you also need to be prevented from supplying energy into the grid during those times.
 
Does the MSP have branch or feeder breakers, or is it just the service disconnect breaker? If the former then it needs to meet busbar rules; if the latter, it doesn't.

Does SolarEdge have Power Control Systems certification? If so you might be able to set the maximum combined output of the inverters to 56A and then your Panel 'C' is okay as is.

The rating of the 200A breaker for the inverter combiner panel does not figure into the calculations. But also, it does not need to be 200A with the inverters shown. It could be 80A, or if you want room for expansion 125A is probably a lot more cost effective than 200.
 
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