Head scratcher with 120 rule and 320amp 2 circuit panel for Enphase system

[whackit]

Hi Folks,

Always appreciate the help here when I'm stumped. I have a weird situation that I'm not sure how to make sense of. Homeowner looking to add a large PV setup with battery backup. Service enters house in a Milbank meter main that has only 2 breakers spots that are filled with 1x100amp breaker and 1x200amp breaker that feed two downstream sub panels. The 200A panel is for the main house and the 100A panel is for a garage/guest house. We want to install a Enphase system controller in between the main panel and the 200A sub to backup main house. The main 200A breaker in the service meter would cover both the solar and the sub panel (there is also a 200amp breaker on sub and they can be fit on both sides of the Enphase control if necessary). See attached image for clarity, apologies for iPad drawings from truck.

So how does the 120% rule apply here (or does it not?). Are the two breakers in the service panel considered "opposite ends" of it's buss even though they are right next to each other? Doesn't the 200A breaker in the main panel which is handling both the solar and the grid loads protect the buss as it is handling the full current?

Not sure why this is stumping me (and a few others). Any insight appreciated.

 

[pv_n00b]

You don't say what version of the NEC you are on but this is more of a feeder tap situation. That might be why it is not clear. So 2020 NEC 705.12(B)(1) applies and not the 120% rule for busbars. It will help a lot if the 200A panel has a main breaker.

 

[whackit]

We are under 2017. The 200A panel does have a main breaker. Also both sides of the Enphase control can be equipped with a 200A breaker.

 

[don_resqcapt19]

We are under 2017. The 200A panel does have a main breaker. Also both sides of the Enphase control can be equipped with a 200A breaker.

As long as the side connected to the 200 amp subpanel has a 200 amp breaker, you are good. 705.12(B)(2)

 

[whackit]

So the Enphase control box is considered the tap and protection is only necessary on both ends of the feeder? Am I reading this correctly?

 

[texie]

I agree with don. The code wants to limit the current on the portion of the feeder between the "tap" and the load panel to the rating of the feeder and the panel as it is fed from 2 sources. The main breaker in the panel prevents this.

 

[jaggedben]

For those not familiar, the Enphase system controller is a listed assembly with provisions for breakers for all connections to it. PV and ESS breakers are required. Usually the grid side connection does not need a breaker but the backed up panel does. As far as I've seen Enphase is a little vauge about the latter, referring you to your local code. It makes sense to apply the feeder tap rule if you're on 2014 NEC or later.

 

[jaggedben]

To answer part of the original question, no part of 705.12(B) applies to busbars on the supply side of the service disconnecting means. Only 705.12(A) applies on the supply side. If the OP wanted to connect the PV to the 200A subpanel without installing the system controller, then the 120% rule could apply to the subpanel, but it wouldn't change how much is allowed to be backfed through the 200A breaker in the meter main.

 

[wwhitney]

To answer part of the original question, no part of 705.12(B) applies to busbars on the supply side of the service disconnecting means. Only 705.12(A) applies on the supply side.

While that division makes sense, as I think I've pointed out before, the text of 705.12(B) does not actually exclude service equipment or busbars. So a literal reading of 705.12(B) would be that when a load side interconnection is made, any service conductor busbars are still subject to 705.12(B)(2)(3), at least when the equipment is "capable of supplying multiple branch circuits or feeders, or both."

Cheers, Wayne

 

[ggunn]

While that division makes sense, as I think I've pointed out before, the text of 705.12(B) does not actually exclude service equipment or busbars. So a literal reading of 705.12(B) would be that when a load side interconnection is made, any service conductor busbars are still subject to 705.12(B)(2)(3), at least when the equipment is "capable of supplying multiple branch circuits or feeders, or both."

Cheers, Wayne

The title of 705.12 is "Load-Side Source Connections", which applies to everything in 705.12. Service busbars are not load side; they are covered by 705.11, "Supply-Side Source Connections". It doesn't make sense to try to apply the 120% rule to a busbar that has no overcurrent protection. It already has virtually infinite current available to it; adding 50A or whatever to infinity is still infinity. It doesn't change anything.

 

[jaggedben]

While that division makes sense, as I think I've pointed out before, the text of 705.12(B) does not actually exclude service equipment or busbars. So a literal reading of 705.12(B) would be that when a load side interconnection is made, any service conductor busbars are still subject to 705.12(B)(2)(3), at least when the equipment is "capable of supplying multiple branch circuits or feeders, or both."

Cheers, Wayne

The wording could be improved to convey the intent, to be sure. Just take my word for it that the title of the section conveys the scope.

That said it's also impossible to apply most of the interesting rules to service conductors (wire or bus) because for example they they are neither feeders nor do they have an 'overcurrent device protecting' them, nor could they have feed through conductors. So that nixes application of 705.12(B) except for maybe 3(5).

Arguably 705.12(A), (C) (D) and (E) could be moved to an independent section though.

Edit: sorry, I used 2020 NEC references in this post, forgot the OP said 2017. Just to be more confusing.

 

[wwhitney]

Just take my word for it that the title of the section conveys the scope.

Ggunn made the same comment, but that's part of my point. The title is "Load Side Connections" not "Load Side Equipment." So even it is scope limiting (which is reasonable), it could cover service equipment on installations with "Load Side Connections."

I agree that's not the intent.

Cheers, Wayne

 

[ggunn]

Ggunn made the same comment, but that's part of my point. The title is "Load Side Connections" not "Load Side Equipment." So even it is scope limiting (which is reasonable), it could cover service equipment on installations with "Load Side Connections."

I agree that's not the intent.

Cheers, Wayne

Any connections in an MLO MDP connected directly to the utility revenue meter are not load side connections, they are supply side connections, and any load breakers in such a MDP are service disconnects. It seems pretty straightforward to me, although one AHJ I deal with doesn't agree and tries to apply 705.12(B)(3)(3) to PV interconnections in MLO MLB panels irrespective of the fact that there is no OCPD protecting the busbar. It makes no sense to me but mine is not to reason why...