Sub-refeed as interconnection

[Solar Noob]

I am confused as I can't find too much information on this interconnection method one of my coworkers suggested I use for a job. There are two sub panels attached to the main panel by breaker, and so his strategy to interconnect would be to refeed the MSP (200A main, 200A buss) into a new 200A main 400A buss subpanel (that is replacing an existing sub) and feed the other existing sub through the bigger subpanel. He claims that NEC 705.12 permits this method of interconnection (as we are landing a 90A breaker in the new sub) as it follows the 120% rule, as the new sub provides us 280A of backfeed from the solar. Is this a correct interpretation or is his thinking flawed?

 

[Solar Noob]

Here's the before and after with his suggestion.

 

[ggunn]

Here's the before and after with his suggestion.

If you are interconnecting in the (E) subpanel on the lower left, you must comply with 705.12 in all three panels; is that what you are asking?

 

[Solar Noob]

We are landing the solar in the new 400A panel. My confusion is whether or not the 120% rule allows us to backfeed 280A of solar back to the 200A rated MSP. It doesn't make sense to me electrically at least.

 

[Solar Noob]

If you are interconnecting in the (E) subpanel on the lower left, you must comply with 705.12 in all three panels; is that what you are asking?

Sorry you're usually the first to respond to my posts and I always word my questions in a confusing way. I am still learning how a lot of this works, so I apologize for my ignorance.

 

[wwhitney]

If you change the MSP to supply just a single 200A feeder, then 705.12 is not a problem for the MSP. If 125% of your inverter output current is 90A (you mentioned a 90A breaker), then you could take that 200A feeder to an MLO panel with a 300A bus, and on that bus put a 90A breaker for the PV, and two 200A breakers to refeed the existing subpanels. [Or just replace one of the existing 200A subpanels with the new 300A panel.]

Or you could land the 200A feeder on a 200A main breaker panel, which could replace one of the existing subpanels, with feed thru lugs to supply the other existing subpanel. Then you can do a feeder interconnection to the 200A feeder, either at a splice box between the 200A main breaker panel and the 200A MSP, or using the wiring space at the top of the 200A main breaker panel, if there's enough room. That avoids the need for a 300A bus.

Cheers, Wayne

 

[jaggedben]

We are landing the solar in the new 400A panel. My confusion is whether or not the 120% rule allows us to backfeed 280A of solar back to the 200A rated MSP.

No, it doesn't. Not by itself.
You still have to comply with one rule or another in the MSP.

There are other rules besides the 120% rule. Such as the 'sum of all breakers' rule that comes immediately after the 120% rule on the code. You might be able to comply with another rule in the MSP to make your coworkers suggestion work. But we need to know more details.

It doesn't make sense to me electrically at least.

Exactly.

BTW it seems strange on a number of levels that you have a 200A MSP feeding two 200A subpanels. If that info is correct, then at least one of the 200A subpanels must be oversized. I might be looking into doing load calculations to downsize one or both breakers for the subpanels. For example if you could downsize them both to 100A then perhaps you could backfeed one of them without installing a new panel.