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Discussion Starter · #1 ·
Residential scenario (maybe I should post it there)
800 sq ft house, 40-amp backfeed breaker coming, quoting new service. See diagram (haha) below.

OUTSIDE
200-amp meter can
200-amp outdoor main breaker panel, 8 spaces with Feed Thru lugs
PV, 37.5 amps input to 40-amp back-feed breaker in main panel
SER from feed thru lugs (either 2/0 or 4/0 to panel in basement

INSIDE
150-amp MB panel (with "main breaker") configured as sub-panel (G & N separate)

Would this be compliant?
* looking at my own diagram, I'm thinking the backfeed breaker should be at the bottom, maybe.*

Handwriting Rectangle Font Parallel Drawing
 

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Not as I see it. Read 705.12 Load-Side Source Connections. You will find the location you have the backfeed breaker in should be opposite the utility. Maximum inverter size with a 200 amp buss and a 200 amp main is 7.6 kW. You can get smaller mains for loadcenters that allow you to increase the size of the inverter(s). I have installed 175 amp mains in 200 amp panels, actually ordered them that way.
 

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Magic Smoke Remover
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[OP:] Could conceivably use a 150-amp branch circuit breaker in the main panel and an MLO in the basement.
I would like this better since it would keep the backfed breaker at the full other end of the grid, but I’m not sure on the actual code part of this. If you fed-thru, you’re not really feeding the bottom end of the loaded buss.
 

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Not as I see it. Read 705.12 Load-Side Source Connections. You will find the location you have the backfeed breaker in should be opposite the utility. Maximum inverter size with a 200 amp buss and a 200 amp main is 7.6 kW. You can get smaller mains for loadcenters that allow you to increase the size of the inverter(s). I have installed 175 amp mains in 200 amp panels, actually ordered them that way.
Some QO stuff has a 225 buss with 200 main. Had to order it last time but I think everything is that way now…
 

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I would like this better since it would keep the backfed breaker at the full other end of the grid, but I’m not sure on the actual code part of this. If you fed-thru, you’re not really feeding the bottom end of the loaded buss.
You are feeding at the opposite end of that buss. Granted, if all your load is inside, current is going past your interconnect breaker. But code doesn’t really address that.
 

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The first Square D distributor I checked with, looking for a "solar ready" meter/main/dist combo, said they had nothing "solar ready"
Don’t know what solar ready is. The first time I had to order a panel with a smaller main, the salesman said they didn’t exist. I told him they do and to dig deeper. When he called me back he was now the expert! 😆
 

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Discussion Starter · #13 ·
Don’t know what solar ready is. The first time I had to order a panel with a smaller main, the salesman said they didn’t exist. I told him they do and to dig deeper. When he called me back he was now the expert! 😆
I believe it refers to a panel with a main breaker and an overrated buss bar, e.g. 200-amp main and 225-amp buss bar.
 

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I would like this better since it would keep the backfed breaker at the full other end of the grid, but I’m not sure on the actual code part of this. If you fed-thru, you’re not really feeding the bottom end of the loaded buss.
(2)
Where two sources, one a primary power source and the other another power source, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the power-source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the ampacity of the busbar. The busbar shall be sized for the loads connected in accordance with Article 220. A permanent warning label shall be applied to the distribution equipment adjacent to the back-fed breaker from the power source that displays the following or equivalent wording:
WARNING:
POWER SOURCE OUTPUT CONNECTION—
DO NOT RELOCATE THIS OVERCURRENT DEVICE.

The warning sign(s) or label(s) shall comply with 110.21(B).

If it were me, I would install the interconnect inside at the bottom of a 200 amp panel.
 

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Magic Smoke Remover
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Plus, the max going past is only 150, not 200.
Worked on a house yesterday that still had the 30 amp main fuse box. Yeah, the dryer and ac were double tapped off the meter but still, with gas appliances, what’s the actual load in the summer time while doing laundry? 45 amps total maybe? I bet that it never gets anywhere near even the 150…
 

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(2)
Where two sources, one a primary power source and the other another power source, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the power-source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the ampacity of the busbar. The busbar shall be sized for the loads connected in accordance with Article 220. A permanent warning label shall be applied to the distribution equipment adjacent to the back-fed breaker from the power source that displays the following or equivalent wording:
WARNING:
POWER SOURCE OUTPUT CONNECTION—
DO NOT RELOCATE THIS OVERCURRENT DEVICE.

The warning sign(s) or label(s) shall comply with 110.21(B).

If it were me, I would install the interconnect inside at the bottom of a 200 amp panel.
What's the science behind keeping the other power source being at the opposite end of busbar? I remember reading some overheating issues but don't remember why.
 
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What's the science behind keeping the other power source being at the opposite end of busbar? I remember reading some overheating issues but don't remember why.
If you were pulling all 200 amps plus say 40 from your solar, and that 40 “comes in” at the top, you’d overload the buss “in the middle”. If the solar “comes in” at the bottom it’ll go back out to the breakers at the bottom. I think that’s the idea anyway. KCL or something like that.
 
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