Grid Tied Solar PV Backfeed in subpanel - is this right?

[tiredfather]

Hi all,

New homeowner with Grid Tied Solar PV. From when the solar PV was installed, there was an addition of a subpanel to support a hot tub.

Question is - does it matter where the solar backfeed is installed? I have been browsing forums but couldn't find a clear answer. Sorry if a dumb question and thanks for taking a look; just a new homeowner trying to figure out if everything is legit!

Exterior with main panel, Enphase Envoy IQ, 17 Canadian Solar panels (cs6u-330p) + 17 enphase iq7+ micro inverters, and lower subpanel
Screenshot 2022-08-23 113919.jpg

Main panel (goes to indoor distribution (one in garage (pictured), one inside (not pictured), then lower subpanel is fed from garage distribution)


Lower Subpanel has pool and solar breakers
Screenshot 2022-08-23 113804.jpg

Garage subpanel w/distribution (this is where I'm confused - solar breakers are here, too. Is this right?)
Screenshot 2022-08-23 114145.jpg

[Ampster]

If I undertstand it the main panel goes to the garage subpanel which feeds the subpanel below the main panel. Presumably the breaker labeled solar in the garage is feeding the subpanel under the main panel that contains breakers for solar and hot tub.

[solarix]

In our area, the utility will not allow loads on a solar circuit. The solar should be fed directly into a bottom slot on the garage sub-panel.

[tiredfather]

Thanks for the replies. I'm curious if the location of the solar breaker affects the solar feed of what it's powering? I'm guessing no but wanted to confirm to make sure the solar is feeding the entire house, not just certain breakers.

solarix thank you for the info. Seems like an easy fix so will add that to the list of projects this weekend.

[Ampster]

... I'm curious if the location of the solar breaker affects the solar feed of what it's powering? I'm guessing no but wanted to confirm to make sure the solar is feeding the entire house, not just certain breakers....

Fortunately solar generation will flow to the nearest loads in your home before any remaining generation flows to the grid. The laws of Physics insure the the grid only sees net generation.

EDIT. It is also important, as mentioned below by solarix, that the backfeed breaker be at the correct location on the bus bar.

[solarix]

Using the bottom location on the breaker panel is a code requirement. The code allows backfeeding the load side of a bussbar as long as the backfed breaker is 20% or less of the the buss rating and if the breaker is on the far end of the buss from the main breaker. The assumption is that some of the intervening loads will absorb enough of the power to prevent over powering the buss. In a worst case analysis, if you draw 200a in from the main breaker and add 30a from your solar, with a lot of circuits on - you could have more than 200a flowing in the bussbar and melt it. That would be bad.

[bcroe]

Using the bottom location on the breaker panel is a code requirement. The code allows backfeeding the load side of a bussbar as long as the backfed breaker is 20% or less of the the buss rating and if the breaker is on the far end of the buss from the main breaker. The assumption is that some of the intervening loads will absorb enough of the power to prevent over powering the buss. In a worst case analysis, if you draw 200a in from the main breaker and add 30a from your solar, with a lot of circuits on - you could have more than 200a flowing in the bussbar and melt it. That would be bad.

If the solar feed is from the opposite bussbar end from the main breaker,
the 2 sources cannot add up and melt the buss, because current flowing
in opposite directions would cancel. Throw a huge load in the middle, and
the 2 sources would feed their share thru different parts of the bussbar.
Bruce Roe

[tiredfather]

Thanks for the info! Appreciate it

[Ampster]

If the solar feed is from the opposite bussbar end from the main breaker,
the 2 sources cannot add up and melt the buss, because current flowing
in opposite directions would cancel. Throw a huge load in the middle, and
the 2 sources would feed their share thru different parts of the bussbar.
Bruce Roe

If there were 220 Amps of loads and 20 Amps of solar coming on to the bus bar the main breaker would not trip because it would only see 200 Net Amps. I am not saying that the bus bar would melt but is it possible that the bus bar could be carrying over 200 Amps in some spots depending on location of breakers?

[bcroe]

If there were 220 Amps of loads and 20 Amps of solar coming on to the bus bar the main breaker would not trip because it would only see 200 Net Amps. I am not saying that the bus bar would melt but is it possible that the bus bar could be carrying over 200 Amps in some spots depending on location of breakers?

It is entirely dependent on the location of the breakers. If the main PoCo
breaker and the solar source breaker were next to each other at one end,
the bus between them and the loads would see the total current. But if
the PoCo and solar breakers are at extreme opposite ends, there is no
point on the busbar where they can add up.

You could say, if the entire load was taken from a single tab on the busbar,
that tab would carry the total. But there is no single snap in load breaker
capable of anything like 220A. Even my 100A breaker uses a pair of tabs,
not a single.

My inverters feed thru 3 boxes to get to the PoCo meter over 300 feet,
each is wired PoCo at the top, Inverter source at the most bottom.
Bruce Roe