# Supply Line Maximum Backfeed

I was reviewing wire gauge sizes and maximum amperages, and have the following question if anyone can answer:

Is the gauge size of the line side wire (from PGE for instance) based on both the current to the property, and that produced by the array?

For instance, if a supply is 3/0 wire, rated at 90 degrees, has a maximum amp rating of 225A, and a 200A main breaker in the main panel, then is the maximum backfeed amperage:

1. 25 amps (subtract the 200A maximum feed from the total max)
or
2. 225 amps

It makes sense to me that if there is no load from the property, then the feed amps are zero, so the pv would have a maximum allowable of 225 amps backfeed on the 3/0 wire.

Conversely, if the property is consuming the entire pv production, then it is not sent through the line side and the max current is still 225 A from the line side.

I don't see a scenario when the current is additive on the line.
Any ideas here? Thank you.

Re: Supply Line Maximum Backfeed

Of course, there is the 10 kW limit for "home sized" solar GT systems... Above that amount, the utility could make you jump through a whole bunch of hoops (at least in our area).

Normally, with a unified meter/main panel, we are only allowed to install up to 20% of the main panel rating worth of solar arrays... Of course, you can play some games.

A 200 amp panel * 20% would be 40 amps maximum (with 200 amp breaker).

If a 200 amp panel with a 125 Amp breaker was installed, you could install solar up to:
• 2000 amps * 20% + (200amp - 125 amp breaker) = 40a + 75a = 115 amps of Solar GT
If you were allowed a line side tap (not something done often in home settings, from what I read here)--Perhaps you could talk somebody into 225A of solar... But then you run into the issue where the power from the Utility Transformer is additive with the power from the Solar GT connection... Techincally, the link between the utility+solar gt could exceed the ratings of the cabling itself.

Of course, this now gets into the whole question of what is the supply side rating of a pole transformer... I have always assumed it to be 10kAmps maximum. If you add 10kA transformer rating + XXX Amp of GT solar, you could exceed the Maximum Interrupt Rating of the standard breakers. In commercial systems (meter is installed very close to line transformer), I understand they have to use special fuses with >10,000 MAI rating... I could see somebody possibly requiring that if there was a lot of solar on a single transformer drop that was shared with other service drops.

Now you are probably in the realm of those big utility/power engineers and out of my league.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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Re: Supply Line Maximum Backfeed

#1 is correct, but in reality it is 20% or 40a.

now you may be a tad confused about the wire portion as it has only to do with the max power for your dwelling's consumption as there is no way you will sell more than that.

the real point of interest is the buses in the main breaker panel as this is where everything does intermingle. you would have the power from the grid present up to 200a and the power produced by any gt solar setup making its power available also on your buses. the only way to sell more with that same breaker panel is to limit what the grid can send into your home so this means reducing the size of the main breaker so that the total power seen on the buses will not exceed their maximum capability. for instance putting a 150a main breaker there would allow the 50a difference plus an extra 20% as buses are rated 20% higher than the service rating is. in the case of the 200a service this is 40a extra so the total capability to sell to the grid is 90a. i think your wires can handle that. remember if you have loads on in the house that the gt inverter will essentially be supplying that power and will only export what you don't use.
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Re: Supply Line Maximum Backfeed

The issue is your interconnect. If not a line side tap then the bus of the breaker panel is the limit, You are only allowed in aggregate to do 120% of the buss rating. So a 200 amp rated panel/breaker you can back feed a 40 amp limit unless you derate the line side breaker in the panel. I had such a situation with mine, where the breaker was a 60amp from the solar backfeed (the max output rating on the inverters was 46amps) and I had to derate the panel breaker to 175amps to keep under the 120% limit.

I was told that the Code people would only approve a line side tap on a commercial install, but never on a residential install. Why? who knows, a code monkey thing but it is spelled out pretty clearly in the NEC on how it needs to be done. YMMV with your code monkeys.
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Re: Supply Line Maximum Backfeed
solar_dave wrote: »
I was told that the Code people would only approve a line side tap on a commercial install, but never on a residential install. Why? who knows, a code monkey thing but it is spelled out pretty clearly in the NEC on how it needs to be done. YMMV with your code monkeys.

I am sure that is dependent on where you live. Some places require a separate backfeed utility meter.
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Re: Supply Line Maximum Backfeed
RCinFLA wrote: »
I am sure that is dependent on where you live. Some places require a separate backfeed utility meter.

Yeah mine does as well but couldn't the line side tap have that meter as well. BTW my utility only asks for my production meter reading annually.