PV System Limits based on Service Panel Size

BB.

Re: PV System Limits based on Service Panel Size

You probably will need to review the size of main panel needed by code for your home...

But, the typical method to begin sizing:

45.8 amps * 1/0.8 NEC breaker/wiring derating = 57.25 ~ 60 amp minimum breaker + branch wiring rating.

Next, if you do not want a large panel--You can down size the main breaker from the meter to the next size down. For example, 200 amp panel with 175 amp main breaker:

200 amp panel * 1.2 "over size for GT inverter" - 175 Amp main breaker = 65 Amp available for GT inverter

So--That would be one way to go. I am not sure that the AHJ would have problems with derating the main breaker unless the 175 Amp service is too small for your home and connected loads/circuits (heavy A/C and/or heat pump, electric heating/water heater/stove, etc.).

-Bill

ronaldmaustin

Re: PV System Limits based on Service Panel Size

BB. wrote: »

You probably will need to review the size of main panel needed by code for your home...
. . .
-Bill

Thank you. This explains the .8 factor, which I think some others with questions in this thread may have overlooked. I suppose I will call the power company (So Cal Edison) and start by seeing if the line drop supports the 200 amp panel. Then I will call the code inspector who is already reviewing my solar permit app to see if there is any issue with a 175 main breaker, as the home was doing just fine with the 100 amp breaker before.

While the 125 amp panel would not appear to give me room for the required 60 amp breaker, people here have posted about a 150 amp bus with a 100 amp main breaker. I am unclear on what that's all about or how one determines what sized bus is in a particular panel, or how to convince an inspector that a 125 amp panel has a 150 amp bus. Maybe I just misunderstood the whole 150 amp bus thing.

solar_dave

Re: PV System Limits based on Service Panel Size

ronaldmaustin wrote: »

While the 125 amp panel would not appear to give me room for the required 60 amp breaker, people here have posted about a 150 amp bus with a 100 amp main breaker. I am unclear on what that's all about or how one determines what sized bus is in a particular panel, or how to convince an inspector that a 125 amp panel has a 150 amp bus. Maybe I just misunderstood the whole 150 amp bus thing.

The manufacturer of the panel can give you the buss bar rating.

inetdog

Re: PV System Limits based on Service Panel Size

dfigu wrote: »

Hi Bill, et al,

sorry to revive this older thread. I have a older house with 100 amp panel. For this case I am assuming the quick answer would be to divide by 2 (i.e. 3840W max)?

Just for my sake, how do you get from 240 A to "a pair of 40A breakers", and is the dereating 80% of 40 to get down to 32A ? What about the second 40A?

thanks!

dave

A pair of 120V 40A inverter circuits, or one two-pole 40A breaker for a 240V inverter input maybe?
Or two inverters each capable of producing 20A of output under the NEC formulas and each connected to a 40A breaker?
If there are breakers closer to the inverters than the backfed breakers in the main panel, you can add up the sizes of those breakers for the 120% calculation instead of adding the ratings of the back-fed breakers in the main panel.

inetdog

Re: PV System Limits based on Service Panel Size

niel wrote: »

dfigu,
for your panel that would be a correct assumption as a 100a panel has buses rated to 120a. i have no idea what the rest of your questioning is and i strongly suspect you are highly confused about some aspect of it. anyway, if you stay with the 100a main breaker that means that you can feed 20a of solar to it because the buses are rated for it. if the main breaker were to be say 75a then this adds extra capacity that the panel can be fed with solar. 120a-75a=45a in solar power. of course where the solar is inputted to the main panel it will do so with a breaker of the proper current rating the pvs may produce. does this clear it up for you?

Actually, in terms of real life equipment, a 100A load center (panel) will either have 100A buses or will have 125A buses. Never seen one with 120A. The problem is that often the label on the panel and even the manufacturers standard catalog listing info will not tell you what the bus rating actually is. If the same part number bus bars are used in both a 100A and a 125A panel, that would be good evidence. Failing that, you are stuck talking to the manufacturer's tech support to try to get the answer.

ggunn

Re: PV System Limits based on Service Panel Size

FWIW and if no one has mentioned this yet, in many jurisdictions you can interconnect through a supply side tap for residential PV systems. In that case you are only limited by the size of the service as to how much inverter current you can backfeed into it.