Solar question

JoeyD74

Senior Member
I have a service upgrade I am doing, two family house. 2nd floor unit has solar system already installed to the 100 amp panel. It’s tapped onto the line side of the main breaker, wires come from a 2 circuit can with a 2 pole 50 amp breaker. Why are they tapped to the line side, between meter and panel vs just going into the panel on a 2 pole 50 amp breaker?
 

GoldDigger

Moderator
Staff member
Because without other unlikely details, the combination of the 100A main and the max inverter output times 1.25 will be greater than 120% of the 100A rating of the panel bus.

Sent from my XT1585 using Tapatalk
 

JoeyD74

Senior Member
The wires are #6thhn from the main tap to a 2 circuit can with a 50 amp breaker in it. I’m assuming the solar system output can’t be more than 50 amp, is that correct?
 

jaggedben

Senior Member
Inverter output actually shouldn't be more than 40A. Inverter output circuits are required to be sized at 125% for continuous output.
 

JoeyD74

Senior Member
Inverter output actually shouldn't be more than 40A. Inverter output circuits are required to be sized at 125% for continuous output.
When I change the panel, should I tap back onto the main feed before the breaker or just put a 2 pole in the panel? I could upsize the wire from the existing 2 pole 50 the solar company installed but the whole system is wired in #6
 

Adamjamma

Senior Member
If you are changing the panel, and used a 200 amp panel with a 100 amp main breaker, as I understand things, the soar could simply run into the panel then without needing to be line side. But, I am still learning so am probably wrong...
 

romex jockey

Senior Member
705.12(A) or (B)., possibly B 1,2,3

methinks it describes a line vs load tap

i'm always needing some guidance in this article of the code

~RJ~
 

jaggedben

Senior Member
When I change the panel, should I tap back onto the main feed before the breaker or just put a 2 pole in the panel? I could upsize the wire from the existing 2 pole 50 the solar company installed but the whole system is wired in #6
If you use a 125A panel with a 100A breaker then you can put the 50A breaker in the panel. Or you can leave it as a tap.

Without knowing what else is involved in your scope of work it's a little hard to say what would be best.
 

ggunn

PE (Electrical), NABCEP certified
When I change the panel, should I tap back onto the main feed before the breaker or just put a 2 pole in the panel? I could upsize the wire from the existing 2 pole 50 the solar company installed but the whole system is wired in #6
125% of the total maximum inverter current plus the rating of the main breaker cannot exceed 120% of the panel busbar rating, and if that condition is satisfied the backfed PV breaker must be located at the opposite end of the busbar from the main breaker. If by some chance the two sum to 100% or less of the busbar rating, you can land the PV breaker anywhere in the panel. If they sum to more than 120% of the busbar rating, then it has to be a line side connection.

If you are changing out the panel anyway you could change it to one with a higher busbar rating but with the same main breaker rating in order to comply with the 120% rule. Changing the wire size from the PV would be irrelevant.
 

Adamjamma

Senior Member
If you use a 125A panel with a 100A breaker then you can put the 50A breaker in the panel. Or you can leave it as a tap.

Without knowing what else is involved in your scope of work it's a little hard to say what would be best.
Hmm..like that...
where I am at, the difference between the 125 amp panel and the 200 amp panel is so small, compared to the 100 amp panel.. that I just always run the 200 amp panels and they usually have 225 amp buses, depending on this quarters supplier to the island..lol...

the number of positions on the panel is more what my cost is than the amps size...
 

JoeyD74

Senior Member
125% of the total maximum inverter current plus the rating of the main breaker cannot exceed 120% of the panel busbar rating, and if that condition is satisfied the backfed PV breaker must be located at the opposite end of the busbar from the main breaker. If by some chance the two sum to 100% or less of the busbar rating, you can land the PV breaker anywhere in the panel. If they sum to more than 120% of the busbar rating, then it has to be a line side connection.

If you are changing out the panel anyway you could change it to one with a higher busbar rating but with the same main breaker rating in order to comply with the 120% rule. Changing the wire size from the PV would be irrelevant.
The new panel will be a 200 amp main breaker panel, not sure on the buss bar rating I’ll look but I doubt it’s enough to avoid a line tap on the solar.
If I understand correctly, the panel could possibly overload the buss bar by having the ability to draw 200 amp from the breaker as well as the rating of the solar system.
 

jaggedben

Senior Member
The new panel will be a 200 amp main breaker panel, not sure on the buss bar rating I’ll look but I doubt it’s enough to avoid a line tap on the solar.
If I understand correctly, the panel could possibly overload the buss bar by having the ability to draw 200 amp from the breaker as well as the rating of the solar system.
Do the math, man. Read 705.12 and/or what's been written above.

120% of busbar minus the main breaker is what you have left for the solar plus.

With a 200A panel and 200A main breaker:
(1.2*200)-200= 40A left for solar, not enough for you

225 panel with 200A main breaker:
(1.2*225)-200= 70A left for solar, enough for you

200A panel with 175A breaker:
(1.2*200)-175= 65A left, also works.
Seriously consider this option. Almost no dwelling in my experience really needs 200A, especially one that was living on 100A. Are you really adding more than 75A of calculated load?

Or just redo the tap. Whatever's easier and cheaper for you.

(BTW, it's very unlikely the panel would actually be overloaded, but the code is conservative.)
 

JoeyD74

Senior Member
Do the math, man. Read 705.12 and/or what's been written above.

120% of busbar minus the main breaker is what you have left for the solar plus.

With a 200A panel and 200A main breaker:
(1.2*200)-200= 40A left for solar, not enough for you

225 panel with 200A main breaker:
(1.2*225)-200= 70A left for solar, enough for you

200A panel with 175A breaker:
(1.2*200)-175= 65A left, also works.
Seriously consider this option. Almost no dwelling in my experience really needs 200A, especially one that was living on 100A. Are you really adding more than 75A of calculated load?

Or just redo the tap. Whatever's easier and cheaper for you.

(BTW, it's very unlikely the panel would actually be overloaded, but the code is conservative.)
They are adding two Mitsubishi systems, multihead hyperheats. Have existing electric stove, electric dryer, electric hot water heater. I’ll look at the buss rating and do the math, I appreciate the help.
Every home owner thinks they need 200 amp though
 
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