PV exceeds max allowable backfeed. True?

Adamjamma

Senior Member
So, and remember please that I am asking as One studying code, not trying to do something wrong, but trying to learn,
if you have a 200 amp panel, and are going to be feeding it with 100 amp breaker from the inverters, the inverters would have to be sized at 75 or eighty amps?
The main breaker for 175 amps
all to keep maximum amps below 240?

but, does this mean you size the wire fore the feed at 200 amps or 250 amps?
and , the wire for the inverter backfeed, is that sized for 80 amps, 100 amps or 125 amps...

all these 125 quotes are getting me confused...
 

jaggedben

Senior Member
So, and remember please that I am asking as One studying code, not trying to do something wrong, but trying to learn,
if you have a 200 amp panel, and are going to be feeding it with 100 amp breaker from the inverters, the inverters would have to be sized at 75 or eighty amps?
The main breaker for 175 amps
all to keep maximum amps below 240?

but, does this mean you size the wire fore the feed at 200 amps or 250 amps?
and , the wire for the inverter backfeed, is that sized for 80 amps, 100 amps or 125 amps...

all these 125 quotes are getting me confused...
You size inverter output circuits and overcurrent devices to 125 percent of inverter nameplate max output. If the inverter labels add up to 80 amps, you size the circuit for 100A with 100A breaker.

If you have a 200A panel then under the NEC rule in question (See 705.12 (B)(2)(3)(b)) you would be allowed to have utility breaker plus 125%-of-inverter add up to 120% of the panel busbar. 120% of 200A = 240A. Thus 240A-100A(inverter)=140A remaining allowed for utility. In that case you'd have to downsize the breaker to the next standard size of 125A.

Under a different rule 705.12 (B)(2)(3)(c) you could avoid downsizing the main breaker if all other breakers in the panel add up to less than 100A.
 
Why are you trying to avoid a suspply side connection so bad? I do these almost every day and it's really easy. Just get some ilsco insulation piercing taps rated for your wire size, a 100amp fusible disco and call it good.

Sent from my SM-G955U using Tapatalk
 

Zee

Senior Member
A slightly lower amp rated micro-inverter might get you under 32 Amps total.....then when you apply 125% continuous you 'll be under 40A.
As long as the micro works with your panel W, V and A.


But I typically just swap out the main brkr. to 175A. Easy.
 

jaggedben

Senior Member
Why are you trying to avoid a suspply side connection so bad? I do these almost every day and it's really easy. Just get some ilsco insulation piercing taps rated for your wire size, a 100amp fusible disco and call it good.
If that's possible, great. On a lot of meter/main combos it's not possible.
 

Coppersmith

Senior Member
Why are you trying to avoid a suspply side connection so bad? I do these almost every day and it's really easy. Just get some ilsco insulation piercing taps rated for your wire size, a 100amp fusible disco and call it good.
Well, the real reason is because the solar contractor made a mistake and told the HO that it could be done in a subpanel. He then recommended me to the HO to install the subpanel. The HO hired me to do the install and it's complete. Now the solar contractor's engineer (who got involved late in the project) is saying "No no no, you need a line side tap." I'm trying to save him some embarrassment by finding a way to use the load side connection.
 

pv_n00b

Senior Member
Well, the real reason is because the solar contractor made a mistake and told the HO that it could be done in a subpanel. He then recommended me to the HO to install the subpanel. The HO hired me to do the install and it's complete. Now the solar contractor's engineer (who got involved late in the project) is saying "No no no, you need a line side tap." I'm trying to save him some embarrassment by finding a way to use the load side connection.
Never try to ask forgiveness from an engineer. If an engineer is going to be part of the project bring them in early and ask permission. Makes life much simpler.
 

Coppersmith

Senior Member
Never try to ask forgiveness from an engineer. If an engineer is going to be part of the project bring them in early and ask permission. Makes life much simpler.
You're preaching to the choir. I just do what they pay me to do and offer advice when I see trouble brewing. I could not foretell this situation due to my ignorance of the PV section of NEC. I now am a lot more knowledgeable. (I don't admit ignorance lightly. :))
 

ggunn

PE (Electrical), NABCEP certified
Never try to ask forgiveness from an engineer. If an engineer is going to be part of the project bring them in early and ask permission. Makes life much simpler.
Well, if that "forgiveness" means letting a code violation slide, the engineer would have no business granting it. I certainly wouldn't.
 

jaggedben

Senior Member
Sorry I misread something. But why don’t you get rid of two modules?
That's an option, too, technically. But no one wants to present that option to the customer unless one has a solid clause in the contract that allows for charging more for other options. I don't think I've ever proposed it when a supply side connection was feasible.
 
That's an option, too, technically. But no one wants to present that option to the customer unless one has a solid clause in the contract that allows for charging more for other options. I don't think I've ever proposed it when a supply side connection was feasible.
Have you looked at 705.12(D)(2)(3)(c)?
 

pv_n00b

Senior Member
Well, if that "forgiveness" means letting a code violation slide, the engineer would have no business granting it. I certainly wouldn't.
Sometimes what looks like a code violation can be turned into an Alternative Means and Materials exception by a good engineer and AHJ working together. I've saved a couple of projects doing this when the designer or contractor screwed up an installation and it would be costly to go back and make it code compliant. But sometimes there is no way to save it and make it safe. In that case, someone just has to eat it and make it right. Plus the tears of a designer or contractor who gets in this position because they don't quite know what they are doing taste like rainbows. :)
 
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