PV exceeds max allowable backfeed. True?

Coppersmith

Senior Member
I'm working with a solar contractor who asked me to install a 100 amp subpanel so he could backfeed several strings of panels with micro inverters. The subpanel will also have circuits for a PV monitor and a heat pump. I placed the 100 amp subpanel breaker in the furthest slots away from the main breaker. His engineer says this arrangement is not code compliant. Is this true? Could someone give me the code reference.

From engineer:

The panel is a 200 Amp panelboard with a 200 Amp breaker.
A load side connection, via a backfed breaker in the sub-panel, would not be code compliant.

The continuous current of the X22 microverters is 1.33 amps.
1.33 amps x 26 microinverters= 34.58 amps
34.58 amps x 1.25= 43.23 amps

The maximum allowable backfeed if the main is a 200 A with 200 A main breaker is 40 amps.

I would have to draw this as a "supply side connection" with an additional sub-panel for PV combining.





 

jaggedben

Senior Member
Probably true, on the main point. See 705.12 (B)(2)(3)(b). (2017 NEC reference, but true under earlier codes too.)

It is possible that 705.12 (B)(2)(3)(c) can be used but unlikely if the 200A panel is the main panel board for the house.

A better option than a supply side connection may be to downsize the main breaker to 175A, which a load calculation usually allows.
 

ggunn

PE (Electrical), NABCEP certified
I'm working with a solar contractor who asked me to install a 100 amp subpanel so he could backfeed several strings of panels with micro inverters. The subpanel will also have circuits for a PV monitor and a heat pump. I placed the 100 amp subpanel breaker in the furthest slots away from the main breaker. His engineer says this arrangement is not code compliant. Is this true? Could someone give me the code reference.
It's true. In the 2014 NEC it's 705.12(D)(2)(3)(a). In 2017 they moved it under 705.12(B).
 

Coppersmith

Senior Member
Probably true, on the main point. See 705.12 (B)(2)(3)(b). (2017 NEC reference, but true under earlier codes too.)

It is possible that 705.12 (B)(2)(3)(c) can be used but unlikely if the 200A panel is the main panel board for the house.

A better option than a supply side connection may be to downsize the main breaker to 175A, which a load calculation usually allows.
It's true. In the 2014 NEC it's 705.12(D)(2)(3)(a). In 2017 they moved it under 705.12(B).
So if I am reading NEC 2017 705.12(B)(2)(3)(b) correctly, the total amps on a busbar cannot exceed 120% of its capacity. If the panel has a 200 amp busbar, then 240 amp total is allowed. My current total is 125% of the solar backfeed (43.23 amps) plus the value of the main breaker (200 amps) which is 243.23. By changing the main breaker to something smaller (probably 150 since I don't think 175 exists) it would be compliant. Correct?

I need to verify that the busbar is rated 200 amps. It's an Eaton BR panel. It says 200 amps max, but I'm not sure that is the buss rating. It might be the MB rating and the buss is higher.
 

ggunn

PE (Electrical), NABCEP certified
So if I am reading NEC 2017 705.12(B)(2)(3)(b) correctly, the total amps on a busbar cannot exceed 120% of its capacity. If the panel has a 200 amp busbar, then 240 amp total is allowed. My current total is 125% of the solar backfeed (43.23 amps) plus the value of the main breaker (200 amps) which is 243.23. By changing the main breaker to something smaller (probably 150 since I don't think 175 exists) it would be compliant. Correct?
Correct. To achieve compliance you can lower the main breaker rating or you can raise the busbar rating. Either way works. If you cannot find definitive documentation that says otherwise, you must assume that the busbar rating is the same as the main breaker rating.
 

jaggedben

Senior Member
So if I am reading NEC 2017 705.12(B)(2)(3)(b) correctly, the total amps on a busbar cannot exceed 120% of its capacity. If the panel has a 200 amp busbar, then 240 amp total is allowed. My current total is 125% of the solar backfeed (43.23 amps) plus the value of the main breaker (200 amps) which is 243.23. By changing the main breaker to something smaller (probably 150 since I don't think 175 exists) it would be compliant. Correct?

I need to verify that the busbar is rated 200 amps. It's an Eaton BR panel. It says 200 amps max, but I'm not sure that is the buss rating. It might be the MB rating and the buss is higher.
Eaton and some other manufacturers make some 'solar ready' panels that have 225A busbars and 200A MCBs. Those allow 70A of continuous PV backfeed. But if you didn't ask for a 'solar ready' panel at the supply house I think it's unlikely you ended up with one.
 

kwired

Electron manager
I thought one of the biggest points is the interlock?
The systems in question won't back feed into the grid if they don't sense incoming grid power. Kind of hard to back feed into grid otherwise if it worked like a standby generator works. One of the primary purposes of grid tying is to sell energy back to the POCO if you are producing it but not using it at the time.
 

Adamjamma

Senior Member
Ok. The systems I normally see are grid replacements to cut electric bill as not allowed to co generate in many areas in Jamaica..l the paperwork as well makes it too costly for many people.
 

ggunn

PE (Electrical), NABCEP certified
The systems in question won't back feed into the grid if they don't sense incoming grid power. Kind of hard to back feed into grid otherwise if it worked like a standby generator works. One of the primary purposes of grid tying is to sell energy back to the POCO if you are producing it but not using it at the time.
I don't know that I would call selling energy back to the POCO a "primary purpose" of grid tied solar. Grid tied solar is much less expensive than standalone solar because it does not require batteries to run. Most grid tied solar systems produce less energy over time than the customer uses, so the question of selling energy back to the utility is moot.
 

kwired

Electron manager
I don't know that I would call selling energy back to the POCO a "primary purpose" of grid tied solar. Grid tied solar is much less expensive than standalone solar because it does not require batteries to run. Most grid tied solar systems produce less energy over time than the customer uses, so the question of selling energy back to the utility is moot.
Ok maybe poor wording, the primary purpose is to lessen the amount paid for energy from the POCO. There may be times when you are producing more then you are using - at those times you are putting power back into the grid, and your metered amount does decrease when this is happening. POCO probably doesn't mind this as long as you are not ending up with more put back then you have taken over a billing period. Those that do put more back on then they take on a regular basis likely need to have different contract then a typical "consumer account" does, and maybe even end up needing to produce a set amount to fulfill that contract.
 

ggunn

PE (Electrical), NABCEP certified
Those that do put more back on then they take on a regular basis likely need to have different contract then a typical "consumer account" does, and maybe even end up needing to produce a set amount to fulfill that contract.
Actually, for residential systems they usually don't. The contracts are generally worded so that both cases (customer produces more energy than he uses, and customer produces less energy than he uses) are taken into account. What the POCO will pay for the "excess generation", though, is highly variable, ranging from full retail price per kWh to absolutely nothing.
 

kwired

Electron manager
Actually, for residential systems they usually don't. The contracts are generally worded so that both cases (customer produces more energy than he uses, and customer produces less energy than he uses) are taken into account. What the POCO will pay for the "excess generation", though, is highly variable, ranging from full retail price per kWh to absolutely nothing.
Unless you have a metering system that tracks demand (in or out) only thing that matters is if the net at the end of billing cycle is positive or negative.
 

jaggedben

Senior Member
Unless you have a metering system that tracks demand (in or out) only thing that matters is if the net at the end of billing cycle is positive or negative.
Well, no. Smart meters can separately count imports and exports and rate schedules can be arranged to treat them differently. For examples in California now, new systems pay some fees and taxes on imports and no longer get the equivalent credit on exports.
 

kwired

Electron manager
Unless you have a metering system that tracks demand (in or out) only thing that matters is if the net at the end of billing cycle is positive or negative.
Well, no. Smart meters can separately count imports and exports and rate schedules can be arranged to treat them differently. For examples in California now, new systems pay some fees and taxes on imports and no longer get the equivalent credit on exports.
That is what I said (or at least intended to say).

My power supplier uses smart meters on nearly all services. Even single family dwellings - no details are on billing other then total use but if I have a customer concerned about high bills I can get demand history from the POCO on these services. Grid tied PV systems are nearly non existent in my immediate area AFAIK, but I'm sure the typical meter used would at least show if there were any overall net "exporting" going on if you were to pull up demand data.
 
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