PV sub panel maximum load

[Greg1707]

I have a PV setup with the inverter feeding to a six space 100 amp sub panel. The sub panel has a 45 amp breaker back fed by the inverter. In the sub panel there is a 30 amp 2 pole breaker supplying the dryer. The sub panel is connected to the main 200 amp panel with a sixty amp breaker.

INVERTER---------45 amp breaker---------2 empty spaces-------30 amp breaker(dryer)--------------200 amp main panel 60 amp breaker

The cable is NM 6-3

Can I add additional load to the sub panel? What is the existing load on the sixty amp breaker?

 

[Greg1707]

photo

photo

Here is an additional photo

 

[pv_n00b]

You can add more CBs and more load up to what the 60A CB can carry. The existing load on the 60A CB is the dryer.

 

[jaggedben]

Put another way... The existence of the solar has nothing to do with the allowable load. The only caveat to that is that it prevents you, in this case, from upsizing the wire and breaker feeding the sub. Well, you could upsize it to 70A, actually, but no further.

 

[Greg1707]

Put another way... The existence of the solar has nothing to do with the allowable load. The only caveat to that is that it prevents you, in this case, from upsizing the wire and breaker feeding the sub. Well, you could upsize it to 70A, actually, but no further.

Thank you for your posting. I am unfamiliar with PV. What is the limitation of 70 amps? Why is, say a 90 or 100 amp breaker with appropriate sized conductors feeding the 100 amp sub panel not an option?

 

[jaggedben]

Thank you for your posting. I am unfamiliar with PV. What is the limitation of 70 amps? Why is, say a 90 or 100 amp breaker with appropriate sized conductors feeding the 100 amp sub panel not an option?

Typically because of something traditionally called the "120% rule", which states that the sum of the utility side breaker and the PV output (125% of inverter nameplate) cannot exceed 120 percent of the busbar rating. So in your case it would seem to be 120A-45A leaves 75A left, and that's not a standard breaker so therefore go down to 70.

There is another rule now that allows the panel to have a 100A breaker if the PV plus all load breakers don't exceed the panel rating. But this does not help in your case, as the load breakers would be limited to 55A total.

I'm actually confused by the 45A inverter breaker you described, that does not seem correct for any SolarEdge inverter that I'm familiar with. So it's possible that when we actually look at the inverter spec sheet that you get a slightly different answer. Same rules apply though.

Review 705.12(B) for the rules, particularly 705.12(B)(2)(3). Replace '(B)' with '(D)' for code references before the 2017 NEC.

 

[ggunn]

Thank you for your posting. I am unfamiliar with PV. What is the limitation of 70 amps? Why is, say a 90 or 100 amp breaker with appropriate sized conductors feeding the 100 amp sub panel not an option?

Keep in mind that the PV contribution to the current in the panel applies to the MDP as well. Every bus that the PV inverter output is connected to, either directly or indirectly all the way back to the service, has to be qualified under 705.12. If you have too much inverter current to connect it to the load side of the MDP, going through a sub panel will not help you.

 

[Greg1707]

Tesla Charger

Tesla Charger

Typically because of something traditionally called the "120% rule", which states that the sum of the utility side breaker and the PV output (125% of inverter nameplate) cannot exceed 120 percent of the busbar rating. So in your case it would seem to be 120A-45A leaves 75A left, and that's not a standard breaker so therefore go down to 70.

There is another rule now that allows the panel to have a 100A breaker if the PV plus all load breakers don't exceed the panel rating. But this does not help in your case, as the load breakers would be limited to 55A total.

I'm actually confused by the 45A inverter breaker you described, that does not seem correct for any SolarEdge inverter that I'm familiar with. So it's possible that when we actually look at the inverter spec sheet that you get a slightly different answer. Same rules apply though.

Review 705.12(B) for the rules, particularly 705.12(B)(2)(3). Replace '(B)' with '(D)' for code references before the 2017 NEC.

This is very helpful. So, to disclose all the information--the customer wants to install a 50 amp circuit for his new Tesla. The main panel has no vacant spaces. That is why I am considering the PV sub panel. From what I understand from your helpful posts, I can install the 50 amp breaker for the Tesla in the PV sub panel. And then run #4 conductors to the main panel and install a 70 amp breaker? All will be good to go?

 

[jaggedben]

A 50 and a 30 both feeding from a 70? Seems borderline. Might trip if he does laundry while charging the car.

What is the inverter model number? Label should be on the right side of it.

 

[electrofelon]

Those little square D 6/12 panels have an odd busbar arrangement where one is fed from one side and the other is fed from the opposite side. How do we treat the "opposite ends of busbar" clause with these?

 

[GoldDigger]

Those little square D 6/12 panels have an odd busbar arrangement where one is fed from one side and the other is fed from the opposite side. How do we treat the "opposite ends of busbar" clause with these?

Where by fed from the opposite side you mean from the opposite end of that bus bar, so that one two pole breaker is fed from the start of one bus and the end of the other?
Sounds to me like you cannot apply the 120% rule in such a panel if you are trying to make a 240V connection. You need to replace the panel or qualify under one of the other backfeed rules.

 

[electrofelon]

Where by fed from the opposite side you mean from the opposite end of that bus bar, so that one two pole breaker is fed from the start of one bus and the end of the other?

Yes. I agree the busbar cant be qualified under the "120%" criterion.

 

[jaggedben]

You guys are right. I forgot about that.

 

[electrofelon]

You guys are right. I forgot about that.

Those panels nearly screw me up every time I work in one.