Adding PV to an existing sub-panel

[bcl]

I have a 125a MLO meter pack w/ a 100a breaker feeding theelectrical panel inside the house, and two more spaces taken up by a 2-pole 50aGFCI breaker that I’m assuming feeds a hot tub. Additionally, there is a feedertap that goes to an unknown load. Not having been on site (I’m looking atsite-survey pics), and assuming that the feeder tap is code compliant (which isdifficult to tell from the pics), I want to make sure I do this right. I’m notopposed to service changes, but I also don’t want to upsell someone justbecause I can’t figure out how to make something work with their current system…
1. Regardless of PV, if I wanted to add a sub-panelin order to get rid of the feeder tap, I’d do a new load-calc based on 220.83,to make sure I could add a subpanel without having to do a service change. Orjust do like most guys do, and add a 50a sub-panel assuming that the sum of thenew loads on the sub panel and the loads inside the house are not going to exceedthe ampacity of the bus-bar before they trip their individual breakers. Thisseems pretty straightforward, right?

2. If I wanted to feed solar into the new subpanel(705.12(D)), do I need to only account for the size of the bus-bar in the subpanel, rated at 125a, and protect it according to 705.12(D)(2)(3)? It seemslike I also need to upsize the wire feeding the sub-panel to the calculated loads+ 125% of the solar load ((705.12(D)(2)(1)(a)). I realize these individualscenarios are easier to work out, but it confuses me when I start thinkingabout trying to size conductors based on the loads + interconnected systems. I’verun into this before when trying to figure out if and how much PV I can add toan existing electrical panel on a substructure.
If these questions are answered in a separate thread, pleasefeel free to direct me to it. Thanks!

 

[Carultch]

I have a 125a MLO meter pack w/ a 100a breaker feeding theelectrical panel inside the house, and two more spaces taken up by a 2-pole 50aGFCI breaker that I’m assuming feeds a hot tub. Additionally, there is a feedertap that goes to an unknown load. Not having been on site (I’m looking atsite-survey pics), and assuming that the feeder tap is code compliant (which isdifficult to tell from the pics), I want to make sure I do this right. I’m notopposed to service changes, but I also don’t want to upsell someone justbecause I can’t figure out how to make something work with their current system…
1.Regardless of PV, if I wanted to add a sub-panelin order to get rid of the feeder tap, I’d do a new load-calc based on 220.83,to make sure I could add a subpanel without having to do a service change. Orjust do like most guys do, and add a 50a sub-panel assuming that the sum of thenew loads on the sub panel and the loads inside the house are not going to exceedthe ampacity of the bus-bar before they trip their individual breakers. Thisseems pretty straightforward, right?

2.If I wanted to feed solar into the new subpanel(705.12(D)), do I need to only account for the size of the bus-bar in the subpanel, rated at 125a, and protect it according to 705.12(D)(2)(3)? It seemslike I also need to upsize the wire feeding the sub-panel to the calculated loads+ 125% of the solar load ((705.12(D)(2)(1)(a)). I realize these individualscenarios are easier to work out, but it confuses me when I start thinkingabout trying to size conductors based on the loads + interconnected systems. I’verun into this before when trying to figure out if and how much PV I can add toan existing electrical panel on a substructure.
If these questions are answered in a separate thread, pleasefeel free to direct me to it. Thanks!

If you interconnect at a subpanel, then 705.12(D) applies to BOTH the subpanel, and the main panel from which it is fed. In NEC2014, you only need to account for 125% of the total inverter continuous current, instead of the interconnection breaker. So 32A of inverters through a 40A breaker at a 100A MCB subpanel fed from a 100A breaker in the main panel, means that you use the 40A number in both locations.

 

[ggunn]

If you interconnect at a subpanel, then 705.12(D) applies to BOTH the subpanel, and the main panel from which it is fed. In NEC2014, you only need to account for 125% of the total inverter continuous current, instead of the interconnection breaker. So 32A of inverters through a 40A breaker at a 100A MCB subpanel fed from a 100A breaker in the main panel, means that you use the 40A number in both locations.

I didn't examine the OP in detail but if I understand your response correctly (100A bus fed by a 100A breaker and needing a 40A breaker for PV), it won't fly under 705.12(D)(2)(3)(b), i.e., the 120% rule. A maximum of 20A (16A of inverter current) is all the subpanel can take.

It might qualify under 705.12(D)(2)(3)(c), where the sum of all breakers both load and supply, discounting the rating of the breaker protecting the bus from the utility side, total less than 100% of the bus rating.

 

[Carultch]

I didn't examine the OP in detail but if I understand your response correctly (100A bus fed by a 100A breaker and needing a 40A breaker for PV), it won't fly under 705.12(D)(2)(3)(b), i.e., the 120% rule. A maximum of 20A (16A of inverter current) is all the subpanel can take.

It might qualify under 705.12(D)(2)(3)(c), where the sum of all breakers both load and supply, discounting the rating of the breaker protecting the bus from the utility side, total less than 100% of the bus rating.

What I meant is that all panels between the service point and the point of interconnection, need to comply with 705.12. You can't pop a subpanel out of a main panel with no capacity for the PV, and expect 705.12(D) to only apply to the subpanel.

 

[jaggedben]

1.Regardless of PV, if I wanted to add a sub-panelin order to get rid of the feeder tap, I’d do a new load-calc based on 220.83,to make sure I could add a subpanel without having to do a service change. Orjust do like most guys do, and add a 50a sub-panel assuming that the sum of thenew loads on the sub panel and the loads inside the house are not going to exceedthe ampacity of the bus-bar before they trip their individual breakers. This seems pretty straightforward, right?

Yes I would just install a subpanel fed by a 50A breaker, assuming the solar breaker is 50A or less. The solar is not a load and does not need to be added to calculated load. Relocate the hot tub GFCI breaker to the subpanel. If that feeder tap is a violation you could perhaps fix it by also moving that circuit to the new sub, although you have a problem with pierced insulation. If the AHJ is tough on details, maybe ask them first.

2.

If I wanted to feed solar into the new subpanel(705.12(D)), do I need to only account for the size of the bus-bar in the subpanel, rated at 125a, and protect it according to 705.12(D)(2)(3)?

It seemslike I also need to upsize the wire feeding the sub-panel to the calculated loads+ 125% of the solar load ((705.12(D)(2)(1)(a)).

If you relocate the hot-tub breaker instead of the subpanel feeder, then no. You are installing a new feeder that is fed from opposite ends, and the rule explicitly applies to other than that situation.

 

[ggunn]

What I meant is that all panels between the service point and the point of interconnection, need to comply with 705.12. You can't pop a subpanel out of a main panel with no capacity for the PV, and expect 705.12(D) to only apply to the subpanel.

And yes, absolutely, that is correct.

 

[bcl]

Think I've got it...

Think I've got it...

Thanks for the replies, everyone. I think I'm getting it, but I was confused about how to size PV to MLO panels. But it just occurred to me that maybe I'm trying to apply the wrong code. If I land a solar breaker on a MLO panel, then doesn't that just count as a supply side connection? 705.12(A). As long as I don't put more than 6 solar breakers on that thing (690.13(D)), and I don't exceed the ampacity of the busbar, am I good to go?

So in order to do this right, I would make sure that my load center feeder is large enough for its loads + 125% of the PV current (705.12(D)(2)(1)). I would size the OCP to account for the loads in the panel board, but also make sure that it is not smaller than 125% of PV ampacity. Finally, I would make sure that the bus bar ampacity complies with 705.12(D)(2)(3). For example:

(23) Enphase M250 = 23 amps

Load center OCP has to be at least large enough to account for the hot tub, 50 amps

Feeder for load center is 50 + (23 * 1.25) = 78.75 amps (#3 THWN or #4 if the inspector can see the 75 degree lugs)

125 amp bus bar is good under any of the requirements of 705.12(D)(2)(3).

 

[ggunn]

Thanks for the replies, everyone. I think I'm getting it, but I was confused about how to size PV to MLO panels. But it just occurred to me that maybe I'm trying to apply the wrong code. If I land a solar breaker on a MLO panel, then doesn't that just count as a supply side connection? 705.12(A). As long as I don't put more than 6 solar breakers on that thing (690.13(D)), and I don't exceed the ampacity of the busbar, am I good to go?

So in order to do this right, I would make sure that my load center feeder is large enough for its loads + 125% of the PV current (705.12(D)(2)(1)). I would size the OCP to account for the loads in the panel board, but also make sure that it is not smaller than 125% of PV ampacity. Finally, I would make sure that the bus bar ampacity complies with 705.12(D)(2)(3). For example:

(23) Enphase M250 = 23 amps

Load center OCP has to be at least large enough to account for the hot tub, 50 amps

Feeder for load center is 50 + (23 * 1.25) = 78.75 amps (#3 THWN or #4 if the inspector can see the 75 degree lugs)

125 amp bus bar is good under any of the requirements of 705.12(D)(2)(3).

I recommend you consult with your AHJ. CPS in San Antonio, for instance will not let you land a PV breaker in an MLO panel, period. Not even if it is protected by external OCP. We had a customer who wanted an external bladed fused disco ahead of his MLO panel and a PV breaker in the panel, and CPS rejected it. No main breaker, no PV.

And yes, I did argue with them about it but I got nowhere.

 

[jaggedben]

Fwiw, I've always been able to convince AHJs that a breaker in an mlo panel is a supply side connection. Your milage may vary.

 

[ggunn]

Fwiw, I've always been able to convince AHJs that a breaker in an mlo panel is a supply side connection. Your milage may vary.

The best I have been able to do in Austin is to get them to let me mount a fused AC disco next to it and tap the busbars.