Question regarding AC wiring and sub-panels with a grid-tied system

[seropaul3]

I am designing a roof-mounted 16 kW grid-tied system for my home and want to avoid unnecessary wiring expense. The solar panels will be installed on a detached garage/barn ( 36 panels ) and connected to a string inverter. The distance from the garage to the service entrance location in the house is approximately 100'.

One possible config would be three strings of 12 panels each, giving a VOC max of 500 volts and a max DC current of 32 amps.

Initially, I was considering running the DC from the solar panels to a point near the service entrance and install the inverter close to the main breaker box. This would entail trenching, laying new conduit, punching another hole in my foundation, running 100' of 8 gauge (minimum), etc.

However, there is an existing sub-panel in the garage that is fed off the main house panel with a 240volt, 100amp circuit over 2 gauge wire. If my calculations are correct, the string inverter AC output would be well below the load carrying capacity of the existing wire/breakers/busbar.

Would it be possible to install my inverter in the garage and tie it into the sub-panel, thereby avoiding the expense of running new lines from the garage to the house?

Any help would be greatly appreciated!

 

[hwy17]

This is a 120% rule problem.

16kW doesn't fit on a 100 amp panel. You can replace the panel with a 200 amp panel with a 100 amp main breaker.

But then your main panel probably still has a 120% problem too.

 

[seropaul3]

This is a 120% rule problem.

16kW doesn't fit on a 100 amp panel. You can replace the panel with a 200 amp panel with a 100 amp main breaker.

But then your main panel probably still has a 120% problem too.

Would this still be an issue if I use a 12 kW inverter? I'm thinking that would output at most 50 amps into the sub-panel. I should note that the sub-panel is fed split-phase 240.

 

[hwy17]

Would this still be an issue if I use a 12 kW inverter? I'm thinking that would output at most 50 amps into the sub-panel. I should note that the sub-panel is fed split-phase 240.

A 100 amp panel with a 100 amp main can accept a max of 20 amps solar breaker input, if the panel has load breakers on it.

 

[seropaul3]

Thanks to your helpful feedback, I've dug a bit deeper into the 120% rule and think I have a solution...

1. Replace the 100 amp breakers with 60 amp breakers, derating the sub-panel
2. Replace the 200 amp main house breaker with a 175 amp breaker, derating the house panel.
3. Backfeed the solar inverter into the sub-panel with a 60 amp breaker.
4. Choose an inverter with a max output of 48 amps (11.5 kW)

If my math is correct, the total current being fed into the house panel would be 235 amps (less than the 240 allowed) and the sub panel would be at 120 amps (right at what is allowed).

Of course, the backfeed breakers would be positioned at the other end of the busbar in each panel.

Would this work?

 

[zanydroid]

Would this work?

Yes

0. Are you sure you have the correct busbar ratings for the 120% rule?

1. It's a subpanel, those take less skill to swap than installing DC solar. Be bold. If you swap to 200A bus there are no concerns. If you swap to 125A bus you can reduce derate to 80A. You can also consider feeding it into the subpanel feeder (IE from the top), this would require you to add a small spa panel, can be tiny 4 position one, to provide OCPD for the inverter. And you need to do 705.12(A) feeder / feeder tap calculation. I believe with small cable / conduit investment + the $70 spa panel you can keep existing barn subpanel at 100A.

(EDIT: for the 705.12(A) approach you will need to add a 100A or whatever OCPD to the top of the barn panel. Not allowed to do this lug fed)

Of course if you don't care about the derate, this is academic.

2. Makes sense
3. Makes sense
4. Makes sense

If my math is correct, the total current being fed into the house panel would be 235 amps (less than the 240 allowed) and the sub panel would be at 120 amps (right at what is allowed).

This isn't really what's happening physically, but the math is correct for the 120% rule. It's more like, the total possible current fed into the house panel would be 100% * 175A [non-continuous calculation] + 125% * 48A [continuous calculation] = 235A, which is less than 120% that the relevant 705.12 section says is safe.

 

[seropaul3]

Many thanks to both of you for responding to my post and answering my questions.