Powerwall Install Design

[Bpaston]

Looks like that is correct. The Powerwalls are designed as such that they never export to the grid. So they would not need to be accounted for in the main bus 120% rule for back feeding. So it looks like as of now with 60amp max of solar running through the Enphase aggregator I can keep the 200 amp service disconnect.

View attachment 408162

 

[wwhitney]

This issue of not exporting to the grid is a red herring. The 120% rule (or one of the other options in NEC 705.12) applies to any panel that is fed from more than one power source. Cr0ntab's inspectors simply dropped the ball and missed the violation, his arrangement does not comply with the NEC.

Cheers, Wayne

 

[cr0ntab]

While I do value @wwhitney's posts here, he's quite knowledgeable, I would defer to your AHJ. They are the source of truth.
My dad got the same setup permitted with two powerwalls and 50A of solar backfeed, he's in LA county (vs me in Riverside county) his inspectors passed it too.

In reality these are just two anecdotes, run your "ideal" setup with the 200A main by your AHJ and let them make the call.

The 120% rule (or one of the other options in NEC 705.12) applies to any panel that is fed from more than one power source.

This seems to imply that each powerwall is an additional source at 30A a pop.

225A * 1.2 = 270A

2x powerwall @ 30A = 60A

That means with two powerwalls, 225A bus and a 200A main breaker you're limited to 10A of solar backfeed? That doesn't seem right.

 

[wwhitney]

225A * 1.2 = 270A

2x powerwall @ 30A = 60A

That means with two powerwalls, 225A bus and a 200A main breaker you're limited to 10A of solar backfeed? That doesn't seem right.

Yeah, that's how the 120% rule works. The only other option to protect the panel bus is for the sum of all the breakers (on each leg) excluding the main breaker to be 225A or less. But if the panel doesn't qualify under that rule, it needs to meet the 120% rule. That's why you'll often see the main breaker downsized to 175A or 150A in a situation like the above.

AHJs certainly make mistakes, your situation is one example.

Cheers, Wayne

 

[SoundDaTrumpet]

Yeah, that's how the 120% rule works. The only other option to protect the panel bus is for the sum of all the breakers (on each leg) excluding the main breaker to be 225A or less. But if the panel doesn't qualify under that rule, it needs to meet the 120% rule. That's why you'll often see the main breaker downsized to 175A or 150A in a situation like the above.

AHJs certainly make mistakes, your situation is one example.

Cheers, Wayne

I would be curious in what the design paperwork submitted to the city shows. There a chance that the electrician didn't build to the prints.

I have been in conversation with the Tesla Energy Design Center, who actually talks to homeowners about the very technical inquiries. Sizing the main breaker down to 175A or 150A is their Step #2. If that doesn't provide enough backfeed, Step #3 is the 'sum of all breakers' method @wwhitney mentioned. Here the Design Center said they start adding subpanels for this to work. I reserve judgement until I know more details. @cr0ntab perhaps the gateway is on a 125A and the (AC, EV, etc.) rests on a 100A sub or sums to 100A.

 

[cr0ntab]

The only other option to protect the panel bus is for the sum of all the breakers (on each leg) excluding the main breaker to be 225A or less.

[USER]@cr0ntab[/USER] perhaps the gateway is on a 125A and the (AC, EV, etc.) rests on a 100A sub or sums to 100A.

Both of you would be correct here, I completely forgot about that.

I almost always see the 120% rule mentioned so I forget about the other options.

The gateway is indeed on a 125A breaker.

 

[RoBoRaT]

I just got 10kW solar and 2 PW installed awaiting AHJ and Power company inspectiob, and PTO. My main panel has 225 BUS, orig 200A breaker derated to 175A.

See attached 120% rule calc...with 2 PW noted at 41.66 A which limited my inverters to 32A.

I am in Riverside county, under SCE.
My installer did not budge when I asked for bigger inverter ...so I was stuck with 32A to meet code.

My DC/AC ratio is 1.41, costing me about 350 kWh clipping (projected in PVwatts).

Is there any way to use a bigger inverter and still meet code?

 

[SoundDaTrumpet]

I just got 10kW solar and 2 PW installed awaiting AHJ and Power company inspectiob, and PTO. My main panel has 225 BUS, orig 200A breaker derated to 175A.

See attached 120% rule calc...with 2 PW noted at 41.66 A which limited my inverters to 32A.

I am in Riverside county, under SCE.
My installer did not budge when I asked for bigger inverter ...so I was stuck with 32A to meet code.

My DC/AC ratio is 1.41, costing me about 350 kWh clipping (projected in PVwatts).

Is there any way to use a bigger inverter and still meet code?

View attachment 408645

Your system is optimal from the installer's viewpoint. You have 95A available for generation. DC/AC ratio is acceptable according to the manufacturer. Larger inverter will force your installer to spend $1500 on a new inverter, re-submit all plans, and interconnect agreement. (1) Ask them for a cash gift card for the difference, or (2) Hit the brakes and ask for a few more panels (>1.55 DC/AC ratio) because you plan to buy another EV. Paying them more will change the installers viewpoint. What they will do is simply de-rate the main breaker to 150A.

My clipping was less severe. $50. Fast forward to today, I plan to add 1kW more panels (grandfathered NEM 1.0), and will be raising ratio from 1.12 to 1.37. Kinda wished I had a bigger inverter.

 

[cr0ntab]

Is there any way to use a bigger inverter and still meet code?

I see two options:

1) Derate your main breaker further and adhere to the 120% rule
2017 NEC 705.12(B)(3)(b)

You would need to do a load calculation and submit that to make sure this option is available to you.

I assume you have a 7.6kW inverter that puts out 32A

The next size up is a 10kW inverter that puts out 42A

Given that you have a 225A bus here are the backfeed limitations:

200A main = 70A backfeed
175A main = 90A backfeed
150A main = 120A backfeed

2x Powerwall = 41.66A * 125% = 52A
10kW Inverter = 42A *125% = 52.5A

Total = 105A backfeed, which means you can drop your main breaker down to 150A and you're good.

2) Similar to how I'm setup, use the sum of all breakers rule.
2017 NEC 705.12(B)(3)(c)

My tesla gateway is on a 125A breaker in my main panel.

The bus is rated for 225A of power and the sum of all my breakers is less than 225A

225A - 125A means that I can still load that main panel with up to 100A of loads

Please double check my numbers @wwhitney @SoundDaTrumpet

 

[RoBoRaT]

Your system is optimal from the installer's viewpoint. You have 95A available for generation. DC/AC ratio is acceptable according to the manufacturer. Larger inverter will force your installer to spend $1500 on a new inverter, re-submit all plans, and interconnect agreement. (1) Ask them for a cash gift card for the difference, or (2) Ask for more panels because you plan to buy another EV. Paying them more will change the installers viewpoint. What they will do is simply de-rate the main breaker to 150A.

My clipping was less severe. $50. Fast forward to today, I plan to add 1kW more panels (grandfathered NEM 1.0), and will be raising ratio from 1.12 to 1.37. Kinda wished I had a bigger inverter.

I had 3 more panels ~1kW, about $ 2.9K removed from my contract. My installer said the power company will not allow more than 10.7kW for my situation.

De-rating to 150A meant removing the NEMA 14-50 outlet and cutting the Tesla HPWC to 50A - which was somewhat ok.
I was not aware of other constraints that prevented derating to 150A to get bigger inverters. My installer explained them at one point but I cant recall specific reason why not derate to 150A.

So, I asked for stuff and they gave me front skirt and bird abatement mesh, no charge.

O' well - I kind of accepted the clippings. 350 kWh per year @ $0.10 per kWh is not so bad.

 

[SoundDaTrumpet]

175A main = 90A backfeed
Please double check my numbers @wwhitney @SoundDaTrumpet

Looks good, except: 175 main (breaker) = 95A backfeed (with 225A bus bars)

 

[SoundDaTrumpet]

I had 3 more panels ~1kW, about $ 2.9K removed from my contract. My installer said the power company will not allow more than 10.7kW for my situation.

De-rating to 150A meant removing the NEMA 14-50 outlet and cutting the Tesla HPWC to 50A - which was somewhat ok.
I was not aware of other constraints that prevented derating to 150A to get bigger inverters. My installer explained them at one point but I cant recall specific reason why not derate to 150A.

So, I asked for stuff and they gave me front skirt and bird abatement mesh, no charge.

O' well - I kind of accepted the clippings. 350 kWh per year @ $0.10 per kWh is not so bad.

It is always good to get the full story from people we pay. In the grand scheme, not bad considering it's only 1-2% of your total annual generation with your panels brand new. When I get around to adding the 1kW, at least I have your numbers to go by... (off topic) my installer did offer replace & upgrade my inverter to a Solar Edge HD, which has 2% better DC-AC efficiency. Not sure if this requires the optimizers to go along with this to make the 2% happen. The increase efficiency does push the CEC AC power past the 1kW limit for NEM 1.0 grandfathering. If anyone has insight on the optimizer question & NEM 1.0, please chime in.

I was really big on load calculation. Having a NEMA 14-50 already installed, Tesla installed a HPWC and didn't cut it down to 20A (based upon my precise load calculation). I asked the AHJ, and simply said: (1) we checked the load calcs back in office, (2) you have 200A service, you are good to go. I would not be surprised if Tesla submitted a load calc along with a whitepaper with justification (j/k). I live next to the Factory so my AHJ is to be trusted. I guess they expect the main breaker to simply pop, except I have no main breaker and the HPWC is on the supply side, so the service conductors will just have to pinch off (ha!) when overloaded.

 

[Bpaston]

This issue of not exporting to the grid is a red herring. The 120% rule (or one of the other options in NEC 705.12) applies to any panel that is fed from more than one power source. Cr0ntab's inspectors simply dropped the ball and missed the violation, his arrangement does not comply with the NEC.

Cheers, Wayne

Wayne,

Do you say it is a red herring because it really does export to the grid, or because the NEC code is written for such technology as of yet?

 

[Bpaston]

So it looks like I have three options to choose from without having to upgrade the meter/main, which to me is out of the question. I want to shoot for a clean install and not have to redo stucco on a new home. In option one and two it looks like I am limited to two PWs no matter what I do as I want around 7kW solar as a minimum. The goal would be to be net zero and we are currently using around 12kW a year, but I don’t think a 7kW system will get me there. Option two may not be feasible based off of load calculations since when I run the math on my own I come up with about 173.5 amps, not sure the AHJ will allow it. The third option I will have to read up on, because I don’t know if it is even possible to line side tap the current meter/main that I have and be able to pass an inspection. I have posted my revised diagram using option 1. In a perfect world I would want the room for 3 PWs and up to 60A of solar but not sure if it is even possible or cost effective.

1st option: Derate service disconnect breaker to 175A and have room for 2 PWs and 30A (7.2kW) of solar. 265A < 270A OK
2nd option: Derate service disconnect breaker to 150A and have room for 2 PWs and 60A (14.4kW) of solar max. 270A = 270A OK
3rd option: Line side tap between meter and main disconnect to power Gateway, backed up loads, and solar. Non backed up loads remain in meter/main panel.