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'whole home backup' confusion

jgbaum

Member
Aug 25, 2021
20
11
San Diego
I recently had a system installed (16kW, 2PW+, backup gateway 2) and all of my paperwork references 'whole home backup'. Basically, they pulled all of my loads from my main panel into a subpanel connected to the gateway. The only breaker in my main panel now is a 100A going to the gateway. What's odd to me is that they had me upgrade my main panel and service (which required significant trenching and $$$) from 100A to 200A, but they're only feeding this subpanel (which is capable of 225A) with 100A. There are several new loads I'd like to add in the coming months (EV chargers, electric dryer, water heater), which my project advisor was aware of, but there doesn't seem to be the bandwidth to support these in the subpanel. Am I mistaken? Should I ask Tesla to update the wiring/breaker to 200A?

My understanding (which may be wrong!) is that in order to be powered by my PV system or PWs when the grid is out, loads would need to be in the subpanel as no electricity would flow to the main.

I've included the wiring diagram below. Any insight would be much appreciated.

Thanks!


n91ktoodr6i71.png
 
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Yonki

Member
Supporting Member
Mar 31, 2015
596
1,761
Pacific Grove, CA
They did get that right. But the scrimping on materials is contrary to the approved plans, so I'd be inclined to tell them their install isn't done until they provide the proper 200A feeders.

Cheers, Wayne
Well the change to 100A was an on-the-fly change at the time of install, where they called in and cleared it with an engineer, and I verbally approved at the time. The revised plans show the 3 AWG.

I should be fine with a 100A limit and can re-run thicker wires and bigger breakers if I ever need to. But I feel a lot better now that you've also blessed the 3 AWG - thanks.
 
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slcasner

Active Member
Supporting Member
Feb 20, 2011
1,393
939
Sunnyvale, CA
I should be fine with a 100A limit and can re-run thicker wires and bigger breakers if I ever need to. But I feel a lot better now that you've also blessed the 3 AWG - thanks.
My plans as submitted for permit showed 4 AWG with 100A breaker, but the city properly called them on it and required a change to 3 AWG.
 
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wwhitney

Active Member
Nov 2, 2017
1,019
1,385
Berkeley, CA
My plans as submitted for permit showed 4 AWG with 100A breaker, but the city properly called them on it and required a change to 3 AWG.
Oddly enough, whether that is correct depends on the size of your service. If the service is 100A (upstream most breaker), then #4 is fine for a 100A circuit. While if it is larger than 100A, #3 is required.

This is the result of two NEC rules: the first is that for most common residential service sizes, the service conductors only have to be sized at 83% of nominal. The reasons for this are a little unclear, it may be a recognition that the NEC's load calculations are particularly conservative for residences. Or it may be related to the fact that conductor ampacities are based on 3 current carrying conductors, but a 120/240V supply has only 2 current carrying conductors (in this context, the neutral is not counted, as it only carries the unbalanced current; i.e. you can't get 100A simultaneously on all 3 conductors.)

That rule means that a 100A residential service only has to be #4 Cu, not #3 Cu. And there is another rule that says a feeder never has to be larger that the service conductors.

A side effect of all this is that performing a service upgrade could require downsizing some feeder breakers for feeders that no longer get to take advantage of the 83% factor.

Cheers, Wayne
 
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It's crazy they put 100A source into the gateway on a home with a 200A main panel service. The gateway 2 supports 200A source input, 200A output to the backup panel, and the optional internal panel board can support 200A with 6 spaces with 125A breaker in each spot. The only reason I can think of to do this is cost savings on breakers and wires hoping no one notices or cares. See the specs here:
Tesla even shows how a 400A service can be split into two 200A systems: see bottom of this page

 
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jgbaum

Member
Aug 25, 2021
20
11
San Diego
Just an update that Tesla finally sent an electrician to our house to scope things out and has agreed to update the wiring & breaker to the subpanel! The fix will be scheduled for some time next week. If, for some reason, it doesn't happen I will post about the experience here. Otherwise, I'm grateful to Tesla for resolving this issue.
OK so they halfway addressed my issue today. They upgraded the breaker and wiring running from the main panel to the gateway to 200A. However, the breaker feeding the subpanel is still 100A. There were 2 reasons they said they couldn't upgrade the breaker to the subpanel. The first reason was that the permits that were approved were for a 100A subpanel. The second reason was that if I add additional loads to the backup panel, the batteries could become overloaded and automatically shut down.

The permitting issue seems like it would easily be addressed and I don't think it would prevent my own electrician from upgrading that breaker if I asked. As to the second issue, they asked me to call in to discuss adding additional loads with the engineers, so I'm trying to make that happen.
 
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OK so they halfway addressed my issue today. They upgraded the breaker and wiring running from the main panel to the gateway to 200A. However, the breaker feeding the subpanel is still 100A. There were 2 reasons they said they couldn't upgrade the breaker to the subpanel. The first reason was that the permits that were approved were for a 100A subpanel. The second reason was that if I add additional loads to the backup panel, the batteries could become overloaded and automatically shut down.

The permitting issue seems like it would easily be addressed and I don't think it would prevent my own electrician from upgrading that breaker if I asked. As to the second issue, they asked me to call in to discuss adding additional loads with the engineers, so I'm trying to make that happen.

At least it seems to be moving in the right direction. Your permits already call for 100amp breaker from main panel to gateway. If you change it out to 200amp, wouldn't that call for a repermit already? For my project, my advisor said any changes in the breaker size would need to go through design and permit stage again.

This is my timeline so far:
9/7-8: Solar + powerwall install. They installed a 110a breaker from main panel to gateway. My plans called for 100a breaker but they didn't have any on them.

Email - Talked with my advisor afterwards trying to get everything updated to 200a

9/16: Another crew came back to switch out the 110a breaker to 100a. They had no idea I was talking to my advisor to get it updated. They looked at my main panel and gateway and said everything can support 200a, but they don't have the breaker sizes with them. The largest one they had 125a, so they ended up using that for now.
They also noticed that the distribution panel that the first crew installed only supports 125a main and 100a loads. So if I were to upgrade to 200a, they need to switch out that panel. Also, I had 125a going from that panel to my subpanel which exceeds the rating of the panel, so they swapped that out to 100a. The subpanel needs to be switched out to support 200a main and 125a loads.

Email - Told my advisor about the changes and waiting on 3.5 weeks with no response now.
 
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wwhitney

Active Member
Nov 2, 2017
1,019
1,385
Berkeley, CA
If the gateway is rated for 200A, how can it be fed by 200A from the grid and 50A X 2 for the powerwalls/solar (300A total)?
The rules for installing the Gateway require a maximum 200A breaker at all the locations that could otherwise cause more than 200A to flow through the Gateway.

For example, the Gateway has 2 sets of lugs for connection on the backup side of the internal disconnect.. You could connect a load panelboard to one set of lugs, and a generation (Powerwalls/PV) panelboard to the other set. If your load panelboard were a 300A panelboard, you could indeed draw 300A through the Gateway in the above scenario and overload that double set of lugs.

But the load panelboard is limited to 200A, with a 200A main breaker in it, to ensure that no more than 200A flows out of the Gateway to that panelboard. That 200A could come all from the grid, or 100A from the grid and 100A from the generation panel, or any combination in between, but the total current through the Gateway is limited to 200A.

Cheers, Wayne
 
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