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Adding 2nd Powerwall 2, problems with overloading main panel bus bars

Bitjockey

Member
Jun 5, 2015
100
80
Toronto, ON, Canada
Our first Powerwall 2 was installed back in 2018 and now my 2nd Powerwall 2 is being installed. I've been been told by my installer (not Tesla, as I'm in Canada where installs are done by certified 3rd parties) that since each Powerwall has a 30A breaker, along with my 7.3kW solar array, I would be exceeding the capacity of my 200A main panel bus bars. To avoid this problem, they suggest that I reduce the capacity of the breaker in my main panel.

So it seems that the installer is configuring the 2 Powerwalls so that my backup load capacity is doubled (ie. 10kW vs 5kW). But what I really wanted from installing the 2nd Powerwall was to double my backup storage so that we could endure longer grid outages. I don't care about having 10kW of simultaneous backup loads, I'm happy with 5kW.

Does any one here know whether the Powerwalls could be configured in a "round robin" configuration, so that in a backup situation, when one Powerwall is close to being depleted it switches over to the 2nd Powerwall? If this is possible then my installation delay would be eliminated, as they're now stuck waiting for back-ordered breakers which may take months to arrive during this pandemic supply chain nightmare.
 

wwhitney

Member
Nov 2, 2017
858
1,121
Berkeley, CA
To answer your question, I'm not familiar with all the Power Control System options currently available in the PW setup screens; it may be possible to reconfigure the (2) PWs for no more than joint 24A continuous output/input at all times. Then the question is whether your electric code recognizes that as sufficient basis for sizing conductors and breakers. The US code (NEC) added that allowance starting with the 2020 version.

What size(kW) is the AC output on your solar inverter? That's more important than the DC kW rating of your array.

Cheers, Wayne
 

Bitjockey

Member
Jun 5, 2015
100
80
Toronto, ON, Canada
wwhitney, thanks for your input; as the installation is at my vacation home, I can't answer that right now, but I've got a 7.3kW solar array that was installed in 2018 along with my first Powerwall. So if the two Powerwalls could be provisioned to limit their combined output to what the original Powerwall could output, I think I'd be OK. I've sent my inquiry to my installer as well as the Powerwall master distributor in Canada.
 

SaveOurPlanet

Member
Jan 6, 2018
237
140
San Francisco
I would love to understand this stuff. When I was looking for option to charge my car in 2018, one electrician told me I have to put my existing 50A steam shower breaker in a transfer switch with a new 40A breaker for the Tesla wall connector, so either steam shower or charge car because I already maxed out on my 125A service. Another electrician told me I could just add a new 30A breaker for a nema 14-30 to charge my car at 24A and still use my steam shower at the same time. They both are Tesla recommended electricians and both quoted me 2,000.00 to do the work, of course I went with the nema 14-30 option. Both electricians recommended against 200A service upgrade as PG&E would charge 10k to 15k for the upgrade and the wait time could be up to 6 months. I got very nervous when I ordered my 6.5 kW solar roof + 2 PWs but Tesla came out twice before the install and never said a word about the service panel. They added a load panel with 3 breakers - a 125A to my existing sub-panel for the house, a 60A for the 2 PWs and a 40A for the Tesla inverter, so how does that work?
 

wwhitney

Member
Nov 2, 2017
858
1,121
Berkeley, CA
They added a load panel with 3 breakers - a 125A to my existing sub-panel for the house, a 60A for the 2 PWs and a 40A for the Tesla inverter, so how does that work?
If your topology is: 125A meter/main -- Gateway -- new panel with 125A breaker for existing subpanel with loads, 40A PV breaker, and (2) 30A Powerwall breakers (or a 60A breaker to a later panel with only (2) 30A Powerwall breaker), then I infer that the new panel must be at least a 200A rated panel. It is served by more than one power source connection (grid plus the inverters), and serves loads, so it has to comply with the 120% rule or its ilk. A 200A bus complies with the 120% rule, which requires the inverter breakers to be on the opposite end of the bus from the grid connection: 125A (grid breaker) + 40A + 60A (inverter breakers) < 120% * 200A (bus rating). With a 225A bus, it would comply with the "100%" rule, which places no restriction on breaker location: 125A + 40A + 60A = 225A.

Cheers, Wayne
 
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sorka

Well-Known Member
Feb 28, 2015
8,042
5,866
Merced, CA
Both electricians recommended against 200A service upgrade as PG&E would charge 10k to 15k for the upgrade and the wait time could be up to 6 months. I got very nervous when I ordered my 6.5 kW solar roof + 2 PWs but Tesla came out twice before the install and never said a word about the service panel. They added a load panel with 3 breakers - a 125A to my existing sub-panel for the house, a 60A for the 2 PWs and a 40A for the Tesla inverter, so how does that work?

I had a main service panel upgrade for the same reason but in my case what your electricians suggested wouldn't have worked because my loads exceeded the bus rating by a lot more than your scenario.

Tesla did the MSP upgrade for $4K. The only thing PGE did was turn the main service off before the meter(basically the pulled the meter out) and then put it back in at the end of the day.
 

Bitjockey

Member
Jun 5, 2015
100
80
Toronto, ON, Canada
I heard back from the Powerwall distributor today, and they stated:

The issue can be summarized as follows. The “Backup Panel” has three incoming energy sources: Powerwall (2x 30A of potential), solar (40A of potential), and the grid (100A of potential via the 100A main breaker). The backup panel is rated for 125A and electrical panel buses allows for 125% of the amperage capacity meaning it can actually handle 156A. The issue is that 156A of capacity is less than 30+30+40+100=200A of generation. This violates the Canadian Electrical Code meaning the install would not pass an electrical inspection. Even though there might not be 200A of load on that panel, the standards/governing regulations argue that since there could be, there is a risk.

So it seems to me that our code follows NEC 705.12(B), at least in the 125% regard.

The installer is proposing that we reduce the incoming breaker on the backup panel from 100A to 50A, which would result in 30+30+40+50 = 150A, within the guidelines.

But since our electrical utility only really credits us for about 50% of what we send back to the grid, we typically run in Self-Powered mode at 50%, increasing it to 80% or more when a storm approaches.

That means that if the grid is up and we charge our Telsa, and turn on the electric stove, and the hot tub comes on, and the batteries are above their minimum SoC, and it's a sunny day, the Gateway will try to source energy from the backup panel and we will easily trip the proposed puny 50A breaker and kill our generation abilities.

I can't see how a dedicated "generation panel" would solve the problem, but maybe I'm obtuse.

If I knew adding a second Powerwall would cause all of this, I would never have had it installed. Is my use-case unique, or has Tesla not figured out how to make additional Powerwall capacity keep within the current ratings of just one Powerwall?
 

h2ofun

Active Member
Aug 11, 2020
2,482
423
auburn, ca
I heard back from the Powerwall distributor today, and they stated:

The issue can be summarized as follows. The “Backup Panel” has three incoming energy sources: Powerwall (2x 30A of potential), solar (40A of potential), and the grid (100A of potential via the 100A main breaker). The backup panel is rated for 125A and electrical panel buses allows for 125% of the amperage capacity meaning it can actually handle 156A. The issue is that 156A of capacity is less than 30+30+40+100=200A of generation. This violates the Canadian Electrical Code meaning the install would not pass an electrical inspection. Even though there might not be 200A of load on that panel, the standards/governing regulations argue that since there could be, there is a risk.

So it seems to me that our code follows NEC 705.12(B), at least in the 125% regard.

The installer is proposing that we reduce the incoming breaker on the backup panel from 100A to 50A, which would result in 30+30+40+50 = 150A, within the guidelines.

But since our electrical utility only really credits us for about 50% of what we send back to the grid, we typically run in Self-Powered mode at 50%, increasing it to 80% or more when a storm approaches.

That means that if the grid is up and we charge our Telsa, and turn on the electric stove, and the hot tub comes on, and the batteries are above their minimum SoC, and it's a sunny day, the Gateway will try to source energy from the backup panel and we will easily trip the proposed puny 50A breaker and kill our generation abilities.

I can't see how a dedicated "generation panel" would solve the problem, but maybe I'm obtuse.

If I knew adding a second Powerwall would cause all of this, I would never have had it installed. Is my use-case unique, or has Tesla not figured out how to make additional Powerwall capacity keep within the current ratings of just one Powerwall?
I do not believe a generation panels solves this. I have one, and had to replace my 200 breaker with a 125, since my solar had a 75 amp breaker
 

miimura

Well-Known Member
Aug 21, 2013
6,266
5,896
Los Altos, CA
I heard back from the Powerwall distributor today, and they stated:

The issue can be summarized as follows. The “Backup Panel” has three incoming energy sources: Powerwall (2x 30A of potential), solar (40A of potential), and the grid (100A of potential via the 100A main breaker). The backup panel is rated for 125A and electrical panel buses allows for 125% of the amperage capacity meaning it can actually handle 156A. The issue is that 156A of capacity is less than 30+30+40+100=200A of generation. This violates the Canadian Electrical Code meaning the install would not pass an electrical inspection. Even though there might not be 200A of load on that panel, the standards/governing regulations argue that since there could be, there is a risk.

So it seems to me that our code follows NEC 705.12(B), at least in the 125% regard.

The installer is proposing that we reduce the incoming breaker on the backup panel from 100A to 50A, which would result in 30+30+40+50 = 150A, within the guidelines.

But since our electrical utility only really credits us for about 50% of what we send back to the grid, we typically run in Self-Powered mode at 50%, increasing it to 80% or more when a storm approaches.

That means that if the grid is up and we charge our Telsa, and turn on the electric stove, and the hot tub comes on, and the batteries are above their minimum SoC, and it's a sunny day, the Gateway will try to source energy from the backup panel and we will easily trip the proposed puny 50A breaker and kill our generation abilities.

I can't see how a dedicated "generation panel" would solve the problem, but maybe I'm obtuse.

If I knew adding a second Powerwall would cause all of this, I would never have had it installed. Is my use-case unique, or has Tesla not figured out how to make additional Powerwall capacity keep within the current ratings of just one Powerwall?
I am not an expert, but my understanding is that the Generation Panel does help with this problem. All of the generation goes in a new 200A panel. 30+30+40=100A. That panel is connected to one set of lugs on the Tesla Gateway. If your main panel is converted to a sub-panel, it is connected to the other set of backup lugs on the Gateway. The main breaker then goes directly to the Grid lugs on the Gateway. Now none of the panel buses have dual sources, so the 125% rule does not apply.
 
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wwhitney

Member
Nov 2, 2017
858
1,121
Berkeley, CA
Now none of the panel buses have dual sources, so the 125% rule does not apply.
When the Gateway is supplied from a main panel that is emptied of breakers other than for a feeder to the Gateway, that panel it still supplied by two sources--the grid and the inverters behind the Gateway. It just doesn't have any other connections, and there is no way to overload the bus with just two connections. However, since the panel is capable of additional connections (just pop in a breaker), in the US it is still subject to (2017) NEC 705.12(B). [Whereas if the main panel were just a disconnect without space for distribution, it would be exempt, as it is not designed for more than 2 connections.]

Such an arrangement obviously does not comply with the 120% rule typically (unless you had, say, a 100A service, downsized the main breaker to 60A and supplied the Gateway with a 60A breaker). However, the NEC has a "100% rule" as well, which says the bus is protected if the sum all breakers, excluding the main, does not exceed the rating of the busbar. That is based on the idea that current in = current out. An emptied main panel complies with the 100% rule.

I was finally able to track down the equivalent section in the Canadian Electrical Code (CSA C21.1-18), section 64-112. It does not have a 100% rule, so that strategy is not compliant with section 64-112. All distribution equipment capable of more than 2 connections and with more than 1 power source connection has to comply with the 120% rule, even when it only has 2 connections. [And in Canada, the 120% rule is changed to a 125% rule for dwelling units.]

A single line diagram would help me understand what the OP has and what the OP would like to do. Based on the response, it sounds like the Backup Panel actually has a 100A grid connection and would contain 40A, 30A, and 30A inverter breakers. That obviously would require a 200A busbar. But if that is the only problem, it would be possible to add a 200A panel with the 100A grid connection, the 40A, 30A and 30A inverter breakers, and a 100A or 125A breaker for a feeder to the Backup Panel. In that combiner panel the sum of the sources does not exceed the busbar rating, so it would comply with 64-112 without restriction on breaker placement. The backup panel would then have only a single combined power source connection, so it would be subject to restriction under 64-112.

Of course, that only addresses the one panel, and without a single line diagram, I'm not sure what is happening upstream and whether there would be other issues with 64-112.

Cheers, Wayne
 

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