Sub Panels with Large Loads

[jimmyz80]

We moved into a new (to us) house in El Dorado Hills, CA a few months ago and finally pulled the trigger on ordering some Powerwalls to shield ourselves from PG&E power outages. Based on my initial research I ordered three Powerwalls, mainly for the increased output current so we can have our air conditioning backed up. It's a 5-ton unit, with an LRA of 153 so I'm hoping a soft-start unit will bring the LRA down to a level that's compatible with three Powerwalls.

My bigger question is how I should expect Tesla to go about designing what gets backed up and what doesn't. Currently the house is wired like this:

Main Panel

• Main panel is a combined Square D SC3040M200F, which means there's no good way to insert the Powerwall Gateway between the meter and the main load center. It's all one unit on the wall.
• Two 100A breakers, each feeding a separate 100A sub panel on each floor.
• 50A breaker feeding my two Tesla Wall Connectors for EV charging. Don't care about backing up.
• Some miscellaneous 15A and 20A breakers for home office outlets, one bathroom, closet, etc. Don't really care about backing these up, but it would be nice.
• 35A back-fed breaker from the 7.28kW Solar City system. I know this will get moved to a panel behind the Powerwall Gateway.

Upstairs 100A Sub Panel

• 15A and 20A breakers for nearly all of the outlets and lights in our main living spaces, including kitchen appliances. Must be backed up.
• 30A breaker for kitchen ovens. Must be backed up.
• 50A breaker for AC compressor. Must be backed up.

Downstairs 100A Sub Panel

• Lights and outlets for downstairs rooms. Don't care about backing up.
• Gas furnace / blower for AC. Must be backed up.
• Gas tankless water heaters. Must be backed up.
• 50A breaker for spa. Don't care about backing up.
• 40A breaker for pool pumps. Don't care about backing up.

In summary, I don't care about backing up the downstairs sub panel except it has a couple of critical loads on it. I don't know whose bright idea it was to split the HVAC system across two sub panels.

Is it likely that Tesla would OK this installation for whole home backup, and just put both of the sub panels behind the Powerwall Gateway? Will they only approve backing up the more important upstairs sub panel, and it'll be up to me to relocate the downstairs HVAC and water heater circuits to the upstairs sub panel?

Curious what everyone thinks who's been through this process before, and how you would have handled this dilemma.

Thanks!

 

[gpez]

I'm probably not going to be super helpful but here we go.

We used a Tesla certified installer and put the solar in at the same time as the Powerwall planning wise (Powerwall came 6 months after solar was actually put in but everything was wired ahead of time). We went with 1 Powerwall just for critical loads backup only. Our setup is simpler than yours - 200a main panel located in the garage with all house loads, including high amp circuits like the oven, dryer, A/C, and 15-50 EV outlet. The installer added 2 panels outside of the garage but on the same wall as the original panel: a production panel and a non-critical loads panel. They then moved the high amp circuits to the non-critical loads panel and wired all 3 (critical loads, non-critical loads, and production) panels to the backup gateway.

For your setup I'd venture to guess that 3 PWs is plenty for true "whole home" backup and they'd move the generation circuits out of your main panel and in to a generation panel that is then wired to the gateway. That's the simple and naïve configuration which is likely the cheapest and easiest in terms of equipment and install costs as well as backing up everything in your house - drawback of course is that during an outage you have more stuff to manage to ensure you don't drain your Powerwalls (would suck if your spa and pool drained your PWs overnight ).

 

[getakey]

welcome to EDH. We moved here almost 2 yrs ago. Just installed 3 PWs in January. Got solar right after we moved here.
I have whole house backed up. 2 separate AC units

 

[jimmyz80]

welcome to EDH. We moved here almost 2 yrs ago. Just installed 3 PWs in January. Got solar right after we moved here.
I have whole house backed up. 2 separate AC units

Thanks! Out of curiosity, who did you use for the install? Tesla directly, or a third party? I originally got my design process started with Tesla, but then yesterday reached out to V3 Electric since some neighbors have used them. They seem knowledgeable and helpful on the phone, but some of their online reviews are horrible. Not to mention using them would only save me about $1,500, but also hold up like $10k for many months with the SGIP claim processing.

 

[getakey]

I used Citadel for the Solar. Excellent. Highly recommend, but they don't do Tesla
Used Infinity for the PW. Mixed results. Had 2 different teams. 1 Team was very good, but they couldn't finish install due to missing part. 2nd team not so good. Had to have 1st team come back to fix. Now working very well. I'm completely off of peak rates.

 

[aesculus]

My bigger question is how I should expect Tesla to go about designing what gets backed up and what doesn't.

You have limited control here. As far as the subpanels go it's all or nothing. If you require anything in a sub panel to be backed up then the whole panel must be backed up. You cannot pick and choose circuits here. This can only be done at the main panel.

Note anything you don't want backed up will stay with your main panel and all the other circuits will be put into a new panel that is behind the PWs and therefore backed up.

My home has 5 sub panels with one sub panel (the heat pump units) as a sub of a sub. They will crater the PWs if they turn on at the same time or are under heavy load. But I have no choice to have them backed up because most of the house is on the same major circuit from the PW. So I have to manage turning these off if I go into backup mode.

I am working on an automated solution for this. There are some options. I sure wish there was a commercial offering but my install is very complex physically I am not holding my breath for one to ever be created that will meet my needs of being wireless, long distance and only affect the compressor side of the control.

 

[cwied]

@jimmyz80 - your setup sounds similar to mine. Tesla did a whole-house backup for me with two Powerwalls. They moved all the circuits from the main panel including the two subpanel circuits into a separate load panel. They then put a 200 amp breaker in the main panel leading to the Gateway. The new panel is fed by the Gateway.

As long as you don't have any breakers more than 60 amps, two Powerwalls should actually be enough for a whole-house backup. I think with three you'll have no troubles.

 

[jimmyz80]

@jimmyz80 - your setup sounds similar to mine. Tesla did a whole-house backup for me with two Powerwalls. They moved all the circuits from the main panel including the two subpanel circuits into a separate load panel. They then put a 200 amp breaker in the main panel leading to the Gateway. The new panel is fed by the Gateway.

As long as you don't have any breakers more than 60 amps, two Powerwalls should actually be enough for a whole-house backup. I think with three you'll have no troubles.

I assume the 60A breaker threshold is only for branch breakers? Ie. the 100A feeder breakers for sub panels wouldn't fall into this category?

 

[aesculus]

I assume the 60A breaker threshold is only for branch breakers? Ie. the 100A feeder breakers for sub panels wouldn't fall into this category?

It's much more complicated than this. For example:

60-amp 240-volt circuit: 60 amps x 240 volts = 14,400 watts

Two Powerwalls can handle (2 * 5,000 = 10,000 watts). So technically one 60 amp breaker would toast two powerwalls if it was totally loaded. But generally you only rate them at 80%, so it would be 11,520 watts, which is still too much. Three would be OK.

But one would not expect all 60 amps to be drawn at one time, especially if its a sub panel with other circuit breakers on it. In fact if you added up all the ratings on all of your circuit breakers I would bet it vastly exceeds the 200 amps of your panel rating. Again they are designed for flexibility and not for constant load. Most of the breakers in your house rarely see a small fraction of their rated load. But devices with motors can have a higher starting load then running load and this is another factor too.

So the real concern is what breakers are downstream of the powerwalls that are controlling dedicated devices like AC units, hot tubs, pumps, dryers, ovens etc. Adding these up becomes the real question for the draw on the powerwalls. And taking into consideration the starting loads too.

BTW if you do exceed the maximum the PWs will trip off line. It's not a safety issue. You would have to go find those devices (like your AC units) and turn them off manually to avoid the PWs trying to restart and trip off again.

 

[jimmyz80]

It's much more complicated than this. For example:

60-amp 240-volt circuit: 60 amps x 240 volts = 14,400 watts

Two Powerwalls can handle (2 * 5,000 = 10,000 watts). So technically one 60 amp breaker would toast two powerwalls if it was totally loaded. But generally you only rate them at 80%, so it would be 11,520 watts, which is still too much. Three would be OK.

But one would not expect all 60 amps to be drawn at one time, especially if its a sub panel with other circuit breakers on it. In fact if you added up all the ratings on all of your circuit breakers I would bet it vastly exceeds the 200 amps of your panel rating. Again they are designed for flexibility and not for constant load. Most of the breakers in your house rarely see a small fraction of their rated load. But devices with motors can have a higher starting load then running load and this is another factor too.

So the real concern is what breakers are downstream of the powerwalls that are controlling dedicated devices like AC units, hot tubs, pumps, dryers, ovens etc. Adding these up becomes the real question for the draw on the powerwalls. And taking into consideration the starting loads too.

BTW if you do exceed the maximum the PWs will trip off line. It's not a safety issue. You would have to go find those devices (like your AC units) and turn them off manually to avoid the PWs trying to restart and trip off again.

Yes this is the common knowledge that I've always operated on, and why we have residential load calculation formulas to size panels and conductors. This is why I was surprised by the simple comment of just looking at breaker size. I'm fine with having to run downstairs and turn off the breaker for the pool/spa - hopefully I can figure out some sort of an automation solution as well.

If the Powerwalls do get overloaded in the off chance that the AC, Spa, and Pool all kick on at the same time, can they be restarted during a grid failure? It would suck to have them go offline, force the solar inverter offline as well, and then not be able to get back online until the grid recovers. I'm guessing there's a way to cold start them and keep the gateway circuit open from the grid, but I don't know for sure.

 

[cwied]

I assume the 60A breaker threshold is only for branch breakers? Ie. the 100A feeder breakers for sub panels wouldn't fall into this category?

That is correct. I actually have one 125 amp subpanel and one 100 amp subpanel.

I realize that 30 amps per Powerwall is more than the Powerwall can theoretically handle, but that seems to be the rule of thumb Tesla uses for sizing the installation. That's what the rep who came out to the site visit told me, anyway. In practice my biggest breaker is 40 amps, so it wasn't an issue anyway.

I believe the Powerwalls restart automatically after a few minutes if they get overloaded. If they shut down because they run out of energy, they also try to restart themselves during sunlight hours to recharge from solar (see "Powerwall at low charge" here: Backup-Only | Tesla Support)

 

[aesculus]

If the Powerwalls do get overloaded in the off chance that the AC, Spa, and Pool all kick on at the same time, can they be restarted during a grid failure?

I have not experienced this yet. All my outages happened when the major loads were offline. But according to the installers the system will auto try (every 15 mins?) to restart.