Single Powerwall Backup Entire 200amp panel?

[Vines]

This is a question we face all the time as designers and installers. Sometimes a customer wants a smaller backup system than we think they need. Going forward, our hands may be slightly tied in a jurisdiction strictly adhering to the code.

In 2023 the California customer may be further confused because we now have to charge extra money for the smaller backup system to install a separate non-backup loads subpanel to isolate those loads the jurisdiction will not allow.

I like @CrazyRabbit suggestion to only consider the full load of those items which will come on automatically but that isn't how the load calculation section (article 220) is written from my experience. Even with the optional methods, most of our customers will not honestly justify less than 60A of calculated demand, if they do not have an EV.

Back to the OP point, if you want you could ask Tesla to install a non-backup subpanel and pay the additional cost so that your AC and other large circuits will never be on backup power. To me, this is paying more to remove flexibility from your system but this may be preferable to the average user.

Questions like this make my job fun, and it will be interesting to see how the AHJ community treats this requirement.

 

[BGbreeder]

Sure, because with a manual transfer system, the proper way to use it is to (a) shut off all the branch breakers (b) do the manual transfer (c) turn on the breakers that you want to power. So the user is the load management system that will keep the connected load below the power source rating. In theory.

Also, in the 2023 NEC cycle, there was a proposal (Public Comment No. 2120-NFPA 70-2021) to add a section to 702.4(B)(2) which would read:

"(c) Protected Equipment. Where the standby source(s) has demonstrated through its listing that in overload conditions it shuts down safely without causing breakers to operate or loads to be energized outside of suitable voltage and frequency limits, the standby source capacity shall be permitted to be less than the maximum connected load."

The panel rejected the change with the response "The panel understands the concept of the commenter but having an undersized standby source with manual load shedding creates problems for system reliability. Having manual load shedding in a requirement for automatic transfer does not correlate."

As the proposed section (c) describes behavior closely matching the PW overload behavior, and the proposal was rejected, it is clear that the intent is that the PW behavior alone is not sufficient to comply with 702.4(B)(2), and a load management system is required. A rule most installs break.

With a free account on nfpa.org, you can read all the details within their TerraView system by going to "Next Edition" on the page nfpa.org/70, and then to First Draft Report or Second Draft Report.

Cheers, Wayne

That is a rather insightful refusal, and does appear to go to the heart of their thinking.

If I am understanding this language, it would suggest that the compliant ESS would have to pass a set of article 220 load calculations, and those calculations would be done at 5kW/Powerwall, 7kW/Powerwall?

Either way, it would seem like a number of ESS installations are not going to have the instantaneous capacity required by Article 220, or am I missing something?

@Vines did the requirement to meet the load calculations for an automatic transfer switch change the minimal number of Powerwalls that your company is installing?

Color me curious...

Much as I bang my head against the wall reading codes sometimes, I do have sympathy for those tasked with drafting around the edge cases trying to keep the rest of us safe, and the train on the tracks.

All the best,

BG

 

[wwhitney]

So, in theory, if you can configure the Tesla Powerwall to require a manual transfer on power outage it would be compliant?

Sure. But unless AHJs start enforcing 702.4(B)(2) in the way discussed, seems like you could ignore this issue. Since (1) not many people are going to research rejected proposals to determine CMP intent and (2) once the NEC is adopted as law, the CMP intent may not control, the words as written are what's important.

Cheers, Wayne

 

[Vines]

That is a rather insightful refusal, and does appear to go to the heart of their thinking.

If I am understanding this language, it would suggest that the compliant ESS would have to pass a set of article 220 load calculations, and those calculations would be done at 5kW/Powerwall, 7kW/Powerwall?

Either way, it would seem like a number of ESS installations are not going to have the instantaneous capacity required by Article 220, or am I missing something?

@Vines did the requirement to meet the load calculations for an automatic transfer switch change the minimal number of Powerwalls that your company is installing?

Color me curious...

Much as I bang my head against the wall reading codes sometimes, I do have sympathy for those tasked with drafting around the edge cases trying to keep the rest of us safe, and the train on the tracks.

All the best,

BG

Currently, we make the assumption that the customer is responsible for managing the connected load. We mostly install larger systems that have decent capacity. Even still, many current customers would not pass honest load calcs for the system they would buy if it were to be whole home backup. We will have to start using actual demand data and dataloggers more often.

I see that the code is not moving in support of this direction, and so it's on my list to talk about with some friendly AHJ's and continue to take the same trainings that the inspectors take.

The code didn't impact our decision to allow 2 Powerwall systems, it was more a matter of that we had so much work in the 3 or more Powerwall systems that we had to manage our resources. SGIP rebates falling away had a larger impact.

Going forward, maybe more systems will integrate smart breakers into the gateway like FranklinWH is doing. They also have generator integration built in which is allowed to recharge batteries. It's my favorite Tesla clone, and they seem to have decent financial backing.

Across the industry, we will see more SPAN type panels for load management, or to have manual reconnection be required. We will see more partial home backup systems as well as more "creative" load calculations.

Maybe in the future, NEC Article 220 will take into account the effect of the PV source factored against the AC load demand, and provide some reduced calculations more appropriate to optional backup systems. As it stands, I cant see that Article 220 recognizes PV much at all except as to limit your options within 220.87.ex.

 

[George75]

Thanks for the responses, I think I can live with it if the power goes out and I'm home flipping a few breakers to stop heavy load items over taxing the system.

I ran it last night of the powerwall and when I got up it had 11% remaining out of the 48% it stated out with as that sun set. I think it'll be more than fine if I had an outage.

I have seen in a few videos the option to control your cars charging while on solar/powerwall. Looks really useful if there was an outage to default car charging to off or a lower rate. I don't have that option in my app therefore I assume it's controlled by a Tesla charger. I'm using the mobile charger on a 14/50 outlet.

 

[Vines]

Thanks for the responses, I think I can live with it if the power goes out and I'm home flipping a few breakers to stop heavy load items over taxing the system.

I ran it last night of the powerwall and when I got up it had 11% remaining out of the 48% it stated out with as that sun set. I think it'll be more than fine if I had an outage.

I have seen in a few videos the option to control your cars charging while on solar/powerwall. Looks really useful if there was an outage to default car charging to off or a lower rate. I don't have that option in my app therefore I assume it's controlled by a Tesla charger. I'm using the mobile charger on a 14/50 outlet.

Update your app if you haven't already. The setting is under settings->ev charging.

This will stop your car from charging if the battery is under a certain percentage. To have your EV charge rate adjust with the solar PV and your home load will take a bit more doing.

 

[jjrandorin]

I have seen in a few videos the option to control your cars charging while on solar/powerwall. Looks really useful if there was an outage to default car charging to off or a lower rate. I don't have that option in my app therefore I assume it's controlled by a Tesla charger. I'm using the mobile charger on a 14/50 outlet.

Its not controlled by the charger, however it appears to be related to which tesla you have. When they released the feature (which, to be clear, is controlling charging when OFF GRID, it has no impact when on grid whatsoever), it was originally only available for model 3 / Y with S and X coming "later".

If you have a model 3, it should show up at some point.. but to repeat, its "limiting car charging while off grid".

 

[Vines]

Its not controlled by the charger, however it appears to be related to which tesla you have. When they released the feature (which, to be clear, is controlling charging when OFF GRID, it has no impact when on grid whatsoever), it was originally only available for model 3 / Y with S and X coming "later".

If you have a model 3, it should show up at some point.. but to repeat, its "limiting car charging while off grid".

Good point I should have made that part clear in my post. I am pretty sure that it should work the same for another Tesla after 2015ish when they plug into my charger. I think there is some PLC signal the Powerwall sends to the charger or the car when off-grid to trigger this behavior.

 

[ucmndd]

Its not controlled by the charger, however it appears to be related to which tesla you have. When they released the feature (which, to be clear, is controlling charging when OFF GRID, it has no impact when on grid whatsoever), it was originally only available for model 3 / Y with S and X coming "later".

If you have a model 3, it should show up at some point.. but to repeat, its "limiting car charging while off grid".

I have an ancient 2016 Model S and charge with the mobile connector on the 14-50. I have the EV charging option and slider in the app, so it should work with pretty much any Tesla vehicle and first-party charging solution at this point.

Like others have reported, it took many weeks after initial system commissioning before all of the options randomly showed up in the app.

 

[Zinc_Saucier]

I have an ancient 2016 Model S and charge with the mobile connector on the 14-50. I have the EV charging option and slider in the app, so it should work with pretty much any Tesla vehicle and first-party charging solution at this point.

Like others have reported, it took many weeks after initial system commissioning before all of the options randomly showed up in the app.

I’m not sure I’d use the presence of the slider as proof that it works. Have you tried testing it off-grid? I have the slider in my app as well, but no Tesla EV nor Tesla charger.

 

[ucmndd]

I’m not sure I’d use the presence of the slider as proof that it works. Have you tried testing it off-grid? I have the slider in my app as well, but no Tesla EV nor Tesla charger.

I've tested it, it does what it claims. Good point though.

 

[Vines]

I've tested it, it does what it claims. Good point though.

My understanding is that it will also work for another Tesla, even if you do not own it. THat is what makes me thing there must be some power line communication going on between the EV and the gateway.

 

[George75]

That's what I was hoping for, the option to eventually show up. I'd already asked Tesla and they didn't have any information on it. I'm happy with being able to restrict charging if I'm off grid.

Has anyone had issues with their solar being set to self consumption mode before PTO and it still randomly exports to the grid. It did this yesterday and today it got stuck like that. It kept sending everything to the grid and refused to send anything to the powerwall. The only way to stop it was to turn everything off and on again.

 

[BGbreeder]

My understanding is that it will also work for another Tesla, even if you do not own it. THat is what makes me thing there must be some power line communication going on between the EV and the gateway.

Musk companies take some very idiosyncratic routes to design and programming from where I sit. I would not say that it is necessarily a bad thing, just that I think that it is hard to extrapolate from what other folks have done to what Tesla did, or might do.

While this is Tesla that we are talking about, so anything is possible, I would observe that Tesla's other products don't seem to use power line communications, even when they might make sense (e.g. remote current meters). I would bet (WAG here) that they might be using WiFi/cellular and perhaps geolocation to do it. The Tesla charger is on the local WiFi network, so it "knows" where it is, and could provide information to the car(?). Doesn't the charger have to tell the vehicle what the max power limit is anyway?

No inside knowledge on this one.

All the best,

BG

 

[morph3ous]

Musk companies take some very idiosyncratic routes to design and programming from where I sit. I would not say that it is necessarily a bad thing, just that I think that it is hard to extrapolate from what other folks have done to what Tesla did, or might do.

While this is Tesla that we are talking about, so anything is possible, I would observe that Tesla's other products don't seem to use power line communications, even when they might make sense (e.g. remote current meters). I would bet (WAG here) that they might be using WiFi/cellular and perhaps geolocation to do it. The Tesla charger is on the local WiFi network, so it "knows" where it is, and could provide information to the car(?). Doesn't the charger have to tell the vehicle what the max power limit is anyway?

No inside knowledge on this one.

All the best,

BG

I think they are using slight changes to the frequency. That is why it would only work off-grid. On-grid the frequency is controlled by the grid.

Just a guess. If they were using power line communications then it should also be able to work while on the grid.

 

[jjrandorin]

The Tesla charger is on the local WiFi network, so it "knows" where it is, and could provide information to the car(?). Doesn't the charger have to tell the vehicle what the max power limit is anyway?

It cant be wifi though, because only the tesla gen 3 wall connectors have wifi in them. The gen 1 and 2 ones dont, and there are probably at least as many gen 2s installed as 3s since they sold gen 2s for years. It also works with the mobile connector that (used to) come with the car, and that definitely doesnt have wifi.

I have only tested this "off grid slow / stop charging" setting once. My wall connectors are not backed up, but I do have a 14-50 outlet that is on the backup side. I tested this feature using my tesla mobile connector, and the 14-50 adapter for my mobile connector that came with my car when I bought it. It worked as I expected, slowing the charging speed down when my powerwalls hit the reserve I had set for that setting, which was 80% for that test.

I am curious how they do it, and why they originally only had 3 and Y vehicles before releasing it wider. Definitely something implied there, but I am not sure what.