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Adding a circuit to back up load center post-install

We just got our PTO from PG&E on our system last week, and because our gas hot water heater just failed, I would like to add a 30A circuit for a heat pump hybrid water heater. I need some help to understand what type of load calculation I may need to show the building department to add another circuit. My primary concern is that the backup load center has a bright red placard stating “Caution do not add new loads”, and I’m just not sure how that affects how an inspector evaluates an additional circuit.

Specifics:
6.46 kW solar, two Powerwall system, whole home backup (Square D panel fed by 125A breaker)
No AC, gas-fired furnace, gas dryer (for now), gas water heater (RIP). Our only 240V load is the mobile charger for our Model 3.

We basically maxed out our S and W facing roof area with panels knowing that we would initially be a net exporter, but planned to electrify NG appliances over time to export less. I let our project advisor know that the only reason I was willing to go with the Tesla plan of filling our roof was because we were planning on this electrification of more appliances. In the PG&E PTO application, Tesla included their estimate of 20% increase of electricity usage (though I think that Tesla includes whatever they need to get an oversized system approved by PG&E without really considering the implications of a usage increase with a whole home backup).

We plan on running the water heater in heat pump only mode (1.2 kW). We’re replacing our 50 gallon gas heater with a 65 gallon model because of the longer recovery time for operating in this mode. We will also use a higher tank temp. with a hot water tempering valve to further minimize the effects of the heat pump-only mode recovery time. If this still isn’t enough, we may need to operate it in “Energy Saver” mode (4.5 kW), but I never see us needing to operate it in “High Demand” mode (5.7 kW)

Our background level usage is around 0.7 kW, with spikes up to approx. 2 kW. Outside of charging activity, I’ve only seen a single short spike over 3 kW in the last two weeks. Since that the two PowerWall configuration provides 10 kW continuous, with 14 kW peak, I don’t see any problem with adding this load. I realize that during an outage we may have to manage our usage a little more carefully, but given our excess solar production and the ease of adjusting the energy usage of the hot water heater via app control, I don’t see our energy demands even remotely approaching system limits.

Given the difficulty of getting our project advisor to return calls while our project was active, I’m not confident that I can get information from Tesla in a reasonable time frame. I was hoping folks in this forum might have gone through a circuit expansion, or have other experience that could provide guidance on what calculation or information our AHJ might want to see at inspection that would allow them to be comfortable with an additional 30A circuit in the panel (given the placard warning).
 

h2ofun

Active Member
Aug 11, 2020
3,704
894
auburn, ca
We just got our PTO from PG&E on our system last week, and because our gas hot water heater just failed, I would like to add a 30A circuit for a heat pump hybrid water heater. I need some help to understand what type of load calculation I may need to show the building department to add another circuit. My primary concern is that the backup load center has a bright red placard stating “Caution do not add new loads”, and I’m just not sure how that affects how an inspector evaluates an additional circuit.

Specifics:
6.46 kW solar, two Powerwall system, whole home backup (Square D panel fed by 125A breaker)
No AC, gas-fired furnace, gas dryer (for now), gas water heater (RIP). Our only 240V load is the mobile charger for our Model 3.

We basically maxed out our S and W facing roof area with panels knowing that we would initially be a net exporter, but planned to electrify NG appliances over time to export less. I let our project advisor know that the only reason I was willing to go with the Tesla plan of filling our roof was because we were planning on this electrification of more appliances. In the PG&E PTO application, Tesla included their estimate of 20% increase of electricity usage (though I think that Tesla includes whatever they need to get an oversized system approved by PG&E without really considering the implications of a usage increase with a whole home backup).

We plan on running the water heater in heat pump only mode (1.2 kW). We’re replacing our 50 gallon gas heater with a 65 gallon model because of the longer recovery time for operating in this mode. We will also use a higher tank temp. with a hot water tempering valve to further minimize the effects of the heat pump-only mode recovery time. If this still isn’t enough, we may need to operate it in “Energy Saver” mode (4.5 kW), but I never see us needing to operate it in “High Demand” mode (5.7 kW)

Our background level usage is around 0.7 kW, with spikes up to approx. 2 kW. Outside of charging activity, I’ve only seen a single short spike over 3 kW in the last two weeks. Since that the two PowerWall configuration provides 10 kW continuous, with 14 kW peak, I don’t see any problem with adding this load. I realize that during an outage we may have to manage our usage a little more carefully, but given our excess solar production and the ease of adjusting the energy usage of the hot water heater via app control, I don’t see our energy demands even remotely approaching system limits.

Given the difficulty of getting our project advisor to return calls while our project was active, I’m not confident that I can get information from Tesla in a reasonable time frame. I was hoping folks in this forum might have gone through a circuit expansion, or have other experience that could provide guidance on what calculation or information our AHJ might want to see at inspection that would allow them to be comfortable with an additional 30A circuit in the panel (given the placard warning).
My new stickers say do not add loads without an engineer involved. Even the inspector from county said just put EV charging into the generation box. Better to hire an expert, IMO
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Moderator
Nov 28, 2018
14,691
18,830
Riverside Co. CA
We just got our PTO from PG&E on our system last week, and because our gas hot water heater just failed, I would like to add a 30A circuit for a heat pump hybrid water heater. I need some help to understand what type of load calculation I may need to show the building department to add another circuit. My primary concern is that the backup load center has a bright red placard stating “Caution do not add new loads”, and I’m just not sure how that affects how an inspector evaluates an additional circuit.

Specifics:
6.46 kW solar, two Powerwall system, whole home backup (Square D panel fed by 125A breaker)
No AC, gas-fired furnace, gas dryer (for now), gas water heater (RIP). Our only 240V load is the mobile charger for our Model 3.

We basically maxed out our S and W facing roof area with panels knowing that we would initially be a net exporter, but planned to electrify NG appliances over time to export less. I let our project advisor know that the only reason I was willing to go with the Tesla plan of filling our roof was because we were planning on this electrification of more appliances. In the PG&E PTO application, Tesla included their estimate of 20% increase of electricity usage (though I think that Tesla includes whatever they need to get an oversized system approved by PG&E without really considering the implications of a usage increase with a whole home backup).

We plan on running the water heater in heat pump only mode (1.2 kW). We’re replacing our 50 gallon gas heater with a 65 gallon model because of the longer recovery time for operating in this mode. We will also use a higher tank temp. with a hot water tempering valve to further minimize the effects of the heat pump-only mode recovery time. If this still isn’t enough, we may need to operate it in “Energy Saver” mode (4.5 kW), but I never see us needing to operate it in “High Demand” mode (5.7 kW)

Our background level usage is around 0.7 kW, with spikes up to approx. 2 kW. Outside of charging activity, I’ve only seen a single short spike over 3 kW in the last two weeks. Since that the two PowerWall configuration provides 10 kW continuous, with 14 kW peak, I don’t see any problem with adding this load. I realize that during an outage we may have to manage our usage a little more carefully, but given our excess solar production and the ease of adjusting the energy usage of the hot water heater via app control, I don’t see our energy demands even remotely approaching system limits.

Given the difficulty of getting our project advisor to return calls while our project was active, I’m not confident that I can get information from Tesla in a reasonable time frame. I was hoping folks in this forum might have gone through a circuit expansion, or have other experience that could provide guidance on what calculation or information our AHJ might want to see at inspection that would allow them to be comfortable with an additional 30A circuit in the panel (given the placard warning).

The sticker that says " do not add loads" means no electrician is going to look at that and add loads to it. You will likely need to engage with whoever installed it, if thats tesla or otherwise, or plan for whatever it is not to be in the backed up loads panel.
 
Tesla installed the system. Frankly the idea of going without hot water for the time frame it would take Tesla to return an email or phone call, let alone address the situation terrifies me.

It doesn't seem realistic that everyone with a back up load center is now dependent on Tesla for every addition to that panel no matter how small, but perhaps that's in fact the case. I was hoping that someone that has been through the situation has some experience to share.
 
With just two heavy loads, car charging and proposed hybrid, no dryer, no mention of electric range, what would the total load be for the two, charger and hybrid?
I seriously doubt you would come close to full loading at any one time. PG&E doesn't wire size for full load, 125A or perhaps not even 100A main feed cables.

ps my son and I installed a hybrid in his Chandler house early in year, most of the time it doesn't use full current load.
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Moderator
Nov 28, 2018
14,691
18,830
Riverside Co. CA
Tesla installed the system. Frankly the idea of going without hot water for the time frame it would take Tesla to return an email or phone call, let alone address the situation terrifies me.

It doesn't seem realistic that everyone with a back up load center is now dependent on Tesla for every addition to that panel no matter how small, but perhaps that's in fact the case. I was hoping that someone that has been through the situation has some experience to share.

My understanding is there are to be no additions to that panel.... /shrug.

There was a post here from someone who tried to add loads to the backup loads panel and no one would touch it. I think it might have been @holeydonut but I am not sure.

I am not even close to knowing how AHJ operate, but in general, common sense says that if the "red sticker says do not add loads, why is a load being added here?" hurdle would have to be overcome by someone in a professional capacity who was wiling to do that.

edit: here is the thread I was thinking about, where @holeydonut couldnt get a tesla wall connector added to the backup side:

 
Last edited:

h2ofun

Active Member
Aug 11, 2020
3,704
894
auburn, ca
My understanding is there are to be no additions to that panel.... /shrug.

There was a post here from someone who tried to add loads to the backup loads panel and no one would touch it. I think it might have been @holeydonut but I am not sure.

I am not even close to knowing how AHJ operate, but in general, common sense says that if the "red sticker says do not add loads, why is a load being added here?" hurdle would have to be overcome by someone in a professional capacity who was wiling to do that.
glad mine sticker added without engineering approval
 
With just two heavy loads, car charging and proposed hybrid, no dryer, no mention of electric range, what would the total load be for the two, charger and hybrid?
I seriously doubt you would come close to full loading at any one time. PG&E doesn't wire size for full load, 125A or perhaps not even 100A main feed cables.
I think there are really two scenarios to consider:

Grid power available, so we care about the 125A breaker in the load center:
Maximum conceivable load would be something like 32A (mobile connector) + 24A (water heater, high demand mode) + 6kw/25A (twice the maximum load I've ever seen for the rest of the house) = 81A. This is well under the continuous load rating for a 125A breaker. Realistically, the hot water heater shouldn't be recovering much when the car is charging at midnight, and I'm planning on running the water heater in a mode that draws closer to 5A, so real world this would be more like a

Power outage, so running from Powerwalls:
We would definitely set the water heater to run in heat pump mode (1.2 kW), and would probably not charge the car (or would only charge minimally). I'm not sure exactly how the PW and Model 3 communicate, but I'd be surprised to learn that it charges at full 32A. I could manually adjust the charging rate if I had to.

In summary, I don't see a scenario where we would be taxing either the 125A panel or the Powerwalls (barring us being stupid during a power outage). I'm just now sure how to formally share this with AHJ (or if they even care about the placard).
 
I
Realistically, the hot water heater shouldn't be recovering much when the car is charging at midnight, and I'm planning on running the water heater in a mode that draws closer to 5A, so real world this would be more like a
I think I backspaced over part of what I was writing, and I don't see an 'edit' button. This should read "so real world this would be more like a 45-50A peak load."
 
I am not an expert. However, I looked into this for our setup and was told that the sticker meant that an electrician/engineer had to get involved for any proposed circuit modifications to show that the current draw with the new circuit(s) would still be within the limits. When I looked at the spreadsheets, they were a mix of actual loads and predicted loads, and not something that seemed to fall into the "typical" simple NEC 120% rule. Beyond my pay grade, at any rate.

So...you might want to replace the water heater with a cheap gas one while you get the permits and paperwork straight for an electric water heater, dryer, stove, etc. I don't know of a quick way through this. Sorry.

Maybe @Vines or @wwhitney know someone locally who can step up.

All the best,

BG
 
My primary concern is that the backup load center has a bright red placard stating “Caution do not add new loads”, and I’m just not sure how that affects how an inspector evaluates an additional circuit.
It sounds like a possible 120% rule issue and it may be at max. You may need an electrician. Most of the prior discussion has been about load calcs but the 120% rule is different when back feed is involved. Conceptually it says the sum of the breakers cannot exceed 120% of the busbar rating. I believe the 120% rule is more restrictive than a load calc and a good electrician might be able to figure it out. The third possibility is it is the Tesla rule that they only allow one 30 Amp breaker. If so that may affect the warranty on the Powerwall.

Can you look on the label and determine the busbar rating? Then add the breakers and see if there already os a 30 Amp breaker. That may help clarify which issue is involved.
Solutions could be a sub panel or power WH from main panel if that doesnt have sticker.
 
Last edited:

h2ofun

Active Member
Aug 11, 2020
3,704
894
auburn, ca
I am not an expert. However, I looked into this for our setup and was told that the sticker meant that an electrician/engineer had to get involved for any proposed circuit modifications to show that the current draw with the new circuit(s) would still be within the limits. When I looked at the spreadsheets, they were a mix of actual loads and predicted loads, and not something that seemed to fall into the "typical" simple NEC 120% rule. Beyond my pay grade, at any rate.

So...you might want to replace the water heater with a cheap gas one while you get the permits and paperwork straight for an electric water heater, dryer, stove, etc. I don't know of a quick way through this. Sorry.

Maybe @Vines or @wwhitney know someone locally who can step up.

All the best,

BG
Yep, that is what my sticker says
 
The busbar rating is 225A, The total amperage of breakers in the backup load center (excluding the 125A backfeed one) is 330A. I can remove a 35A of breakers that go nowhere useful (long story, but think older house that had the loads from circuits removed during a remodel, but the breakers are still present). The installation electrician moved all circuits to the backup load center including these two that were turned off. I could also move the 50A circuit for the car charger; if push came to shove, I'd rather have hot water than be able to charge my car during an outage.

I'm willing to get an electrician or engineer involved, but I'm not quite sure what type of engineer or electrician can "overrule" the guidance on the placard. Is it literally any licensed electrician, or is there a Powerwall qualified subset of electricians, or a certification I should ask for?
 

h2ofun

Active Member
Aug 11, 2020
3,704
894
auburn, ca
The busbar rating is 225A, The total amperage of breakers in the backup load center (excluding the 125A backfeed one) is 330A. I can remove a 35A of breakers that go nowhere useful (long story, but think older house that had the loads from circuits removed during a remodel, but the breakers are still present). The installation electrician moved all circuits to the backup load center including these two that were turned off. I could also move the 50A circuit for the car charger; if push came to shove, I'd rather have hot water than be able to charge my car during an outage.

I'm willing to get an electrician or engineer involved, but I'm not quite sure what type of engineer or electrician can "overrule" the guidance on the placard. Is it literally any licensed electrician, or is there a Powerwall qualified subset of electricians, or a certification I should ask for?
I would used just a licensed electrician. They can tell you if they can do the job and if not, who to go to
 
So...you might want to replace the water heater with a cheap gas one while you get the permits and paperwork straight for an electric water heater, dryer, stove, etc.
That thought occurred to me as well, but, ... yuck. I installed solar, and drive electric (partially) and am replacing a gas water heater with an electric heat pump model to save the planet, damn it! The idea of buying and installing a hot water heater only to immediately junk it, seems counterproductive (but unfortunately perhaps pragmatic).
 

h2ofun

Active Member
Aug 11, 2020
3,704
894
auburn, ca
That thought occurred to me as well, but, ... yuck. I installed solar, and drive electric (partially) and am replacing a gas water heater with an electric heat pump model to save the planet, damn it! The idea of buying and installing a hot water heater only to immediately junk it, seems counterproductive (but unfortunately perhaps pragmatic).
Mine died the other day, and decided to just go with another propane water heater. I probably should of gone with electric, but ..... Now, I do have solar hot water panels too. So I hope at least in the summer no hot water heater is even needed since the tank is at 150 degrees.
 
The busbar rating is 225A, The total amperage of breakers in the backup load center (excluding the 125A backfeed one) is 330A
I am not sure I understand which panel has the 225 Amp rating? That sounds like a main panel? 120% of that is 270 so 330 exceeds that so I am guessing that you could put the water heater there until an electrician figures it out. I have a heat pump water heater to so I agree with your swap out. Besides, with the NOX regs there are no longer "cheap" gas water heaters. in California at least.
I agree it would be futile, frustrating or take forever. Find a good electrician. It could be as simple as moving one breaker. The electrician just needs to figure out what is driving the limitation. It has to be one of the following;
  1. Tesla rule about onec39 Amp circuit per Poweewall.
  2. Load caculation
  3. 120% rule.
 

holeydonut

Active Member
Supporting Member
Jun 27, 2020
3,286
2,666
East Bay NorCal
My understanding is there are to be no additions to that panel.... /shrug.

There was a post here from someone who tried to add loads to the backup loads panel and no one would touch it. I think it might have been @holeydonut but I am not sure.

I am not even close to knowing how AHJ operate, but in general, common sense says that if the "red sticker says do not add loads, why is a load being added here?" hurdle would have to be overcome by someone in a professional capacity who was wiling to do that.

edit: here is the thread I was thinking about, where @holeydonut couldnt get a tesla wall connector added to the backup side:



Lol if there's a screw up involving disconnects or loads, I'm the guy :p

Yeah, getting extra things added to the backup loads side poses two separate challenges.

1) The NEC load calc to see if adding your load there poses risk to the particular way your home is designed (the service size, max amps of each sub-panel, etc).
2) Whether someone wants to deal with the legal liability of messing with things on the backup side of a scary energy gateway.

The NEC load calc is simple enough. Just about any person who works in a electrician shop will be able to come in and see if your sub panels and loads allow it. I did a load calc and showed I was fine under the NEC rules, but the electricians didn't care.

Because they only care about legal risk. They don't want to deal with others tech... so they won't just land a breaker wherever if it's behind the gateway. They would only touch upstream slots; which is annoying since the upstream slots weren't metered.
 
That thought occurred to me as well, but, ... yuck. I installed solar, and drive electric (partially) and am replacing a gas water heater with an electric heat pump model to save the planet, damn it! The idea of buying and installing a hot water heater only to immediately junk it, seems counterproductive (but unfortunately perhaps pragmatic).
I hear you. Yuck. But what is a warm shower worth?

You could probably find a home for your old "new" water heater via CL, so you aren't actually junking it...

All the best,

BG
 
They would only touch upstream slots; which is annoying since the upstream slots weren't metered.
If what you mean by upstream is the main panel, that could be a workaround. I am not sure what you mean by they "weren't metered" but i assume you mean not metered by Tesla's meter. Otherwise If there is a way to have some slots ahead of the utility meter I am sure many would be interested.
:) :)
 

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