Connecting new Heat Pump system to PowerWall 2

[UCIBtheG]

Hoping for some reliable electrical advice from someone on the forum.

I just had installed a new Carrier Infinity heat pump system, replacing a central A/C condenser and gas furnace. During initial walk-through, I explained that my old air conditioner system was connected to a 40 amp circuit, and I wanted the new system circuit breaker to be moved from our outside panel to the critical load panel backed up by my (one) PowerWall 2. I explained that there is a strict requirement on the panel that all loads be 30A or less. He made a call and said that the new system would work, as it consists of an air handler and a heat pump, each of which would work off 30A breakers. He confirmed two open slots on the panel and said all would be fine to move forward.

Fast forward to system installation, and the installer informed me that the air handler unit was fine to connect to the Powerwall panel, but the heat pump shows "Max Circuit Breaker 40A" on the manufacturer sticker. Installer said that if we were to connect it to a 30A breaker on the Powerwall panel, it would not pass inspection. The sticker also shows "Minimum Circuit Amps 24.4A", but installer said that the inspectors always go by the Max value. His proposed solution was to leave it connected to the same 40A circuit that the old air conditioner was on, get a successful inspection, then move it to the Powerwall critical load panel after that.

My wife is not at all comfortable with this seemingly "cheating" the system. If the system were to cause a fire, we'd be worried that the insurance company would deny coverage if they determined circuit changes were made post-inspection. Not to mention we have a newborn in the house and just don't want that splinter of worry in the back of our mind.

After doing some research (here), it seems to me that there shouldn't be an issue with inspection passing the new unit running off the 30A critical load panel, and having a 40A fuse at the new heat pump unit outside. From what I gathered, inspectors' biggest concern is making sure that the wiring gauge isn't too small in relation to the amperage passing through it. Since we'd be moving the 40A wiring (assuming it would be 8 AWG) over to the Powerwall panel to a 30A breaker, wouldn't it pass inspection configured as I originally requested? I'm really hoping someone here could give me a definitive answer. It's time for us to pay for the system, and we're in a bit of a standoff now until this issue gets sorted out.

Hopefully that all makes sense, please let me know if any clarifications are needed.

 

[wwhitney]

It's fine to run equipment labeled "MCA 24.4, MOCP 40A" off a 30A circuit, if it will reliably start. There's a chance that the equipment could trip a 30A breaker on startup (and possibly intermittently), but trying that configuration is not a code violation.

If the equipment does trip a 30A breaker on startup, it's unlikely 1 PW would be able to start the equipment during a grid outage, so the wrong equipment was selected by the HVAC people. An inverter driven compressor would have been a better choice, as typically they have a lower starting current requirement than 1 or 2 speed equipment.

Cheers, Wayne

 

[arnolddeleon]

Hrmm, when I upgraded to a heat pump on a property my HVAC installer actually downgraded the circuit breaker to 30A and 30A disconnect because apparently the local inspector prefers the smallest possible breaker that supported the equipement even though the old circuit was 40A and all the wiring was good for 40A.

I don't think there are safety issues with the proposal from your installer says the random guy from the Internet. You have to trust your installer to some degree, the inspector is not going to review everything they do.

 

[UCIBtheG]

It's fine to run equipment labeled "MCA 24.4, MOCP 40A" off a 30A circuit, if it will reliably start. There's a chance that the equipment could trip a 30A breaker on startup (and possibly intermittently), but trying that configuration is not a code violation.

If the equipment does trip a 30A breaker on startup, it's unlikely 1 PW would be able to start the equipment during a grid outage, so the wrong equipment was selected by the HVAC people. An inverter driven compressor would have been a better choice, as typically they have a lower starting current requirement than 1 or 2 speed equipment.

Cheers, Wayne

Wayne, thanks so much for your response. Just confirming some acronyms, MCA would be Minimum Circuit Ampacity, which is indeed 24.4 on my unit. The other two related lines on my unit are "Max Fuse" and "Max CKT-BKR", both 40A. I'm assuming MOCP (from your post) is something like Maximum Over Current Protection? Is that the same thing?

I'm cautiously optimistic that the Carrier Infinity they selected won't trip the 30A circuit. It is listed as having safe-start built into the unit, and the unit fan does start very slowly and quietly. It is a 5 stage unit.

 

[wwhitney]

I'm assuming MOCP (from your post) is something like Maximum Over Current Protection? Is that the same thing?

Yes

I'm cautiously optimistic that the Carrier Infinity they selected won't trip the 30A circuit.

Well, give it a try on a 30A circuit. If it works, no problem. MOCP is a maximum, not a requirement.

Cheers, Wayne

 

[CrazyRabbit]

Watch this video:

Air-Conditioning, Circuit Sizing [440.4(B), 2020 NEC] | The first time you’re called upon to size a conductor and overcurrent device for central air-conditioning and you look at the nameplate requirements that... | By Mike Holt Enterprises | Facebook

656 views, 68 likes, 3 loves, 13 comments, 32 shares, Facebook Watch Videos from Mike Holt Enterprises: The first time you’re called upon to size a conductor and overcurrent device for central...

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You will not be able to run your AC unless solar is producing, one powerwall is not sufficient.
It would not be “legal” to add you AC to your backup loads panel as your backup source is not sufficient to power it.

 

[wwhitney]

You will not be able to run your AC unless solar is producing, one powerwall is not sufficient.

I don't think you can say that for sure just based on the info in the OP, but there's certainly a good chance of that being the case. So it will also require testing.

Cheers, Wayne

 

[nwdiver]

I don't think you can say that for sure just based on the info in the OP, but there's certainly a good chance of that being the case. So it will also require testing.

Cheers, Wayne

Looks like that compressor has a VFD so there won't be any starting current. I have a similar sized AC on a 4.4kW inverter and it works great.

 

[UCIBtheG]

Some further electrical info on the unit, mine is the row corresponding to 36-30 on the leftmost column. There is no LRA value, which I believe is the key to it not popping the breaker at startup, correct? I know I'll find out for sure once it is connected and I try running it from a simulated outage situation. But until the installer comes back to take care of wiring it up to the 30 amp breaker on the PW subpanel, this looks like it should work?

One last question, should the fuse associated with the heat pump be changed from a 40A to 30A to match the breaker? Or does it not even matter since it is at least 30A and within the prescribed 40A max? I certainly don't want it to fail inspection over a silly oversight like that if it matters. Whatever fuse will give me the best odds to pass inspection is what we'll go with.

 

[wwhitney]

Some further electrical info on the unit, mine is the row corresponding to 36-30 on the leftmost column. There is no LRA value, which I believe is the key to it not popping the breaker at startup, correct?

LRA provides an indication of peaking starting current, which can control both the minimum size breaker that will hold at startup, and whether the PW(s) can start the unit off-grid. I'm 98% sure that "LRA N/A" means it's an inverter driven unit that will have little or no starting current surge, and so with an MCA of 24.4A, it would start fine on a 25A breaker and the PW will be able to start it fine.

One last question, should the fuse associated with the heat pump be changed from a 40A to 30A to match the breaker? Or does it not even matter since it is at least 30A and within the prescribed 40A max?

If you have a 30A breaker supplying a 40A fused disconnect, that's OK. The 30A breaker will generally control (barring a fuse/breaker trip curve mismatch). The fuses in the disconnect are then optional, the disconnect could be unfused, but there's no need to change it.

BTW, the table you posted has the wrong info in the 60C column, the 60C ampacity of #12 Cu is 20A, and of #10 Cu is 30A, so the wires sizes in that whole column should be bumped up one size (+2 AWG). [Back in the 2008 NEC, the 60C ampacity of #12 was 25A, so if the table was published 12 years ago, only the 48-30 / 60C entry would be wrong. But that doesn't apply currently.] The upshot is that if your unit is wired with NM cable, or has an LFNC/LFMC whip (that is more than 9% of the total circuit length, or more than 10' total) it should be #10; if it's wired with a 75C rated wiring method then #12s are fine.

Cheers, Wayne

 

[UCIBtheG]

My installer has indicated they'll go ahead and change the heat pump over to the subpanel on a 30A breaker, but that I need to be ready to argue my case with the city and/or inspector if it doesn't pass. I think we've vetted this out thoroughly enough that it shouldn't come to that. But if it does fail, what then? Is there some kind of appeal system if I get a clueless inspector? Or perhaps a way to request someone knowledgeable in advance that has seen a similar scenario before?

Wayne, I wish you could just come out and take care of the inspection I'm sure whoever does come out would be lucky to know half what you (along with several others that peruse this forum) know in these matters.

 

[wwhitney]

My installer has indicated they'll go ahead and change the heat pump over to the subpanel on a 30A breaker, but that I need to be ready to argue my case with the city and/or inspector if it doesn't pass.

You don't need to worry too much. From what I've heard, the typical contractor may hear something like an engineer or designer saying to them "in this case, I want the equipment OCPD to match the maximum listed on the nameplate" or "it's best to use the maximum listed on the nameplate to minimize chance of a callback" but then internalize that as "code requires the maximum listed on the nameplate," which is not true. The inspector should know the difference.

If the inspector raises an issue, I suggest politely asking what code section governs the OCPD size and that the choice of 30A would violate, perhaps "so that we can study it to understand the requirements." And then depending on how the conversation goes, point out that NEC section 440.4(B) specifies that the nameplate provides the "maximum rating of the branch-circuit short-circuit and ground-fault protective device," and that maximum is not the same as the only choice.

https://up.codes/viewer/california/ca-electric-code-2022/chapter/4/equipment-for-general-use#440.4_(B)

Cheers, Wayne

 

[jjrandorin]

I certainly cant speak to any of the technical aspects, but I do know that even if this works (and it sounds like it might), @UCIBtheG isnt going to want to use that system for very long if off grid. I certainly get wanting options, especially when off grid, but I also think sometimes people underestimate how much power these things use, vs how relatively little storage power is in one powerwall.

 

[wwhitney]

I do know that even if this works (and it sounds like it might), @UCIBtheG isnt going to want to use that system for very long if off grid.

Depends on the HVAC demand and average COP for the conditions. 1 W = 3.41 BTUs/hr, and 1 ton = 12,000 BTUs/hr, so 1 ton = 3.5 kW. If the HVAC demand during an outage is only 1 ton, and the system can do an average COP of 5, then under those mild conditions, the average power draw is only 700W. While if the HVAC demand is 3 tons, and the system can only do an average COP of 2.5, then the average power draw is 4.2 kW. [The 36 in the OP's model number indicates 36,000 BTUs/hr nominal rating, i.e. 3 tons.]

In other words, if the system can modulate efficiently, which I would hope an inverter driven system can do, then what matters is the HVAC demand at the moment, not the size of the equipment.

Cheers, Wayne

 

[nwdiver]

Depends on the HVAC demand and average COP for the conditions. 1 W = 3.41 BTUs/hr, and 1 ton = 12,000 BTUs/hr, so 1 ton = 3.5 kW. If the HVAC demand during an outage is only 1 ton, and the system can do an average COP of 5, then under those mild conditions, the average power draw is only 700W. While if the HVAC demand is 3 tons, and the system can only do an average COP of 2.5, then the average power draw is 4.2 kW. [The 36 in the OP's model number indicates 36,000 BTUs/hr nominal rating, i.e. 3 tons.]

In other words, if the system can modulate efficiently, which I would hope an inverter driven system can do, then what matters is the HVAC demand at the moment, not the size of the equipment.

Cheers, Wayne

Yep. I have a 2 ton unit. My total off-grid load when it's running is usually ~900w so the HVAC is probably ~700w. Variable drive compressors are AWESOME!

 

[UCIBtheG]

I certainly cant speak to any of the technical aspects, but I do know that even if this works (and it sounds like it might), @UCIBtheG isnt going to want to use that system for very long if off grid. I certainly get wanting options, especially when off grid, but I also think sometimes people underestimate how much power these things use, vs how relatively little storage power is in one powerwall.

I have a fair bit of experience with the PowerWall, having lived with it for over 4 years now. HVAC can certainly chew through an ESS, as the heat wave in early September proved. On the hottest days (115+ degrees peak), I set the PW mode to Self-Powered up until the 5-8pm super peak period to keep it from draining the PW during the peak period. My previous (2004 vintage) A/C unit ran non-stop, total house load ~4kW draw. With my 10% reserve, it was just enough to cover the three hours of super peak. That's about a worst-case scenario.

As discussed above, the new unit is a 5 stage, and when running at the lowest stage, I see ~1kW total load. Of course, the lowest stage isn't going to keep me at 78 degrees indoors when it's 115 outside, but at least the option to modulate draw is there. The same goes for a power outage scenario (rare in my neck of the woods); with the variable speed compressor I can try to manage load to make it through to when solar is available or when the power company estimates restoration.

This new system is all part of my long-term plan to completely get off natural gas and cheerfully tell PG&E to cut me off. All that are left now are the water heater (I'm strongly eyeing the new 120V Rheem heat pump water heater that just became available a couple months back) and an induction range to replace the gas range. There are big rebates on both of those last I looked.

 

[Steve2498]

I have a fair bit of experience with the PowerWall, having lived with it for over 4 years now. HVAC can certainly chew through an ESS, as the heat wave in early September proved. On the hottest days (115+ degrees peak), I set the PW mode to Self-Powered up until the 5-8pm super peak period to keep it from draining the PW during the peak period. My previous (2004 vintage) A/C unit ran non-stop, total house load ~4kW draw. With my 10% reserve, it was just enough to cover the three hours of super peak. That's about a worst-case scenario.

As discussed above, the new unit is a 5 stage, and when running at the lowest stage, I see ~1kW total load. Of course, the lowest stage isn't going to keep me at 78 degrees indoors when it's 115 outside, but at least the option to modulate draw is there. The same goes for a power outage scenario (rare in my neck of the woods); with the variable speed compressor I can try to manage load to make it through to when solar is available or when the power company estimates restoration.

This new system is all part of my long-term plan to completely get off natural gas and cheerfully tell PG&E to cut me off. All that are left now are the water heater (I'm strongly eyeing the new 120V Rheem heat pump water heater that just became available a couple months back) and an induction range to replace the gas range. There are big rebates on both of those last I looked.

What is the model # of that Rheem heat pump water heater?

 

[UCIBtheG]

What is the model # of that Rheem heat pump water heater?

The 40 gallon is PROPH40 T0 RH120, and the 50 gallon PROPH50 T0 RH120 (note these require a dedicated 15A circuit; there are other 120V models that do not). More info at Rheem Releases 120V Plug-In Heat Pump Water Heater That Can Be Plugged Into Typical Outlet

 

[UCIBtheG]

Well, the saga continues. Here's the e-mail I got this morning from the project manager. Any ideas what information on the PW critical load panel they might need besides that the breakers all be 30A or less? Would they looking for a panel model number or something?

"Hello sir, I just got off the phone with the Elk Grove building department and they are requiring the MOCP of 40amp breaker to be installed but he said we can resubmit to the planning review information on your panel box showing that the max breaker size is 30 amps and request that the electrical be installed on that panel box.

Can you email me information on that panel box so I can get our permit department to resubmit to the planning dept what we are doing thanks"

 

[UCIBtheG]

I sent the project manager a link to the Powerwall data sheet, will send some pics of the panel tonight. This isn't supposed to be this hard.