Tesla Powerwall neutral

sketchy

Senior Member
Location
MN
Hey all, wondering about how to size the neutral on these. Would they be considered to have a balanced load on the AC output? Or depending on demand could phase A potentially draw more than phase B? I'm leaning toward the latter but because there is an inverter inside I'm confused on whether the neutral could be downsized. This will come into play when more than 1 powerwall is installed.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Most Powerwalls are installed with a Backup Gateway (transfer switch), so they are capable of running in standalone mode. As such they need a neutral, sized as you would for any feeder for the loads behind the Backup Gateway (sometimes the whole house, sometimes not).

Cheers, Wayne
 

jaggedben

Senior Member
You need a full neutral for each Powerwall branch circuit because it just so happens that for 30A OCPDs the code does not allow it to be smaller than the EGC, which in turn is not allowed to be smaller than the ungrounded conductors.

For a panel aggregating multiple Powerwalls you might be allowed to size it to the maximum unbalanced load from the backed up loads. However, it seems to me that the time to calculate that to the satisfaction of any inspector who might question it is never worth the money saved on wire in residential situations.
 

jaggedben

Senior Member
I don't know anything about the specs on a powerwall but my gut tells me they shouldn't even need a neutral.
And your gut is totally wrong. Without neutrals to the Powerwalls there would be no 120V when the system disconnects from the grid. It's not provided anywhere else. I beleive they do it by having two synced 120V inverters in each unit, although it might be an autotransformer.
 

sketchy

Senior Member
Location
MN
I think I'll be safe downsizing the neutral one size down from the hots. That will cover 99% of our installs.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I think I'll be safe downsizing the neutral one size down from the hots. That will cover 99% of our installs.
To the extent that you have known 240V loads that have been part of the off grid load calculation for the powerwall, that seems justified. If you are only relying on balance between unrelated 120V loads, I am not so sure, especially given that some manual load shedding may be part of the usage plan.

Sent from my XT1585 using Tapatalk
 

ActionDave

Moderator
Staff member
Location
Durango, CO, 10 h 20 min without traffic from wing
Occupation
wire pulling grunt
And your gut is totally wrong. Without neutrals to the Powerwalls there would be no 120V when the system disconnects from the grid. It's not provided anywhere else. I beleive they do it by having two synced 120V inverters in each unit, although it might be an autotransformer.
I pictured them as a big bunch of batteries that needed 240V for charging, I didn't consider needing a neutral to bring 120V power back to the house.
 

jaggedben

Senior Member
I pictured them as a big bunch of batteries that needed 240V for charging, I didn't consider needing a neutral to bring 120V power back to the house.
And to be fair, the way they do it isn't the only way to provide a neutral. But obviously one has to be provided somehow.
 

caribconsult

Senior Member
Location
Añasco, Puerto Rico
Occupation
Retired computer consultant
Yes, you do need a neutral and the output of the Tesla Gateway system has a neutral otherwise you wouldn't be able to get 120v AC, which is what the majority of your house uses. It's 240 across the two hot sides and 120 (or therabouts) from one hot to neutral. Downsizing the neutral cable doesn't sound like a good idea...whatever is coming out of the positive side has to return via the neutral.
 

jaggedben

Senior Member
Theoretically the neutral for a panel containing multiple Powerwalls could be made smaller according to 220.61. However this would require a complicated calculation against the backed up loads, and I can't imagine it would ever be worth the time to do that just to save $5 on a wire size. (Especially since the calc might end up telling you that couldn't downsize most of the time anyway.) Maybe if you were installing six Powerwalls and the feeder was 100ft, but there are probably still better things to do with that time, like sell more Powerwalls.

A more interesting, and more likely in real life case, would be a 'generation panel' that contains no loads but contains energy storage and also solar inverter(s) that do not require a neutral, or that qualify under 705.95(B)*. For example if the circuit requirements for two Powerwalls and the solar inverter were 30A + 30A = 40A respectively, then the feeder hots would be rated for 100A but the neutral could be rated for 60A. And that calc takes less time than it takes to state the result.

*705.95(B) changed to 705.28(C)(2) in the 2020 NEC.
 

caribconsult

Senior Member
Location
Añasco, Puerto Rico
Occupation
Retired computer consultant
It all depends on how much work you want to do to save $5 worth of wire. But even if you have a 230 device (like a stove) connected right to your two hot sides, you still need a neutral for return, right?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
If you have pure 240V loads, then the neutral is not needed to feed the loads. A minimal neutral would still be required for bonding/ground reference.

-Jon
 

Pscanlin

Member
Location
Houston, Texas
Occupation
Electrical Designer
Hey all, wondering about how to size the neutral on these. Would they be considered to have a balanced load on the AC output? Or depending on demand could phase A potentially draw more than phase B? I'm leaning toward the latter but because there is an inverter inside I'm confused on whether the neutral could be downsized. This will come into play when more than 1 powerwall is installed.
Tesla power wall 2 have a 22 amp output. we only #10 awg thwn between PW2 and back up panel
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Tesla power wall 2 have a 22 amp output.
That's a bit simplistic. The data sheet does say "real power, max continuous: 5 kW", which is 21 amps at 240V. But it also says "apparent power, max continuous: 5.8 kVA", which is 24 amps at 240V. And it also has 10s peak numbers of 7 kW and 7.2 kVA. The latter is, of course, 30 amps at 240V.

Cheers, Wayne
 

mikeames

Senior Member
Location
Germantown MD
Occupation
Teacher - Master Electrician - 2017 NEC
But even if you have a 230 device (like a stove) connected right to your two hot sides, you still need a neutral for return, right?
The only appliances that are strictly 240 in a house are the AC compressor and the hot water heater. The stoves and dryers generally have 240v elements but required the neutral for the other stuff, motors, electronics, etc... Unless you have old school 3 wire appliances.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
The only appliances that are strictly 240 in a house are the AC compressor and the hot water heater. The stoves and dryers generally have 240v elements but required the neutral for the other stuff, motors, electronics, etc... Unless you have old school 3 wire appliances.
A. Also, most air-handlers are now 240v.

B. The old-school 3-wire appliance circuits did have a neutral; what was lacking was a separate EGC.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I didn't know that. I thought it was the other way around. (y)
No, it was an allowance for the neutral, as a current-carrying conductor, to also do grounding duties. That's why the limitations such as service cable or insulated neutral, and supplied from main panel, where they knew the neutral-ground bond was made.
 
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