208V charging?

[pepperoni]

I recently had a 14-50 outlet installed in my garage, and noticed that my Tesla was reporting charging voltage between 207-210V, not 240V as I had expected. A 30V drop would naturally be concerning, so I did what any self respecting engineer would do and measured the phase voltage. The screen shot from my oscilloscope below shows the two lines are 120 deg out of phase, not 180, as though they are coming off of a three phase system. This perfectly explains the 208V that my car sees. I should add that I live in an attached townhouse, and not in a commercial building where one might encounter three phase power.

So my question to all the home electricity experts out there is how common is this? Should I be surprised to see 120 phase on residential power? Is it anything I should ask to get looked at? I’m more curious than anything.

 

[jmaddr]

Very interesting. I'd say it uncommon for individual residential units.
What does your panel look like? If you panel has two busses like is common, and if you measure the voltage from bus to bus, does it show the same 120 degree shift?

 

[nwdiver]

So my question to all the home electricity experts out there is how common is this?

Super common anywhere that gets 3 phase power. That's one thing that can REALLY make commercial L2 charging SUUUUCK. Not only do you get 30A(like charge point) but it's usually 208v @ 30A.

208v is basically the 3 phase equivalent of 240v (using 2 of 3 phases). It's 120v 120 degrees out of phase (208v) instead of 120v 180 degrees out of phase (240v, 2 of 2 phases).

 

[Earl]

I've never seen or heard of this at a US residence. Very surprising.
Is it new construction? Old construction? Condo? Converted retail or industrial establishment? Anything else about the house that is out of the ordinary?
Is the service from underground or from overhead poles?
A photo of the interior of the electrical service panel would be interesting.

 

[Rocky_H]

shows the two lines are 120 deg out of phase, not 180, as though they are coming off of a three phase system.

Yes, that's a 3 phase voltage system.

So my question to all the home electricity experts out there is how common is this? [...]Is it anything I should ask to get looked at?

Pretty common, and no, you definitely don't need to get it looked at.

Should I be surprised to see 120 phase on residential power?

Welllll...depends on the meaning of "residential".

Is it new construction? Old construction? Condo? Converted retail or industrial establishment? Anything else about the house that is out of the ordinary?

Already answered this:

I should add that I live in an attached townhouse

In a standalone single family standard house, it would be crazy unusual to see 3 phase going to those. But for a lot of kinds of multi-unit dwelling buildings, those basically are kind of commercial properties, so apartments and townhouses are very commonly 3 phase supplied.

 

[EfficientWatts]

Super common anywhere that gets 3 phase power. That's one thing that can REALLY make commercial L2 charging SUUUUCK. Not only do you get 30A(like charge point) but it's usually 208v @ 30A.

208v is basically the 3 phase equivalent of 240v (using 2 of 3 phases). It's 120v 120 degrees out of phase (208v) instead of 120v 180 degrees out of phase (240v, 2 of 2 phases).

Not that this will happen in the US (if ever), but I wish there could be 240/415V (ala Europe and almost everywhere else) at least in commercial settings as a supported standard. Right now as I know it 277/480 is used for lighting and some heavier equipment. If 240/415 was used then we could get better L2 charging that would be a little more efficient, plus commercial settings may be able to take advantage of this standard more so than other areas. Of course then there's the question of what would happen to all the 277/480V equipment unless one wanted to support 3 standards, which I why I don't think this would happen unless there's a really good reason to, and so far it seems 208V seems to be enough.

 

[pepperoni]

Very interesting. I'd say it uncommon for individual residential units.
What does your panel look like? If you panel has two busses like is common, and if you measure the voltage from bus to bus, does it show the same 120 degree shift?

I've never seen or heard of this at a US residence. Very surprising.
Is it new construction? Old construction? Condo? Converted retail or industrial establishment? Anything else about the house that is out of the ordinary?
Is the service from underground or from overhead poles?
A photo of the interior of the electrical service panel would be interesting.

Several of you asked, so here is the panel. The EV power exits the panel in the lower left. Let me know what you see!

Our home was built in 2003. As I said, it is a townhome, with four attached units. I guess I’m surprised that it would be receiving 3 phase power. What for? The AC?

 

[nwdiver]

Several of you asked, so here is the panel. The EV power exits the panel in the lower left. Let me know what you see!

Our home was built in 2003. As I said, it is a townhome, with four attached units. I guess I’m surprised that it would be receiving 3 phase power. What for? The AC?

View attachment 791179

2 of 3 phases 120/208 can be fed just like both phases of 120/240 (Some things might need a little more current). It was likely easier to put a transformer on each phase and balance those 3 phases to each unit as opposed to one large 120/240 transformer. So it's unlikely anything is actually running on 3 phase power. Everything is just using 2/3 phases.

 

[RTPEV]

Several of you asked, so here is the panel. The EV power exits the panel in the lower left. Let me know what you see!

Our home was built in 2003. As I said, it is a townhome, with four attached units. I guess I’m surprised that it would be receiving 3 phase power. What for? The AC?

But your unit is not receiving 3 phase power, just 2 out of the 3 phases that is feeding the entire property.

Basically you have 3-phase power coming into the property and then 2 out of the 3 phases are distributed to each individual unit.

 

[pepperoni]

Thanks everyone, I learned a few new things from this thread! First, it is not uncommon for densely packed residential units (like my townhouse) to each receive two out of three connections to a 3-phase power. In fact, in my learnings elsewhere on the internet, most of NYC’s residential units are powered this way. Apparently my neighborhood in Santa Clara, too. Second, @nwdiver points out that it is easier (cheaper?) this way for the electric company when installing transformers in a neighborhood with attached units.

I guess now I’m feeling a bit shortchanged, knowing that my oven could be heating faster and car charging 15% faster had I been on a 240v system. Ignorance was bliss.

 

[Genie]

I guess now I’m feeling a bit shortchanged, knowing that my oven could be heating faster and car charging 15% faster had I been on a 240v system. Ignorance was bliss.

Not worth being bothered, wherever we live the power brought into our homes is what it is and not much to be done about it.

 

[Genie]

Mind if I ask why so many 20A breakers in that panel? I usually see 15A for 120V circuits.

 

[ucmndd]

Mind if I ask why so many 20A breakers in that panel? I usually see 15A for 120V circuits.

It’s my experience that most houses built in the last 20 years or so have bumped up to 20 amp light/plug circuits vs. 15.

RE 120/208, pretty common in North America for multi-tenant residential. 3 phase to the site transformer, then 2 legs fed to each individual unit.

 

[pepperoni]

Mind if I ask why so many 20A breakers in that panel? I usually see 15A for 120V circuits.

I have not a clue. I can see no difference between the rooms on 15s vs those on 20.

 

[terranx]

Not really related to your original post, but I'm noticing that 50A circuit with the wires exiting through the bottom left of the panel - the neutral connection appears to be connected to your ground bus. That should probably be connected to the neutral bus instead? If that's the one going to your garage I suppose it doesn't really matter in the grand scheme since Teslas don't use the neutral. But still might be worth fixing.

 

[pepperoni]

Not really related to your original post, but I'm noticing that 50A circuit with the wires exiting through the bottom left of the panel - the neutral connection appears to be connected to your ground bus. That should probably be connected to the neutral bus instead? If that's the one going to your garage I suppose it doesn't really matter in the grand scheme since Teslas don't use the neutral. But still might be worth fixing.

Yes, I saw that too. I’ve been wondering whether it is worth making a fuss about that. The Mobile Connector doesn’t use the neutral, so in a practical sense it makes no difference to me.

 

[EfficientWatts]

Yes, I saw that too. I’ve been wondering whether it is worth making a fuss about that. The Mobile Connector doesn’t use the neutral, so in a practical sense it makes no difference to me.

Since there are 3 wires you should technically have a 6-50 outlet instead since that uses only 3 wires. There are 6-50 adapters available.

Not sure what this means in terms of code/compliance/insurance/etc but something to keep in mind.

 

[terranx]

Since there are 3 wires you should technically have a 6-50 outlet instead since that uses only 3 wires. There are 6-50 adapters available.

Not sure what this means in terms of code/compliance/insurance/etc but something to keep in mind.

I think it's using the conduit as ground which can be code legal if done right.

 

[EfficientWatts]

I think it's using the conduit as ground which can be code legal if done right.

But the 3rd wire is already hooked up to ground as you noted,, unless you're talking about moving the neutral to the neutral bus and using the conduit as the actual ground in order to "properly" wire it. Can't speak to the code compliance of such a setup (starts to remind me of a 10-x outlet); I guess personally I would feel more comfortable putting a 6-50 outlet on there, but that's just me.

 

[terranx]

But the 3rd wire is already hooked up to ground as you noted,, unless you're talking about moving the neutral to the neutral bus and using the conduit as the actual ground in order to "properly" wire it. Can't speak to the code compliance of such a setup (starts to remind me of a 10-x outlet); I guess personally I would feel more comfortable putting a 6-50 outlet on there, but that's just me.

It's hooked up to ground, but likely the installer meant it to be a neutral (notice it's tagged white).

Or it could be like you're saying and the neutral pin at the plug isn't hooked up to anything - in which case, I agree, switching to a 6-50 would be the best option.