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Electrician confusion with 14-50 outlet

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Hey folks - I'm waiting on my model S to be delivered so figured I'd prep by getting the nema 14-50 outlet installed ahead of time. I called several electricians but it appears that they strategy for installing is inconsistent.

I have a main panel that has a 125A breaker that feeds to subpanel, and a 40A for AC. 2 electricians said they could install a 50/40 quad breaker over the AC breaker, while a few other said they could not and had to pull from the subpanel. Does anyone know why some would suggest it but others don't?

Attached are some pics of the main panel and subpanel. Note that I plan to charge only at night when family is sleeping with low load on the power supply.
 

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...40A for AC. 2 electricians said they could install a 50/40 quad breaker over the AC breaker...

The service disconnects say 125 Amperes and 40 Amperes.

I assume the electrician is talking about the 40 A circuit. If so, they should not place a 50 A breaker there (the circuit for NEMA-14-50 is 50 A, not 40 A. The maximum should be 40 A.

It seems that you have a panel that has 2 big empty slots that looks like a big rectangular hole at the bottom left and another 2 big empty slots that are covered at the bottom right. It seems that the panel is 125 A. If the city and the electricians can calculate the load and think it is safe and not overloading the 125 A circuit, a 50 A breaker should be easily installed there for your NEMA-14-50 outlet.
 
Right, I was wondering if you could load a 50A onto that 40A breaker for AC unit. 2 electricians said it was totally fine, so was thinking sure why not but others said no so it's really confusing.

Note that for the other panel, those empty spots don't have anything behind it. I don't think it can attach to any poles since it's just empty space if that makes sense.
 
Electricians are cool like that in that they all come up with different answers to the same set of conditions.

I see no logical reason why you couldn’t do a 50/40 quad breaker in place of your 40 amp AC breaker if that’s a more convenient place to source the load. But I’m also not an electrician. So, you know, take that for what it’s worth.
 
Your two main service breakers, I believe, are fixed mounted and not replaceable with quad breakers. I'm not sure 50/40 combos are even in existence. Also, GFI type breaker may be required for EV charging in your area. Best bet is to have a load calc done on paper (or logged with a meter) and use the space in the panel for your EVSE.
 
Full-disclosure: not an electrician, just asked questions while having work done to main panel of two homes to add NEMA 14-50.

I would imagine replacing your main panel double-pole 40 with a double-pole 40 / double-pole 50 quad breaker (they do seem to exist) would be dependent on if the wire size between your main panel and the meter is rated for the increased load...

...but your main panel setup already has me confused since I was always taught (but never actually verified if it is required by code) that a panel has to have a master-cutoff-breaker that sits between the meter and everything on the panel, so I would have expected a 175/200A breaker overseeing your AC and subpanel breakers.
 
If the 40A breaker is for your AC, then you will have to do without AC if you want to use it for your car. EV charging requires a dedicated circuit.

So then the 125A breaker services your main panel, and you appear to have room to install a dual-pole breaker. The 125A service is not sufficient to install a 50A circuit for your car. Assuming you plan to use the Mobile Connector (MC) then a 50A circuit it not needed as the MC will max out at 32A when charging thus requiring a 40A circuit. Install a 40A breaker and a 14-50 outlet, and be use to mark the outlet as 40 amps. This is allowed by Code.
 
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BTW, even if they make 40/50 quad breakers (which I assume they do) you cannot use it since the breaker will need to be GFIC. The only way I know of to get around this with a 40/50 is to install a Wall Connector instead of a 14-50 outlet. With the Wall Connector you will charge at 40A on the 50A circuit.

Also, if you go forward with an outlet consider a 6-50 instead of a 14-50. EV chargers do not utilize the neutral wire so you can save yourself some money by going with a 6-50 outlet. Whatever outlet you choose, make sure it is commercial grade!