This old NEMA 10-30 receptacle runs to a 2-pole 30A breaker via 8-gauge stranded wire. Can I replace the receptacle with a NEMA 14-50, replacing the breaker with a 2-pole 40A?
(I want to plug in a car charger with a NEMA 14-50 plug that specifies 40A.)
Update: Here is a photo of the panel. I'll have to go back to double-check the wire sizes, but the NEMA 10-30 is being served by the bottom-right-most 30A 2-pole.
OK, this panel has a big problem with adding circuits. This is a "Split Bus" panel employing the "Rule of Six" to avoid the very high cost (at the time) of breakers over 60 amps. It has 6 main breakers and does not have one main breaker.
These have been outlawed, at least in the context of having the sum of breakers exceed service size. And for a very good reason: Nothing prevents service and panel overload except careful adherence to a service load calculation (discussed below), and "nobody" does that when adding additional loads.
So, the first order of business here is to determine how much capacity headroom exists for EV charging -- and if it's not available, load management will be required (again see below).
On the wires to that 30 amp breaker, it looks like the builder (to cut costs and simplify stock) just grabbed a spool of 6 AWG aluminum SE-U cable, and used it for all large loads. Given the age, it looks like the relatively brittle old AA-1350, so don't handle it any more than you need to.
I would put an "extension box" over top of that junction box, then splice to copper wires to land on a subpanel or hardwire EV station. It's a pity the ILSCO Mac Block Connector is so scarce, it's the perfect splice for the job (#6 to <=#6 and reasonably compact). The ground splice can be a bare 2-lug connector (much cheaper).
You cannot do a 14-30 or 14-50 here except by bending the "retrofit ground" rule beyond its breaking point, claiming the SEU mesh was neutral all along, and retrofitting a ground. However, that would be bad news for your Load Calc, since a socket must count as full value and 100% in the Load Calc. You can't adjust charge rate downward per NEC 625.42 or operate on dynamic load management 750.30.
We now return you to my original post.
It's a real problem when people buy the "charger" and then say "well now I need to wire this into my house". In that case, pretty much count on running a whole new circuit.
But it's actually much, much worse than that. Often when you go to run that circuit, of course you are obliged to do a NEC 220.82 Load Calculation on the electrical service... and on houses with old 10-30 sockets, you tend to find there is not enough electrical capacity. So now the "Fallacy Of Sunk Costs" poisons the picture: to support the charger you already bought, you need either a $1000 "dumb load shed device" or a $5000 service upgrade. And people get swept along, thinking "surely any other charger will also need this".
That is simply not true. There are affordable solutions for any electrical panel or circuit, however the choice of "charger" must follow from that.
The panel's capacity must first be considered via a load calculation per NEC 220.82. That is a wall of text, but Sacramento made a really good worksheet out of it. Go River Cats A's! https://www.cityofsacramento.gov/content/dam/portal/cdd/Building/Forms/CDD-0213_Electrical-Load-Calculation-Worksheet.pdf
Next, consider your actual charging needs - and Technology Connections has a video to cover that in detail. https://www.youtube.com/watch?v=Iyp_X3mwE1w
Now if the needs won't fit in the service capacity, we have one more play BEFORE things start getting expensive. That is dynamic load management.
Almost none of the above will play well with random chargers. Specific ones must be chosen - though the selection may be broad (or not).
On the cabling, that circuit is almost certainly 10 AWG wire with only 30 amps nominal capacity, allowing 24 amp charging. It cannot support a charger with a 14-50 plug. However some "travel chargers" are made with selectable plugs or smaller plugs.
Or wall units could be set to a 24 amp or 16 amp amperage as necessary to fit the service and/or circuit.
