Can I use 90 amp breaker and #4 wire to connect to two 14-50 50 amp outlets? Or I need two 60 amp separate breakers for those two outlets? Those two 14-50 outlets are for EV car charging. My concern of using one 90 amp breaker is the 50 amp rated outlet could be overrun. I'm planning to use both outlets together. The total amps should be under 90 amp. Could the outlet be overloaded?
Stop the press! The Tesla Wall Connector does exactly that
Leave it to Tesla. They HAVE come up with a way to do exactly what you want. It's called the Tesla Wall Connector. They are designed to have 2 (or up to 16) share 1 circuit. They have datacomm lines between chargers that will make them automagically adjust (video, cued up to 16:16) to prevent overloading the circuit.
Apparently, the Tesla Wall Connector can support everyone else's EV that takes a J1772 connector (with an adapter). Do your diligence on this; but the people saying "yes" sound smarter than the people saying "no". Regardless, there's also the ClipperCreek system that does the same thing.
It's not a perfect solution - it can't prioritize your guest's vehicle over your own, for instance - but it will auto-balance, when EV #1 reaches full, it will increase current to EVs #2 and #3.
The video also makes a drive-by mention of other competitor EVSEs also doing this trick, so that is something to research too. Since you didn't mention any model of EVSE, and we're not a shop-for-me site, that's as far as I can take that.
When wiring these, you bring the 90A cable down to a large (e.g. 6" x 6") junction box, then use any proper wiring method (e.g. #4-14 Polaris connectors) to split out the supply wire to the two EVSEs. Follow the UL-approved instructions in making this connector, including down-breakering e.g. to 60A, or hardwiring if it says that, which it certainly will. Don't defy the instructions and use sockets - or you could create a serious safety hazard. And don't mix EVSEs that aren't matched, and you must use the data communication lines and switch settings.
Can't do it
You cannot put a 50A socket on a 60A breaker. (typo?)
You cannot put 2 sockets on a breaker that equals the sum of the sockets. That's basically the same as not using any breakers at all. With rare exception, breaker size must exactly match socket size.
Also you're not allowed to have two 50A receps on one circuit even if it was breakered 50A.
You need a subpanel
If you want to run one line, you'll need to go to a subpanel. Then, you have two 50A breakers in the subpanel that go to your sockets.
However, "the total amps should be under 90 amps" isn't good enough. You can't run feeders at 100% of their load. The feeder size needs to be 125% of the expected loadm and since you expect to run two chargers at once. So for instance if you have two 40A chargers that take 50A breakers (125% of 40A), then you have 80A of actual load and must run feeder for 125% of that (100A).
Why #4 copper wire? Do you already have that, or are you just trying to chintz out on the cable vs. the #3 you'd need for proper 100A? Because hey, aluminum wire. It's the right stuff for a feeder of this size, Large feder like this has always been the right stuff, and it never had the problems of small branch circuit wiring: the subpanel lugs are aluminum for Pete's sake. (why create a dissimilar metal problem?) It has always been safe even in the old AA-1350 alloy. The new AA-8000 alloy is gold standard. It's also 1/3 the price of copper.
You'll need #1 aluminum wire for 100A, which is readily available in THWN or XHHW, but hard to find in MH and other cable types, so you can just go 2/0 in that case. It's still dirt cheap compared to #4 copper. It will give you some headroom to be able to run other stuff besides just the chargers.
Also in the "don't chintz" out" department, don't shortchange yourself on the panel. Spend a few bucks extra and get a nice big panel so you can add other circuits as you get the inspiration. Because you'll want to do that. People who DIY subpanels tend to also DIY other upgrades later.
But remember - EVSEs are programmable
An EVSE is nothing but a relay and a tiny microcontroller that sends a signal that tells the EV how much current it can safely draw. It is the EV's job to comply with that request. The conversation goes like this:
EVSE: Hey, anyone, I can serve 40 amps. Anyone? Anyone? Car: Oh, hi. 40A, alrighty then. Please turn it on. EVSE: SNAP Power is on.
That first part is not hardwired, it is a "software" announcement. It is possible for EVSE manufacturers to design their EVSE so you can program it to any value you please.
A breaker that serves EVSE's must be 125% of that value. So for instance:
- 40A EVSE -- 50A breaker
- 32A EVSE -- 40A breaker
- 72A of combined EVSE loads -- 90A breaker
So maybe you can use that #4 after all. Still, I'd sell it on Craigslist and get aluminum. Copper is so valuable that it'll probably be a net profit after you buy the Al wire! And then you won't be painted into a corner by limitations. Which is how you got to the "full panel" situation in the first place... why repeat that?
Why be limited by electricity? Electricity serves us, not the other way around. This stuff is cheap.
I myself would run 2/0 aluminum off a 125A breaker (because both are price/availability sweet spots and the 125 is the biggest breaker that will fit in 2 spaces) then run dual 50A breakers to 40A chargers.