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I have a sub panel mounted next to my main panel that is supplied with a 90amp breaker. I would like to add a 50 amp breaker to handle a 14-50 plug for an EV charger.

My question is can the sub panel handle the load without overloading? Other than the pool pump, the rest of the breakers handle outlets and switches.

There are four open slots on the right and three on the left.

I installed a 14-50 receptable using 8/3 Romex and fed it into the box. I paused when I thought I may not have enough amps in this panel.

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    Highly related info on EV charging. Also, what does your load calculation say? (Search the internet on how to do a proper load calc - it doesn't involve adding up the amps listed on the breaker handles...)
    – FreeMan
    Nov 8, 2023 at 17:06

3 Answers 3

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There are five issues here:

8/3 Romex for a 50A Circuit

Look at an ampacity table and you will find that 8 AWG Romex (a.k.a., NM-B) is only rated for 40A. You can use it with a 14-50 receptacle, but only because a 40A receptacle can be used for both 40A and 50A circuits. But you can't use it with a 50A circuit, because 50A > 40A. And you can't use it with 40A continuous for an EV because continuous requires a derate - 40A continuous needs a 50A circuit. So if all the other issues are resolved and you still want to use a 50A circuit/40A actual (whether 14-50 or hardwired) then you need to change to either 6 AWG NM cable (good for up to 55A) or 8 AWG individual wires (e.g., THWN). Wires have to be installed in conduit, which is easy enough for a receptacle right below the panel - just connect it to the panel with a nipple.

14-50 plug for an EV charger

No. Just no.

Install a hardwired EV charger. That has a bunch of benefits:

  • Eliminates a common point of failure - the receptacle. Many cheaper receptacles, even though UL/ETL-listed, simply don't handle the continuous (several hours at a time) load of EV charging very well.
  • Removes the GFCI requirement. While not in place in all jurisdictions yet, the latest NEC requires GFCI protection for 240V receptacles. That is not required for a hardwired EV charger because that already includes built-in GFCI protection. In addition, the built-in GFCI protection includes an automatic reset feature, which is (by definition) not the case with an expensive 240V GFCI breaker.

50A (40A actual) Circuit

Most people do not need that large a circuit for EV charging. It gets complicated, but basically most people can charge daily on a 30A (24A actual) or even 20A (16A actual) 240V circuit. That lowers the impact on the panel and the utility service, which can make quite a difference.

Panel Load Calculation

You need to do an actual Load Calculation based on all the area served by the panel (i.e., the area served by general purpose receptacle and lighting circuits) and all specific loads (pool pump, HVAC, cooking equipment, etc.) served by the panel. That will give you a number. Subtract that from the feed to the panel to see how much is available to add to the panel for EV charging. It might be 50A. It might be a lot less.

Service Load Calculation

Do the same type of calculation for the main utility feed (i.e., the main panel). See how much that leaves for the subpanel. It might be 90A (and then the Load Calculation for the subpanel is what really matters) or it might be a lot less, in which case this overrides the subpanel Load Calculation.

If and only if all Load Calculations show that you can spare 50A (40A continuous) for EV charging, then you can proceed to install your 50A/40A charger (preferably hardwired). If the number is lower then you install with that lower number, or you look at load shedding options.

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    Thank you so much for your detailed reply. It definitely gives me a lot to think about. Right after this post, I did realize that I incorrectly stated the 8/3 vs the 6/3 actual cable. I am already looking into a hard wired EVSE charger as in the short time I have read about it, it seems to have multiple benefits versus the standard charger that comes with the vehicle. Reason I decided on a 50A was due to the specs on the back of the charger. I did look into a lower breaker, like the 40A but I did not know enough to go in that direction. Nov 9, 2023 at 3:22
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It seems like everyone who buys an EV gets handed a particular pack of misconceptions by the dealer and by people on the Internet. They almost never think about what you thought about, which is called "provisioning": Making sure the supply equipment has the capacity.

To give you some background info, the whole 14-50 RV-park plug is because early EVs were given "travel unit" EVSEs to allow them to outrange the then-quite-poor Supercharger network. Here's CGP Grey scheming to do exactly that at 11:15 and doing it at 26:14. Tesla did it so everyone copied it. What they didn't do was provide the array of alternate plugs for 15A, 20A, 30A level 2 charging, all of which are fine for 95% of EV drivers who plug in every night.

And an important note on RV parks is they use RV-park grade sockets like the Hubbell or Bryant. Cheaper sockets like Leviton, ELEGRP, Legrand etc. are imagined for ranges which are plugged in once every 10 years and see intermittent loads. Experience is the cheapies tend to melt spectacularly under continuous EV loads. The RV park quality is actually a larger diameter - 2.45" instead of 2.2" (which drives people crazy trying to find cover plates). Note in the video at 26:39 where the socket is a bit small for the hole on the commercial RV stand - cheapie :)

Your house is not a gas station. Recharging at home is not a "transaction" like buying gas or using a DC fast charger (things you do as rarely as possible since it's a PITA), so shift out of that mentality. Home charging is a "state of being" - if the car is home it's plugged in. We call it ABC - Always Be Charging. (Except you set the car's console to only charge during the cheap electric rates, if you have a ToU rate plan).

Of course for that to make sense, charging is best setup to be super easy - a 2-second flick of the wrist every time you come and go. The "travel unit" experience tends to be all about draping cords over and around things, stowing them, handling dirty or wet cords, and that gets old quick. As such, I'm nudging you in the direction of a "wall unit" style that is mounted where most convenient for charging. If you want to get from your current junction box to there, try Legrand Wiremold surface conduit.

Another reason I advise a wall unit is most of them the onboard DIP switches or other adjustment, which lets you make the unit fit for a 15A, 20A, 30A, 40A, 50A or 60A circuit, depending on what your panel can support.

I think the other answers cover most of the other stuff I might say.

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  • Thank you for taking the time to write back. I really appreciate it. All the responses definitely highlights things that I need to be aware of. As someone new into the EV world, I want to make sure I learn as much as I can. So once again, thank you. Nov 9, 2023 at 3:15
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Instead of using a "dumb" plug-in EV charger you can upgrade to a smart EVSE which has current probes you can put on the feed of your panel. That will let the EVSE throttle down the car's current consumption when the other loads kick on. That will make sure that panel isn't getting overloaded.

Otherwise You will have to do a actual load calculation, look at all the appliances on that panel and sum up the listed power consumption in Watts of the devices likely to be on at the same time. Then divide that by 240. If it's less than 35A you are fine.

If it's more then you can very likely set the EVSE to draw less current statically.

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  • Thank you so much for the feedback. I will definitely look into the EVSE. In the few minutes of reading on them I can see how it can benefit and potentially safeguard the vehicle and older homes with aged or revised panels over multiple owners. Nov 9, 2023 at 3:11

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