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I recently saw this load-sharing product - NeoCharge that automatically shares power between something like a dryer and an EV on one outlet on one circuit. It’s really great how load management seems to be the future of controlling various high-powered electrical devices.

While looking at installing an EV charger, I was thinking about the future. The best current place for an EV charger in my garage is in the back center of the garage, which will work great for cars in the garage. That said, if I have family visiting, they won’t easily be able to charge their future EVs unless we left the garage door open and they parked within like 4 feet of the garage (or we moved cars around as they charge, which isn’t an optimal solution).

This got me thinking about load sharing as it relates to multiple EV charging locations. For example, it would be great if there was a solution similar to the NeoCharge that would allow 2 outlets (one at the back of the garage and one on the side of the garage) on the same circuit in a code-compliant way that would only allow one to charge at a time.

I believe there are provisions in the code for having multiple outlets on a dedicated appliance circuit when the items share a space and couldn’t possibly be used at the same time (i.e. an EV in a garage shop with a table saw or something that couldn’t be used unless the EV was pulled out of the garage), but I’m not sure that helps here.

Along the same lines, I know that there are mechanical switches for generator backfeeds that physically don’t allow the backfeed to be on and the main breaker to be on at the same time, but I was wondering if there’s anything like that for manual load sharing?

For example, some sort of device similar to the HOMCGK2C that could make it so that only one of two breakers could be on at the same time or something like that. So that if I wired an outlet to the side of the garage and the back of the garage (in this example on two different breakers), then when I have family over that need to charge outside, I could flip the switch to the outside breaker, but in normal use, it would power the other breaker for the back of the garage.

Update - to clarify what I realized from @manassehkatz-Moving 2 Codidact's answer is that I'm looking for a solution that doesn't involve 2 EVSEs, but rather involves 2 14-50 receptacles so that the person charging outside can use something like a mobile charger to charge since I don't think it makes sense to purchase a second EVSE for the relatively infrequent number of times that the outdoor one would be used.

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Honestly I would solve that by positioning the EVSE. If there are 2 separate garage doors, "between the doors" is a favored location. Otherwise midway down the side of the garage, back of the door mechanism so the far car can be charged by draping the cord over the nearer car. They would have to unhook the EVSE to leave. Alternately you could screw bicycle hooks to the ceiling joists and flick the EV cord up onto the bicycle hooks to clear the other car. Now the EVSE is close enough to the garage door.

But let's step back and take a bigger picture view.

Laying down the basic facts.

People starting into EVs tend to be pre-programmed with huge misunderstandings about how home charging works. These misunderstandings lead to bad decisions. Some of it comes from experience with other devices, or social media, or social media, or our #1 enemy: every car comes with a free TRAVEL EVSE for opportunity charging on the road, and comes with the plugs found on the road such as the sockets at RV parks for RVs. The thought was (mind you, this was 2011 and there was no charging network) that you could plan travel around RV parks. That's silly now.

So if you want to provision a NEMA 14-50 for guests who might have an RV or EV... go for it. Your idea of a mutually excluding interlock isn't dumb at all, and could be done using "side by side" generator interlocks such as ECSBPK02, QO2DTI or Eaton CHML. No need for silicon tech.

However, we can do better than that. Let's continue with the education, and let me just bang high points so you can skip what you already know.

  • An EV doesn't need 60A, 50A or even 40A. Really. In fact 20A will suffice.
  • Most houses can't handle 50A of additional load! You need to do a NEC Article 220 Load Calculation (not some other method) on your house's service to find out what is possible. The previous paragraph is what makes home EV charging possible.
  • ALL level 1 and 2 charging uses the same protocols and ALL are interchangeable. Tesla only uses a different connector shape, and a simple adapter made of plastic and copper takes care of that.
  • The charger is on the car actually. The EVSE is nothing but a smart GFCI which also tells the car how many amps it can get from this socket right now (through the miracle of 1980s era microcontroller tech).
  • Certain EVSEs can talk to each other and dynamically distribute a single current allocation across multiple EVs. This is huge.
  • The DC fast charging network is quite well-developed actually. Especially for Teslas. Thus, people don't necessarily bring their "travel" chargers with them anymore, so a 50A socket may be useless to them.
  • A charge rate which makes sense at home (e.g. 20A circuit replacing 100 miles in 10 hours; 140 miles in 14 hours) does not necessarily make sense as a "destination charger" for travelers who might need to replenish 200+ miles in a single night.
  • Currently with gas cars, do you stop at the gas station every single evening or morning so you are 100% full at the start of your day? No, you do not do that? Well, how has that worked out? Same principle applies to an EV battery. If you skip a night to let a guest charge, you're probably fine.

Your interlock idea will work, but...

The problem is you are putting a NEMA 14-50 socket on one side of the interlock. And that means you need to provision 50A for that socket in your NEC Article 220 Load Calculation. And that means most likely you'll be getting a service upgrade which is likely to be at least a couple grand - more if it pushes you above 200A service.

But if it does work out, you can simply establish a subpanel with a $30 sliding-plate interlock between two breakers, such as Siemens ECSPBK02, Eaton CHML or Square D QO2DTI. Choose the panel make that fits the interlock - not every manufacturer has a $30 interlock.

But here's the other gotcha: GFCI. If you pull your permit after your state adopts NEC 2020, then you will probably need a $150 GFCI breaker on the RV outlet.

Hardwired EVSEs do not need GFCI protection since they are themselves GFCI receptacles. Think about it. Travel units can't be hardwired, but almost any wall-unit can. This little fact has a big impact on total price since by the time you buy a 50A GFCI and a QUALITY socket, you're most of the way to the cost of a wall unit, which needs neither.

Don't be skittish. Take advantage of Power Sharing tech.

This got me thinking about load sharing as it relates to multiple EV charging locations. For example, it would be great if there was a solution similar to the NeoCharge that would allow 2 outlets (one at the back of the garage and one on the side of the garage) on the same circuit in a code-compliant way that would only allow one to charge at a time.

They're way past that. Two matching EVSEs can talk to each other and coordinate sharing of a single allocation of power. They need to be hard-wired.

...And I gather the root problem is you're still "tire kicking" EVs and want to retain the option to angrily return the car to the lessor and tell them to keep it. So you don't want to sink a lot of cost in house-side infrstructure? Because we get a lot of that.

The answer is that yellow Romex that Alec mentioned at 32:55. Run a common 20A circuit to the garage - you'll use that for lots of things. For now, punch it down to a 240V breaker and use a NEMA 6-20 socket in the standard box. Order a $40 NEMA 6-20 dongle for that free travel EV and the dongle tells the car "20A circuit" (240V implied). You'll charge 140 miles in 14 hours and that's enough for getting comfortable with EVs.

Once you are comfortable, I would advise pulling the trigger on one EVSE which is equipped with Power Sharing technology - Tesla Wall Connector, CrippleCreek HCS, etc. Later if you see the need, you can buy the second one and implement Power Sharing between them. Maybe you can make do with one - if switching cars proves burdensome then get the second one and implement Power Sharing.

Note also that if you buy EVSEs that allow you to program the circuit ampacity... you can simply continue to use the 20A circuit you already wired. Just remove the socket and extend it to a suitable location for the EVSE. Then, build up experience charging two EVs on 20A. For most people, this will work. If it doesn't for you and having to top up at DC fast chargers become burdensome, then simply run a larger circuit and just reprogram the EVSEs. (Unfortunately CrippleCreek and a few others do not provide a way to reprogram, so avoid those).

Position your EVSEs with a thought for guests

Your default idea was to have one EVSE at the back of the garage, positioned to charge either car. A good start, but...

I would be choosy about positioning of two EVSEs (or even one) to allow for the possibility of guest charging, i.e. by running the cord outside. Obviously that favors positioning the EVSE near the garage door or near a side/back door depending on layout. We already covered the part where all EVSEs use the exact same connector except for shape, and cheap adapters take care of that.

As such, if your visitor knows you have an EV, **it is likely they expect to use your EVSE and wouldn't even use a 50A outlet. A lot of people don't even realize those things are travel EVSEs and simply leave them plugged into the wall at home and it would not occur to them to bring it. DC fast charging and 60A-class destination charging is now highly prolific, and there's no real need to limp into a KOA Kampground anymore to charge.

Installing an RV outlet for convenience of guests with RVs or travel EVSEs should only be done if a) your house can support the ampacity, b) you are willing to comply with state GFCI requirements, and c) you expect their guests to bring RVs or their travel EVSEs.

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    If a person has 320A service in a house with gas everything, IE tons of spare capacity, would you be more sympathetic with installing a larger charger on day 1? The gear may cost more but if you're paying for a day or two of labor by an electrician and assistant, it seems wiser to get it done up front? You don't fill your gas tank every night but sometimes you can go top it off in 5 minutes if you feel the need. Why lose that if you don't have to? (Not 5 minutes, but not 14 hours either.)
    – jay613
    Jan 23, 2023 at 15:34
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    @jay613 Oh sure, if you've got the power to spare, go for it. It's a tiny bit harder on the battery but whtevz. My aim is to break people out of the assumption they MUST have that much power, because that makes people afraid to buy EVs. Jan 24, 2023 at 5:53
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Note that with any significant EV charging you should do a load calculation to see how much power is actually available. Just like you can't write a check for a large amount simply because you have blank checks (you need money in the account), you can't install big circuits in your panel simply because you have empty spaces (you need a large enough utility service to power everything).

Share2

There is actually a much better solution. This is a common enough issue, that the EVSEs (which is the official name for what almost everyone calls a "charger") are actually designed to talk to each other and share the load. If they are on a 40A circuit, 32A usable (continuous use derate) then if you connect one car it will get 32A but if you connect two cars then each will get 16A. But even better, if one is at 80% and the other is at 20%, most of the power will go to the 20%.

The key word to look for is Share2. Many brands of EVSE support it, including Tesla and Clipper Creek. Note however that this is EVSE, not receptacle. This is not for using the portable/travel charger cables that come with your car - this is for permanently (or nearly permanently, most can be plug/receptacle connected rather than hardwired, but that doesn't make sense to me) installed equipment on a circuit dedicated to EV charging. But it solves the problem far more elegantly than what I would call "external" load sharing because it 100% avoids circuit/panel overloads and it optimizes for the cars' actual needs.

Smaller Circuits

Yes, 14-50 is considered a "typical" EV charging receptacle. That is because it is standard for RVs. But most of the time, most people don't need that much power. You can put multiple 14-50 receptacles on a circuit, but then you have to seriously worry about load sharing because if they both go full force (40A each) that is a huge 80A load, and your house service may have serious problems with that.

But most people, most of the time, don't need that much. You could install 2 x 30A @ 240V or even 2 x 20A @ 240V circuits. That would put your maximum load at 60A (48A continuous) or 40A (32A continuous) which would be much less likely cause a real problem. 20A @ 240V is actually enough to charge most cars for typical daily usage. You don't need to charge your car in 1 or 2 hours - you need to charge it over 8 to 10 hours. And most people, most of the time, are not starting at 10% battery charge.

The only catch is that if you do this then you need to have the appropriate adapter/cable for your car. Tesla, and many others, have cables available for a variety of receptacles. Tesla rates a 20A 240V (NEMA 6-20) circuit as adding 12 to 15 miles of range per hour, so an 8 hour charge adds 96 to 120 miles of charge - more than most people drive in a typical day.

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    Thank you for the reply! I'm realizing now that I should have clarified that I am referring to receptacles and not EVSEs specifically, for the simple reason that I don't want to have to buy 2 EVSEs just to have a second charing location for guests, so I don't think something like Share2 would work. I was hoping to just have a 14-50 outlet in both locations so that someone could use for example their mobile charger with 14-50 adapter to charge.
    – will74
    Jan 22, 2023 at 15:31
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You don't actually need an interlock for this - just a subpanel.

Have your 50A breaker supply go to a 100A or 200A subpanel.

Install 50A (GFCI since you don't want to use hard-wire EVSEs - factor that additional expense in and you might find a hardwire EVSE with its built-in self-resetting GFCI looks like less of a burden) breakers in the panel to service each receptacle.

If you overload, the breaker feeding the subpanel trips. If you want to put an interlock on the two breakers you can, but you don't need to.

It would also be perfectly reasonable to supply only 30A for "infrequent guest usage."

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  • Oh, yeah that's interesting. Is there some type of provision for a circuit/recepticle that is used very infrequently in a load calc? For this example, or for like some other tyle of appliance in a home that might only get used every 2-6 months or something like that?
    – will74
    Jan 22, 2023 at 19:30
  • @will74 an interlock such as ECSBPK02, CHML, QO2DTI can be put between two adjacent branch circuit breakers. Jan 22, 2023 at 21:27
  • Thank you! It's too bad that my panels are Homeline, which they don't make the same type of interlock for (unless I do another new subpanel and buy one that has the interlock) -- although it sounds like from answers below that I'll probably go another route than the mechanical interlock.
    – will74
    Jan 24, 2023 at 18:43
  • You don't need an interlock, as it says right here in this answer.
    – Ecnerwal
    Jan 24, 2023 at 20:55

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