Setting up a dual charger capable circuit in the garage

My idea is if I'm going to run a dedicated power line from my basement panel to the garage anyway, I might as well make it capable to run two chargers simultaneously, since it's a two car garage and in 3 years it could be possible that both of the cars in our household need to plug in.

I would probably have a sub panel located in the garage with two separate fuses (50A each?) that goes to a NEMA 14-50 plug into which I'd connect the EVSE.

Has anyone here done this already? Anything to look out for?

That should work as long as you've sized your wiring appropriately. But, you could get by with a smaller circuit if you hardwired the units. You need to go to a 50A branch circuit because you're feeding a 50A capable plug, but assuming your talking about installing 30/32A EVSEs, if you hardwire them, you only need a 40A branch circuit for each verses 50A. The smaller wire called for is less expensive. With a 50A circuit, you could hardwire or plug in a 40A unit, though, based on the 125% rule that applies to USA branch circuits for this type of device.

What I need is a beginners guide on to how to do this in a forward thinking way so I don't have to hire an electrician again in only a few short years.

I'm already in thinking 40A EVSE. Are 30/32A enough to max out the i3? What about Tesla (should the long rumored Model 3 appear in a few years as promised)?

Ok so I see the i3 maxes out at 30A currently, but still I'm thinking long-term...

The current Tesla units can use an 80A EVSE (from what I understand), so you'd need a 100A circuit specifically for it. Who knows what later versions will be able to accept. Rumors are they may be considering inductive charging, but likely that's at least a few years away, but probably would still be able to be plugged in to use the current infrastructure. Now, I do not know too many houses that have a spare 100A or more on their panels. Keep in mind that upgrading the service sometimes incurs a higher base electrical rate, even if you don't use it. And, depending on the local transformer and wire, it could end up quite expensive to install as well.

Ok so you've made it clear that I could go too far as well as not far enough.

My house has 200A service, but I suppose I probably don't have a full 100A to spare.

With a 200a service you probably do have well over 100a spare if you charge in the middle of the night when the rates are lower anyway. The job of the breaker is to protect the wire, not ration the electricity. Your total 200a panel can have 400a of breakers, as long as you don't try to max them all out at once.

Find a wire sizing guide on the web (e.g., www.csgnetwork.com/wiresizecalc.html), enter the length of your wiring run, and put in a large enough wire to handle at least 100a as that would give you two medium sized chargers. #4 copper would carry 150a for 25' which is probably as much length as you need to your garage sub-panel.

From your sub-panel you'll probably only run 20' to each charger and #8 would carry 100a.

A Tesla will take 80a, which is a 100a circuit, the remaining 50a circuit can easily charge an i3 or equivalent. So running both chargers flat out at 3am should not be a problem unless you've got some big consumers in your house that are running at 3am.

The key thing is to use large enough wire, error on the larger size since it lowers the line loss. That said, #4 wire is a pain to pull!

Thanks for the tip on the wire size calculator. Yeah, I see what you mean about spare amps now.

FWIW, not everyone recharges late at night! You get home from work, the car's nearly empty, it's baking outside, and the a/c is running at full tilt, you take a shower and have an electric WH (maybe more than one, or say worst case, an electric tankless that may need 80A all by itself), and the electric stove is running full tilt for a nice dinner...dedicating a lot of amps to one big user may just pop your main breaker! Throw in the frig just started, someone turned on your 2Hp disposer, and is running the dishwasher and maybe dryer and washing machine. It's easy to actually need that 200A service and not have room for a 'spare' 80A or more in our plush American homes, especially in one where someone can afford a big Tesla and maybe another EV.

jadnashuanh said:

FWIW, not everyone recharges late at night! You get home from work, the car's nearly empty, it's baking outside, and the a/c is running at full tilt, you take a shower and have an electric WH (maybe more than one, or say worst case, an electric tankless that may need 80A all by itself), and the electric stove is running full tilt for a nice dinner...dedicating a lot of amps to one big user may just pop your main breaker! Throw in the frig just started, someone turned on your 2Hp disposer, and is running the dishwasher and maybe dryer and washing machine. It's easy to actually need that 200A service and not have room for a 'spare' 80A or more in our plush American homes, especially in one where someone can afford a big Tesla and maybe another EV.

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Yes, you can always construct a scenario that will not work. Congratulations you are awarded a "glass half empty".

The OP is Spinball, not "everyone". My sense is that Spinball can do the math for his house today, and again in the future. He has only one electric car today and is just trying to size the wire to run to his garage with an eye on the future.

Boatguy said:

My sense is that Spinball can do the math for his house today, and again in the future. He has only one electric car today and is just trying to size the wire to run to his garage with an eye on the future.

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There might be no "authority" to prevent those with private homes from potentially overloading their main panels by installing too many circuits or circuits that are too large. Do electrical contractors decide how large additional circuits can be, or will they install whatever the homeowner wishes, likely after making a recommendation?

For those of us living in apartments where electrical permits must be obtained before installing our EVSE circuits, building departments decide the sizes our EVSE circuits based on the capacity of our main panels and the appliances being used. A neighbor purchased a Tesla Model S and wanted to install a 40 amp EVSE circuit off of his 100 amp main panel. I had allowed him to charge his Model S on my 20 amp EVSE circuit (i.e., 208 v. @ 16 amps) which he considered to be very undesirable for his 85 kWh battery pack But the Building Department would issue a permit for the installation of no more than a 30 amp circuit after an inventory of all of his appliances, so he is limited to charging his Model S at 208 v. @ 24 amps. Fortunately, he doesn't drive very far on a typical day and really can't because we live on a smallish island, so he should be OK.

A responsible homeowner and/or electrical contractor will certainly obtain a permit, but inspectors and their interpretation of the code varies greatly from one jurisdiction to another!

The overriding concept is that electrical circuit "sizing" is determined by the size of the wire. The purpose of a circuit breaker is to protect the wire (and thus the building) from becoming an electric oven instead of just carrying electricity. Any circuit can be overloaded, but it will be shut down by the breaker to protect the wire. The main panel is just another circuit protected by another breaker.

Rationing power among the consumer is both a design and usage problem. Plug two hair dryers (have daughters?) into the same circuit and you'll likely trip a breaker. Charge your cars at 2am and draw 120a (Tesla + i3) and you're not likely to have much problem on a circuit designed for 150a from a 200a panel. There is a reason that rates are lower at night. Or as janashuanh points out, charge both cars at 2pm on a hot day with everything in the house running and you'll probably trip the main breaker.

It's no different than having a car that will do 100mph. Drive 100mph in San Francisco and you'll go to jail, drive 100mph across east Montana and you'll be passed by another car. Just because it will go 100mph doesn't mean you want to do it all the time.

So who gets to decide what circuits are installed in a private home? The homeowner or the government via building codes? If it is the homeowner, will contractor liability laws allow a certified electrician to install circuits drawing more than the main breaker? Or will only outlaws have outlaw circuits?

Should we as a society trust homeowners to properly manage their own power loads? Think of the children!!! Think of what could happen!!

Most homes have more amperage in breakers than their main one, and that's not a problem. But, one still has to be rational about what your base load is, and the likelihood of not having enough on the main to satisfy your current demand. SOme devices also have a pretty big startup current spike. Run them long enough and eventually, at some point, they will all try to start at the same time...when you get close to the max draw, you could pop your main breaker. Unless you were using a sophisticated fast reading current monitoring device, you'd never know what happened.

Take a modern kitchen...if you read the installation manuals for many of the devices there, they each want their own breaker...so:
- electric stove 30-60A depending on type and number of burners and ovens
- refrigerator 15A on average unless it's a huge built-in
- counter outlets - requires two 20A circuits, or 40A
- dishwasher 15A
- garbage disposal 15A
- lighting circuit 15A
- vent hood 15A

That's 175A at the worst case and it's just one room. Throw in an electric dryer 30A, an electric WH 30A (or more if a high output), central ac (20A or more, more if you have multiple systems), 20A required for a bathroom, and likely more than one.

The point being, 'saving' enough power to go much more than what an i3 requires may take some power management in the home, and trying to recharge a Tesla on an EVSE at full possible rate is a major commitment. Now, throw in your pool pump, and if you're on a well, maybe another 20A (with a good turn-on surge), and maybe you begin to get my point. Most places can accommodate a 40A circuit for an EV. A lot of homes cannot support much more. My sister's home is in great need of a panel upgrade, and has trouble with what's in the house now, let alone trying to put in an EVSE!

Some things in your home run periodically 24-hours (refrigerator, heating/a-c, maybe electric WH if that's what you use). Then, there are a lot of small loads from those things that never turn off (most tvs with a remote and other similar appliances, but those don't add up to much). How often do you have every light in your home on? Maybe more often if you have children, but it all adds up. Then, consider the 80% rule...you should not have a constant load that is more than 80% of the breaker protecting it. Shorter duration power draws are fine. A vehicle's EVSE, though, counts towards that 80%.

People want bigger batteries for longer range...few think about how they are going to get the power to do that, or if they use what they have available, how long it will take! One reason why I think longer range vehicles will tend to be hybrids...it only takes minutes to 'recharge' it with gasoline. Unless Tesla fields their battery swap tech, the fastest you can refill one of those approaches an hour. That puts a big crimp on covering lots of miles in a day since you have to stop where the charging stations are, not when you would like to, pretty much like you can with a gas station. Some people like to drive off of the interstates - often more interesting things to see and do there. Unless it is REALLY popular, you're not going to find a Tesla Superstation around. Lots of luck with long distance on an EV in that situation. A hybrid with a big battery makes a lot more sense...stay EV most of the time, but use the ICE to augment it or take you the rest of the distance without major delays.

jadnashuanh said:

The current Tesla units can use an 80A EVSE (from what I understand), so you'd need a 100A circuit specifically for it. Who knows what later versions will be able to accept. Rumors are they may be considering inductive charging, but likely that's at least a few years away, but probably would still be able to be plugged in to use the current infrastructure. Now, I do not know too many houses that have a spare 100A or more on their panels. Keep in mind that upgrading the service sometimes incurs a higher base electrical rate, even if you don't use it. And, depending on the local transformer and wire, it could end up quite expensive to install as well.

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Inductive charging is not so far away; in fact, it is here for some vehicles. I inquired about the i3 a couple months ago, and they said it was in the works but not ready yet.

https://www.pluglesspower.com/

I've been gathering quotes for 100A sub panel with two 50A circuits ending in 14-50 outlets. Still deciding on a EVSE.

as usual Boatguy seems to give great guidance. I would only add most Tesla owners use a 50 amp circuit. Even though ours will charge at 80 there isn't the need. 50 amps is fine. with a 280 range we simply haven't seen the issue to spend all the money to upgrade.
Our garage has a 100 amp main so i installed a 50 amp for the tesla and a 40 amp for the i3, only because I had a 40 amp breaker and wire. The moment Tesla ships the model 3 and I can get rid of the i3 I will switch the 40 amp circuit to 50 amps. Even with all the problems of the i3, it is a handy little commuter car. If we are out running around and burn thru %80 of the batteries in the morning, it only takes 3 to 4 hours to charge it back up with a good charger, I use the Clipper Creek model 40. Now it looks like Washington state will not be getting and fast charge, DC public chargers reliable home charging is key. In the last three weeks we have had the need for public charging of the i3 as we were away from the house. Of the fifteen or so charging stations we attempted to use, twelve or so of the J1772 chargers were not in operations, the Chademo chargers were fine and Leaf cars were coming and going. BMW seems to blown it in picking the combo dc standard. Even if they help Charge now put in 100 from Portland to San Deigo, its little help.