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60A or 100A circuit breaker? Is a 100A really adequate for charging two Tesla cars?

60A or 100A circuit breaker? Is a 100A really adequate for charging two Tesla cars?

Should I install the wall connector on a 60A or a 100A circuit breaker?

Currently, we only have a Model X 75D, whose charger capacity is 48A. So a 60A would be adequate.
But installing the 100A only costs ~$200 more than the 60A. So I'm thinking what's the advantage of a 100A over a 60A.
The obvious advantage is that 100A would be useful if we have another Tesla car with larger charger capacity, say MX P100D (72A) (but not likely in my family's case).

And one of the electricians told me that charing 2 Tesla cars would be easier with a 100A circuit breaker. It sounds about right at first, but I thought about it twice and came up with some doubts:

0. Let's say our second Tesla car will be a long range M3 (charger capacity: 48A).
1. If we want to charge the 2 cars at the same time, the combination charger capacity of our 2 cars will be 48A + 48A = 96A. Which means that a 100A circuit breaker won't be safe.
2. If I daisy chain 2 wall connectors, would I be able to take the advantage of the full 100A (80A for safety)? (where can I find more info about daisy chaining 2 wall connectors?)

Forgive my ignorance about electricity, correctly me if I'm wrong.

bob | 20 augustus 2018

If you use two HPWCs you can interconnect them. When interconnected it will only allow one to be active at any given time. We have a MS and MX set up this way. The MS is typically used for around town errands (max 60 or 80 miles a day). MX I use for business and can easily do 200 miles a day a couple times a week. We only have 100 amps to the whole house (1972 vintage home). Cost to up the service to the house is non-trivial. So I've got a 50 amp breaker hosting 2 HPWCers. The MS is set to start charging at 12:05 am (cheaper rates) and the MX is set to start at 1 am. Most nights the MS is done topping off to its set 70% capacity before 1 am. But if not the MX will gracefully wait for the MS to finish before it starts charging.

wang5150 | 20 augustus 2018

The master HWPC is set to 80 amp max for the 100 amp line. It will use the 80 amp available to divide up between the 2 HWPCs.

I have an 100 amp line connected to 2 HWPC that are daisy-chained. I have a Model X 75D with 72 amp charger and a Model 3 with 48 amp charger. I can hook both up at night to charge at the same time. The HWPC will auto-throttle the max of 80 amps of charging amps between the 2 cars so if the model 3 finished charging sooner, it would charge the Model X at the full 72 amp available.

Hope this helps!

jjgunn | 20 augustus 2018

https://forums.tesla.com/forum/forums/tesla-wall-charger-model-3-what-am...

Found a thread that can help. Post below is from 9-10 months ago. If you're planning on charging 2 cars at same time - you're going to need a bigger boat....err umm I mean more AMPS :-)

https://www.electriciantalk.com/f9/ev-sub-panel-neutral-size-no-ulnbalan...

especially page 2 - don't cheap out if you're hooking 2 HPWC's to the same circuit
https://www.electriciantalk.com/f9/ev-sub-panel-neutral-size-no-ulnbalan...

shaneqi | 20 augustus 2018

@wang5150

If I want to let a electrician do the wire and the first wall connector. Can I just install the second in the future by myself? Without an electrician, without new permit? I assume there will be configuration adjustments needed, can I do that also myself?

wang5150 | 20 augustus 2018

@shaneqi, I originally only had 1 installed and had the electrician come back later after I got the Model 3 to install the second. They didn't need to pull permit and was much easier/cheaper for the second install.

They had to connect a communication wire between the 2 HWPCs and configure the master HWPC to work with the slave unit. I assume someone with minor electric work skills can probably manage this just fine but I chose to have a professional do the second install to make sure it was working properly with the correct conduits, wire gauges, etc.

shaneqi | 20 augustus 2018

@wang5150 makes sense, thank you so much for sharing!

bob | 20 augustus 2018

@shaneqi first, admittedly I know enough to be really really dangerous... I wired my own. The instructions are pretty easy to follow. It's just a matter of setting some dip switches on both units and wiring the interconnection cable (I think that was an 18 GA wire - can't recall off the top of my head). The manual specs the wire size for the interconnection. It's basically a network cable between the two devices so they can talk to each other.

shaneqi | 20 augustus 2018

Wow @bob, you’ve got a pair of handy hands there. Thanks for sharing!

I’ve decided to go with 100A wall connector. And care about the second wall connector later.

bp | 21 augustus 2018

With the current battery packs (75/100 for the S/X and smaller for the 3 SR/LR), you should be able to charge overnight using Tesla's 14-50 connector (providing up to 40A) or a Gen 2 UMC (providing up to 32A).

At least for overnight charging, it usually won't be required to charge at more than 40A.

Charging faster should only be needed when a lot of driving is done during the day, and a quick charge is needed to do additional driving for the rest of the day (typically only needed for road trips - and that would be done on the supercharger network - not at home).

justin.rogers77 | 21 augustus 2018

We have a MS 75D and X P90D and charge both with the UMC on a 50amp circuit, up to 40amps when needed, each car has a 70 mile daily commute and never have a problem charging both each evening.

TeslaTap.com | 21 augustus 2018

Another advantage of going with 100 amp circuit to begin with, even if only charging at 48 amps is the wires will remain cooler and you'll save a tiny bit of electricity every time you use it.

davidahn | 21 augustus 2018

+1 on 100A circuit, dual HPWCs. I did not know about the "master" "slave" arrangement, that's pretty cool. Sounds like it has the smarts to throttle you if you set both vehicles to 80A. I wish the mobile connectors would throttle you instead of giving you an error if you ask for more than 32A.

Rocky_H | 22 augustus 2018

@shaneqi, First off, there is no safety concern to any of this, no matter the capabilities of the cars. You are adding up things like 48A + 72A, and seeing that it is over 100, and thinking it’s going to blow things up. That’s not how that works. First of all, to explain how this works, I need to address this:

@bob, Quote: “If you use two HPWCs you can interconnect them. When interconnected it will only allow one to be active at any given time.”

No. One at a time is definitely NOT how that works. OK, now back to @shaneqi’s issues—you are stating it as if the car can ONLY charge at its maximum level of 48A or 72A. That’s not true. They can charge at lower rates, and they do, with these linked wall connectors. It’s a requirement that they can only feed a continuous current of 80% of the breaker rating, so 48A from a 60A circuit or 80A from a 100A circuit. But the wall connectors will continue to adjust the current flow between the two cars to keep the total below the total limit. So even if the cars have maximum capabilities of 48A and 72A, the two connectors on a 60A circuit may be allocating 20A and 28A to the two cars to make sure it stays within the proper limits.

Even with a 60A circuit, that should still handle two cars pretty well for overnight charging, but a lot of people would prefer 100A.

shaneqi | 22 augustus 2018

I got two quotes from electricians. They both have all the pictures they asked.

(Tesla Wall Connector's cost excluded)
One is $3300 for 100A (because of upgrading the house from 200A to 300+A), $600 for 60A.
The other is $1500 for 100A, $1100 for 60A. (this electrician didn't say upgrading the house is needed, even they've seen my panel's picture).

I thought our house wouldn't need upgrade in order to install the 100A charger, because it's a new built house, the builder should've considered things like EV charging. So I wanted 100A, since it only cost ~$200 more than 60A.

My current and the only plate is a 200A one. (https://imgur.com/a/FWWUUlz)
Do you guys think installing a 100A charger on a total 200A panel is safe?

After the electrician told me 100A charger would require upgrade of the house's electric, I realized that if charging cars takes 80A, AC takes 40A, and add up oven, stove, etc, it would be dangerous when a lot of machines are running at the same time. Or at least the breaker would be tripped. Did I do the math wrong?

If that's correct, I guess I will go with 60A, because it's safe, and way cheaper comparing to upgrade the house's electricity.

@Rocky_H, thank you for the explanation, it dispelled my concerns of having 60A, I think it will work fine.

khanhvn | 22 augustus 2018

AFAIK you don't need 300A panel to support 100A charging circuit, especially if you charge the cars at night when most of the appliances are not running. My panel is 200A and I installed 60A charging circuit and now regret that I didn't go higher to 100A or at least 80A.

I'd suggest at least run 80A or 100A wire for future proof. You can use 60A breaker for now, but if you change your mind later then just change the breaker which is much cheaper than rewire the circuit.

Rocky_H | 23 augustus 2018

@shaneqi, Let me show some quick ballpark numbers of why a 60A circuit probably still works fine for two cars. As a reference point, the Tesla site says a Model X at 48A will get about 30 miles per hour of charging from that. For an 8 hour overnight, that is 240 total miles you could refill between the two cars. Do you really run both of your cars down that low that they would need more than 240 miles refilled in a night? And probably one of them could get in 10 or so hours of charging if it's home through some of the evening before the 8 hours overnight.

And you have more than just the two choices of 60 or 100. You could go with 70 or 80 amp if they would fit within your calculation.

The other thing you could check is with the load calculation. Electric code allows in a load calculation that you can designate two large loads to be time-offset and then only count the larger of the two instead of adding both. So if you are going to run your electric car charging after midnight, the oven won't be running after midnight, and you could just count one of those. So then you may be able to count a 70 or 80 amp circuit for charging if the 50A oven load is not having to be counted in addition to that.

bob | 23 augustus 2018

I also question if charging with lower amperage vs 72A is better for the battery life? Logic being it should be easier for the cooling system to keep the battery temp down with a slower charge. We know (according to Tesla) that constant Supercharging isn't good for the batteries. Isn't it logical to think that 'trickle' charging is better than fast charging. If so, It becomes a balance of how much time the vehicle will be idle vs the average charge needed.

Also @ Rocky_H switching off between chargers is how I've seen our system work. My guess is that might have been when we only had 30 amps charging 2 vehicles. But I'll also admit that typically I'm asleep while the cars are charging I'm not in the garage watching the green LEDs on the HPWCs. I do appreciate your clarifying my earlier statements.

shaneqi | 23 augustus 2018

Here's the calculation from the electrician that said that the 200A panel needs to be upgraded for 100A charger:

https://imgur.com/lSqdQ3x

I don't 100% understand the calculation, the result is that the total service load after installing a 100A charger is 268A, while it's 204A for 60A charger. (my only panel is a 200A one)

I think the calculation is a little bit conservative, it added up HVAC, oven, dryer, and the charger together (even with an additional 25% of the charger). It's not likely we would ever run those appliances at the same time.

But on the other hand, this calculation seems like an 'NEC standard' that the city inspector will run during the inspection. Which means the electrician and I don't wanna break it.

I'll go head asking the electrician that if 100A would break city code and if 80A or 70A is an option.

Rocky_H | 24 augustus 2018

@bob, Quote: “I also question if charging with lower amperage vs 72A is better for the battery life?”

That will make no difference at all from the battery’s perspective. Remember the scale of power we’re talking about here. At a Supercharger, they run at 100kW+, and when the battery is getting full, we whine and groan about how very very slowly it’s going when it’s down to 30kW and 20kW as it is tapering down the power level.

How much power is that 72A home charging? It’s only 17kW. All home charging from AC circuits is so very low power that the battery pack won’t care. So don’t bother to look toward the battery, but maybe look toward whether you are running any of your wiring or charging equipment near their maximum current levels. It might be a little beneficial to them to be turned down a little from their maximum rating to keep them cooler and be a little less stress on their components.

jjgunn | 25 augustus 2018

This thread has some magnificent information. Everyone should read this thread.

jpcollins9 | 27 augustus 2018

While I have 200 amp service to my house, I've an auxiliary box in my garage with 100 amp service (fed by the 200 amp main box) that primarily handles the washer/dryer and some kitchen appliances. I've two 240/50 amp charging plugs for my two Teslas. I stagger the charging times and both charge daily. If the first hasn't finished charging when the timer for the second wants to start, the outer ring for the second car will turn red until the first is finished then will start charging. It is about as low hassle as it can be.

khanhvn | 29 augustus 2018

@shaneqi
Looks like your electrician's calculation is wrong. You need to add 25% extra amp for the largest motor (NEC 430.25 code), but the Tesla charger has NO motor. The car has a couple of motors, but the charger DOES NOT. And the car motors are not running when parking I guess :)

I'd suggest to find another electrician for a second opinion.

khanhvn | 29 augustus 2018

Another mistake on line 13: the max load of the charger is NOT 100 Amp. It should be 100A x 80% = 80A max load on a 100A circuit. So he over-estimated your max load by at least 45A : 25A (wrong NEC 430.24) + 20A (wrong max charging rate)

shaneqi | 29 augustus 2018

@khanhvn thank you for pointing out such tiny mistakes from the calculation!

dysert2 | 30 augustus 2018

I have installed two daisy chained Tesla wall chargers for charging my MX p100D and MS p100D. Cost me an arm and a leg to do it. Required a new feeder line from the pole, a new mast, and a new 3 pole meter. Separated the main existing 200 amp panel from the sub panel and upgraded the sub panel to a standalone 200amp panel and it has the 100 amp breaker and surge protector for the two Tesla wall units. I can charge both cars simultaneously and the designated master Tesla unit manages the charge going to both cars simultaneously. If I am only charging one vehicle it will charge at the max rate. Both the S and the X get around 40-45 miles per hour of charge, For most folks this is overkill and probably more than we needed as well but since this was all new to us, we wanted maximum efficiency for charging. We charge both cars to about 80% capacity and usually let them go to 15-20% before recharging. The system works great. Max charge time for a single car the way we drive is 3-4 hours once per week or so. Both cars at once overnight easily and faster if we don’t let them go below 25-30 %.

alpeshpatelmd | 30 augustus 2018

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