Dual HPWC thoughts

chuckd said:

Since getting our P85D, my Mercedes (and my other ICE cars for that matter) just feel clunky and noisy. So I'm pondering trading one out for another P85D or 'maybe' a model X.

Problem is, my garage has a 100 amp service (separate garage structure), and getting additional power would be a huge pain. My current HPWC is on an 80 amp circuit, and limited to 60 amps with the dip switches.

If I add another HPWC, I'd have to limit both chargers to something like 40A. That just adds more time!

What do you think if I simply set the charge schedules on both vehicles to significantly different times, so that they won't fire up together? Think this can work? I realize that having two 80 amp breakers in a panel with a 100 amp main is probably not the greatest idea though.

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Strangely enough, code has something to say about this. I'm sure FlasherZ will give you the chapter and verse, but I believe you are not allowed to have two 80A breakers with permanent connections to 60A continuous load consumers (why 60 and not 64? 80% continuous load is permissible) in a 100A panel. From the way my electrician explained it to me, code allows to assume alternating use on outlets, but not on permanently wired consumers. But I haven't verified that this is true

After talking to my electrician (I'm in the same boat, 100A panel in the garage) I now have two chargers, one 40A, one 32A in my garage. This allows both the Leaf and the Model S to charge. I still schedule them at different time, just out of an abundance of caution. Once the Model X replaces the Leaf (and the ICE minivan) this should still be sufficient. The cars usually are parked at least 8h over night. So that allows for > 70kWh on one charger and >60kWh on the other. There's no way we'll drive that much. Currently we tend to charge about 20-25kWh over night for the Tesla and 12-20kWh for the Leaf.

chuckd said:

...I realize that having two 80 amp breakers in a panel with a 100 amp main is probably not the greatest idea though.

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Agreed, because of safety concerns. Can you have one 80 A at 240 V for one of the HPWCs, and one 20 Amp at 240 V for the other? Then, put the new 80 A HPWC at a place where both Model S cars can charge quickly from it if needed (not simultaneously). The other Model S can just charge at the slower rate.

chuckd said:

Since getting our P85D, my Mercedes (and my other ICE cars for that matter) just feel clunky and noisy. So I'm pondering trading one out for another P85D or 'maybe' a model X.

Problem is, my garage has a 100 amp service (separate garage structure), and getting additional power would be a huge pain. My current HPWC is on an 80 amp circuit, and limited to 60 amps with the dip switches.

If I add another HPWC, I'd have to limit both chargers to something like 40A. That just adds more time!

What do you think if I simply set the charge schedules on both vehicles to significantly different times, so that they won't fire up together? Think this can work? I realize that having two 80 amp breakers in a panel with a 100 amp main is probably not the greatest idea though.

Click to expand...

I wouldn't do it....but I'll let other more expert folks chime in.

Why not just add a 14-50 outlet and use the UMC for one car? You'll save some money, do it safely, you'll still have full 'overnight' charging available on both cars and you can use the HPWC for either car whenever you need a faster charge (assuming you have dual chargers).

If you think you can sequence or behavior manage the loads effectively, just add a load shedding control to break the pilot signal in one of the HPWCs in the event you exceed your feeder capacity. This way, if both started to ramp up to max, one would be cut off. Here is an example of a load shedding control: GeneratorLoadSheddingController The generator aspect isn't applicable to you, but the operation is the same. Your feeders are your limiting factor instead of a generator supply rating.

FWIW If I was in your shoes, though, I would just investigate the cost of getting new feeders to your garage. It might be cheaper than you think.

It is legal to size the feeder for use by only one at a time, if you are configuring the cars to charge at different times.

HPWC's are considered installer-configured, so you must use the loads configured in each HPWC. Even if you set the cars to a reduced amount, it's the HPWC configuration that matters when it comes to code. This means that an HPWC capable of 80A charging must be figured at 100A (125% continuous load rule) in calculations.

Article 220 covers feeder sizing for subpanels. 220.40 requires that the feeder must not be less than the sum of the loads. However, 220.60 says "where it is unlikely that two or more noncoincident loads will be in use simultaneously, it shall be permissible to use only the largest load(s) that will be used at one time". This means that if it is likely that you will only charge one at a time, it is ok to put 2 100A loads on a 100A feeder (EDIT: providing no other loads on that panel).

chuckd said:

So you're saying that right now, I could probably put in a 100A circuit in that panel and boost my HPWC up to 80A currently (that is, with only one car/charger)?

Not sure what would happen if my garage door closes during charging. That might get close to the max.

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Sorry, I edited to clarify the post above. That assumes no other loads on that panel (in my garage, the home's service panel handles lighting and receptacle loads, while a special sub-panel handles the HPWC's + NEMA 14-50 for charging).

If you want to use the general lighting & accessory outlet method (220.12), you need 1A per every 40 sq ft, then general-purpose receptacles are included in that figure.
If you want to use the small appliance and laundry loads method (220.52), you need 12.5A per 120V receptacle circuit.
If you want to use the specific load method (220.14(A)), you need to add up the loads listed on the appliance nameplates (garage door opener, common tools, etc. that would be operating while charging).

In your case, a 100A feeder to the garage should not have an HPWC configured for 100A circuit (80A charging) because you have to account for other loads. It would probably work without tripping the breakers but wouldn't be code-legal.

I have a variation of this in my Boulder garage. There, I have a dedicated 200 Amp, 240 Volt, Split-phase panel for car charging. The outlets, lights, etc in the garage come from another panel in the house. The charging panel contains three breakers, a 100 Amp breaker for an HPWC set at 80 Amps, a 90 Amp breaker for a Roadster HPC that can pull 70 Amps, and a 50 Amp breaker to a 14-50 that can pull 40 Amps. Normally, I leave the Model S connected to the HPWC set at 56 Amps, the Roadster set at 40 Amps, and a UMC on the 14-50 could draw 40 Amps. Normal usage is then 56 + 40 + 40 = 136 Amps of 160 Amps that I could draw from the panel. If I ever want to increase the Roadster or Model S draws, I check what the other chargers are doing. Because the three loads are in adjacent garage spots, the coordination is pretty easy. I have raised the Model S to its full 80 Amps and used all three ports at once at a full 160 Amps on the panel. At 160 Amps, the panel has a nice 60 Hz hum going, but no problems after an hour long test.

It looks like this configuration and article 220.60 mean this is even a reasonable install according to code...

Cottonwood said:

I have a variation of this in my Boulder garage. There, I have a dedicated 200 Amp, 240 Volt, Split-phase panel for car charging. The outlets, lights, etc in the garage come from another panel in the house. The charging panel contains three breakers, a 100 Amp breaker for an HPWC set at 80 Amps, a 90 Amp breaker for a Roadster HPC that can pull 70 Amps, and a 50 Amp breaker to a 14-50 that can pull 40 Amps. Normally, I leave the Model S connected to the HPWC set at 56 Amps, the Roadster set at 40 Amps, and a UMC on the 14-50 could draw 40 Amps. Normal usage is then 56 + 40 + 40 = 136 Amps of 160 Amps that I could draw from the panel. If I ever want to increase the Roadster or Model S draws, I check what the other chargers are doing. Because the three loads are in adjacent garage spots, the coordination is pretty easy. I have raised the Model S to its full 80 Amps and used all three ports at once at a full 160 Amps on the panel. At 160 Amps, the panel has a nice 60 Hz hum going, but no problems after an hour long test.

It looks like this configuration and article 220.60 mean this is even a reasonable install according to code...

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The only exception is that you must treat your HPWC as 100A and your HPC as 90A if they're configured to 100A and 90A circuit values, regardless of what you normally set the car as; the NEC does not permit user-configurable loads, only installer-configured loads. But you're legal in a sense that your HPC + HPWC won't exceed the 200A value, if you don't anticipate using your NEMA 14-50 at the same time as them both.

Ok we are talking extreme what if's!!! Simply designate one car as primary and the other car that charges at between 3am to 7am. The primary car can plug in anytime prior to 11pm and suck down maximum power to be recharged.

In extremely rare off case where the primary car will charge outside of this time frame, you can modify the HPWC with a switch to over-ride the internal dip switch to charge at 40amps until morning!!!!

As an example of NEC 220.60, I have two HPWCs installed, both on 100A breakers for 80A charging. They're installed in a sub-panel with a 200A main fed with #3/0 copper wire. So, definitely sufficient power to run them both simultaneously. However, that is the max allowed with just those two things running, technically. There are other loads (all smaller 120V loads) on that panel. Since my local inspector is pretty reasonable, we agreed that NEC 220.60 applied for the purpose of calculations for the other loads in the panel. YMMV on this, however, depending on your inspector, and honestly with two EVs and two chargers if I were an inspector I probably wouldn't let 220.60 fly, especially if the panel main wasn't at least the sum of the two charger breakers. In my case I was able to show the inspector the average load on that panel without the HPWCs, and even with both running simultaneously it would only be a few % above what the NEC would normally allow (all small 120V loads for one section of the house), and for likely only a short time.

I personally would not try to use 220.60 to install two HPWC beyond the capacity of the panel, however. In the OP's case, one HPWC @ 100A (80A charging) + other loads would be illegal per the NEC, but depending on the other loads two HPWC on 80A breakers (60A charging) might be legal using 220.60. However, violating 220.60 in this case (running both at 60A) would pop the sub-panel breaker. Basically, any configuration that easily allows a user to trip a large breaker is a bad idea, IMO, even if it is technically legal. Again, though, with two EVs and two chargers I personally don't think 220.60 should apply in most cases anyway.

Also, keep in mind that while sitting parked and plugged in the Model S will occasionally engage the HPWC and top off the battery, especially if it is very cold or hot in the garage or wherever the cars are. And, it is possible this happens for both cars at the same time. I've had my P85D and my fiance's P85 start charging at nearly the same time to top off their packs after sitting for a couple of days. So while my configuration can handle both running at once without tripping the 200A breaker, technically shouldn't be doing so per NEC 220.60. If in the example above (two HPWC on 80A breakers on a 100A subpanel pulling 120A) the panel breaker would have tripped.

*shrugs*

My two cents: If you're going to have two HPWCs, you might as well make the effort and put out the money to get a dedicated 200A panel for them and run them both on 100A breakers.

If you set both HPWCs to 40A you can still charge both cars fully overnight so why worry?

billarnett said:

If you set both HPWCs to 40A you can still charge both cars fully overnight so why worry?

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Yeah, can't one not recuperate charge quickly enough w/both at 40 amps?

Are both cars actually being driven THAT much daily that it's an issue?

Charging both overnight at 40A would be an 80A draw from the panel with a 100A main. Since both will be charging at night at the same time (since charge time will be longer for sure) there is no way the NEC 220.60 exception would apply, and the panel would be at max capacity with just the chargers before adding in any other loads. So, this wouldn't legally work for the OP either.

wk057 said:

Charging both overnight at 40A would be an 80A draw from the panel with a 100A main. Since both will be charging at night at the same time (since charge time will be longer for sure) there is no way the NEC 220.60 exception would apply, and the panel would be at max capacity with just the chargers before adding in any other loads. So, this wouldn't legally work for the OP either.

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Why is it ok for you to have two 80A loads on a 200A panel but not ok for the OP to have two 40A loads on a 100A panel? (I'm assuming the other loads are trivial.)

billarnett said:

Why is it ok for you to have two 80A loads on a 200A panel but not ok for the OP to have two 40A loads on a 100A panel? (I'm assuming the other loads are trivial.)

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Technically, if I use them at the same time it's not OK. I was able to convince the inspector that they wouldn't be used concurrently, however, which is 99% the truth since there is almost never a case where we'll both need to charge at the same time. The other loads are trivial, but if the two HPWC were used at 80A (100A breakers) simultaneously, any additional load would violate the load calculation and technically be illegal.

In the matter of "both cars charging overnight at 40A" that would definitely violate NEC 220.60. You'd have an 80A continuous load (calculates at 100A) on a 100A panel. Unless it is a dedicated panel with no other circuits, it would be illegal.

I personally don't think 220.60 should apply to EV charging and may actually suggest this to the NFPA.

wk057 said:

Technically, if I use them at the same time it's not OK. I was able to convince the inspector that they wouldn't be used concurrently, however, which is 99% the truth since there is almost never a case where we'll both need to charge at the same time. The other loads are trivial, but if the two HPWC were used at 80A (100A breakers) simultaneously, any additional load would violate the load calculation and technically be illegal.

In the matter of "both cars charging overnight at 40A" that would definitely violate NEC 220.60. You'd have an 80A continuous load (calculates at 100A) on a 100A panel. Unless it is a dedicated panel with no other circuits, it would be illegal.

I personally don't think 220.60 should apply to EV charging and may actually suggest this to the NFPA.

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So the OP would be OK if he had no other loads concurrent with the two 40A chargers? Couldn't he reasonably argue that the other loads are unlikely to be on during the charging cycles? We're pushing the envelope here I guess. But given the OP's situation it seems like he doesn't have any other good options. I suppose he could limit the HPWCs to 35A each leaving room for 10A of other loads. That would still be sufficient to charge both cars overnight except when they're almost down to zero.

billarnett said:

So the OP would be OK if he had no other loads concurrent with the two 40A chargers? Couldn't he reasonably argue that the other loads are unlikely to be on during the charging cycles? We're pushing the envelope here I guess. But given the OP's situation it seems like he doesn't have any other good options. I suppose he could limit the HPWCs to 35A each leaving room for 10A of other loads. That would still be sufficient to charge both cars overnight except when they're almost down to zero.

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Would depend on what those loads are, sure. But if they're unrelated to charging the car (and thus them being active or inactive has nothing to do with the charging of the cars) it would be difficult to argue that they'll be unlikely to be on concurrently for the 220.60 exception.

I'll stick with my original suggestion and that would be to get a dedicated 200A panel for two 80A (100A breaker) HPWCs.