Dual HPWC Install

Looks good. We're about ready to put our second one in for Model X - more on that in a bit...

A couple of notes for others who see this:

* First, your choice of 90 degC #2 is required in this case, because when you have more than 3 current carrying conductors in a single raceway, table 310.15(B)(3)(a) requires derating to 80%; 90 degC #2 is rated at 130 amps, which gets derated to 104 amps (just enough to match the 100 amp requirement for 80A continuous loads). It's also a good demonstration of the difference between 90 degC rated conductors and 75 degC rated termination. You're allowed to consider the 90 degC column for conductor derating in a conduit, even though your breakers would consider a maximum of the 75 degC column (since most breakers are rated only to 75 degC). You would not be able to use 4 #3 conductors from the panel because of the derating requirement in a single conduit, and #3 @ 90 degC is 110A, derating would make it good for only 88A - not even good for the 90A rating on the old HPWC's.

* Second, the junction box must be accessible; it bears repeating because it's not seen in this picture (but I note you talked about it being above a drop ceiling, which is legal as long as it's securely attached to framing members or wall surface above the drop ceiling).

* Finally, NEC 625's disconnect requirement was primarily designed for commercial EVSE's with OSHA-style tag-out/lock-out requirements. Most inspectors I know agree that it's crazy for home installs where breakers can control it. The AHJ ultimately is the determiner of whether it's reasonable or not. This is why you never hear electricians or "ultimate authorities" on the NEC, because of local amendments and interpretation.

Now, for my install, I have to do a bit more. I have a considerably longer run and with a second HPWC I'm beginning to bump up against the 200A main's rating on my non-generator panel, as my pool equipment, hot tub, machine shed, and current HPWC (along with some other loads in the home) are all from that panel. So, I plan to do something a bit different. I'll bring in a third service entrance conductor to a 200A main disconnect in the basement (to meet the requirement of all building disconnects being co-located), then place a new 200A panel above my doors in the garage (where my current 125A subpanel is located). From there I'll make the runs down to the HPWC's (and, if I am bored enough, a bell box with a surface-mounted 14-50 outside).

Great comments! Thanks!

FlasherZ said:

* First, your choice of 90 degC #2 is required in this case, because when you have more than 3 current carrying conductors in a single raceway, table 310.15(B)(3)(a) requires derating to 80%; 90 degC #2 is rated at 130 amps, which gets derated to 104 amps (just enough to match the 100 amp requirement for 80A continuous loads). It's also a good demonstration of the difference between 90 degC rated conductors and 75 degC rated termination. You're allowed to consider the 90 degC column for conductor derating in a conduit, even though your breakers would consider a maximum of the 75 degC column (since most breakers are rated only to 75 degC). You would not be able to use 4 #3 conductors from the panel because of the derating requirement in a single conduit, and #3 @ 90 degC is 110A, derating would make it good for only 88A - not even good for the 90A rating on the old HPWC's.

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Correct me if I am wrong here, but if you use really good insulation (XHHW-2), this derating is not required. This exception is under the table that you cite.


From looking at Tesla Supercharger plans, they call out XHHW-2 insulation, I believe for this reason.

wk057 said:

My setup is two 200A panels, and the loads were reconfigured during phase 1 of my off-grid solar project so that the two chargers could be put in one panel along with only generic 120V loads of the first floor. The total of loads in the other panel, mostly 240V loads like HVAC and the 2nd floor subpanel for its 120V loads, fit nicely from a load perspective in their own 200A panel.

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That's pretty typical. I'm a bit hamstrung here in that my 2 panels are split based on standby (25 kW genset) vs. grid-only. So by far, the heaviest loads tend to be on the non-generator panel. My meter pan can take up to 5 sets of lugs, though, so I'm good if I need to add more. My basement tends to look a bit crazy with all the solar, electrical, and landline equipment:

FlasherZ said:

That's pretty typical. I'm a bit hamstrung here in that my 2 panels are split based on standby (25 kW genset) vs. grid-only. So by far, the heaviest loads tend to be on the non-generator panel. My meter pan can take up to 5 sets of lugs, though, so I'm good if I need to add more. My basement tends to look a bit crazy with all the solar, electrical, and landline equipment:

View attachment 70996

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Is that a DS1 line card chassis on the far left? Our setups are similar, minus the solar. I've got two 200A entrance feeds, one through a transfer switch with a ~20kW genset, so like you I re-wired the panels a few years ago to determine what was on the generator. My current HPWC is going to my grid-only panel. I'm going to need to run a second line from the garage to the basement for the model X.

I had talked to dominion about a second entrance feed and meter for just the garage and they said no. It's unfortunate, as the transformer is closer to the garage. My service entrance is on the opposite side of the house and I have a finished basement. Running the line for the first HPWC was expensive and required about 15 holes in the ceiling etc.

Cottonwood said:

Great comments! Thanks!



Correct me if I am wrong here, but if you use really good insulation (XHHW-2), this derating is not required. This exception is under the table that you cite.

Exception: Type XHHW-2 insulated conductors shall not be subject to this ampacity adjustment.​

From looking at Tesla Supercharger plans, they call out XHHW-2 insulation, I believe for this reason.

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You're looking at the text of 310.15(B)(3)(c), not 310.15(B)(3)(a). The text of 310.15(B)(3)(c) refers to raceways subject to sunlight heating on rooftops, which has a new XHHW-2 exception as of NEC 2014. Table 310.15(B)(3)(a) refers to adjustment factors for conductors in a raceway. Page 70-158 is a hellish page in a section that's already difficult to read.

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andrewket said:

Is that a DS1 line card chassis on the far left? Our setups are similar, minus the solar. I've got two 200A entrance feeds, one through a transfer switch with a ~20kW genset, so like you I re-wired the panels a few years ago to determine what was on the generator. My current HPWC is going to my grid-only panel. I'm going to need to run a second line from the garage to the basement for the model X.

I had talked to dominion about a second entrance feed and meter for just the garage and they said no. It's unfortunate, as the transformer is closer to the garage. My service entrance is on the opposite side of the house and I have a finished basement. Running the line for the first HPWC was expensive and required about 15 holes in the ceiling etc.

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Yes, there's a 25-pair cable coming into the house (demarc up top) and that's an 12-slot box for cards (HDSL2 used here).

I talked about a second meter and they said I'd be charged for a second service - so I buried my own conduit (at roughly $500/year meter charge, I can bury a lot of conduit, can even pay for the directional boring to make it easy!).

The mess you see on the bottom of those joists near the panels is the result of moving circuits around to arrange circuits for the genset panels. There are two fairly deep work boxes to move these circuits between panels. If you look closely you can see a few things wrong with the picture, done by the previous owner. One example is the use of schdule 40 PVC water pipe instead of electrical conduit.

That's a nice setup... and 64 kW is really great. My plant consists of 4 different arrays of panels, total of 18.5 kW. I don't have enough roof space to go above 30 kW.

There's another 9 kW array on the shed roof, not pictured there.

Both of my Teslas have had the dual-onboard charger and I've been debating installing a HPWC at my beach house. Outside of the convenience of the plug, have you found cases where having an 80A charge going (instead of 40A) have been critical?

anxman said:

Both of my Teslas have had the dual-onboard charger and I've been debating installing a HPWC at my beach house. Outside of the convenience of the plug, have you found cases where having an 80A charge going (instead of 40A) have been critical?

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Originally, I figured there would be - as I go on trips frequently, I find that at least 3-4 times a year, I come back from the airport and we want to leave for my in-laws' (140 miles away) as soon as practical. This was before they installed chargers at the airport, and since then I haven't found a case where it was necessary.

That said, having a charge cord in the garage that doesn't have to be coiled up and put in the car all the time is the best reason to have something there. I need to have a cord with me for a few places I stay. And with the difference being only $100 between a UMC bundle and a wall connector, I'd opt for the wall connector every time.

FlasherZ said:

Originally, I figured there would be - as I go on trips frequently, I find that at least 3-4 times a year, I come back from the airport and we want to leave for my in-laws' (140 miles away) as soon as practical. This was before they installed chargers at the airport, and since then I haven't found a case where it was necessary.

That said, having a charge cord in the garage that doesn't have to be coiled up and put in the car all the time is the best reason to have something there. I need to have a cord with me for a few places I stay. And with the difference being only $100 between a UMC bundle and a wall connector, I'd opt for the wall connector every time.

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I used my UMC as my home charger on a 14-50 for about a year. A few weeks ago, I pulled the trigger on a HPWC (partly because they cut the price, but mostly because my daughter managed to zap herself plugging the UMC into the wall).

The HPWC is really a much nicer way to charge than the UMC. I agree that it's worth a lot to not have to coil the UMC and put it in the car. Interestingly, I also get a slightly faster charge from the HPWC than I did from the UMC, even with just a single charger. The UMC would show voltage a few ticks below 240, but with the HPWC I am consistently right on 240 (or a bit above).

wk057 said:

You'll see slightly less voltage drop, and hence slightly more wattage at the car, using the HPWC vs the UMC at 40A. The UMC has pretty thin wire inside (#10?) compared to the HPWC (#6). At 40A you should see virtually no drop (~1%) using the HPWC.

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I recall that for flexibility purposes, the UMC has 2x 14AWG per leg. A no-no if used for infrastructure, but ok in appliances if it passes the tests.

Am I reading this correctly? Your garage is above a finished basement?

Isn't that against code everywhere? As the powers that be want to ensure that GASoline (remember that oily sh..) cannot drip down there.

wk057 said:

Ah right. Forgot about the parallel conductors in the UMC. The HPWC complies with NEC 625.17 and 400.5(A)(2) if I recall correctly, which gives it a 99A rating using 90C #6 conductors.

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The HPWC uses a very high-quality cable with conductors that are actually rated for 105 degC operation (at least the first cable I came across). The NEC doesn't recognize anything greater than 90 degC.

Even then, there will likely be a debate as to whether the NEC can even claim authority over the WC as it is an appliance. The NEC attempts to claim it by saying that the car is the ultimate appliance and the WC is merely an attachment point (e.g., an outlet).

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GreenT said:

Am I reading this correctly? Your garage is above a finished basement?

Isn't that against code everywhere? As the powers that be want to ensure that GASoline (remember that oily sh..) cannot drip down there.

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Not necessarily - it's the same in my case... the north wall of my house structure and basement is the south garage wall. The space just above the sill where the joists sit provides room for the conduits to go through the wall.

Great post we are adding a second HPWC as well and this helps understand some options. Right now I have a 14-50 and an a HPWC dropped into a separate Tesla only panel in the garage.