HPWC wire gage

Atebit · Sep 2, 2015

I'm building a house & I had my electrician run a dedicated 100A circuit for my HPWC. In my case, the service entrance & breaker panel is on the side of the house opposite the garage. So the total run of the wiring is probably around 80 feet or so. I know he used copper 3 AWG as called for in the install guide, but for a run that long I'm concerned about voltage drop and heat buildup along the run.

I looked up up the resistance/foot of 3 AWG and came up 0.016 ohms total assuming 80 feet and 0.020 ohms total for 100 feet. So worst case that works out to about a 1.6V drop assuming 100 feet and the HPWC drawing 80A. Doesn't sound bad, but that's still 128W of heat. But then again, given it's a straight run and not coiled up, I guess it's really only 1.28W/ft...

Soolim · Sep 2, 2015

It is a one time cost, if it makes you feel more comfortable, why not just upsize it, and use #2 AWG copper conductor? Wires are cooler, lower voltage drop.

KJD · Sep 2, 2015

The Tesla home charging FAQ has a section on wire size for the HPWC and it has some good detail on the subject.
FAQ: Home Tesla charging infrastructure QA

For wire-in-conduit, 3 AWG THHN (dry locations) or THWN (wet locations) is sufficient to carry 100 amps, but many electricians will use 2 AWG to ensure there is enough headroom, and it’s carried more widely by supply houses. The ground wire must be a minimum of size 6 AWG.

wk057 · Sep 2, 2015

While I prefer #2 for the HPWC, #3 at 80 ft / 80A / 240V is only about 1.2% voltage drop (~3V). It should be fine, and meets code.

llavalle · Sep 2, 2015

If you decide to go with #2, it's going to be a VERY tight fit in the HPWC. #3 is already pretty tight in there, I can't really imagine #2 fitting.

But maybe you can run #2 up to your disconnect and #3 from the disconnect to the HPWC? That's what I did. I ran 00 gauge Aluminium wires from my main panel to a subpanel (125amp) and from the sub panel to the disconnect. Then #3 copper for the HPWC. Way cheaper than having 80ft of #2 or #3 copper.

Here's what mine looked like during installation :

wk057 · Sep 2, 2015

I couldn't find any pics, but I have two at my place wired with #2. It's not too bad and not really any harder to do than with #3.

llavalle · Sep 2, 2015

wk057 said:
I couldn't find any pics, but I have two at my place wired with #2. It's not too bad and not really any harder to do than with #3.

I stand corrected.

I just looked up the Standard size (diameter) for AWG :
3 AWG : 0.2294in
2 AWG : 0.2576in

Difference : 0.0282inches, between 1/64in and 1/32in more... I was under the impression it was more than that.

FlasherZ · Sep 2, 2015

#2 fits with a little elbow grease and cursing. Not the easiest in the world, but not the most difficult I've wired either.

Soolim · Sep 2, 2015

llavalle said:
Here's what mine looked like during installation :

View attachment 92829

Trust that when the installation was completed, the aluminum ground conductor was bonded to the disconnect switch enclosure just like the copper ground wire is.

llavalle · Sep 2, 2015

Atebit · Sep 3, 2015

Thanks for the replies & info! My electrician wired it similarly to llavalle's setup...2 AWG aluminum from the service entrance to the sub-panel in the garage & then 3 AWG copper from the sub-panel to the HPWC.

Soolim · Sep 3, 2015

Atebit said:
Thanks for the replies & info! My electrician wired it similarly to llavalle's setup...2 AWG aluminum from the service entrance to the sub-panel in the garage & then 3 AWG copper from the sub-panel to the HPWC.

llavalle said:
I ran 00 gauge Aluminium wires from my main panel to a subpanel (125amp) and from the sub panel to the disconnect. Then #3 copper for the HPWC. Way cheaper than having 80ft of #2 or #3 copper.

QUOTE]

Your installation is not the same as Llavale, he used #00 aluminum @ 135A ampacity to the subpanel. Your electrician save money by running #2 AWG Aluminum @ 90A ampacity, when compared to #3 AWG copper @100A ampacity. Hence it does not improve on voltage drop or energy loss. It will be different if the #2 AWG is copper @ 115A.

llavalle · Sep 3, 2015

Soolim said:
Your installation is not the same as Llavale. Your electrician save money by running #2 AWG Aluminum @ 90A ampacity, when compared to #3 AWG copper @100A ampacity. But it does not improve on voltage drop or energy loss.

I was about to say the same. 00AWG or 2/0AWG is not the same as 2AWG. Are you sure he used 2AWG AL wires? not 2/0AWG ?

In order of big to small :

0000(or 4/0) > 000(or 3/0) > 00(or 2/0) > 0(or 1/0) > 1 > 2 > 3 > 4... and so on.

Ref : American wire gauge - Wikipedia, the free encyclopedia

FlasherZ · Sep 3, 2015

Atebit said:
Thanks for the replies & info! My electrician wired it similarly to llavalle's setup...2 AWG aluminum from the service entrance to the sub-panel in the garage & then 3 AWG copper from the sub-panel to the HPWC.

#2 AL is only good for 90A circuit rating. 80A charging requires 100A circuit & feeder rating. I hope you charge at no more than 72A (the limit with 90 degree rated feeders and circuits), or you have an illegal install.

Cottonwood · Sep 3, 2015

FlasherZ said:
#2 AL is only good for 90A circuit rating. 80A charging requires 100A circuit & feeder rating. I hope you charge at no more than 72A (the limit with 90 degree rated feeders and circuits), or you have an illegal install.

Because there is no 90A CB/72A Charging DIP setting on the new HPWC's, #2 AL will require the HPWC to be set to 80A CB/64A Charging.

I hope that your electrician used 2/0 or 00 AL which has twice the aluminum and half the resistance of #2 AL.

Atebit · Sep 7, 2015

Thanks for the info, I'll check. The rough in is still exposed so I should be able to tell by looking at the jacket directly instead of getting second hand info.

Atebit · Sep 8, 2015

So I was able to get over there today. The sheath states "Style R XHHW-2 3 CDRS 2 AWG 1 CDR 4 AWG". According to http://www.allenelectric.com/referencedata/ampacity.htm, XHHW-2 is rated for 100A, unless I'm reading it incorrectly, which is entirely possible since I'm not an electrician. So I think I'm ok?

FlasherZ · Sep 8, 2015

Atebit said:
So I was able to get over there today. The sheath states "Style R XHHW-2 3 CDRS 2 AWG 1 CDR 4 AWG". According to http://www.allenelectric.com/referencedata/ampacity.htm, XHHW-2 is rated for 100A, unless I'm reading it incorrectly, which is entirely possible since I'm not an electrician. So I think I'm ok?

That's the conductor temperature rating for the 90 degC column.

There are two factors to consider:

1 - the temperature of the conductor itself. As you noted, XHHW-2's insulation is rated at 90 degC. You're good there because of the rating to 100A.
2 - the temperature at terminations - where wire is bolted to breakers, receptacles, junctions, etc. This is where you have a problem - most equipment that you'll find today is rated only at 75 degC for those terminations. All new breakers and receptacles that you buy today will likely be rated at 75 degC, with 90 degC limited to special applications (that you will pay $$$ for). This is what limits you in this case to 90A maximum circuit rating (NEC 110.14(C)).

Atebit · Sep 8, 2015

Thanks for the detailed info, Flasher. I will pass this information on to my electrician. Here's my perspective: prior to pulling any wiring I provided him with the HPWC install guide that specified 3 AWG CU for 80A charging. He chose to use something else without consulting me. So if he now has to spend more money for the subpanel, breakers, etc. that's on him. I'm sure my builder will support me in this.

I'll google around to see what I can find, but if you'd be able to fast-track me to the components that meet the required specifications that would be helpful.

FlasherZ · Sep 8, 2015

Atebit said:
Thanks for the detailed info, Flasher. I will pass this information on to my electrician. Here's my perspective: prior to pulling any wiring I provided him with the HPWC install guide that specified 3 AWG CU for 80A charging. He chose to use something else without consulting me. So if he now has to spend more money for the subpanel, breakers, etc. that's on him. I'm sure my builder will support me in this.

I'll google around to see what I can find, but if you'd be able to fast-track me to the components that meet the required specifications that would be helpful.

#1 ALuminum is good to 100A with 75 degree terminations, and #3 CU (copper) likewise.

Be careful - your electrician might try telling you that what he did is fine because the car will never charge at more than 80A, so 90A is plenty good. This is incorrect! NEC 210.19(A)(1)(a) says otherwise - the circuit conductor rating must not be less than 125% of the continuous load. NEC 625 says all EV loads are treated as continuous loads, 80A * 125% = 100A. The conductor and terminations must be rated at 100A.

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