max size wire for the HPWC

max size wire for the HPWC

Does anyone know the max gauge wire the HPWC will accept. The installation manual mentions the ground
wire will only accept up to #4AWG, but they don't explicitly talk about the main contacts. Normally for 100
amp service you need #3 wire in Cu. But for a long run, #2 will have less voltage drop. Anyone know
what those lugs will take?

mthanos | January 26, 2013

Sorry about the broken lines. I haven't posted that often.

Brian H | January 26, 2013

Don't use the Enter/Return key, except for new paragraphs. Line breaks are automatic.

JackB | January 26, 2013

I used #4 for the hot wires and there was not a lot of extra space. I doubt that #2 copper will fit.


Alex K | January 26, 2013

@JackB | JANUARY 26, 2013: I used #4 for the hot wires and there was not a lot of extra space.

The installation instruction say, "For most branch circuits of 100A, use 3 AWG (26.7 mm2), 167°F (75°C) copper wire. Ground wiring can be a maximum of 4 AWG. If your ground wiring does not fit into the bus, use a wire nut or other suitable connection method. For installations less than 100A, use conductors that are sized according to local electrical codes."

hammy16 | January 27, 2013

I used 3AWG and it was the max the terminal would take.
I can confirm that the ground will only take up to 4 AWG

It is a tight fit to get every thing connected..

BiffandSully | June 7, 2014

Reviving this thread to ask a question about my HPWC install.

We have a long run from the breaker box to the garage. Want to put the HPWC on a 100 amp breaker (90 would be ok too). The run will go from the breaker box up the outside of the house through conduit, into the soffit and through an attic space then into the garage. Box is located at the farthest point possible from the garage. Our electrician has proposed using #2 SER cable.

I don't know the difference between AWG wire and SER. Will 2 SER fit? Is it overkill?

RFD | June 7, 2014

My electrician used #2 aluminum, works fine.

tga | June 7, 2014

AWG is a wire sizing standard ("American Wire Gauge"). SER is a cable type ("service enterance, round"), but the conductors are still sized using AWG.

SE cable has an oval profile, with the conductors laid parallel to each other. SER has the conductors twisted to give a round profile.

SE/SER cable is rated for external use and underground (but I'm pretty sure it must be in conduit if underground). It is commonly used for connections from the meter to the main breaker (hence the name). It is also used to connect subpanels inside a house and high current branch circuits (like a HPWC).

It is available in aluminum and copper. Copper is generally not used; it is significantly more expensive. Aluminum is safe if installed correctly (using non-oxidizing grease and properly tightening connectors), provided the breaker and device are rated for AL wire.

"#2 SER" is probably 2-2-2-4 SER (or 2-2-4, if you can get it without a neutral), which is a common aluminum size, using AWG2 current conductors (2 hots and a neutral) and AWG4 for a ground wire.

Depending on how you (or more importantly, your inspector) read the latest electrical code, AWG2 AL is good for a 90A or 100A breaker. Ampacity ratings depend on both the guage (wire diameter) and metal - thinner (higher gauge #) has a lower current capacity. Aluminum has a lower capacity than copper at the same size (AL has a higher resistance than CU), so AL wire has to be a bit bigger than CU for the same current capacity.

BiffandSully | June 7, 2014

Thanks RFD and tga.

So it sounds like 2 aluminum SE will be ok. How big of a deal is it if the electrician connects it to a 100 A breaker first, and if the inspector balks, then change to 90A?

One other thing that gives me pause is that we have a swimming pool with a 100A subpanel. The heat pump is on a 50A breaker. The electrician says we'll be ok but should avoid running the heat pump and HPWC at the same time. This strikes me as a rather soft-wired solution. Will it pass inspection? House has 200A service.

tga | June 7, 2014

Changing the breaker after the fact is maybe 10 minutes. Easy-peasy. But you will probably pay for the additional labor and the breaker.

You (or the electrician) might want to check with your electrical inspector and ask if this combination will pass. Better to do your homework up front than find out later and have to get him back to redo it.

Figuring out the max loads you can run is a bit of an art - it's based on maximum expected loads. The size of the breaker feeding the subpanel isn't really the issue; it's the size of the loads/breakers off the subpanel that really matter. If you can show that the car is charging at night, and the pool filter/heater/whatever won't compete, you should be fine. You can probably add a timer to the pool filter/heater to enforce that if need be (ie, shut off the pool loads from midnight-6am, then set the car to start charging at midnight).

But again, the a good electrician will know all of this, handle everything correctly up front (wire/breaker sizing and load calcs) and you'll have no probablem with the inspection. The more work that electrician has done in your town, the more familiar he will be with your inspector's "quirks". | June 7, 2014

I'd use a 100 amp breaker - as you will need it to handle 80 amps of the HPWC. The way the electrical code works is the maximum continuous current draw is 80% of the breaker rating (in the USA, Europe is different).

Now if you need to go to a lower current, it's easy to do at the HPWC with dip switch settings. It allows 40 to 80 amps in 10 amp increments. You can also use less current by just setting the current option within the car.

I doubt the inspector will balk, but one never knows. With the pump on (40 amps) and HPWC at 80 amps, you'll have 40 amps left for everything else. This shouldn't be much of a problem as you'll likely set up charging for midnight, which means very few other items will be active while the car is charging.

Remember that 80% derating, so your house handles 160 amps continuous rating, and a peak current of 200 amps. It's not even clear the pump is 40 amps continuous, but the breaker would allow it.

Kimscar | June 7, 2014

I installed #2 AWG. It fits just fine. I would never use aluminum wiring. Expansion and contraction of aluminum wire creates a phenomenon known as cold creep. When warm expands and contracts when coo this leads to loose connections. Heat fire. Stick with copper.

BiffandSully | June 7, 2014

Copper wire will be much more expensive, no? Especially for my extremely long run, I'm not sure I can stomach the cost of Cu. I am already looking at $1950 for the install using #2 SE cable. Have others been ok with #2 aluminum? Adding dual chargers and now the HPWC has turned out to be a very costly last-minute decision which I probably didn't need for my commute which is between 40 and 60 miles round trip.

tga | June 7, 2014

Copper is ~4x more expensive than alumininum for the same size (but note that the copper will have somewhat higher amperage capacity).

If aluminum wire is a fire hazard, why is it allowed by the NEC?

Again, most homes have AL wiring from the meter to the main panel. The problem occurs when aluminum wires are not pre-cleaned and coated with anti-oxidation grease and the connections are not tightened to spec. Loose/dry connections allow oxidation to form, which has a high resistance, causing heating and expansion at the terminal, which loosens the connection, aggravating the problem.

Do it right, and no oxidation will form at the connection. No oxidation and the connection will not heat/fail. You need oxidation for the problem to occur.

Having said that, if it will keep you up at night, spend the extra $ for CU, if only for peace of mind. If I'm doing the work myself, I'd use copper, because I don't trust that I have the skills to do AL safely. Put with a pro doing it, nothing to worry about.

Kimscar | June 7, 2014

@tga is also correct about oxidation. The prep he mentions is great for oxidation. However that doesn't fix the expansion and contraction of the wire. In many areas aluminum wire is illegal for housing. My cost was about $1700.00 to set me up using 2AWG copper. Let's say total run about 70 feet. Also 100 amp disconnect added in garage for setup. There is no doubt that copper is much more expensive than aluminum. In the end if your area allows either copper or aluminum and you go aluminum I would check the connections once a year to see it is still properly torqued.

BiffandSully | June 7, 2014

I just heard back from the electrician, in the bid he spec'd #2 SE copper wire. So it will fit right? The HPWC installation calls for #3 AWG copper which if I understand will be smaller diameter? Those that have #2 copper does it go into the unit ok?

BiffandSully | June 7, 2014

ok I re-read the thread and Kimscar used #2 AWG, so if my electrician's "#2 SER copper" is the same size I guess it will fit ok. Interesting though that further up in the thread hammy 16 says that 3AWG is the max the terminal will take.

sule | June 7, 2014

I used 3AWG for all, was fine. They just allow less for ground as it really isn't supposed to carry all the load anyway - something is wrong if it does.

GabrielB | June 7, 2014

My garage is diagonally opposite from the main panel, so I had a long run 75+ feet, and I had to go through the attic. In the winter #3AWG would be ok, but if I happen to need to charge during the day in the summer, the attic could be at or above 140degF and at that temperature NEC code max continuous load on #3AWG is 78.1Amps. #2AWG is 92.3Amps.
I chose a 100Amp breaker, #2AWG Romex (outer wrap stripped when transitioned to metal conduit) with 4AWG ground. ALL COPPER.
#2AWG Copper is the max size that will fit L1 & L2 in the HPWC.
I do not think #2AWG Aluminum will fit the L1 & L2 HWPC terminals.
Please avoid aluminum wiring. It is bigger, stiffer, and oxidation paste and torqueing are very critical and may require extra service as the years go by.
At 80Amps charge, connections do get quite warm in the cold winter and get very hot in the summer. Unless I am in a hurry, I usually set the charge to 60 Amps overnight. Less stress on my house wiring and HPWC, longer life.

Short answer: #2AWG Copper will fit L1&L2. #2AWG Aluminum will not.

BiffandSully | June 7, 2014

THANK YOU GabrielB. Very helpful. I think I've put it together now. My electrician was right in wanting to use #2 copper. I don't know if SER cable is similar to Romex, but I'll assume he knows what he is doing and stop nitpicking.

Love this forum! (cooking now, have had 2 martinis. cheers)

Rocky_H | June 9, 2014

The thought that was occurring to me was if you're that on the edge about your wiring size and the cable run length, how about setting it up to not be at the max current and breaker size? Use an 80 amp breaker, and set the HPWC to use 60 amps. That's 75% usage. 60 amps is a fairly high rate, but it makes it much less worry about problems with your setup.

scott.mcphee | July 25, 2014

2AWG is 2AWG, whether that is rendered in Cu or Al.

If 2AWG Cu fits, then so would 2AWG Al, no?

melfont | July 25, 2014

I too would not use Al wire. there have been issues with Al/Cu junctions. If the terminals on HPWC are Cu or brass, there could be a problem. Also, I would not use wire nuts on Any wire larger than AWG 10. You can do your long run with AWG 2 Cu wire then use a junction box or pull box to transition to AWG 3 (and AWG 4 for ground) for the short connection to the HPWC.

Bubba2000 | July 25, 2014

Al wire can be a fire hazard. Why take the chance after spending so much on the car - and house? I would go with AWG #2 CU sheated with a ground included. It is good for at least 120A. External exposed cable should be in a 1.5" conduit Need large radius conduit in the corners. 100A breaker.

Regardless, I charge at 20A. Ran 80A a couple of times when I was helping somebody charge up and get on the way. Since I charge only to 75% every day, I use hi Amp only when I have to leave town to top off the car.

Prefer large margin of safety.

akikiki | July 25, 2014

Bubba2000, where you been? Seems like its been a long time since you posted to any threads? We've missed you.

Charger | July 26, 2014

Maybe I can explain a little. I am a licensed electrician in WA and I know a little about this. #2 IS #2 regardless of copper or aluminum, however copper is more conductive and carries more current than aluminum, so for the same ampacity you will need larger size aluminum wire. As stated previously, #3 copper type SE is the minimum size that will carry 100A, however it is not as common as #2 and may cost more. #2 copper Type SE cable is good for 115A; #2 aluminum Type SE = 90A. I ALWAYS use copper in my family's, friend's and my own home doesn't matter the cost. Aluminum works but it is a cheap replacement- my own opinion. Take if for what you will. Here is an ampacity chart that shows size, type, and ampacity of wire types.
If it were in my house, #2 copper and 100A breaker. When I get my model S, I already have a NEMA 14-50 plug and I plan on using that. I hope I helped and wasn't too confusing.

sule | July 26, 2014

@Bulldogger81: +1
@melfont: +1 on junction points. Different metals touching can cause oxidation, that can cause increased resistance, that can cause heating and all that, well, can cause charging problems and fire.

Furthermore there is one more thing to think about: length of the cable and Ohm's law.

The electrical resistance of conductors is proportional to their length. The coefficient (resistance per length) is different for different materials. Copper is better.

The resistance is somewhat inversely proportional (complicated due to surface effects) to the area of a cross section of the conductor. Conductor of twice the DIAMETER and same length and material will have four times less resistance.

Based on the above you can calculate the resistance of the entire run.

Now, the resistance itself takes a "power tax" - you can think of it as a road roll :). That "tax" is is the resistance multiplied by the square of the current (amperage). Keep in mind that the total conductor length is twice the length of the cable - one coming "in", another "out". Given same length, diameter and current that power tax is proportional to the resistance coefficient of the material and will be greater (tax, loss) for Aluminium. It manifests itself in normal conditions as HEAT (potential fire hazard). It also manifests itself as a voltage drop at the other end - a product of the total resistance and the amperage).

I just don't like Aluminium for this. At all.

hammy16 | July 26, 2014

I confirm that #3 is the biggest wire the main HPWC terminals will accept.

electronk | July 26, 2014

I'm a Canadian Professional Electrical Contractor. Some items to give consideration to with respect to Copper versus Aluminum, from an Electrical Code perspective are first that the lugs are rated to accept copper or aluminum (a Cu/Al should be stamped on the lug somewhere)and second that the lugs are rated for the conductor size to be used. Note that the written literature for the HPWC specifies copper wire, but on page 13 torque values are offered that include Aluminum wire.

I use a 70 amp branch circuit for my HPWC, with #4 copper. If long distance was involved, to include underground wiring, I wouldn't hesitate to use aluminum in the form of USEB (similar to SER). USEB has Aluminum phase conductors with a copper concentric 'neutral' which could serve as the equipment ground for the HPWC since the neutral isn't used. I'd be inclined to install a junction box next to the HPWC, or disconnect, and transition there to copper to feed the HPWC. This strategy could be employed to address the aluminum/copper issue as well as the wire gauge issue (transition from #2 aluminum for example to #3 copper into the HPWC. Again, proper lugs are necessary (and they aren't hard to find)to deal with both copper and aluminum wires.

Hope this helps.

Kimscar | July 26, 2014

@hammy16 #2 AWG Copper works just fine, it's what I have in my HPWC.

2kids10horses | July 26, 2014

I'm sorry I don't know the size wire my electrician used. I insisted on copper. He used three separate pieces of a braided copper. For the neutral, the copper braid was too fat to attach directly into the HPWC without modification. He stripped about an inch of insulation off, unbraided the copper, then rebraided the wires into two "ends" that were able to attach into the HWPC. The two hot wires were no problem. I'm running at 100 amps.

Fortunately, I had a very short run from my service box to the HPWC. The copper wire is very thick. I think braided copper can carry more amps than a single strand.

My lightening rods are grounded with huge braided copper wires.

Brian Vicars | November 10, 2015

I know that I am opening up an old post but I think it is important.

Tesla has decided to install a 48 amp charger in the Model X SUV instead of the 40 amp available for the model S. But unlike the Model S, the SUV will not have an upgrade option to a dual charger. The HPWC instruction manual is very specific about the wire requirement for the installation of the 80 amp continuous circuit. It appears that a significant number of installations have been performed incorrectly causing a fire safety issue. Tesla therefore has decided to restrict the maximum charging rate of the Model X to 48 amps with no upgrade option.
I would anticipate that this change will also be adopted by the Model S in the near future. Say goodbye to 80 amps continuous home charging.

tes-s | November 10, 2015

It appears that a significant number of installations have been performed incorrectly causing a fire safety issue. Tesla therefore has decided to restrict the maximum charging rate of the Model X to 48 amps with no upgrade option.

Is that your speculation, or do you have a Tesla source? I have not heard of incorrect installations of HPWC, or any fire safety issues. This is a very standard electrical installation.

If they are in fact only offering the single 48amp charger, my speculation is they did not have room for dual chargers, so as a compromise made the single charger a little bigger. Not anticipating a change to the MS charging. Just a guess.

davidahn | November 10, 2015

I also have #2 copper, 236-238V 79A consistent, average 59 MPH charge rate. Snug fit. Ground wire is #4.

hammy16 | November 10, 2015

My HPWC can take up to #3 on the main terminals. #4 on the ground
#3 is not easy to route/connect but possible. Found a few links on folks using
#2 but maybe they upsized the connector. We have had our HPWC since Nov 2012
so is one of the very first ones.