Wiring size and Wall Connector

[MYPWR]

This post is to get feedback from others who have considered or did increase the wire size to compensate for high temperatures.

I have installed my Tesla gen3 wall connector a year ago in my garage. My layout is a 50 foot run from the breaker panel. I used 30 feet of 3/4" NM PVC liquid tight conduit in attic and 15 feet of 3/4" electrical PVC conduit down the walls. Both conduits are rated 90 degrees Celsius . After charging sessions the electrical PVC conduit at the wall connector gets hot. Not seen any temps over 120 degrees at the surface of the electrical PVC conduit. I use the Wall Monitor app to verify the power and temps and do see temps hitting 175 degrees at the MCU. I do have it configured for 48 amps with a 60 amp breaker. I am using two THHN 6 AWG copper wires and a single THHN 10 AWG for Neutral.

I am moving soon and have been rethinking my wiring setup. I want to reduce the heat and have a plan to up the wiring size. My thinking is for the next house the wall connector will be about 30 feet away from the breaker box in the garage and I'm going to switch to two THHN 4 AWG copper wiring and single THHN 8 AWG copper for the Neutral. Running the THHN in 1 inch electrical PVC conduit or maybe 1 inch EMT conduit. The wiring price increase is not an issue.

 

[MYPWR]

I have been watching Youtube and reading about the effect of ambient air temperature on the THHN wire. Industry rating of THHN wire (19 strand) is calculated at ambient air temp of 86 degrees F. If you have THHN #6 awg rated at 90 degrees C (194 deg F) in an area that will have 110 degrees ambient air temps like a garage or attic. The amp rating drops. For ambient air of 110 degrees F it drops 22 amps when exposed to those temps. Math:75x.87x.80=52.2 AMPS 52.2 amps is what the rating of a THHN #6 wire exposed to 110 ambient air temp.

Switching to THHN #4 awg wire you will get a change in calculated carrying capacity. Math:100x.87x.80=69.6 AMPS

This is compensating for the high ambient air temps which we here in Florida see most of the year. The Tesla wall connector pulls 48 amps continuously so a 52 AMP calculated capacity is close when the air temp is 110 or higher. This I believe is what I am seeing with the higher temps at the wall connector.

 

[Rocky_H]

The amp rating drops. For ambient air of 110 degrees F it drops 22 amps when exposed to those temps. Math:75x.87x.80=52.2 AMPS 52.2 amps is what the rating of a THHN #6 wire exposed to 110 ambient air temp.

Can you show where you're getting these numbers from? You just say "math" and then put a bunch of numbers without telling what they are, and I just used a temperature derating calculator and get nowhere close to that. I'm going to show my guesses for what these are:

75 is probably the default 75 amp rating at 90 degrees C, which you use as the starting point for 6 gauge THHN wire in a derating calculation.

.87 I guess is the temperature derating factor. 110 F is 43.3333 C temperature. The derating calculator I was using only had 5 degree steps, and for 40 C to 45 C it had derating factors of .88 to .82, so that seems about right.

.80 What is this? Are you already trying to apply the 80% use factor of a continuous load and then saying it is the rating of the circuit? That's not how that's done. If so, you would be applying it twice. A circuit rating is for the full level for intermittent use, and then you make your usage of it at a lower level if it is continuous.

Advanced Wire Ampacity Calculator

This wire ampacity calculator will calculate and determine the maximum allowable ampacity of a conductor for a given wire size.

wiresizecalculator.net

I used this calculator that does both ampacity and temperature derating calculations, and it derated the original 75A down to 65A, which should still be fine for a 60A rated circuit.

Is it for the number of current carrying conductors? I found a table about that, but that rule only starts with more than 3 current carrying conductors, and the .80 factor applies for 4 to 6. A wall connector doesn't use more than three current carrying conductors, so that wouldn't be used. I can't figure out what your .80 factor is.

I'm curious because you sound so confident in this, but it sounds like you are mixing up the use current versus the current rating of the circuit.

 

[MYPWR]

Here is the YouTube video I watched. The first 5 min covers the ambient temperature stuff.

 

[Rocky_H]

Here is the YouTube video I watched. The first 5 min covers the ambient temperature stuff.

Ah, yep, there it is. You're using his second example, which applies a .80 factor for the reduction because of 5 current carrying conductors. A wall connector circuit does not have 5 current carrying conductors, so it will not have that .80 factor. It uses the full 1.0 factor because it's up to 3 current carrying conductors.

These temperature derating issues usually don't impact most residential situations, especially inside garages. The default use of wire in conduit is already being done at a standard rating of the 75 degree column. But you get to start derating calculations from the 90 degree column of the insulation value, so you start higher before bringing it down. And it usually still comes out above the 75 degree value. There is a crossover point of course at some temperature, but that is I think around a 130 to 140-ish degrees F where that would come into play and actually reduce it. So that is sometimes a factor in attic areas, for example.

 

[MYPWR]

Thanks. Next house install I’m switching to THHN #4 for my 2 power wires and going up to 1 inch conduit. Lowering wire temp is what I am going for. Forgot to mention the 60amp breaker does not seem to get as hot as the wall connector side. Already planning out my runs in the garage.

 

[GSP]

@MYPWR - Unless you have only a short “super off peak” time window, consider lowering charging from 48 A to a lower value. Tesla’s PCS has three 16 A circuits and they all have to run at full capacity to get 48 A. The result in higher themperatures and bigger delta-T for temperature cycling. The later causes more mechanical stress in components that expand and contract with heat. I suggest trying 24 A, to run two PCS circuits at 12 A each, or 36-42 A to run three circuits at 12-14 A each.

I charged my 2018 Model 3 at 16 A, the full capacity of one PCS circuit, for years (to share current with my other Tesla). This resulted in failure of that circuit, so now my max charging ampacity is only 32 A. I now charge at 18 A, so each of the remaining two PCS circuits only need to provide 9 Amps.

Good luck with your new home and EVSE install.

GSP