8/2 MC cable -80' run thru attic (40amp charge on 50a breaker)?

[sandsrfr]

I was planning to use a run of 8/2 MC cable I have to install a TWC.

The 8/2 MC cable is rated at 55a-90degree (or 50a-75degree). Per NEC, 80% derate for continuous load means either 44amps (90degree) or 40amps (75degree).

Since Residential breakers (squareD) are rated at 75degrees I assume 40amps is Max allowable charging on #8thhn (in MC Cable)?

Is it code compliant to use 8/2 MC cable for a Tesla wall charger install with a 50amp breaker at the main and provision the TWC to 40amps max charging?

Is there any eating required for attic install and/or an 80' length of run?

 

[howardnj]

^^^
Use a Ca. licensed electrician that will apply and arrange for permits and inspection.
My HPWC in my garage required a 150ft run from my 200 amp basement panel.
The NJ licensed electrician used #2 four conductor aluminum cable to feed the 100 amp sub panel in my garage.
They did not obtain the required breakers and hardware from home depot but use a local electrical supply house.
Don't go cheap and risk burning down your house.

 

[Cosmacelf]

I was planning to use a run of 8/2 MC cable I have to install a TWC.

The 8/2 MC cable is rated at 55a-90degree (or 50a-75degree). Per NEC, 80% derate for continuous load means either 44amps (90degree) or 40amps (75degree).

Since Residential breakers (squareD) are rated at 75degrees I assume 40amps is Max allowable charging on #8thhn (in MC Cable)?

Is it code compliant to use 8/2 MC cable for a Tesla wall charger install with a 50amp breaker at the main and provision the TWC to 40amps max charging?

Is there any eating required for attic install and/or an 80' length of run?

Yes that will work. More info here: Advice for EV Charging in North America

And the voltage drop calculator linked on that page says 8 gauge is fine at 80 feet for your 40A draw.

No issues running MC in an attic. It just can’t be used in a wet location.

 

[jcanoe]

When you provision the TWC for use on a 50 amp circuit the TWC will automatically limit charging to 80% (40A).

MC wire is a good option for indoor installation (never for use outdoors), only dry locations (i.e. not installed or pass through a damp or wet crawl space); above ground only. There is also a restriction against running MC inside hollow concrete block.

 

[davewill]

It will do, but if you think you might ever want to upgrade to a 60a circuit, it would pay to put in 6/2 instead at this point.

 

[sandsrfr]

Yes that will work. More info here: Advice for EV Charging in North America

And the voltage drop calculator linked on that page says 8 gauge is fine at 80 feet for your 40A draw.

No issues running MC in an attic. It just can’t be used in a wet location.

Thank you, that clarifies it for me. Thanks for the links.

Its funny I've asked a few 'engineer' colleagues and you get all sorts of differentiating opinions. One stated you have to use #4 for a 60amp circuit and another said they would just use #6NM-B (really?). I don't think they however follow the 'code' specifics and are using their engineering experience for assumptions on what to use. When you drill down into it the NEC /CEC, it defines a minimum gauge which I trust wasn't just pulled out of thin air.

Regarding the other comments:
I realize the advantages to up-sizing and less resistance in larger wires, but money doesn't grown on trees and I've been given this 8/2MC for free. I am perfectly happy with charging at 40amps for our minimal driving. I could even lower the chargers threshold even more and swap in a lower breaker on this 8/2 if 'required' and still charge up each night more than enough. If I ever need the additional 8amps of charging (40amp to 48amp), I can always purchase and install 6/2 later as its across an accessible attic. No need for a 'licensed electrician' as I'm capable of this (just finished my DIY 8kW solar install on the roof) and get a permit and pass inspection to have that assurances it was done right.

 

[davewill]

How much of that cable to do you have? If you think you'd ever want a second charging circuit (or one for power tools), you could run two of them while you're at it. Even if two 40a circuits is too much for your box, you can still use load sharing to keep the total draw down to, say, 50-60a total.