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Building long distance adapters for dryer and range outlets

My dad recently received his model S (St. Louis area) and wanted to visit me in Champaign, IL (200 mile trip). I figured that the best way for him to charge here would be to hook up to my NEMA 10-50 range outlet, which is 50 feet from the nearest curb. I searched the forums for solutions on how to do this, but I generally found more questions than answers. So I thought I would post my solution here in the hope that it would be useful to others.

What I ended up doing is getting a set of 4 foot replacement appliance cords, connecting a NEMA 14-50R (50A female plug) to the other end and then connecting that to a 30 feet RV extension cord which then plugs into the mobile adapter, for a total length of slightly over 50 feet. This setup pretty much "just worked"; very little voltage drop to speak of (the Tesla reported 245V at 32A charging current) with only the replacement appliance cord getting slightly warm. The only caveat is make sure you know the rating on the breaker that drives that outlet and set the charging current at 80% of that. In my case, the breaker turned out to be rate 40A, so we set the charging current at 32A. Do not try to pull more than 80% of the breaker rating because the house wiring is sized to take a continuous load of at most 80% of the breaker rated current.

Obtaining parts

You can get replacement appliance cords at pretty much any hardware store. I found a cheap place to order them online, ceshowroom.com:

NEMA 10-30P Dryer cord: ~$4
NEMA 14-30P Dryer cord: ~$6
NEMA 10-50P Range cord: ~$8

(The RV cord plugs directly into a NEMA 14-50 Range outlet)

The NEMA 10 series are old style outlets without a ground connector (houses built before 1996)
The NEMA 14 series are new style outlets with a ground connector (houses built after 1996)

Female plugs and the RV extension cord from amazon.com:

NEMA 14-50R plug: ~$15 x 3 = $45
NEMA 14-50 30' extension cord: ~$100

The female plug is the hardest part to find.
There are also 50' 50A cords available, for around $180.

Building the adapters

For the most part, this is pretty straightforward; follow the instructions that come with the female replacement receptacle for cutting, stripping, connecting the wires and installing the strain relief clamp. The tricky part is dealing with the old style cords that don't have a grounding connector. It seems like the paradigm back then was to combine ground and neutral into one wire since the neutral doesn't usually carry much current in 240V appliances (e.g. my range is "grounded" through its neutral connector).

The Model S only needs two power wires and a ground connector, so it can work just fine in this paradigm: just connect the ground to the neutral. For the replacement plug I was using, this amounts to connecting the middle (neutral) cord wire to the bottom ground connecting clamp with a jumper wire that goes around to the neutral connector on top. You can also leave the neutral unconnected since the Tesla doesn't care about it, but don't try to use the adapter with anything else in that case.

Thanks @keithc, This has already been covered extensively in previous threads, and there is a detailed "how to" over at TMC.


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