ClipperCreek HCS-40P #6 or #8 wire?

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The HCS-40 requires a 40 amp circuit, and a 40 amp circuit generally requires #8 wire, unless it's a really long run. However, an HCS-40P requires a 6-50 or 14-50 outlet, which should be on a 50 amp circuit, which would then require #6 wire. I think you technically can install a 50 amp outlet on a 40 amp circuit, but that'd be ill advised. Someday you might take down your HCS-40P and someone could come along with their Tesla or Rav4 and see a Nema 6-50 or 14-50 and assume it's a 50 amp circuit and overstress those #8 wires by charging at a continuous 40 amps.

Here's the HCS-40 manual, which on page 6 specifies to use a 40A breaker for the HCS-40 and a 50A breaker (hence #6 wire) for the HCS-40P. Also, if it's the model with a 14-50 plug, you'll need a circuit with 3 conductors + ground, while with the 6-50 you only need 2 conductors + ground.
 
The price difference between #6 and #8 is trivial compared to replacing the entire line with #6 if you decide to upgrade to a 50A circuit. I generally overprovision things like this. Most of the time, I don't end up needing it.

However, voltage drop is also a concern. You might have a short run, but the thicker gauge might save a volt or two. With a 32A current, this can save you 30-60W of dissipation. This sort of thing can actually save you money. If you used it for 4 hours every day, the ROI on the thicker gauge would be about 2 years (highly dependent on electrical rates).
 
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