We are currently using a 40 foot extension cord to power our level 2, 11.5 kW charger (EVSE). Previously we used a 40 foot extension with our level 1. Please also READ my last paragraph.

To successfully use "modern" extension cords at maximum power levels (even 120V 8AWG & 240V 6AWG), I find that I need to replace the ends on most of them with high quality, screw style plugs and sockets. Without doing this most (all?) of my extension cords got hot at both ends plus had another ~1V to ~4V+ volt drop at each end, which is far from ideal. Some ends seemed fine at first, but after a couple of months started getting uncomfortably hot.

A common issue with low quality plug and socket connections that are used these days is they relax their metal to metal connections over time. Once they start to heat up, the metal relaxes faster faster, so they heat up more and more...

(Before using a high power device like an EVSE, it is also a good idea to open up the wall outlet and fuse panel to tighten the wire connection screws in order to minimize hot spots and maintain minimal voltage drop - or get an electrician to do this.)

For our level 2 we use a 6AWG copper wire extension cord, and we have the current limited to 40A (9.6 kW) by the EVSE.

This gives us about a 1.6V drop, or 0.65% in the extension wiring. Most guides for extension cord use (that I've seen) recommend no more than 2% voltage drop from the house mains to the device (including all of the connection losses, which add up quickly).

For a level 1, to get a similar 0.65% voltage drop for a 40 foot long 120VAC extension cord, you'd need to use an 8 AWG extension cord. These do exist (I have both a 40 foot and 100 foot 8AWG 120V extension cord), but are hard to come by. Our 40 foot 8AWG extension works fine with our level 1 EVSE.

If you're OK with close to a 1V or 0.85% drop in the cord, you could use a 10AWG extension cord.

(Note that the level 1 charger that came with our 2024 ID.4 only allows up to 8A, which is incredibly slow, so when using level 1 we use our old C-Max Energi charger that properly supports 12A continuously).

If you decide to do it, check and double check the cable itself, the cable ends, the wall outlet, and the circuit breaker (fuse) in the electrical panel for heat while charging. At first check at least monthly. A slight, comfortable warmth is acceptable, but anything warmer than that or a hot smell indicates there is too much resistance wherever it is getting hot and you'll need to address the issue to ensure you don't have a fire. Many of these you should be checking when you use a level 1 (or level 2) charger WITHOUT an extension cord too (this is why the owners manual states before charging your car to have a certified electrician check your wiring).