Actually, this is easy, especially if you're talking about Home Depot extension cords.
Take your average NEMA5-15 (15A) wall socket, standard on home interiors everywhere. Maximum continuous load by the NEC is 12A (80% of 15A). Fine. And there's some serious copper wire in your walls so, from the Breaker Panel to the Socket, there's some minimum voltage drop. (Ohm's law: Vdrop = Current * Resistance_of_the_wire.)
Now, go down to Home Depot and buy their Super Duty Extension Cord!
Before breaking it out of its wrapping, look closely for the current rating. You'll discover that There Isn't Any Marking. And that's because Copper Is Expensive, Plastic Is Cheap, and that Super Duty is Super At Profits, but not so much at carrying current. In fact, if one uses that Super Duty Home Depot Special to try and charge your Tesla with the Mobile Connector, one'll swiftly discover that one won't get 12A out of it - the Tesla will see an almighty big voltage drop (thin copper wires). Tesla doesn't know if that's the Vdrop at a socket (in which case, there's something seriously wrong with the socket or house wiring, and making the House Burn Down is considered to be a Bad Thing) or an cheapie extension cord (in which case it'll make the extension cord get hot... maybe too hot); so, not knowing which, the Tesla will drop the charging current by half or something.
I've actually seen this. The solution is to wander over to a place that caters to contractors, like Harbor Freight. And over at Harbor Freight, one will discover a display where they really do label the cords for the current they can carry. And the longer a 15A rated extension cord is, it's 'way more expensive per foot for the longer cables - because they beef up the gauge so the overall voltage drop is minimized, and Copper Is Expensive. If you really want your socks knocked off, check out the prices there for a 20A (that's a 120VAC cord with a right-angle blade on it) that's, say, 300' long. For a contractor with a circular saw, though, it's perfect.
Any of those 15A rated extension cords are 'way more expensive than the junk one gets at Home Depot or similar stores. I do keep one of those Harbor Freight ones in my trunk; and, yes, it actually does deliver 12A at 120VAC without a quibble from the Tesla.
I strongly suspect that getting the voltage too low on your hammer might damage the hammer; or, more likely, light the typical cheapie orange-colored extension cord on fire, or melt the connectors, take your pick. They've probably had More Than Enough of That, hence the pinkie-swear at checkout.
