Wire size for an EV charger circuit

Almost all modern building wire is rated 90 degrees Celsius. Almost no equipment is. Breakers, lugs and charger terminal blocks are typically listed for 60 or 75 degrees, and section 110.14(C) says the circuit is sized to the lowest of those. So the 90-degree column of the ampacity table is not the column you use, and a quote built on it is wrong.The general shape of the rule is that equipment rated 100 amps or less, and conductors of size 14 through 1, is used at the 60-degree column unless the equipment is marked for a higher temperature. Most current breakers and most EV charging equipment are marked 75 degrees. Some are not. The label is on the equipment and on the instructions, and an electrician who is doing this properly has looked at both.The second thing that pulls you back to the 60-degree column has nothing to do with the equipment. NM cable, the flat white or yellow stuff most people call Romex, must be used at 60-degree ampacity regardless of what its conductors are rated. That single sentence is why a garage run in NM cable needs a larger conductor than the same circuit run as individual conductors in conduit.

The 60-amp row is the one that generates arguments. Number six copper carries 65 amps in the 75-degree column and 55 amps in the 60-degree column. Sixty-five clears a 60-amp breaker and fifty-five does not. So the same wire is correct or incorrect depending entirely on what it lands on and how it is run, and both electricians can be telling you the truth.For the 50-amp row the same thing happens one size down: number eight copper is 50 amps at 75 degrees and 40 amps at 60 degrees. Which is why a 14-50 receptacle fed with NM cable is normally run in six-gauge cable, not eight, even though eight would be fine in conduit.

Resistance costs you voltage over distance, and the charger sees less than 240 volts at the far end. That is not a safety problem at the levels we are discussing; it is a slower charge and a warmer conductor. The code addresses it in an informational note recommending no more than three percent drop on a branch circuit and five percent overall. Informational notes are not enforceable, which is exactly why some quotes ignore it and some do not.A good quote designs to three percent. It costs one conductor size on a long run and it is the difference between a charger that delivers what the label says and one that quietly does not.

The code requires aluminium conductors to be an AA-8000 series alloy, which is a different material from the 1960s branch wire that gave aluminium its reputation. For a 60-amp circuit that means number four aluminium against number six copper. For a 50-amp circuit, number six aluminium against number eight copper.On a fifteen-foot run in a garage the material saving is trivial and copper is simpler. On a hundred-and-fifty-foot run out to a detached garage, aluminium is meaningfully cheaper and lighter and it is what many electricians will propose. That is a legitimate proposal, not a corner being cut.The condition is the terminations. Lugs listed for aluminium, marked AL7CU or AL/CU, torqued to the value printed in the equipment, with the manufacturer's requirement on antioxidant compound followed. Aluminium is unforgiving about torque in a way copper is not, and this is the part to ask about.

First: what breaker, what conductor size, and what wiring method. A quote that says sixty-amp circuit and stops has not told you the thing that decides the price. Ask whether it is NM cable, conductors in conduit, or armoured cable, because that determines the size and therefore the material cost.Second: are the terminations at both ends rated 75 degrees. If the answer is yes, number six copper on a 60-amp circuit is correct and you should not be charged for number four. If the answer is no or it depends, number four is correct and it is not padding.Third: how long is the run and was voltage drop considered. Under fifty feet this rarely matters. Over a hundred it always does.Fourth: is the conductor size on the permit application the same as the one in the quote. It should be. That is also the number the inspector checks, which makes it the one number in the whole quote that gets independently verified by someone who does not work for either of you.

Everything above is stripped of the conditions that a real design has to handle: ambient temperature correction, conduit fill and derating when more than three current-carrying conductors share a raceway, box fill, the specific terminals in the specific equipment, and whatever your jurisdiction amended. Those are not footnotes. They are the job.A 240-volt continuous circuit is licensed electrical work in every United States jurisdiction we are aware of, and the failure mode is heat inside a wall you cannot see. Use this to read a quote and to have a better conversation. Do not use it as a cut list.