I live in the U.S. I have a NEMA 14-50 outlet in my garage. The wiring in my wall is 6 AWG copper stranded. I have been trying to calculate the maximum load I can safely run through these wires.

The formula I see online is: [Copper Resistance Value] * [Desired amperage] * [Length of run in feet] / [Allowable voltage drop]

The sites here and here use a value of 12.9 for the resistance factor. The site here uses a value of 11.2. I am not sure which is correct, but I am using 12.9 to be on the safe side.

I want 60 amps. The outlet is 30 ft away meaning 60 ft round trip. A standard voltage drop of 3% is fine. (12.9 * 60 amps * 2 * 30 ft) / (240v * 3%) = 6450.

Now, from reading I believe this number is in a unit called circular mils. I see that 12 AWG wire has a maximum of 6530 circular mils. And low and behold the first two sites I list say 12 AWG is what I need.

Now, the third site has an extra check embedded in the JavaScript. It contains a maximum amperage per wire gauge. It also calculates that the circular mils are fine, however it has to go up to 4 AWG in order to support 60 amps. Its internal code lists 6 AWG as only supporting 55 amps while 4 AWG supports 70 amps. The Wikipedia page on AWG lists the same values.

I am trying to charge an EV off this. The standard here is to deduct 20% of the maximum amperage and run at that rate. So a 60 amp line can charge an EV at a rate of 48 amps. This is exactly what I want. The first two sites say I am fine. The third site says I can only do 55 amps on the circuit which correlates to a 44 amp charge rate on my EV.

My car allows granular control over its charging rate so I can set it up to run either way. The breaker is only 50 amp though I have found 55 amp breakers online. They are rare but doable.

So, can I put in a 60 or 55 amp breaker and charge at a rate of 48 amps? What about putting in a 55 amp breaker and charging at 44 amps? There are no spikes above this, it is a constant charge rate.

You are pretty close to the edge there. If it were me, I’d switch to #4 wire since this is going to basically be a continuous use thing (it’s going to run at high current for long periods of time while your EV is charging). Switch to a 60 amp breaker and a 60 amp outlet.

Don’t forget to swap out your outlet. The one you have is only rated for 50 amps. NEMA-14 outlets are available up to 60 amps.

I have a electric car and got the recommended charger / wiring installed in the garage.

I was wondering if the charger is smart enough to detect supply line voltage drop from 0 load to full load and accordingly adjust the charging current or completely turn it off.

Although not really the best solution, I can see how a charger like that, can mitigate the OPs concern. It may even prevent fires.

I understand that this is not the case for most household equipment, but the EV charger we have seems to have a lot of engineering built into it.

To nitpick the question asked, the answer is gigawatts or any extremely large wattage rating. To simplify, wattage is a power measurement. Current is a flow measurement. Voltage is the pressure measurement. Pressure times flow is power. Power factor and other physical measurements get factored in for true power ratings but most of the time the two biggest factors are current and voltage. Bare 6 guage wire strung off proper insulators can carry 75 amp all day at gigavolts. I do not know if there is a maximum voltage for copper wires but I’d think physics has an answer that I’m to lazy to look for.
The amperage of a wire is also limited by the ambient temperature and the rating of the insulation. A doorbell wire could probably safely carry 20 amps for quite a while if it were uninsulated and strung outdoors in an artic winter. Look at the actual size of the wire in a glass fuse sometime.
In the OP situation, 60 amps through a #6 wire should be doable if the insulation rating is for something higher than 40C. What is the maximum charge rate duty cycle? Is it always 48A or will it drop over the course of the night? How old is the wiring. I’d be leery if it were varnish and paper or even from before the mid 70’s. If newer, I’d throw the proper 60 amp receptacle and breaker on and not give it another thought.