You don't have a 60A subpanel.
I know this because the size of a subpanel is decided by the capacity of the feeder wires... and there is no such thing as 60A wire.
Really. Don't believe me, go look at NEC Table 310.15(B)(16). No wire is 60A.
So, what you actually have is one of these:
- 55A wire - in which a 60A breaker is used due to the "round up" rule in 240.4(B). You can't plan to use more than 55A, though.
- 65A or larger wire, and they just went with a 60A breaker because they are cheaper than 70A breakers.
So your next step is to find out what size and metal of wire you actually have, as well as which insulation type. Then we can figure out the ampacity of the cable, and that tells you how much room you have for EV charging.
Next, a load calculation on the loads served by the subpanel.
There is a standard format for Load Calculations. Do it for the loads which are now served out of the subpanel. That tells you how many amps you are using now.
Subtract one from the other and...
Electric vehicle charging is a "soft setting" anyway!
Given that your amp need is variable, I gather we're charging an EV and you are installing an EVSE (the thing that goes on the wall with a J1772 cord coming off it).
The ampacity of an EV is adjustable (on the fly actually) - here's a technology briefer on how that works.
When you install or "Commission" an EVSE, you configure how many amps it has at the breaker, by cracking it open and setting DIP switches (or an intentionally difficult means using a special WiFi network).
So for instance if you find your circuit is really 55A and you have 13A of load already provisioned, you have 42A "left" for EV charging. You tell the EVSE that. Internally it will multiply that by 80% giving 33.6A, and tell the EV not to draw more than that when it charges.