4 circuits in a conduit is usually fine
With a 120/240 split phase circuit, the neutral isn't counted for conduit fill derate. That means every possible circuit in residential split-phase service has 2 wires. Four circuits have 8 wires.
Now we look on table 310.15(B)3a. We see that 7-9 wires calls for a 70% derate. WAIT.
For THHN, the 70% derate comes off the 90C column of the ampacity table. This is because of section 310.15(B):
the temperature correction and adjustment shall be permitted to be applied to the ampacity for the temperature rating of the conductor...
So we arrive at numbers as follows:
- 14 AWG: 25A x 70% = 17.5A
- 12 AWG: 30A x 70% = 21A
- 10 AWG: 40A x 70% = 28A
You said your draw is 24A.
Now if there were 4-6 wires, the derate would be 80%. That would give you 32A on your #10 wire, and you'd be unquestionably in the clear.
10 AWG is statutorially limited to 30A
Your 10 AWG wire is rated at 35A assuming the two wire terminations are rated at 75C, which is likely. (Less likely: 90C terminations in which case THHN or THWN-2 wire are good for 40A).
However, a separate rule in Code, 240.4(D), arbitrarily limits 10AWG to 30A max regardless. ThreePhaseEel explains why in a comment. It also limits #12 to 20A and #14 to 15A.
Continuous load derates to 125%
625.21 Overcurrent Protection. Overcurrent protection for feeders and branch circuits supplying electric vehicle supply equipment shall be sized for continuous duty and shall have a rating of not less than 125 percent of the maximum load of the electric vehicle supply equipment.
NEC 625.21 designates your EV charger a continuous load. You need to provision cable good for 125% of its 24A load, or 30A.
Do the derates stack?
That is the remaining question. Obviously some of these derates do not stack, for instance the 70% conduit fill derate does not stack with the statutory 240.4 derate. So do we take 125% of 35A, 30A or only 28A?
I have to tell you, my Code fu isn't quite there to answer that question. But I sure know it's a question worth asking, and not just assuming.
It's a wobbler, to be sure, but only a statutory one.
And are you sure that’s a 24A charger and not a 22A charger? Can the software be patched/configured to make it a 22A charger? May be worth hitting up the manufacturer and seeing what they can tell you.
If you can eliminate one of the three other circuits in the conduit, so you have 3 circuits in total, that will reduce the 310.15(B) derate to 80%, which will change that "28A" number to "32A" and again moot the question.
Another way to do that same thing without losing any circuits is to convert two of the three other circuits into a multi-wire branch circuit for the length of its run in the conduit. The entire MWBC counts as 2 wires for 310.15(B) purposes, so again your derate comes up to 80%/32A.