Wiring a 5 ton rooftop unit and a 220 plug
Yeah, that's fine but you have to follow the rules. 

The fill rules

All wires count.  (In switch loops, neutrals also count even if they're not even present). 

There are various conduit fill calculators around the Web and on smartphone apps. 

The derate rules


Grounds don't count. 
If there are 2 or 3 hot wires, neutrals don't count because they only carry differential current 
This means in 120/240V single or split phase, 1 circuit always counts as 2 wires. 
You have 4 wires
NEC 310.15(B)(2) calls for an 80% derate. But wait.
The derate is off the 90C column in 310.15(B)(16), or to be more precise the highest column allowed by the wire type; so for instance THWN (not THWN-2) uses the 75C column.  There is a special rule that NM uses the 90C column even though it is 60C wire, but NM better not be outside.  I presume your wire is THWN***-2***, which is 90C, so here we go: 


THWN-2 #8 gets 55A@90C, it derates 80% to 44 amps. 
THWN-2 #10 gets 40A@90C, it derates 80% to 32 amps. 



Well, that's not so bad.  You are still capped at the 30A and 40A limits, so this doesn't really limit you.

3 circuits (6 wires) uses the same derate.  

4 circuits uses a 70% derate, which will bite you on #10 and #8, (28A and 38.5A), however you get to round up to the next available breaker size, so you dodge a bullet.