Office receptacle circuits, 120 V, two wires each, 1 in. EMT, 40 ft run. Cross-section to scale.
Full load is 20 A per circuit (15 A on 15 A breakers), spread across its receptacles; half is half.
Hot and neutral both carry current, so both count, 310.15(E)(1). The ground does not, 310.15(F).
Each wire keeps the percent in Table 310.15(C)(1) for its count of wires, then the factor in Table 310.15(B)(1)(1) for hot air.
THHN starts at the 90°C column of Table 310.16, held to the 75°C column by 75°C terminals, 310.15(A), 110.14(C)(1)(a)(3).
The wire must be good for its breaker, 240.4. No next size up for multi-receptacle circuits, 240.4(B)(1). 12 AWG copper: 20 A breaker at most, 240.4(D).
EMT is itself a ground, 250.118(A)(4); the green wire is trade practice. One green serves the pipe, 250.122(E).
Two pipes: each circuit stays together, 300.5(B). 10 AWG: the ground stays 12 AWG, 250.122(D).
Neutrals are grouped with their hots in each box, 200.6(B).
Loads run under 3 hours. Longer loads are held to 80% of the breaker, 210.20(A).
Temperatures are estimated by a model: a wire at exactly its allowed amps settles at 90°C. Blue is under 90°C, red is over. Real wire does not glow.
The table cuts in steps, so four circuits to five looks sudden.
Time is sped up. A real pipe run takes much longer to heat.
The 50°C attic is a chosen scenario. Pipes in sun less than 3/4 in. above a roof get a 33°C adder, 310.15(B)(2).
Shared-neutral and three-phase circuits are left out.
Twelve 10 AWG wires plus the ground fill 31% of the pipe, under 40%, Chapter 9 Table 1.