At a 75°C termination rating, a 250 A overcurrent device needs at least 250 kcmil copper. That is the minimum on ampacity grounds — a long run may need more for voltage drop.
Minimum conductor
By material and termination rating
Material
60°C
75°C
90°C
Copper
350 kcmil
250 kcmil
4/0 AWG
Aluminum
500 kcmil
350 kcmil
300 kcmil
Smallest conductor whose published ampacity — capped by the small-conductor overcurrent rule where one applies — reaches 250 A, at the table's basis conditions of 30°C ambient and no more than three current-carrying conductors. Derating for a hotter location or a fuller raceway pushes these up.
Read the 90°C column carefully
The 90°C column above is almost never the answer at the terminals. Breakers, panels, and most equipment are listed for 60°C or 75°C conductor terminations, and the conductor must be sized at the temperature the termination is listed for, not the temperature the insulation can survive. The 90°C column exists mainly as a starting point for derating: you may correct down from it for ambient and conductor count, but the result still cannot exceed what the termination allows.
This trips people up constantly. Your THHN is 90°C-rated wire; your breaker is a 75°C termination. The 75°C column governs.
Equipment grounding conductor
Copper
4 AWG
Aluminum
2 AWG
Sized by
Rating of the overcurrent device ahead of the equipment
The equipment grounding conductor is sized from the overcurrent device, not from the circuit conductors. Where the circuit conductors are upsized — for voltage drop, for instance — the grounding conductor must generally be increased proportionally, a rule this reference does not compute.
Distance
How far 250 kcmil copper runs at 250 A
System voltage
Max run at 3% drop
120 V single phase
135 ft
240 V single phase
275 ft
480 V single phase
555 ft
One-way distance at which the circular-mil voltage-drop estimate reaches 3% of nominal, carrying the full 250 A. Beyond this the conductor needs to go up a size for voltage drop even though its ampacity is still fine.
Source:NFPA 70, National Electrical Code, 240.6(A); NFPA 70, National Electrical Code, Table 310.16; NFPA 70, National Electrical Code, Table 250.122. Published by NFPA.NEC 2023