Determine the conductor ampacity given the following: conductors are 500 kcmil THWN copper; ambient temperature is 125°F; eight current carrying conductors are in the raceway.

Determine the conductor ampacity given the following: conductors are 500 kcmil THWN copper; ambient temperature is 125°F; eight current carrying conductors are in the raceway.

• A. 178.2 amps
• B. 199.5 amps
• C. 294.6 amps
• D. 380 amps

Answer: (A)

Ampacity is not just the conductor’s nominal size; you must adjust for the insulation rating, the ambient temperature, and how many current-carrying conductors share the raceway. For a 500 kcmil copper THWN, use the base ampacity from the 75°C column in the NEC table (THWN is rated for 75°C). Then apply the ambient temperature correction for 125°F (about 52°C) from the same table, which reduces the allowable ampacity. Finally, since there are eight current-carrying conductors in the raceway, apply the derating factor for 7–9 conductors (from the derating table). Putting it together: base ampacity around 385 A, multiply by the ambient correction factor near 0.66, giving about 254 A, and then apply the derating factor of 0.70 for eight conductors to get about 178 A. This aligns with the given answer of 178.2 A.

Ampacity is not just the conductor’s nominal size; you must adjust for the insulation rating, the ambient temperature, and how many current-carrying conductors share the raceway.

For a 500 kcmil copper THWN, use the base ampacity from the 75°C column in the NEC table (THWN is rated for 75°C). Then apply the ambient temperature correction for 125°F (about 52°C) from the same table, which reduces the allowable ampacity. Finally, since there are eight current-carrying conductors in the raceway, apply the derating factor for 7–9 conductors (from the derating table).

Putting it together: base ampacity around 385 A, multiply by the ambient correction factor near 0.66, giving about 254 A, and then apply the derating factor of 0.70 for eight conductors to get about 178 A. This aligns with the given answer of 178.2 A.