Question 1

What is the minimum copper conductor size required for a 20-amp continuous load in a location with 60°C insulation rating?

Accepted Answer

10 AWG (NEC NEC 310.16). Per NEC 210.19(A)(1), continuous load conductor ampacity must be at least 125% of the load. 20A × 1.25 = 25A minimum. Per Table 310.16, 10 AWG at 60°C = 30A, which satisfies 25A. 12 AWG at 60°C = 20A is insufficient.

Question 2

A 30-amp non-continuous load requires which minimum copper conductor size at 75°C rating?

Accepted Answer

10 AWG (NEC NEC 310.16). Per NEC 310.16, Table 310.16, 75°C column: 10 AWG copper = 35A. Since the load is 30A non-continuous, a conductor rated for 30A or higher is required. 10 AWG at 75°C = 35A, which exceeds the 30A requirement.

Question 3

What is the minimum aluminum conductor size for a 40-amp load at 90°C insulation rating?

Accepted Answer

8 AWG (NEC NEC 310.16). Per NEC 310.16, Table 310.16, 90°C column for aluminum: 8 AWG = 45A, which exceeds the 40A non-continuous load requirement. 8 AWG is therefore the minimum conductor size. 6 AWG aluminum at 90°C = 55A is larger than required.

Question 4

A 50-amp continuous load requires which minimum copper conductor size at 75°C rating?

Accepted Answer

6 AWG (NEC NEC 310.16). Per NEC 310.16 and the 125% continuous load rule (210.19(A)(1)): 50A × 1.25 = 62.5A minimum conductor ampacity required. Table 310.16, 75°C column for copper: 6 AWG = 65A, which exceeds 62.5A. Therefore 6 AWG is the minimum required conductor. 4 AWG at 85A is oversized.

Question 5

Determine the minimum copper conductor size for a 100-amp continuous load at 90°C rating.

Accepted Answer

2 AWG (NEC NEC 310.16). Per NEC 215.2(A)(1)/210.19(A)(1), a continuous load requires conductor ampacity of at least 125%: 100 A x 1.25 = 125 A. Table 310.16, 90°C copper column: 3 AWG = 115 A (insufficient), 2 AWG = 130 A, which meets 125 A. Therefore 2 AWG is the minimum; 1 AWG (145 A) is larger than required.

Practice Questions: Conductor Sizing