E2 Conductor Ampacity and Derating 3

Question 1

What is the maximum number of current-carrying conductors allowed in a conduit before a derating adjustment must be applied per NEC 310.15(C)(1)?

Accepted Answer

3

Answer

NEC 310.15(C)(1) requires adjustment factors when more than 3 current-carrying conductors are in a raceway or cable.

Question 2

A 3/0 AWG copper THW conductor is installed in an ambient temperature of 40°C. Using NEC Table 310.15(B)(1), what correction factor applies to the 75°C column ampacity?

Accepted Answer

0.82

Answer

For 75°C rated insulation at 40°C ambient, NEC Table 310.15(B)(1) specifies a correction factor of 0.82.

Question 3

Under NEC 310.15(A)(3), when can an engineer use an ampacity that differs from the NEC tables?

Accepted Answer

When using the engineering supervision method with a qualified engineer

Answer

NEC 310.15(A)(3) permits ampacity to be calculated under engineering supervision using recognized calculation methods as an alternative to table values.

Question 4

A 10 AWG THHN conductor in a 90°C rated conduit system has a terminal rated at 60°C. What ampacity column governs the calculation?

Accepted Answer

60°C column — 30A

Answer

Per NEC 110.14(C)(1), the conductor ampacity cannot exceed the terminal's temperature rating; with a 60°C terminal, the 60°C column (30A for 10 AWG) governs.

Question 5

In a commercial building, a 4-wire, 3-phase circuit with a linear load uses a neutral conductor. How is the neutral counted for bundling adjustment purposes?

Accepted Answer

It is not counted because it only carries unbalanced current from linear loads

Answer

NEC 310.15(E)(1) states that a neutral conductor on a 4-wire, 3-phase wye circuit carrying only unbalanced current from linear loads is not counted as a current-carrying conductor.

(E2) E2 Commercial Electrical Inspector Practice Test

(E2) E2 Commercial Electrical Inspector Practice Test

E2 Conductor Ampacity and Derating 3