E2 Sizing and Protection of Motor Circuits 2 — Questions and Answers
Question 1: Per NEC 430.22, the minimum ampacity for branch circuit conductors supplying a single continuous-duty motor shall be what percentage of the motor's full-load current?
- 100% of FLA
- 115% of FLA
- 125% of FLA (Correct answer)
- 150% of FLA
Correct answer: 125% of FLA
NEC 430.22 requires branch circuit conductor ampacity of at least 125% of the motor's FLA for continuous-duty applications.
Question 2: Which NEC table provides full-load current values for single-phase AC motors used for conductor and protection sizing?
- Table 430.247
- Table 430.248 (Correct answer)
- Table 430.249
- Table 430.250
Correct answer: Table 430.248
Table 430.248 lists FLA values for single-phase AC motors, which are used in place of nameplate values for conductor and overcurrent device sizing.
Question 3: A 5 HP, 230V, single-phase motor has a table FLA of 28A. What is the minimum required branch circuit conductor ampacity?
- 28A
- 32.2A
- 35A (Correct answer)
- 42A
Correct answer: 35A
Minimum conductor ampacity = 125% × FLA = 1.25 × 28A = 35A per NEC 430.22.
Question 4: Per NEC 430.6(A)(1), which current value is used for sizing motor branch circuit conductors and short-circuit/ground-fault protective devices?
- Motor nameplate ampere rating
- Values from Tables 430.247 through 430.250 (Correct answer)
- Locked rotor current rating
- Service factor current
Correct answer: Values from Tables 430.247 through 430.250
NEC 430.6(A)(1) requires using table values from 430.247–430.250, not the motor nameplate, for conductor and short-circuit/ground-fault protection sizing.
Question 5: A 20 HP, 460V, 3-phase motor has a table FLA of 27A. What is the minimum branch circuit conductor ampacity required by NEC 430.22?
- 27A
- 31.1A
- 33.75A (Correct answer)
- 40.5A
Correct answer: 33.75A
125% of 27A = 33.75A is the minimum conductor ampacity per NEC 430.22 for continuous-duty operation.
Question 6: Which NEC table provides full-load current values for 3-phase AC squirrel cage and synchronous motors?
- Table 430.247
- Table 430.248
- Table 430.249
- Table 430.250 (Correct answer)
Correct answer: Table 430.250
Table 430.250 provides FLA values for 3-phase AC motors, including squirrel cage and synchronous types, used for sizing conductors and overcurrent devices.
Question 7: Per NEC Table 430.22(E), motor branch circuit conductors for a 30-minute duty-rated motor must have an ampacity of at least what percentage of the motor's FLA?
- 110%
- 120%
- 150% (Correct answer)
- 200%
Correct answer: 150%
NEC Table 430.22(E) requires 150% of FLA for branch circuit conductors supplying 30-minute and 60-minute duty-rated motors.
E2 Sizing and Protection of Motor Circuits 2
Per NEC 430.22, the minimum ampacity for branch circuit conductors supplying a single continuous-duty motor shall be what percentage of the motor's full-load current?