E2 Sizing and Protection of Motor Circuits 3 — Questions and Answers
Question 1: Per NEC 430.32(A)(1), overload protection for a motor with a service factor of 1.15 or greater must be set to trip at no more than what percentage of the motor's FLA?
- 115% of FLA
- 125% of FLA (Correct answer)
- 140% of FLA
- 150% of FLA
Correct answer: 125% of FLA
Motors with SF ≥ 1.15 or temperature rise ≤ 40°C require overload protection set to trip at no more than 125% of FLA per NEC 430.32(A)(1).
Question 2: For a motor with a service factor of 1.0 and a temperature rise exceeding 40°C, what is the maximum allowable overload trip setting per NEC 430.32(A)(1)?
- 110% of FLA
- 115% of FLA (Correct answer)
- 125% of FLA
- 140% of FLA
Correct answer: 115% of FLA
Motors without SF ≥ 1.15 or temperature rise ≤ 40°C are limited to an overload trip setting of no more than 115% of FLA.
Question 3: A 460V, 3-phase motor has a nameplate FLA of 22A and a service factor of 1.15. What is the maximum overload relay trip setting per NEC 430.32(A)(1)?
- 22.0A
- 25.3A
- 27.5A (Correct answer)
- 30.8A
Correct answer: 27.5A
With SF ≥ 1.15, maximum overload setting = 125% of nameplate FLA = 1.25 × 22A = 27.5A.
Question 4: Per NEC 430.32(C), if the standard overload setting is insufficient to start or carry the load for a motor with SF ≥ 1.15, the maximum allowable trip setting may be increased to:
- 125% of FLA
- 130% of FLA
- 140% of FLA (Correct answer)
- 150% of FLA
Correct answer: 140% of FLA
NEC 430.32(C) permits the trip setting to be increased up to 140% of FLA for motors with SF ≥ 1.15 when the standard setting cannot carry the load.
Question 5: Per NEC 430.37, how many overload sensing elements are required for a 3-wire, 3-phase motor with no neutral conductor?
- 1 element
- 2 elements
- 3 elements (Correct answer)
- 4 elements
Correct answer: 3 elements
NEC 430.37 requires one overload sensing element in each of the three ungrounded phase conductors for a 3-wire, 3-phase motor.
Question 6: Per NEC 430.32(B), a motor with a nameplate marked 'thermally protected' may use the built-in thermal protector to serve as the overload protection, provided the protector is approved for use with the motor and will:
- Trip at exactly 115% of FLA
- Prevent dangerous overheating of the motor (Correct answer)
- Open the circuit within 30 seconds of an overload
- Operate only when the motor draws locked rotor current
Correct answer: Prevent dangerous overheating of the motor
NEC 430.32(B) allows built-in thermal protectors as overload protection when approved for use with the specific motor and capable of preventing dangerous overheating.
Question 7: Per NEC 430.43, where the automatic restarting of a motor after an overload trip could be hazardous to personnel, which type of overload device is prohibited?
- Manual reset overload relay
- Automatic reset overload relay (Correct answer)
- Electronic overload relay with lockout
- Bimetallic relay with manual reset button
Correct answer: Automatic reset overload relay
Automatic reset overload relays are prohibited where automatic restarting after a trip would endanger personnel, as no human action is required to restart the motor.
E2 Sizing and Protection of Motor Circuits 3
Per NEC 430.32(A)(1), overload protection for a motor with a service factor of 1.15 or greater must be set to trip at no more than what percentage of the motor's FLA?