Free E2 Motors and Transformers Questions and Answers 1 — Questions and Answers
Question 1: An inspector is examining a 10 HP, 460-volt, 3-phase squirrel-cage motor. According to NEC Table 430.250, its Full Load Current (FLC) is 14A. What is the maximum permitted rating for an inverse-time circuit breaker used for branch-circuit short-circuit and ground-fault protection?
- 25A
- 40A
- 30A
- 35A (Correct answer)
Correct answer: 35A
NEC 430.52(C)(1) and Table 430.52 permit sizing an inverse-time circuit breaker for a polyphase squirrel-cage motor at a maximum of 250% of the motor's FLC. The FLC from Table 430.250 is 14A. Calculation: 14A x 250% = 35A. Since 35A is a standard size listed in NEC 240.6(A), it is the maximum permitted rating.
Question 2: A dry-type transformer is installed indoors in a commercial building. According to NEC Article 450, which of the following is a general requirement for its installation?
- The transformer must always be installed in a fire-resistant vault.
- A disconnecting means must be located in sight of the transformer, unless it is lockable and its location is field-marked on the transformer. (Correct answer)
- The primary overcurrent protection must not exceed 100% of the transformer's rated primary current.
- Ventilating openings can be blocked as long as the ambient temperature does not exceed 30°C.
Correct answer: A disconnecting means must be located in sight of the transformer, unless it is lockable and its location is field-marked on the transformer.
NEC 450.14 requires the disconnecting means to be in sight from the transformer. However, it provides an exception: the disconnect can be in a remote location if it is capable of being locked in the open position and its location is field-marked on the transformer. Vaults are only required for specific high-voltage or oil-filled transformers, not all dry-types. Primary overcurrent protection is typically allowed to be higher than 100% (e.g., 125%). Blocking ventilation is prohibited by NEC 450.9.
Question 3: When sizing separate overload protection for a continuous-duty motor with a service factor of 1.15, what is the maximum percentage of the motor's nameplate full-load ampere (FLA) rating that should be used?
- 115%
- 250%
- 125% (Correct answer)
- 100%
Correct answer: 125%
According to NEC 430.32(A)(1), a separate overload device for a continuous-duty motor with a service factor of 1.15 or greater shall be selected to trip at no more than 125% of the motor's nameplate full-load current rating. The 115% value is for motors with a service factor less than 1.15 or a marked temperature rise over 40°C. The 250% value relates to short-circuit protection, not overload protection.
Question 4: An inspector observes a dry-type transformer with a horizontal top surface installed in a readily accessible electrical room. What specific marking is required on the transformer's top surface according to NEC 450.9?
- The transformer's kVA rating and voltage.
- A warning to maintain 3 feet of clearance.
- The date of installation.
- A marking to prohibit storage. (Correct answer)
Correct answer: A marking to prohibit storage.
NEC 450.9(A) specifically requires that transformer top surfaces that are horizontal and readily accessible shall be marked to prohibit storage. This is to ensure that ventilation is not impeded by items being placed on top of the transformer. While other markings are required on the nameplate, this specific surface marking is for preventing blocked ventilation.
Question 5: A motor controller for a 480V motor is located in an electrical room, and the motor itself is located 100 feet away on the factory floor, out of sight from the controller. Which of the following is required by NEC 430.102(B)?
- The motor circuit conductors must be oversized by 25%.
- The motor controller must have a horsepower rating.
- A disconnecting means must be installed in sight from the motor and driven machinery location. (Correct answer)
- A second set of overload heaters must be installed at the motor.
Correct answer: A disconnecting means must be installed in sight from the motor and driven machinery location.
NEC 430.102(B) requires a disconnecting means to be located in sight from the motor and the driven machinery location. An exception exists if the controller's disconnecting means is individually lockable in the open position, but the general rule is to have a disconnect within sight (defined as visible and not more than 50 feet away). Since the motor is out of sight, a local disconnect is required.
Question 6: For a transformer rated 1000 volts or less with a primary current of 15 amperes, what is the maximum rating of an overcurrent protective device (OCPD) on the primary side if it is the *only* protection for the transformer?
- 250% of the primary current
- 125% of the primary current (Correct answer)
- 150% of the primary current
- 115% of the primary current
Correct answer: 125% of the primary current
According to NEC Table 450.3(B), for a transformer with a primary current of 9 amperes or more and protected by an OCPD on the primary side only, the maximum rating or setting of that device is 125% of the rated primary current. Note 1 to the table permits the next higher standard size if 125% does not correspond to a standard size, but the base percentage is 125%.
Free E2 Motors and Transformers Questions and Answers 1
An inspector is examining a 10 HP, 460-volt, 3-phase squirrel-cage motor.
According to NEC Table 430.250, its Full Load Current (FLC) is 14A.
What is the maximum permitted rating for an inverse-time circuit breaker used for branch-circuit short-circuit and ground-fault protection?