NEC Questions and Answers - Based on 2011 NEC
December 2013

By Mike Holt for EC&M Magazine

Here’s the follow up to yesterday’s newsletter. This includes all of the answers to the questions sent, so you can see how you did.

Q1. What does the Code require for overcurrent protection sizing for a single motor-compressor air-conditioner? Please provide an example.
A1.  The short-circuit and ground-fault protective device for single motor-compressors must not be more than 175 percent of the motor-compressor current rating. If the protective device sized at 175 percent isn’t capable of carrying the starting current of the motor-compressor, the next size larger protective device can be used, but in no case can it exceed 225 percent of the motor-compressor current rating [440.22(A)].

Question: What size conductor and protection are required for a 24A motor-compressor connected to a 240V circuit?
(a) 10 AWG, 40A        (b) 10 AWG, 60A        (c) a or b         (d) 10 AWG, 90A

Answer: (a) 10 AWG, 40A
Step 1: Determine the branch-circuit conductor [Table 310.15(B)(16) and 440.32]:
24A x 1.25 = 30A, 10 AWG, rated 30A at 75°C [Table 310.15(B)(16)]
Step 2: Determine the branch-circuit protection [240.6(A) and 440.22(A)]:
24A x 1.75 = 42A, next size down = 40A

If the 40A short-circuit and ground-fault protective device isn’t capable of carrying the starting current, then the protective device can be sized up to 225 percent of the equipment load current rating. 24A x 2.25 = 54A, next size down 50A

Q2. What is the Code rule regarding disconnects on the primary side of transformers?
A2. A disconnect is required to disconnect all transformer ungrounded primary conductors. The disconnect must be located within sight of the transformer, unless the location of the disconnect is field-marked on the transformer and the disconnect is lockable [450.14].

Author’s Comment: Within sight means that it’s visible and not more than 50 ft from one to the other [Article 100].

Q3. Does the NEC require transformers to be accessible?
A3. Transformers must be readily accessible to qualified personnel for inspection and maintenance, except as permitted by 450.13 (A) or (B) [450.13]:

Author’s Comment: Dry-type transformers not exceeding 50 kVA with a metal enclosure can be installed above a suspended-ceiling space used for environmental air-handling purposes (plenum) [300.22(C)(3)].

Q4. How do I calculate the minimum size of conductor required by the NEC for a single motor in a continuous duty application? Please give an example.
A4. Conductors to a single motor must be sized not less than 125 percent of the motor FLC rating as listed in Table 430.247 Direct-Current Motors, Table 430.248 Single-Phase Motors, or Table 430.250 Three-Phase Motors [430.22].

Question: What size branch-circuit conductor is required for a 7½ hp, 230V, three-phase motor?
(a) 14 AWG     (b) 12 AWG     (c) 10 AWG     (d) 8 AWG

Answer: (c) 10 AWG
Motor FLC = 22A [Table 430.250]
Conductor’s Size = 22A x 1.25
Conductor’s Size = 27.50A, 10 AWG, rated 30A at 75°C [Table 310.15(B)(16)]

Note: The branch-circuit short-circuit and ground-fault protection device using an inverse time breaker is sized at 60A according to 430.52(C)(1) Ex 1:

Circuit Protection = 22A x 2.50
Circuit Protection = 55A, next size up 60A [240.6(A)]
Circuit conductors that supply several motors must not be sized smaller than the sum of the following [430.24]:
(1)        125 percent of the full-load current of the highest rated motor
(2)        The full-load current ratings of other motors

Question: What size feeder conductor is required for two 7½ hp, 230V, three-phase motors, if the terminals are rated for 75°C?
(a) 14 AWG         (b) 12 AWG         (c) 10 AWG         (d) 8 AWG

Answer: (d) 8 AWG
Motor FLC = 22A [Table 430.250]
Motor Feeder Conductor = (22A x 1.25) + 22A
Motor Feeder Conductor = 49.50A, 8 AWG rated 50A at 75°C [Table 310.15(B)(16)]

Author’s Comment: The feeder overcurrent device (inverse time circuit breaker) must comply with 430.62 as follows:
Step 1: Determine the largest branch-circuit overcurrent device rating [240.6(A) and 430.52(C)(1) Ex 1]:
22A x 2.50 = 55A, next size up 60A
Step 2: Size the feeder overcurrent device in accordance with 240.6(A) and 430.62:
Feeder Inverse Time Breaker: 60A + 22A = 82A, next size down, 80A

Author’s Comment: The “next size up protection” rule for branch circuits [430.52(C)(1) Ex 1] doesn’t apply to motor feeder short-circuit and ground-fault protection device sizing.

Q5. What is the Code requirement for disconnecting means of electric space heating equipment?
A5.  Means must be provided to simultaneously disconnect the heater, motor controller, and supplementary overcurrent devices of all fixed electric space-heating equipment from all ungrounded conductors [424.19].

The disconnecting means must be capable of being locked in the open position. The provision for locking or adding a lock to the disconnecting means must be on the switch or circuit breaker, and it must remain in place with or without the lock installed.

The disconnecting means for fixed electric space-heating equipment with supplementary overcurrent protection must be within sight from the supplementary overcurrent devices [424.19(A)].

Author’s Comment: According to Article 100, within sight means that it’s visible and not more than 50 ft from one to the other.

For fixed electric space-heating equipment without supplementary overcurrent protection, the branch-circuit circuit breaker is permitted to serve as the disconnecting means if the circuit breaker is within sight from the heater or it’s capable of being locked in the open position [424.19(B)].

A unit switch with a marked “off” position that’s an integral part of the equipment can serve as the heater disconnecting means, if it disconnects all ungrounded conductors of the circuit [424.19(C)].

For more NEC Practice, purchase Mike Holt's NEC Exam Practice Questions book based on the 2011 NEC.