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Mike Holt - EC&M Questions & Answers - March 2013
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2011 NEC Questions and Answers - March 2013
Based on - NEC - 2011 Edition

NEC Questions and Answers - Based on 2011 NEC
March 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 is the Code rule for sizing feeder conductors, both ungrounded and neutral? Please show an example.

A1. Section 215.2(A)(1) requires that the minimum feeder conductor ampacity, before the application of ambient temperature correction [310.15(B)(2)(a)], conductor bundling adjustment [310.15(B)(3)(a)], or both, must be no less than 125 percent of the continuous load, plus 100 percent of the noncontinuous load, based on the terminal temperature rating ampacities as listed in Table 310.15(B)(16) [110.14(C)(1)].

Author’s Comment: See 215.3 for the feeder overcurrent device sizing requirements for continuous and noncontinuous loads.

However, there are two exceptions to this rule:

Ex 1: If the assembly and the overcurrent device are both listed for operation at 100 percent of its rating, the conductors can be sized at 100 percent of the continuous load.

Author’s Comment: Equipment suitable for 100 percent continuous loading is rarely available in ratings under 400A.

Ex 2: Neutral conductors can be sized at 100 percent of the continuous and noncontinuous load.

Question: What size feeder conductors are required for a 200A continuous load if the terminals are rated 75ºC?

(a)  2/0 AWG ungrounded conductors and a 1/0 AWG neutral conductor
(b) 3/0 AWG ungrounded conductors and a 1/0 AWG neutral conductor
(c) 4/0 AWG ungrounded conductors and a 1/0 AWG neutral conductor
(d) 250 kcmil ungrounded conductors and a 3/0 AWG neutral conductor

Answer: (d) 250 kcmil AWG ungrounded conductors and a 3/0 AWG neutral conductor

Since the load is 200A continuous, the feeder conductors must have an ampacity of not less than 250A (200A x 1.25). The neutral conductor is sized to the 200A continuous load according to the 75ºC column of Table 310.15(B)(16). 250 kcmil has an ampacity of 255A, and 3/0 has an ampacity of 200A.

The feeder neutral conductor must be sized to carry the maximum unbalanced load in accordance with 220.61 and must not be smaller than the size listed in 250.122, based on the rating of the feeder overcurrent device. The sizing requirements of 250.122(F) for parallel conductors don’t apply [215.2(A)(2)]

Question: What size neutral conductor is required for a feeder consisting of 250 kcmil ungrounded conductors and one neutral conductor protected by a 250A overcurrent device, where the unbalanced load is only 50A, with 75ºC terminals?

(a) 6 AWG    (b) 4 AWG    (c) 1/0 AWG     (d) 3/0 AWG

Answer: (b) 4 AWG [based on Table 250.122]

Table 310.15(B)(16) and 220.61 permit an 8 AWG neutral conductor, rated 50A at 75ºC to carry the 50A unbalanced load, but the neutral conductor isn’t permitted to be smaller than 4 AWG, as listed in Table 250.122, based on the 250A overcurrent device.

The feeder conductor ampacity must not be less than that of the service conductors where the feeder conductors carry the total load supplied by service conductors with an ampacity of 55A or less [215.2(A)(3)].

Also, be aware that the feeder conductors for individual dwelling units or mobile homes need not be larger than service conductors sized to 310.15(B)(7) [215.2(A)(4)].

Informational Note 2: To provide reasonable efficiency of operation of electrical equipment, feeder conductors should be sized to prevent a voltage drop not to exceed 3 percent. In addition, the maximum total voltage drop on both feeders and branch circuits shouldn’t exceed 5 percent.

Informational Note 3: See 210.19(A), Note 4, for voltage drop for branch circuits.

Q2. What are the Code rules for multiwire branch circuits?

A2. A multiwire branch circuit is one that consists of two or more ungrounded circuit conductors with a common neutral conductor. There must be a difference of potential (voltage) between the ungrounded conductors and an equal difference of potential (voltage) from each ungrounded conductor to the common neutral conductor.

A multiwire branch circuit can be considered a single circuit or a multiple circuit [210.4(A)].

To prevent inductive heating and to reduce conductor impedance for fault currents, all conductors of a multiwire branch circuit must originate from the same panelboard.

Author’s Comment: For more information on the inductive heating of metal parts, see 300.3(B), 300.5(I), and 300.20.

Informational Note [to210.4(A)]: Unwanted and potentially hazardous harmonic neutral currents can cause additional heating of the neutral conductor of a 4-wire, three-phase, 120/208V or 277/480V wye-connected system, which supplies nonlinear loads. To prevent fire or equipment damage from excessive harmonic neutral currents, the designer should consider: (1) increasing the size of the neutral conductor, or (2) installing a separate neutral for each phase.

Author’s Comments:
•  See the definition of “Nonlinear Load” in Article 100.
•  For more information, please visit www.MikeHolt.com. Click on “Technical Information” on the left side of the page, and then select “Power Quality.”

Each multiwire branch circuit must have a means to simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates [210.4(B)].

Note: Individual single-pole circuit breakers with handle ties identified for the purpose can be used for this application [240.15(B)(1)].

Caution: This rule is intended to prevent people from working on energized circuits they thought were disconnected.
Multiwire branch circuits must supply only line-to-neutral loads [210.4(C)].

Ex 1: A multiwire branch circuit is permitted to supply an individual piece of line-to-line utilization equipment, such as a range or dryer

Ex 2: A multiwire branch circuit is permitted to supply both line-to-line and line-to-neutral loads if the circuit is protected by a device such as a multipole circuit breaker with a common internal trip that opens all ungrounded conductors of the multiwire branch circuit simultaneously under a fault condition.

Informational Note: See 300.13(B) for the requirements relating to the continuity of the neutral conductor on multiwire branch circuits.

Caution: If the continuity of the neutral conductor of a multiwire circuit is interrupted (opened), the resultant over- or undervoltage can cause a fire and/or destruction of electrical equipment.

The ungrounded and neutral conductors of a multiwire branch circuit must be grouped together by cable ties or similar means at the point of origination [210.4(D)].

Ex: Grouping isn’t required where the circuit conductors are contained in a single raceway or cable unique to that circuit that makes the grouping obvious.

Author’s Comment: Grouping all associated conductors of a multiwire branch circuit together by cable ties or other means within the point of origination makes it easier to visually identify the conductors of the multiwire branch circuit. The grouping will assist in making sure that the correct neutral is used at junction points and in connecting multiwire branch-circuit conductors to circuit breakers correctly, particularly where twin breakers are used. If proper diligence isn’t exercised when making these connections, two circuit conductors can be accidentally connected to the same phase.

Caution: If the ungrounded conductors of a multiwire circuit aren’t terminated to different phases or lines, the currents on the neutral conductor won’t cancel, but will add, which can cause an overload on the neutral conductor.

Q3. How do I size the conductors and protection device for a single motor?

A3. 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)]

Q4. How do I size the conductors and protection device for a feeder to several motors?

A4. 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 and feeder inverse time circuit breaker 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 (for each motor)

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.

Description: 2011 NEC Practice Questions Textbook - 11PQ
For more NEC Practice, purchase Mike Holt's NEC Exam Practice Questions book based on the 2011 NEC.

 

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Comments
  • I really appreciate the Newsletter's educational content. Thanks!

    Sarah Raymer  April 4 2013, 10:19 am EDT
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