NEC Questions and Answers – Based on the 2011 NEC

February 2012

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. Is bonding of gas metal piping systems required by the NEC?

A1. Metal-piping systems such as sprinkler, gas, or air that are likely to become energized must be bonded. The equipment grounding conductor for the circuit that’s likely to energize the piping can ¬¬serve as the bonding means [250.104(B)].

Note 1: Bonding all piping and metal air ducts within the premises will provide additional safety.

Note 2: The National Fuel Gas Code, NFPA 54, Section 7.13 contains further information about bonding gas piping.

Informational Notes in the NEC are for information purposes only and aren’t enforceable as a requirement of the Code [90.5(C)].

Q2. Can low voltage lighting transformers be supplied from a kitchen small appliance branch circuit?

A2. No. The 20A, 120V small-appliance circuits required by 210.11(C)(1) must not supply outlets for luminaires or appliances [210.52(B)(2)], except for a receptacle for an electric clock or gas-fired range, oven, or counter-mounted cooking unit.

Q3. What is the minimum size conductor required by the NEC to supply a single air conditioner? Please provide an example.

A3. Branch-circuit conductors to a single motor-compressor must have an ampacity not less than 125 percent of the motor-compressor rated-load current or the branch-circuit selection current, whichever is greater [440.32].

Additionally, branch-circuit conductors for a single motor-compressor must have short-circuit and ground-fault protection sized between 175 percent and 225 percent of the rated-load current [440.22(A)].

Example: What size conductor and overcurrent device are required for an 18A motor compressor?

(a) 12 AWG, 30A (b) 10 AWG, 50A (c) a or b (d) 10 AWG, 60A

Answer: (a) 12 AWG, 30A

Step 1: Determine the branch-circuit conductor [Table 310.15(B)(16) and 440.32]:

18A x 1.25 = 22.50A, 12 AWG, rated 25A at 75°C [Table 310.15(B)(16)]

Step 2: Determine the branch-circuit protection [240.6(A) and 440.22(A)]:

18A x 1.75 = 31.50A, next size down = 30A If the 30A short-circuit and ground-fault protection 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. 18A x 2.25 = 40.50A, next size down 40A

A 30A or 40A overcurrent device is permitted to protect a 12 AWG conductor for an air-conditioning circuit. See 240.4(G) for details.

Q4. What are the Code requirements for Type NM cable installed in an accessible attic?

A4. Type NM cable installed in accessible attics or roof spaces must comply with 320.23 [334.23].

• On the Surface of Floor Joists, Rafters, or Studs. In attics and roof spaces that are accessible, substantial guards must protect cables installed across the top of floor joists, or across the face of rafters or studding within 7 ft of the floor or floor joists. If this space isn’t accessible by permanent stairs or ladders, protection is required only within 6 ft of the nearest edge of the scuttle hole or attic entrance [320.23(A)].

• Along the Side of Framing Members [320.23(B)]. When Type NM cable is installed on the side of rafters, studs, or floor joists, no protection is required if the cable is installed and supported so the nearest outside surface of the cable or raceway is at least 1¼ in. from the nearest edge of the framing member if nails or screws are likely to penetrate [300.4(D)].

Q5. Is a transfer switch required for portable generators used as optional standby systems?

A5. Yes, a transfer switch is required for all fixed or portable optional power systems [702.5].

Ex: Temporary connection of a portable generator without transfer equipment is permitted where written safety procedures are in place and conditions of maintenance and supervision ensure that only qualified persons will service the installation, and where the normal supply is physically isolated by a lockable disconnecting means or by the disconnection of the normal supply conductors.

Q6. How close can a panelboard be mounted to a sink or other non-electrical equipment?

A6. For the purpose of safe operation and maintenance of equipment, access and working space must be provided about all electrical equipment [110.26].

Equipment that may need examination, adjustment, servicing, or maintenance while energized must have working space provided in accordance with 110.26(A)(1), (2), and (3).

Width of the working space is covered by 110.26(A)(2), and must be a minimum of 30 in., but in no case less than the width of the equipment.

The width of the working space can be measured from left-to-right, from right-to-left, or simply centered on the equipment, and the working space can overlap the working space for other electrical equipment.

In all cases, the working space must be of sufficient width, depth, and height to permit all equipment doors to open 90 degrees.

Q7. What is the Code 10 ft feeder tap rule?

A7. Feeder tap conductors up to 10 ft long are permitted without overcurrent protection at the tap location if installed as follows [240.21(B)(1)]:

(1) The ampacity of the tap conductor must not be less than:

a. The calculated load in accordance with Article 220, and

b. The rating of the device or overcurrent device supplied by the tap conductors.

(2) The tap conductors must not extend beyond the equipment they supply.

(3) The tap conductors must be installed in a raceway if they leave the enclosure.

(4) The tap conductors must have an ampacity not less than 1/10th of the rating of the overcurrent device that protects the feeder.

Note: See 408.36 for the overcurrent protection requirements for panelboards.

Example: A 400A breaker protects a set of 500 kcmil feeder conductors. There are three taps fed from the 500 kcmil feeders that supply disconnects with 200A, 150A, and 30A overcurrent devices. What are the minimum size conductors for these taps?

• 200A: 3/0 AWG is rated 200A at 75°, and is greater than 1/10th of the rating of the overcurrent device, which is rated 400A.
• 150A: 1/0 AWG is rated 150A at 75°, and is greater than 1/10th of the rating of the overcurrent device, which is rated 400A.
• 30A: 8 AWG is rated 50A at 75°, and is greater than 1/10th of the rating of the overcurrent device, which is rated 400A. Anything smaller than 8 AWG can’t be used, as it will have an ampacity of less than 1/10th of 400A (40A).

Q8. What are the rules on using SE cable for indoor feeders?

A8. SE cable used for interior branch circuit or feeder wiring must be installed in accordance with the same requirements as Type NM Cable—Article 334, excluding 334.80 [338.10(B)(4)(A)].

The maximum conductor temperature rating can be used [310.15(B)(2)] for ampacity adjustment and correction purposes, but when installed in thermal insulation the conductors must be sized in accordance with Table 310.15(B)(16) 60°C rated conductor column.

Caution: Underground service-entrance cable (USE) must not be used for interior wiring because it doesn’t have flame-retardant insulation. It would only be permitted in interior wiring when dual listed as wire type in accordance with Table 310.104, such as RHW.

Q9. Does the NEC require that the sum of the service overcurrent protection device ratings be limited by the ampacity of the service conductors?

A9. No. The general rule is that the rating of a service overcurrent device must not be greater than the ampacity of the conductors. However, there is an exception that applies when two to six disconnects are used.

According to 230.90(A), Ex 3, the combined ratings of two to six service disconnecting means can exceed the ampacity of the service conductors provided the calculated load, in accordance with Article 220, doesn’t exceed the ampacity of the service conductors.

Q10. What is the Code requirement in regard to bonding to lightning protection systems?

A10. If a lightning protection system is installed on a building/structure, it must be bonded to the building/structure grounding electrode system [250.106]. The grounding electrode for a lightning protection system must not be used as the required grounding electrode system for the buildings or structures [250.60].

Note 1: See NFPA 780—Standard for the Installation of Lightning Protection Systems, which contains detailed information on grounding, bonding, and side-flash distance from lightning protection systems.

Note 2: To minimize the likelihood of arcing between metal parts because of induced voltage, metal raceways, enclosures, and other metal parts of electrical equipment may require bonding or spacing from the lightning protection conductors in accordance with NFPA 780—Standard for the Installation of Lightning Protection Systems.

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