Stumped by the Code?

Aug 14, 2008 4:25 PM, By Mike Holt, NEC Consultant

All questions and answers are based on the 2008 NEC.

Q. Am I required to secure Type MC cable that is located under a raised floor of an information technology equipment room?

Guidelines for branch-circuit conductors installed under raised floors in information technology equipment rooms.

A. Yes. All wiring methods under raised floors must be secured in accordance with the wiring method requirements [645.5(D)(2)]. For example, Type MC cable with four or less conductors sized no larger than 10 AWG must be secured within 12 inches of every outlet box, junction box, cabinet, or fitting and at intervals not exceeding 6 feet [330.30].

If the room is compliant with 645.4 and the power cables (MC) are listed as part of, or for, information technology equipment, they are not required to be secured in place per 645.5(E)

Q. How many receptacles can be installed on a generalpurpose branch circuit?

A. The NEC doesn't limit the number of receptacle outlets on a generalpurpose branch circuit in a dwelling unit [220.14(J)], but for other than dwelling units, the NEC limit is 13 receptacle outlets each at 180VA per mounting strap [220.14(I).

Circuit VA = Volts × Amperes

Circuit VA = 120V × 20A = 2,400VA

Number of receptacles = 2,400VA ÷ 180VA = 13

According to the NEC Handbook published by the NFPA, receptacle loads are not considered a continuous load; therefore, the calculation is based on 20A, not 80% of 20A.

Q. How many 12 AWG conductors can I install in a 1inch EMT raceway?

A. EMT must be large enough to permit the installation and removal of conductors without damaging the conductor insulation [358.22]. This is accomplished by complying with conductor fill limitations of Table 1 of Chapter 9. Because all of the conductors are the same size, Annex C, Table C.1 can be used to determine the number of conductors (26 conductors).

Although the NEC permits 26 12 THHN conductors in trade size 1 EMT, 310.15(B)(2)(a) requires the ampacity of the conductors to be adjusted when installing more than three currentcarrying conductors in the raceway.