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Code Quandaries: Stumped by the Code? November 2011

Nov 22, 2011 1:26 PM, Mike Holt, NEC Consultant

Your most pressing National Electrical Code (NEC) questions answered

All questions and answers are based on the 2011 NEC.

Q. Does the Code allow the installation of cord-connected equipment above a suspended ceiling?

A. Unless specifically permitted in 400.7, flexible cords must not be concealed by walls, floors, or ceilings, or located above suspended or dropped ceilings [400.8(5)]. Flexible cords are permitted under a raised floor (with removable panels) used for environmental air, because this area isn’t considered a concealed space. See the definition of “Exposed” in Art. 100.

Receptacles are permitted above a suspended ceiling, but a flexible cord isn’t. Why install a receptacle above a ceiling if the flexible cord isn’t permitted in this space? Because the receptacle can be used for portable tools; it just can’t be used for cord- and plug-connected equipment fastened in place, such as a projector (click here to see Figure).

 

Q. Is a supplemental grounding electrode for a single rod, pipe, or plate electrode always required?

A. A single rod, pipe, or plate electrode must be supplemented by an additional electrode that’s bonded to one of the following [250.53(A)(2)]:

1. The single rod, pipe, or plate electrode
2. The grounding electrode conductor of the single electrode
3. The neutral service-entrance conductor
4. The nonflexible grounded service raceway
5. The service enclosure

Ex: If a single rod, pipe, or plate grounding electrode has an earth contact resistance of 25 ohms or less, the supplemental electrode isn’t required.

 

Q. How is the ampacity determined for NM-B cable?

A. Conductor ampacity is calculated on the 90°C insulation rating of the conductor in accordance with Table 310.15(B)(16); however, the conductors must be sized to the termination temperature rating of 60°C [334.80]. Let’s work a sample problem to help you better understand these guidelines.

What size Type NM cable is required to supply a 9.60kW, 240V, single-phase fixed space heater with a 3A blower motor? The terminals are rated 75°C.

Step 1: Determine the total load in amperes:

I = VA ÷ E = 9,600W ÷ 240V + 3A = 40A + 3A = 43A

Step 2: Size the ungrounded conductors and overcurrent device at no less than 125% of the total heating load [424.3(B)].

Conductor/protection size = load × 1.25 = 43A × 1.25 = 53.75A

According to Table 310.15(B)(16), a 6 AWG conductor rated 55A at 60°C, protected with a 60A overcurrent device [240.6(A)] will satisfy these requirements.

If multiple Type NM cables pass through the same wood framing opening that’s to be sealed with thermal insulation, caulking, or sealing foam, the allowable ampacity of each conductor must be adjusted in accordance with Table 310.15(B)(3)(a) [334.80]. This requirement has no effect on conductor sizing if you bundle no more than nine current-carrying 14 or 12 AWG conductors together. For example, three 14/2 cables and one 14/3 cable (nine current-carrying 14 THHN conductors) are bundled together in a dry location, the ampacity for each conductor (25A at 90°C, Table 310.15(B)(16)) is adjusted by a 70% adjustment factor [Table 310.15(B)(3)(a)].

Adjusted conductor ampacity = 25A × 0.70 = 17.50A        

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