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Code Quandaries

Aug 1, 2008 12:00 PM, By Mike Holt, NEC Consultant

All questions and answers are based on the 2008 NEC.

Q. When adjusting the ampacity of a conductor due to conductor bundling, do we use the 90°C column ampacity or the 75°C column ampacity as listed in Table 310.16 for THHN?

A. According to 110.14(C), conductors are to be sized to the lowest temperature rating of any terminal, device, or conductor of the circuit, in accordance with the equipment terminal temperature rating. However, conductors with insulation temperature ratings higher than the termination's temperature rating can be used for conductor ampacity adjustment, correction, or both.

This means conductor ampacity must be based on the conductor's insulation temperature rating listed in Table 310.16, as adjusted for ambient temperature correction factors, conductor bundling adjustment factors, or both. The location of conductors can influence their ampacity as well. For example, THHW is a 90°C conductor in a dry location, but it's a 75°C conductor in a wet location. THHN/THWN-2 is a 90°C conductor, in wet, dry, or damp locations [Table 310.13(A)].

Example: The ampacity of each 12 THHW conductor in a dry location is 30A, based on the values listed in the 90°C column of Table 310.16. If we bundle nine current-carrying 12 THHN conductors, the ampacity for each conductor (30A at 90°C, Table 310.16) needs to be adjusted by a 70% adjustment factor [Table 310.15(B)(2)(a)].

Adjusted conductor ampacity = 30A × 0.70 = 21A

If the conductors are installed in a wet location, the ampacity of 12 THHW conductors is 25A according to the 75°C column of Table 310.16 [Table 310.13(A)].

Adjusted conductor ampacity = 25A × 0.70 = 17.50A, 18A [220.5(B)]

Q. Can Type NM cable be installed in a raceway in a ground floor slab?

A. Maybe. Type NM cable is not permitted to be installed in a wet location [334.12(B)(4)] and according to the definition of a “Location, Wet” as contained in Art. 100, “a raceway in concrete slab that is in direct contact with the earth is considered a wet location.” So in this case, the answer is no, you're not permitted to install Type NM cable in the raceway (Figure).

However, if a plastic vapor barrier is installed between the earth and the concrete slab, then the concrete slab is not in direct contact with the earth and any raceway in a concrete slab is not located in a wet location. Under this condition, Type NM cable is permitted in the raceway.

Q. Does the NEC specify a particular color arrangement for 3-phase systems?

A. No, but electricians often use the following color system for power and lighting conductor identification:

120/240V, single-phase — black, red, and white

120/208V, 3-phase — black, red, blue, and white

120/240V, 3-phase — black, orange, blue, and white

277/480V, 3-phase — brown, orange, yellow, and gray; or brown, purple, yellow, and gray

Q. If the interior “guts” of a panel have overspray from acoustical spray, what do you do?

A. The panelboard (guts) must be replaced because the internal parts of electrical equipment must not be damaged or contaminated by foreign material, such as paint, plaster, cleaners, etc. [110.12(B)].

Q. What are the NEC requirements for installing panels in clothes closets?

A. Overcurrent devices must not be located near easily ignitable material, such as in clothes closets. According to Art. 100, a clothes closet is an uninhabitable room or space intended primarily for storage of garments and apparel.

Q. What are the separation requirements when installing signaling and communications cables in cable trays with Type MC and TC cables?

A. None. However, if the power conductors are installed as individual conductors, then a separation of 2 inches or a barrier is required, in accordance with:

• Coaxial cables [820.133(A)(1)(b), Ex 1]

• Class 2 and 3 cables [725.136(B) and 725.136(I)]

• Communications cables [800.133(A)(2), Ex 1]

• Fire alarm cables [760.136(G)]

• Intrinsically safe systems cables [504.30(A)(2), Ex 1]

• Radio and TV cables [810.18(B), Ex 1]

Q. Can I run Type NM cables in raceways in aboveground wet locations?

A. No, cables installed in raceways in aboveground wet locations must be listed for use in wet locations [300.9], and Type NM cable is not listed for use in a wet location [334.12(B)(4)].

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