NEC Questions Based on the 2008 NEC

May Part 1

 

Here is the follow up to yesterday’s newsletter. This includes all of the answers to the questions sent yesterday so you can see how you did.

 

By Mike Holt, for EC&M Magazine

 

Q1. When adjusting the ampacity of a conductor due to conductor bundling, do we use the 90 degree column ampacity, or the 75 degree column ampacity, when the conductor insulation is THHN?

A1. Conductor ampacity adjustments are based on the conductor temperature insulation rating [110.14(C)]. For example: The ampacity of nine current-carrying 12 THHN conductors in a dry location would be need to be adjusted by a 70 percent adjustment factor [Table 310.15(B)(2)(a)].

Adjusted Conductor Ampacity = 30A [Table 310.16] x 0.70

Adjusted Conductor Ampacity = 21A

 

Q2. Can Type NM cable be installed in a raceway in a ground floor slab?

A2. Maybe. Type NM cable is not permitted to be installed in a wet location [334.12(B)(4)] and a raceway in a concrete slab in direct contact with the earth is considered a wet location [Article 100]. If however, there is a plastic vapor barrier between the earth and the concrete slab, then the raceway would not be located in a wet location.

 

Q3. Does the NEC specify a particular color arrangement for three phase systems?

A3. 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, three-phase—black, red, blue, and white

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

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

 

Q4. If the interior of a panel has overspray from acoustical spray, what do you do?

A4. The panel 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)]

 

Q5. I have a building that has two services from the utility. Do I have to use the same grounding system for both?

A5. Yes [250.58].

 

Q6. What are the NEC requirements for installing panels in closets??

A6.  Overcurrent devices must not be located near easily ignitible material, such as in clothes closets. According to Article 100, a clothes closet is a nonhabitable room or space intended primarily for storage of garments and apparel.

 

Q7. What is the minimum size feeder/service for a construction trailer?

A7. The feeder/service conductors must have a rating not less than 60A [225.39(D) and 230.79(D)].

 

Q8. What is the smallest size wire you can use for a lighting fixture whip?

A8. Fixture wires as small as 18 AWG are allowed to run from the luminaire to an outlet box, as long as the conductors are not over 6 ft long [410.117(C)] and they are connected to a circuit rated not more than 20A [240.5(B)(2)].

 

Q9. Can I use 14 AWG conductor pigtails from 12 AWG, 20A circuit so that I can back-stab the wires to the receptacle?

A9. No. Branch circuit tap conductors are not permitted for receptacle outlets [210.19(A)(4) Ex No.1 (c)].

 

Q10. Can I run Type NM cable in a raceway outdoors on the building?

A10. No. Cables installed in raceways in above ground 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)].