NEC Questions and Answers
April Part 1 of 2
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. What are the rules for installing fluorescent light fixtures in clothes closets?
A1. There are specific clearances that must be maintained for either fluorescent or incandescent luminaires installed in a clothes closet. Here are the rules from 410.8(D):
(1) Surface-mounted totally enclosed incandescent luminaires must maintain a minimum clearance of 12 in. from the storage space.
(2) Surface-mounted fluorescent luminaires must maintain a minimum clearance of 6 in. from the storage space.
(3) Recessed incandescent luminaires with a completely enclosed lamp must maintain a minimum clearance of 6 in. from the storage space.
(4) Recessed fluorescent luminaires must maintain a minimum clearance of 6 in. from the storage space.
Q2. What are the requirements for supporting outlet boxes above/to suspended ceilings?
A2. Outlet boxes can be supported to the structural or supporting elements of a suspended ceiling, if securely fastened by one of the following methods [314.23(D)]
(1) Ceiling-Framing Members. An outlet box can be secured to suspended-ceiling framing members by bolts, screws, rivets, clips, or other means identified for the suspended-ceiling framing member(s).
Where framing members of suspended ceiling systems are used to support luminaires, they must be securely fastened to each other and must be securely attached to the building structure at appropriate intervals. In addition, luminaires must be attached to the suspended-ceiling framing with screws, bolts, rivets, or clips listed and identified for such use [410.16(C)].
(2) Independent Support Wires. Outlet boxes can be secured, with fittings identified for the purpose, to independent support wires that are taut and secured at both ends [300.11(A)].
Q3. What are the requirements for installing conductors from different panels in the same raceway?
A3. Power conductors can occupy the same raceway, cable, or enclosure if all conductors have an insulation voltage rating not less than the maximum circuit voltage [300.3(C)(1)].
Q4. What are the disconnecting means requirements for transformers?
A4. The NEC does not have specific requirements for disconnecting means at a transformer, but overcurrent protection requirements of Article 450 and Article 240 must be followed.
Q5. In what rooms of a home is an electrical panel with breakers prohibited?
A5. Overcurrent protection devices should not be located where they would be subjected to physical damage or in areas where they might create a fire or safety hazard [240.24(C)].
Some specific locations are prohibited in dwelling units:
- Overcurrent protection devices must not be located near easily ignitible material, such as in clothes closets [240.24(D)].
- Overcurrent protection devices must not be located in the bathrooms of dwelling units, or guest rooms or guest suites of hotels or motels [240.24(E)].
Q6. Can Type NM cable in a dwelling unit be used to wire recessed light fixtures that require the circuit conductors to have at least a 90°C rating?
A6. Yes, Type NM conductors are rated at 90°C [334.104], even though the ampacity is required to be selected based on the 60°C column of Table 310.16 [334.80].
Q7. Can I pass conductors from one panel through another panel, and if yes, what are the restrictions?
A7. Yes, cabinets, cutout boxes, and meter socket enclosures can be used as a raceway for conductors that feed through if the conductors do not fill the wiring space at any cross section to more than 40 percent [312.8].
Splices and taps can be installed in cabinets, cutout boxes, or meter socket enclosures if the splices or taps do not fill the wiring space at any cross section to more than 75 percent [312.8].
Q8. What is the minimum distance permitted between the 277/480V panels oppositely facing each other and how is this distance measured?
A8. Two panels facing each other with 277V to ground in each panel fits Condition 3 of Table 110.26(A)(1), and the minimum distance between them must be 4 ft:
Q9. I have noticed that the light fixtures located in the attics and crawl spaces of numerous residences have been damaged unintentionally. Does the NEC require fixtures in these locations to be suitably guarded against accidental damage?
A9. Nothing in the NEC requires guarding of light fixtures in the attics of dwelling units. When installed around combustible material, luminaires are required to have guards to limit the temperature around the combustible material [410.5].
Q10. A 30 kVA 480/208V transformer is fed by a 60A 3-pole breaker from a 480V panel. This transformer secondary then feeds a 120/208V panel which is within 10 ft of conductor length. Is secondary overcurrent protection required for the secondary conductors?
A10. The NEC permits 10 ft of secondary conductors without secondary conductor protection if [240.21(C)(2)]:
(1) The ampacity of the secondary conductor must not be less than:
a. The calculated load in accordance with Article 220,
b. The rating of the device supplied by the secondary conductors or the overcurrent protective device at the termination of the secondary conductors, and
c. Not less than one-tenth the rating of the overcurrent device protecting the primary of the transformer, multiplied by the primary-to-secondary transformer voltage ratio.
(2) The secondary conductors must not extend beyond the switchboard, panelboard, disconnecting means, or control devices they supply.
(3) The secondary conductors must be enclosed in a raceway.
Q11. What size conductor is required for a 3000A service supplied with eight parallel sets?
A11. We must choose individual conductors that are rated at least 375A each (3000A/8 sets), as 240.4(C) does not allow the next size up when the overcurrent device is rated above 800A. This ampacity is selected from the 75°C column of Table 310.16 for conductors over 100A in order to accommodate the terminal temperature rating requirements of 110.14(C)(1)(b).
Copper 500 kcmil is rated 380A in the 75°C column of Table 310.16 and will be satisfactory as long as there are no more than 3 current carrying conductors installed per raceway at an ambient temperature not exceeding 85°F.
Any installation making use of parallel conductors per phase must meet all of the requirements of 310.4 for parallel conductors.
Q12. How do I calculate the feeder/service load for track lighting in a commercial building?
A12. The feeder/service VA load must not be less than 150 VA for every 2 ft of track lighting or fraction thereof [220.43(B)]. Where a feeder or service supplies continuous loads, the minimum feeder or service conductor size, before the application of any adjustment and/or correction factors, must have an allowable ampacity of not less than 125 percent of the continuous load [215.2(A)(1) for feeders and 230.42(A) for services].
Example: What is the approximate feeder/service calculated load for conductor sizing for 150 ft of track lighting in a commercial occupancy?
(a) 10,000 VA (b) 12,000 VA (c) 14,000 VA (d) 16,000 VA
Answer: (c) 14,000 VA
Feeder calculated load - 150 ft/2 ft = 75 units x 150 VA x 1.25 = 14,063 VA
This rule doesn’t apply to branch circuits. There is no limit on the length of track on a single branch circuit. Therefore, the maximum number of lampholders permitted on a track lighting system is based on the wattage rating of the lamps and the voltage and ampere rating of the circuit [410.101(B)]. The maximum load on a branch circuit must not exceed 80 percent of the circuit rating [210.19(A)(1)].
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