NEC Questions and Answers
By Mike Holt for EC&M Magazine
Q1. What are the NEC requirements on grounding gas piping?
A1. Gas piping can’t be used as a grounding electrode [250.52(B)(1)], however, it must be bonded if it is likely to be become energized [250.104(B)]. The equipment grounding (bonding) conductor for the circuit that may energize the piping can serve as the bonding means. So effectively, this means that no action is actually required by the electrical installer!
Q2. Is all electrical equipment required to be listed?
A2. Not all electrical items required or permitted by the NEC are required to be listed, however if the equipment you are using is listed, it must be installed and used in accordance with any instructions included in the listing or labeling requirements [110.3(B)]. There are many products that the NEC does require to be listed such as receptacles [406.2(A)], low voltage lighting systems [411.3], EMT fittings [358.6], grounding and bonding fittings [250.8] and many, many others. Because of this, it may be worth your time to use all listed products, since the NEC requires so many items to be listed. Also, there are some states, such as Washington and Minnesota, that require that all items be listed., so be sure to check any local amendments that your jurisdiction might have.
Q3. I have been told that when conductor ampacity adjustment is required (bundling or ambient temperature), we are to determine the conductor ampacity based on the temperature rating of the terminals in accordance with Table 310.16. Is this true?
A3. No, conductor ampacity adjustment is based on the ampacity of the conductors in accordance with the temperature rating of the conductors. Typically conductors used today are THHN in a dry location, for these circuits, the ampacity is based on the 90°C rating of Table 310.16. [110.14(C)(1)(a)(2) and 110.14(C)(2)(b)(2)].
Q4. How do I size the ground wire for a circuit when the hot wires have been increased in size because of voltage drop?
A4. When ungrounded circuit conductors (hot wires) are increased in size for any reason, the equipment grounding (bonding) conductor must be proportionately increased in size [250.122(B)]. For example, if the ungrounded conductors for a 40A circuit are increased in size from 8 AWG to 6 AWG, the equipment grounding (bonding) conductor must be increased in size from 10 AWG to 8 AWG. This is because the circular mil area of 6 AWG is 59 percent greater than 8 AWG (26,240 cmil/16,510 cmil) [Chapter 9, Table 8]. According to Table 250.122, a 40A circit protection device would require a 10 AWG equipment grounding (bonding) conductor, but it must also be increased the equipment grounding (bonding) conductor in size by 59 percent (10380 cmil X 59% (1.59) = 16,504 cmil), which results in a 8 AWG.
Q5. Can ‘line’ and ‘load’ conductors be installed in the same raceway?
A5. Service conductors cannot be installed in the same raceway with feeder or branch-circuit conductors [230.7], but line and load conductors for feeders and branch circuits can be in the same raceway or enclosure. For example, it is common to use the ungrounded (hot) conductor and the switch leg in the same raceway for a single pole switch, which would be line and load in the same raceway.
Q6. Q6.What type of precautions must I take when splicing aluminum conductors to copper?
A6. Copper and aluminum conductors must not make contact with each other in a device unless the device is listed and identified for this purpose [110.14]. Few terminations are listed for the mixing of aluminum wire and copper, but if they are, they will be marked on the product package or terminal device.
The reason copper and aluminum should not be in contact with each other is because corrosion develops between the two different metals due to galvanic action, resulting in increased contact resistance at the splicing device. This increased resistance can cause overheating of the splice and cause a fire.
Q7. Can I put two wires under a single terminal?
A7. The answer is generally no. However, two conductors can be placed under the same terminal, if the terminal is identified for this purpose, either within the equipment instructions or on the terminal itself [110.14(A)]. Careful, each grounded (neutral) conductor within a panelboard, must terminate to an individual terminal [408.41].
Q8. Am I required to install the secondary circuit conductors of a low-voltage lighting system (less than 30V) in a raceway or cable like power conductors?
A8. If the low-voltage conductors will be concealed or extended through a building wall, they must be installed using any Chapter 3 wiring method such within a raceway, cable, or enclosure, just like power conductors [411.4(A)(1)]. In addition, if the low-voltage wiring is supplied by a Class 2 power source, it can be installed using the requirements of 725.52. This means you can use 150V cable [725.82(G)], boxes are not required, and the Class 2 conductors cannot be mixed with power conductors in raceways or enclosure unless a separated by a barrier [725.55].
Q9. Must the offsets at outlet boxes be counted towards the total amount of bends between pull boxes for EMT?
A9. Yes. To reduce the stress and friction on the conductor insulation, the total amount of bends between pull points must not exceed 360° [358.26]. This includes all bends, even the small ones at the outlet boxes.
Q10. Can 277V circuit conductors be within the same raceway with 120V circuit conductors?
A10. Yes, power conductors of different systems 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)(A)].
