NEC Questions and Answers
June Part 2 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. Can I use NM for wiring inside wooden stud walls of a dwelling unit garage which will be exposed?
A1. If it is judged by the AHJ to be subject to physical damage, the cable must be protected from physical damage by rigid metal conduit, intermediate metal conduit, Schedule 80 rigid nonmetallic conduit [352.10(F)], electrical metallic tubing, guard strips, or other approved means [334.15(B)].
Q2. When are insulated bushings required on raceway terminations?
A2. Where raceways contain conductors 4 AWG and larger that enter an enclosure, the conductors must be protected from abrasion during and after installation by a fitting that provides a smooth, rounded insulating surface, such as an insulating bushing [300.4(F)].
For IMC and RMC, a metal or plastic bushing must be installed on conduit threads at terminations in accordance with 300.4(F), unless the design of the box, fitting, or enclosure is such as to afford equivalent protection [344.46 and 342.46].
Q3. A main distribution panel is located in the main building of an RV park, and it contains numerous 100A breakers, which each feed six RV pedestals. Must a ground rod be driven at each pedestal, or is the grounding at the main distribution panel sufficient?
A3. A pedestal meets the definition of “structure” in Article 100, which is simply “that which is built or constructed”. With that in mind, each feeder you describe serves a structure, and every structure fed by a feeder requires a grounding electrode system [250.32(A)].
Q4. The voltage to ground momentary drops from 110V to 102V when the 240V air conditioner kicks in and the lights dim. Is this a NEC violation?
A4. No. The purpose of the is the practical safeguarding of persons and property from hazards caused by the use of electricity [90.1(A). The doesn’t consider voltage drop to be a safety issue in this application.
Q5. Can manufactured wiring systems which can be unplugged be used for emergency lighting and exit lights?
A5. Sure.
Q6. A single main breaker is installed, then taps supply individual meters with disconnecting means for individual tenants. Should a main bonding jumper be installed at the single main breaker or at each separate disconnect?
Q7. Is it permissible to run heating/air-conditioning duct four ft above a panelboard if the ductwork services the room where the panelboard is located?
A7. No. The footprint space (width and depth of the equipment) extending from the floor to a height of 6 ft above the equipment or to the structural ceiling, whichever is lower, must be dedicated for the electrical installation. No piping, duct, or other equipment foreign to the electrical installation can be installed in this dedicated footprint space [110.26(F)].
Q8. Can the disconnect switch be located next to equipment installed above an accessible drop ceiling or must the switch be at a readily accessible location?
A8. Yes. Switches and circuit breakers used as switches can be mounted above 6 ft 7 in. if they are next to the equipment they supply and are accessible by portable means [240.24(A)(4) and 404.8(A) Exception No. 2].
Q9. How do I size the conductors and the breaker for a motor?
A9. There are two components to this question: Sizing the conductors and sizing the ground fault and short circuit protection. Let’s do this step-by-step:
Step 1: Conductors to a single motor must be sized not smaller than 125 percent of the motor FLC rating as listed in [430.22(A)]:
• Table 430.247 Direct Current Motors
• Table 430.248 Single-Phase Motors
• Table 430.250 Three-phase motors
Step 2:Each motor branch circuit must be protected against short circuit and ground faults by a protection device sized no greater than the percentages listed in Table 430.52 [430.52(C)(1)].
Example: What size branch-circuit conductor, ground fault and short circuit protection device is required for a 71⁄2 hp, 230V, three-phase motor with a nameplate of 19A?
Step 1. Motor Branch-Circuit Conductor Size
Motor FLC = 22A [Table 430.250]
Conductor’s Size = 22A x 1.25
Conductor’s Size = ‑27.5A, 10 AWG, rated 30A at 75°C
[Table 310.16]
Step 2. Branch-circuit short-circuit and ground-fault protection device using an inverse time breaker.
Q10. Is there any allowance in the NEC to locate receptacles just where it is convenient for furniture lay-out or the designer’s wishes, if the required number of receptacles is installed?
A10. No, a receptacle outlet must be installed so no point along the wall space will be more than 6 ft, measured horizontally, from a receptacle outlet [210.52(A)].
Q11. How many conductors can I install in a wireway?
A11. The maximum number of conductors permitted in a wireway is limited to 20 percent of the cross-sectional area of the wireway. When more than 30 current-carrying conductors are installed in any cross-sectional area of the wireway, the conductor ampacity, as listed in Table 310.16, must be reduced according to the adjustment factors listed in Table 310.15(B)(2)(a) [376.22].
Example: 20A circuit. If the wireway has more 32 current-carrying conductors, the ampacity of each conductor is adjusted by a 40 percent adjustment factor [Table 310.15(B)(2)(a)]
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