All questions and answers are based on the 2008 NEC.
Q. Do I need a grounding conductor for a pole light, or is the ground rod good enough?
A. You absolutely need an equipment grounding conductor. Metal parts of electrical raceways, cables, enclosures, or equipment must be bonded together (and to the supply system) in a manner that creates a low-impedance path for ground-fault current that facilitates the operation of the circuit overcurrent device [250.4(A)(5)].
Because the earth is not suitable to serve as the required effective ground-fault current path, an equipment grounding conductor is required to be run with all circuits.
Danger: Because the contact resistance of an electrode to the earth is so high, very little fault current returns to the power supply if the earth is the only fault current return path. What’s the result? The circuit overcurrent device will not open, and all metal parts associated with the electrical installation, metal piping, and structural building steel will become and remain energized.
Q. We are told that we must GFCI protect our drinking fountain, but the receptacle is buried behind the unit. Do we really have to do this?
A. All electric drinking fountains must be GFCI-protected [422.52], but if you are really concerned about accessibility, you can always use a GFCI breaker for protection.
Q. What are the NEC requirements when replacing 15A or 20A, 125V receptacles?
A. See 406.3(D) as follows: Grounding-type receptacles. Where an equipment grounding conductor exists, grounding-type receptacles must replace nongrounding-type receptacles, and the receptacle's grounding terminal must be connected to an equipment grounding conductor in accordance with 406.3(C) or 250.130(C).
Nongrounding-type receptacles. Where no equipment grounding conductor exists in the outlet box for the receptacle, such as old 2-wire Type NM cable without an equipment grounding conductor, existing nongrounding-type receptacles can be replaced in accordance with one of the following:
- Another nongrounding-type receptacle.
- A GFCI-type receptacle marked "No Equip¬ment Ground."
- A grounding-type receptacle, if GFCI protected and marked "GFCI Protected" and "No Equipment Ground."
When existing receptacles are replaced in locations where GFCI protection is currently required, the replacement receptacles must be GFCI protected. This includes the replacement of receptacles in dwelling unit bathrooms, garages, outdoors, crawl spaces, unfinished basements, kitchen countertops, rooftops, or within 6 ft of laundry, utility, and wet bar sinks. See 210.8 for specific GFCI-protection requirements.
Author's comment: GFCI protection functions properly on a 2-wire circuit without an equipment grounding conductor because the circuit equipment grounding conductor serves no role in the operation of the GFCI-protection device. See the definition of "ground-fault circuit interrupter" for more information.
Q. A conduit contains six current-carrying 10 AWG THHN conductors on a roof for a photovoltaic system, where the ambient temperature 94ºF and the ambient temperature add is 60ºF in accordance with Table 310.15(B)(2)(c). How do I determine the conductor ampacity for this condition of use?
A. When conductors are installed in an ambient temperature other than 78°F to 86°F, ampacities listed in Table 310.16 must be corrected in accordance with the multipliers listed in Table 310.16. Where the number of current-carrying conductors in a raceway or cable exceeds three, the allowable ampacity of each conductor, as listed in Table 310.16, must be adjusted in accordance with the adjustment factors contained in Table 310.15(B)(2)(a) and 310.15(B)(2)(c). When both of these conditions exist, you must do both calculations, but you can start with the 90°C ampacity for the calculations [110.14(C)(1)].
Table 310.16 ampacity if 10 AWG THHN is 40A
Ambient temperature correction [Table 310.16] = 0.58, based on 154ºF
Conductor bundle adjustment [310.15(B)(2)(a)] = 0.80, six current-carrying conductors
Adjusted ampacity = 40A x .58 x 0.80
Adjusted ampacity = 18.56 or 19A
Q. We got written up for having our underground conduits too close together. I know you can't bundle cables together, but I didn't think this applied to raceways too. Does it?
A. Actually, it does. In 310.15(B)(2)(b), it states that spacing between conduits, tubing, or raceways must be maintained, even though the Code doesn't say what the spacing is — or how to adjust when the spacing is not maintained. Hopefully, the next Code cycle will have better language for this rule.