Stumped by the Code?
All questions and answers are based on the 2005 NEC.
Q. I recently came across a handhole with a metal cover in the backyard of a house that is used to splice the underground service conductors. I told the utility mechanic the cover needed to be grounded (bonded) because there could be accidental contact with live circuits.
He basically said, “Take a hike,” and that he did not have to ground the metal cover because utility companies follow the requirements of the National Electric Safety Code (NESC), not the National Electrical Code (NEC).
A. According to the NEC, if it's not a utility installation, metal covers and other exposed conductive surfaces of handholes must be effectively bonded to an effective ground-fault current path in accordance with 250.4(A)(5). This ensures electrical continuity and the capacity to safely conduct any fault current likely to be imposed on the covers [314.30(D)].
This is accomplished by bonding the metal parts to an equipment grounding (bonding) conductor that is sized to the circuit protection device in accordance with 250.122.
A ground rod cannot be used for this purpose because the earth contact resistance is so high that very little current would return to the electrical supply source via the earth. Plus, because the utility does not provide overcurrent protection for its secondary conductors, the fault would not be able to be cleared anyway (see Figure).
The bottom line is: Don't ever touch anything metallic that contains utility wiring because the utility is not required to remove dangerous ground-faults in accordance with the NEC [90.4(B)(5)].
Q. What is the smallest conductor required for a 9 kW, 240V single-phase electric space heater that contains a 3A blower motor if the terminals are rated for 75°C conductor sizing?
A. According to 424.3(B), the branch-circuit conductors and overcurrent protection device for electric space-heating equipment must be sized no less than 125% of the total load.
Step 1: Determine the heating load
I = VA ÷ E
I = 9,000VA ÷ 240V = 37.5A
Step 2: Size the conductor at 125% of the total heating load.
Conductor size = (37.5A + 3A) × 1.25
Conductor size = 50.63A (go ahead and round up to 51A)
Therefore, the required conductor for this installation would be 6 AWG, rated 65A at 75°C [Table 310.16].
Q. I have a 25-foot metal light pole on which speakers and stage lighting are mounted. The pole is hollow, and the lighting and audio circuits have been run together inside the pole. I'm in the process of pulling these poles for rehab and need to know if I'm required to separate the audio cables from the power conductors.
A. The output wiring for audio circuits must be installed in accordance with the marking for use with the specific class of wiring method selected. Typically, audio equipment marking indicates that Class 2 wiring methods, in accordance with 725.82, are permitted. According to the requirements of 725.55(A), Class 2 conductors and power conductors must not be placed in any enclosure or raceway with power conductors, unless the power conductors or Class 2 circuit conductors are contained in a Chapter 3 wiring method [725.55(J)]. Just be sure the power conductors are contained in a Chapter 3 wiring method such as UF cable, ENT, Flex, etc., and you're good to go.
Q. When can I mix the control wiring for air-conditioning equipment with the equipment's power conductors in the same raceway?
A. If the remote control circuit conductors are classified as Class 1 — and they are installed in accordance with 600V insulation [725.27(B)] — then they can be in the same raceway with the A/C equipment power conductors [725.26(B)(1)]. But control wiring for most A/C equipment is classified as Class 2, not Class 1 because it's supplied by a Class 2 power supply.
The NEC permits Class 2 circuits to be reclassified as Class 1, if the Class 2 equipment markings on the power supply are removed, and the circuit conductors in the raceway have 600V insulation and are installed per a Chapter 3 wiring method [725.25 and 725.27(B)].
Note: According to the NEC, Class 2 circuits reclassified as Class 1, are no longer Class 2, regardless of the continued connection to a Class 2 power source. In fact, they're also not permitted to be installed with other Class 2 or Class 3 circuits that have not been reclassified as Class 1 [725.55].
Q. I'm entering a building with a 600-pair underground telephone cable, and I need to run this cable a few hundred feet above a suspended ceiling. Can I use EMT for this purpose?
A. No. Communication wires and cables installed within buildings must be listed in accordance with 800.113 and 800.179. However, unlisted communications cable (underground) is permitted if the length of the cable within the building from its point of entrance doesn't exceed 50 feet, and the cable terminates in an enclosure [800.113 Ex. No. 2].
Note: The point of entrance is defined as the point where the cable emerges from an external wall, from a concrete floor slab, or from a rigid metal conduit or an intermediate metal conduit that is grounded (bonded) to an electrode in accordance with 800.100 [800.2]. That's why you can't use EMT.