Your most pressing National Electrical Code (NEC) questions answered. All questions and answers are based on the 2008 NEC
Q. Where is the high-leg conductor really supposed to terminate? I keep seeing it on phase "C" on the electric utility side of the meter, on phase "B" in the service disconnect, and everywhere downstream.
A. Panelboards supplied by a 4-wire, delta-connected, 3-phase (high-leg) system must have the high-leg conductor (which operates at 208V to ground) terminate to the "B" phase of the panelboard [408.3(E)]. Exception: The high-leg conductor can terminate to the "C" phase when the meter is located in the same section of a switchboard or panelboard (see Figure). INSERT Figure 408-03E0 02 Warning: The ANSI standard for meter equipment requires the high-leg conductor (208V to neutral) to terminate on the "C" (right) phase of the meter socket enclosure. This is because the demand meter needs 120V, and it gets it from the "B" phase. Warning: When replacing equipment in existing facilities that contain a high-leg conductor, use care to ensure that the high-leg conductor is replaced in the original location. Prior to 1975, the high-leg conductor was required to terminate on the "C" phase of panelboards and switchboards. Failure to re-terminate the high leg in accordance with the existing installation can result in 120V circuits inadvertently connected to the 208V high leg with disastrous results.
Q. The local inspector says receptacles in walls of a multifamily dwelling must be spaced 24 in. apart from each other when they are installed back to back. Can you help me understand where he’s coming from?
A. Because the walls are fire-resistance rated, the rules of 300.21 apply. Openings in fire-rated walls, floors, and ceilings for electrical equipment must be fire-stopped using methods approved by the authority having jurisdiction (AHJ) to maintain the fire-resistance rating of the fire-rated assembly. Outlet boxes must have a horizontal separation not less than 24 in. when installed in a fire-rated assembly, unless an outlet box is listed for closer spacing or protected by fire-resistant "putty pads" in accordance with manufacturer's instructions.
Q. What are the requirements in locating transformers above a suspended ceiling?
A. Dry-type transformers, rated not more than 50kVA, are permitted above suspended ceilings or in hollow spaces of buildings, if not permanently closed in by the structure [450.13(B)]. Dry-type transformers not exceeding 50kVA with a metal enclosure can be installed above a suspended ceiling space used for environmental air-handling purposes [300.22(C)(2)].
Q. What is the maximum height for a disconnect switch?
A. Switches and circuit breakers used as switches must be capable of being operated from a readily accessible location [404.8(A)]. They must also be installed so the center of the grip of the operating handle of the switch or circuit breaker (when in its highest position) isn't more than 6 ft, 7 in. above the floor or working platform [240.24(A)]. It's also worth pointing out that the disconnecting means for a mobile home must be installed so the bottom of the enclosure isn't less than 2 ft above the finished grade or working platform [550.32(F)].
Q. If a manufacturer states that a multi-motor chiller has a minimum circuit ampacity of 357A, a full load of 343A, a minimum circuit breaker size of 400A, and a maximum circuit breaker size of 400A, can I feed this with 500kcmil?
A. Yes, 500kcmil, having an ampacity of 380A [Table 310.16] would be fine. For a her¬metic refrigerant motor compressor, the rated load current marked on the nameplate of the equipment is to be used in determining the rating of the disconnecting means, the branch circuit conductors, the controller, and the branch circuit short circuit and ground-fault protection [440.6(A)].
Q. Can service conductors (between the meter and the service disconnect) be tapped to feed a fire pump?
A. Yes, a separate service or a connection located ahead of (but not within) the service disconnecting means is allowed as the power source for a fire pump [695.4(A)].
Q. We need to mount a GFCI receptacle a few feet from an A/C unit. Is it acceptable to run the separate circuits serving each of these in the same raceway and use the A/C disconnect as part of the raceway for the GFCI?
A. Cabinets, cutout boxes, and meter socket enclosures can be used as a raceway for conductors that feed through if the conductors don't fill the wiring space at any cross section to more than 40% [312.8].
Q. When calculating box fill, do the mechanical ground and isolated ground count as one or two wires?
A. All equipment grounding conductors in a box count as a single conductor volume in accordance with Table 314.16(B), based on the largest equipment grounding conductor that enters the box [314.16(B)(5)]. Insulated equipment grounding conductors for receptacles having insulated grounding terminals (isolated ground receptacles) [250.146(D)] count as an additional single conductor volume in accordance with Table 314.16(B).