All questions and answers are based on the 2005 NEC.
Q. Can Type MC cable be installed from fixture to fixture without support above a suspended ceiling?
A. Type MC cable can be run unsupported where the cable is not more than 6 feet long from the last point of cable support to the point of connection to a luminaire within an accessible ceiling 330.30(D)(2). Type MC cable fittings are permitted to be used as the last means of cable support (Figure).
Q. What are the requirements on the location of the primary protection device for a transformer? Must it be within sight of the transformer and/or the secondary panelboard?
A. Section 450.3 provides details on sizing the overcurrent protection for a transformer primary, but there are no requirements that the overcurrent protection device be within sight of either the transformer or panelboard, nor other location requirements.
Q. What is the Code requirement for the outdoor clearance height of temporary electrical feeders above the ground at a construction site?
A. First of all, remember that all requirements of the NEC apply to temporary installations unless specifically modified in Art. 590 [590.2(A)].
The following vertical clearances apply to outdoor feeders and branch circuits [225.18]:
(1) 10 feet above finished grade, sidewalks, platforms, or projections from which they might be accessible to pedestrians for 120V, 120/208V, 120/240V, or 240V circuits.
(2) 12 feet above residential property and driveways, and those commercial areas not subject to truck traffic for 120V, 120/208V, 120/240V, 240V, 277V, 277/480V, or 480V circuits.
(4) 18 feet over public streets, alleys, roads, parking areas subject to truck traffic, driveways on other than residential property, and other areas traversed by vehicles (such as those used for cultivation, grazing, forestry, and orchards).
Q. If a raceway contains three 120V circuits — and each ungrounded conductor has its own white wire (instead of a common neutral) — how many current-carrying conductors would the NEC count as current carrying for conductor ampacity adjustment?
A. On a 2-wire circuit, the current is the same on both conductors. Your example raceway contains six current-carrying conductors, and Table 310.15(B)(2)(a) adjustment factors must be applied. Section 310.15(B)(4)(a) has two examples of circuits where the neutral is not considered a current-carrying conductor.
The neutral conductor of a 3-wire, single-phase, 120/240V system, or 4-wire, 3-phase, 120/208V or 277/480V wye-connected system, isn't considered a current-carrying conductor and is not required to be counted for ampacity adjustment.
Q. On which phase of a disconnect must I land the high-leg conductor of a 3-phase, 4-wire delta-connected system?
A. The Code does not specify which phase to terminate the high-leg conductor in disconnects, but the high-leg conductor must terminate on the “B” (center) phase within a panelboard or switchboard [408.3(E)].
It's my understanding that the ANSI standard for meter equipment requires the high-leg conductor to terminate on the “C” (right) phase of the meter enclosure. This is because the demand meter needs 120V, which it gets from the “B” phase.
Warning: When replacing equipment in existing facilities that contain a high-leg conductor, care must be taken 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. A re-termination error can also cause 3-phase motors to run in the opposite direction, which can cause significant damage.
Q. What is the maximum distance I can locate the electric meter from the service disconnect?
A. There is no maximum distance of service conductors when run on the outside of a building or structure. However, once service conductors enter the building, the NEC requires them to terminate in a service disconnecting means placed at a readily accessible location nearest the point of service conductor entry (the NEC does not specify the maximum length within a building) [230.70(A)(1)].
Q. What are the rules for grounding the metal mounting straps of snap switches?
A. Section 404.9(B) requires that the metal mounting yokes for switches, dimmers, and similar control switches must be grounded to an effective ground-fault current path, whether or not a metal faceplate is installed. The metal mounting yoke must be grounded by metal screws, mounting it to a metal box or by connection of an equipment-grounding conductor to the grounding terminal on the yoke.
Q. What is the minimum distance permitted between two 277/480V panels that face each other, and how is this distance measured?
A. Two panels facing each other with 277V to ground in each panel meets Condition 3 of Table 110.26(A)(1), and the minimum distance between them must be 4 feet.