Your most pressing National Electrical Code (NEC) questions answered. All questions and answers are based on the 2008 NEC
All questions and answers are based on the 2008 NEC.
Q. What are the requirements for supporting communications raceways and cables above a suspended ceiling?
A. Communications raceways and communications cable assemblies must be securely fastened in place, and ceiling-support wires or the ceiling grid must not be used to support communications raceways or communications cables [800.24], as shown in the Figure. Raceways and cables are permitted to be supported by independent support wires attached to the suspended ceiling [300.11].
Q. What are the NEC requirements when installing a panel or disconnect directly below another panel or disconnect?
A. Equipment such as cabinets and panels can be located above or below other electrical equipment, but it must not extend more than 6 in. into the equipment's working space [110.26(A)(3)]. Be sure that the height of the center of the grip of the operating handle of any switch or circuit breaker doesn't exceed 6 ft 7 in. in the ON position [404.8(A)].
Q. In an industrial environment, does the NEC require an equipment grounding conductor through metal raceways such as electrical metallic tubing (EMT) or rigid metallic conduit (RMC)?
A. No. The NEC requires all circuits to contain an equipment ground consisting of any one or a combination of the items listed in 250.118. Because EMT and RMC are both recognized as an equipment grounding conductor, an additional equipment grounding conductor of the wire type would not be required.
Q. How do we size the primary overcurrent device and conductors for a 45kVA transformer 480V to 120/208V?
A. The primary winding of a transformer must be protected against overcurrent in accordance with the percentages listed in Table 450.3(B) and all applicable notes.
Step 1: Determine the primary current:
I = VA ÷ (E × 1.732)
I = 45,000VA ÷ (480V × 1.732)
I = 54A
Step 2: Determine the primary overcurrent device rating [240.6(A) and 450.3(B)]:
54A × 1.25 = 68A, next size up 70A, Table 450.3(B), Note 1
Step 3: The primary conductor must be sized to carry 54A continuously (54A × 1.25 = 68A) [215.2(A)(1)] and be protected by a 70A overcurrent device [240.4(B) and 450.3(B)]. A 4 AWG conductor rated 85A at 75°C meets all of these requirements [110.14(C)(1) and 310.16].
Q. We installed fluorescent lighting in a warehouse, and the inspector says that the fixtures must have a “heavy-duty” rating because they are on a 30A circuit. Can we use 20A inline fuses to get around this issue?
A. No. The NEC specifically states that a 30A branch circuit shall be permitted to supply fixed lighting units with heavy-duty lampholders. According to 210.3, the rating of a branch circuit is determined by the rating of the branch-circuit overcurrent device. An inline fuse is a supplementary overcurrent device [Art. 100], and it's not permitted to serve as the required branch-circuit overcurrent device [240.10].
Q. We have interconnected office furniture, and the inspector says we need to have simultaneous disconnect for all circuits that feed the furniture. If we have four circuits supplying office furniture, am I required to have handle ties for all four circuits (4-wire and 2-wire)?
A. No. The requirement is that “multiwire branch circuits” must have a means to simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates [210.4(B) and 605.6]. Individual single-pole circuit breakers with handle ties identified for the purpose, or a breaker with common internal trip, can be used for this application [240.15(B)(1)]. In your example, you must only simultaneously disconnect all of the circuits connected to the 4-wire circuit.
Caution: This rule is intended to prevent people from working on energized circuits they thought were disconnected.
According to Art. 100, a multiwire branch circuit consists of two or more ungrounded circuit conductors with a common neutral conductor. There must be a difference of potential (voltage) between the ungrounded conductors and an equal difference of potential (voltage) from each ungrounded conductor to the neutral conductor.