Here are the latest short answers to questions posted on our Web site. In our discussion, we will cover topics from Secs. 210-52(c)(5), 250-30(a)(3), 250-104(a)(4), 300-22(c)(2), 384-16(a), and 450-13(b); and Art. 600, of the National Electrical Code (NEC).
Q. What is the largest kVA rated dry-type transformer you can install in the space over a hung ceiling used for environmental air-handling purposes?
A. Sec. 450-13(b) permits you to install transformers over a hung ceiling, provided they do not exceed 50kVA, 600V nominal. A commentary on page 523 in the 1999 "NEC Handbook," published by NFPA, states Sec. 300-22(c)(2) permits dry-type transformers in the air space.
Q. Can I install a GFCI receptacle in the backsplash area behind the sink in the kitchen?
A. Yes, however, Sec. 210-52(c)(5) indicates you can place the receptacle not more than 18 in. above the countertop. It also explains you cannot install receptacle outlets in a face-up position in the work surfaces or countertops.
Note: The fact that the receptacle was a GFCI receptacle had no bearing on the answer.
Q. How many exit signs can you install on a branch circuit, and does Art. 600 apply to exit signs?
A. Art. 600 does not apply to exit signs. You can determine the number of exit signs on a circuit as follows:
Number of fixtures = circuit rating x 80%1/ampere rating of fixture
(1 The maximum load on a branch circuit is limited to 80% of the rating of the circuit; see Sec. 384-16(a).)
Example: The number of exit fixtures containing two 25W lamps on a 20A circuit would be?
(a) 11 (b) 23 (c) 31 (d) 38
Answer: (d) 38 Number of fixtures = (20A x 80%)/0.42A* = 38 fixtures
(*Two lamps x 25W = 50W/120V = 0.42A)
Q. I'm installing a 480/120, 208V, 3-phase transformer on the roof of a standard five-story office building with no building steel. Someone told me to ground the transformer to the local water piping system on the roof. Don't I have to ground the transformer within 5 ft from where the water pipe enters the building?
A. In accordance with Sec. 250-30(a)(3), the grounding electrode for a separately derived system (transformer) needs to be as near as practicable to the neutral-to-ground bond for the transformer. The grounding electrode shall be an effectively grounded structural metal member of the structure (which you don't have) or an effectively grounded metal water pipe within 5 ft from the point of entrance into the building (which is on the first floor). If an effectively grounded structural steel member or water pipe isn't available, then you must use the service equipment-grounding electrode.
Note: Sec. 250-104(a)(4) also requires you to bond the grounded (neutral) conductor of the separately derived system to the nearest available point of the interior metal water piping system in the area served by the separately derived system. This means you must bond the neutral at the location where the neutral-to-ground bond is for the transformer to the metal water pipe up on the roof.