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Code Quandaries

Dec 1, 1999 12:00 PM, By Fred Hartwell, Hartwell Electrical Services, Inc.

Topics discussed in our "Code Forum" column involve complicated issues requiring extensive analysis. However, not every Code question warrants such in-depth treatment. Here are the latest short answers to questions posed on our Web site. Coverage includes topics in: Sec. 240-3(b)(1), Sec. 300-37, Sec. 310-10, Sec. 310-15(b)(6), Sec. 336-5(a)(1), Sec. 336-18, Sec. 336-26, Sec. 338-4(a), Sec. 424-9, Table 310-16, and Arts. 336, 338, and 400.

Q. I am writing to see if the NEC prohibits the installation of wiring devices in perimeter convector units or housing for fin tube radiation. Are there restrictions on wiring in this location or for LAN/voice wiring and devices?

Q. An office space has hot water fin tube radiation on the perimeter. The fin tube is enclosed by a sheet metal housing with a fixed section (about 6 in. wide) every 4 ft and removal sections in between. Our client wants to install receptacles and LAN/voice outlets in the fixed section. The receptacles will be located above the heating element (fin tube). The wiring, probably MC cable, is the proposed method. It will be secured to the perimeter wall, again above the heating element. Is this a legal installation, and are there restrictions on the wiring methods?

A. (Editor's Note: We've combined the answer for these two questions.) First of all, these are listing restrictions issues. Some electric baseboard units have factory-made and suitably listed receptacles to go in the ends of this type of heater. The NEC recognizes these in Sec. 424-9. The FPN calls attention to generic listing restrictions that lead to the necessity for such devices; namely that electric heaters can't be located under receptacles. If there isn't a violation of the listing restrictions, either those specific to the heater or the generic ones in the UL Green Book, you can install the receptacles.

No restrictions exist in the NEC with respect to nonelectric circulating hot water heating systems, other than generic provisions like Sec. 310-10, which prohibits the use of a conductor under temperatures that would degrade its insulation.

If I understand your installation, that would be unlikely in the case of Type MC cable with typical THHN insulation. However, if the design temperature exceeds 60 degreesC (140 degreesF), you couldn't use No. 12 Type MC cable for 20A multiple-outlet circuit wiring. Applying the ambient temperature derating factor (0.58) for a higher temperature to the conductor ampacity in Table 310-16 for this wire (30A) yields an adjusted ampacity of 17.4A. Sec. 240-3(b)(1) prohibits rounding up to 20A in this case unless you have each outlet on an individual branch circuit.

With respect to the LAN, the NEC protects against fire and electrocution hazards, and the energy levels in such wiring pose neither threat. Therefore, this is purely a design question.

Q. Can I run a 15kV transformer feed (in conduit) through a telecommunications room?

A. The NEC allows for medium- voltage raceways in areas accessible to unqualified persons as long as the wiring method meets Sec. 300-37. If you use a pull box, be sure to add caution labels and provide a secure fastener.

Q. I have a three-story apartment building with one feeder (2000A, 208Y/120V, 3-phase) going to main switchgear breakers and then through distribution breakers to two sets of apartment metering. From the meters, it's proposed to use SER cable installed above the false ceiling into the apartments and down the drywall to each apartment subpanel. Do Arts. 336, 338, and 400 apply to SE cable? Can I use SER cable between a suspended ceiling and the fixed ceiling out of sight in this case? What other Code rules might apply to the SER cable?

A. Art. 338 applies, since this is the named article for the wiring method. Art. 400 only applies to flexible cords, and has no bearing here. By way of Sec. 338-4(a), however, all of Parts A and B of Art. 336 do apply. This brings in the three-story limitation in Sec. 336-5(a)(1) (which you meet) and the support rules in Sec. 336-18.

Be careful in calculating ampacities for this cable. The 60 degreesC ampacity column limitation in Sec. 336-26 applies to this cable for the first time. For example, to run a 100A feeder to an apartment, you'd need 1/0 Al SER cable; even if the cable carried the typical XHHW designation.

This rule used to be tucked away in Part C of Art. 336, but the 1999 NEC split the 1996 NEC wording, and this part of it ended up in Part B (affecting Art. 338). Don't even think about using Sec. 310-15(b)(6) (old Note 3) because these feeders don't originate in a single-phase distribution.