At EC&M, we know NEC issues are crucial to our readers. That's why we've dedicated a monthly department to answering your latest Code questions and concerns. When you find yourself stumped by the Code, just e-mail your question to firstname.lastname@example.org for future consideration in our Code Quandaries column.
Q. Does the Code allow you to run Class 2, low-voltage cable exposed in occupied public space? Can the cable be run exposed in a non-public workspace, e.g. a mechanical room, etc.?
A. Maybe. Sec. 725-7, which covers mechanical execution of work, specifies that Class 1, Class 2, and Class 3 circuits shall be supported by the building structure in such a manner that the cable "will not be damaged by normal building use." Cables installed in public places, where they could be damaged by normal building use shall be protected or installed in a suitable raceway. Cables installed in mechanical rooms, where they could be damaged by normal building use, must also be protected or installed in a suitable raceway.
The bottom line is that this is a judgment call by the "authority having jurisdiction," which is usually the electrical inspector.
Q. I'm installing a feeder in parallel. Can I use two different types of insulation on the different phases? For example: A phase, THHN; B phase, XHHW; and C phase, THHN.
A. Yes. (Sec. 310-4). The FPN to this section in the 1996 NEC indicates that conductors of one phase or grounded (neutral) conductor were not required to have the same physical characteristics as those of another phase or grounded (neutral) conductor to achieve even current distribution. In the 1999 NEC, this text was relocated from the FPN into a Code requirement for Sec. 310-4.
When conductors are run in parallel (joined together at both ends electrically, thereby forming a single electrical conductor), the currents need to be evenly divided between the individual parallel conductors so that each conductor heats evenly. You accomplish this by ensuring that each of the conductors within a parallel set has the same impedance, material, cross-sectional area, and insulation type - and that all conductors terminate in the same manner.
Example: You can wire a 400A feeder that has a neutral load of 240A with the following: (see Figure, above).
Phase A, two 250 THHN aluminum (rated 410A), 100 ft long.
Phase B, two No. 3/0 THHN copper (rated 400A), 104 ft long.
Phase C, two No. 3/0 THHN copper (rated 400A), 102 ft long.
Neutral, two No. 1/0 THHN aluminum (rated 240A), 103 ft long.
Ground, two No. 3 THHN copper, 101 ft long.
Q. Does the NEC require you to install the primary and/or secondary supply for a transformer in a flexible wiring method?
A. No. Installers use flexible wiring methods for convenience, reduction of audible noise from vibration, and flexibility. Generally, they use a flexible wiring method for the primary and secondary supply for a transformer because it's much easier to install flex than EMT or PVC. However, they could use electrical metallic tubing, rigid nonmetallic conduit, or any other fixed raceway for the transformer primary or secondary supply.
Q. I have encountered a design specification that states: "Pull points shall be provided in each conduit run to conform with the requirements of the NEC." I have always installed "pull boxes" 100 ft from each other. I tried looking this up in the Code but was unable to find a reference to the distance between pull boxes. Does the NEC cover this?
A. No. The NEC does not contain requirements limiting the distance between pull points. Because you may not splice conductors within a raceway [Sec. 300-13(a)], the maximum distance between pull boxes depends on the length of the conductors.
The Code specifies there shall be no more than the equivalent of four-quarter bends (360 total) between pull points. The Code states that "raceway fill" is based on common conditions where the length of the pull and the number of bends are within reasonable limits. Under certain conditions, you should consider a larger size conduit or a lesser conduit fill [FPN to Table 1 of Chapter 9].
Q. For purposes of computing the lighting load for feeders and services, many local engineering firms disregard Table 220-3(a), and instead use 125% of the actual installed lighting load. When the bulk of the light fixtures employ T8 fluorescent lamps with electronic ballasts, such a computation results in a lower load than Table 220-3(a) would require. Does the NEC permit this?
A. No. Sec. 220-3(a) is clear on this topic. The Code states, "A unit load of not less than that specified in Table 220-3(a) for occupancies specified therein shall constitute the minimum lighting load for each square foot (0.093 m superscript 2) of floor area."
Example D3 in Appendix D contains a note that says "where the actual connected lighting load (at 125%) is less than 125% of the load from Table 220-3(a), the minimum lighting load from Table 220-3(a) shall be used in the calculation." If the actual lighting load is greater than the value computed from Table 220-3(a), use 125% of the actual connected lighting load.