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Code Quandaries

Nov 1, 2009 12:00 PM, By Mike Holt, NEC Consultant

All questions and answers are based on the 2008 NEC.

Q. Is it legal to splice conductors in an "LB" fitting?

A. Splices are only permitted in conduit bodies that are marked by the manufacturer with their volume; and the maximum number of conductors permitted in a conduit body is limited in accordance with 314.16(B) [314.16(C)(2)]. Short-radius conduit bodies, such as capped elbows, handy ells, and service-entrance elbows must not contain any splices or taps [314.5].

Example: How many 12 AWG conductors can be spliced in a conduit body that has a capacity of 15 cu. in.? (Figure 314-16C2 01) The answer is 6 conductors (15 cu in. ÷ 2.25 cu in. = 6.67) [Table 314.16(B)]

Q. Can I install a panel in the clothes closet of a house if the working space of 110.26 is maintained?

A. No. Overcurrent devices are not permitted to be located near easily ignitable material, such as in clothes closets.

Q. How do you properly size a generator? Is there a standard somewhere that provides any guidance?

A. Prior to the 2008 NEC, the Code was silent on requiring any types of calculations for optional standby systems. New to the Code in the 2008 edition is 702.5(B)(2), which tells us that the optional standby power source must have adequate capacity to supply the full load transferred, if using an automatic transfer switch.

For a new installation, the load is determined by Art. 220 or an alternate method approved by the AHJ. For existing facilities, the maximum demand data for one yearr or the average power demand of a 15-min. period over a minimum of 30 days can be used to size the power source [220.87].

Non-NEC considerations: Consider the type of load and impedance of the generator; it’s typical to size a generator for UPS services at three times the expected load in order to have the generator impedance low enough so that harmonics is not a problem. The issue with harmonics is due to the alternator size and impedance and not the engine. A less expense approach to this problem is to have an oversized alternator attached to the same size engine or specify an alternator with a lower sub-transient reactance. Generator sizing is complicated, and becomes even more so, when motor loads are involved. As generator performance varies with manufacturer, it’s a good idea to consult the manufacturer and avoid manufacturers who cannot give you an answer.

Q. Does the Code address sharing a neutral for two conductors of the same phase?

A. Well, not really. Section 310.10 is the closest thing you will find, which says conductors must not be used where the operating temperature exceeds that designated for the type of insulated conductor involved. With that said, it would be legal to do this if the neutral was big enough.

Q. I was written up for having my NM cable in the same drilled stud hole as my doorbell wiring. I have been doing this for years. Have I been wrong all this time?

A. No, you haven't been doing it wrong. The section you were probably cited was 725.136(A), which states, "Class 2 and Class 3 circuit conductors and cables must not be placed in any enclosure, raceway, or cable with conductors of electric light, power, and Class 1 circuits." Notice that this rule applies to conductors of electric light and power, not cables of electric light and power.

Q. We typically put the refrigerator for a house on its own circuit, but do I have to do this for commercial as well? These are very small refrigerators, if it makes any difference.

A. There is nothing that requires a refrigerator to be on an individual branch circuit, either for residential or commercial applications.

Q. Does the Code specify a maximum length for a disposal cord?

A. Yes. A flexible cord is permitted for a waste disposal if the cord has a grounding-type attachment plug, the cord length is at least 18 in. and not longer than 3 ft, the waste disposal receptacle is located to avoid damage to the cord, and the waste disposal receptacle is accessible [422.16(B)(1)].

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