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Code Quandaries

Mar 1, 2004 12:00 PM, All, By Mike Holt, NEC Consultant

Stumped by the Code?

Q. How close to a shower stall or hydromassage bathtub can I install a switch?

A. You can't locate switches within the wet location of a tub, shower space, or hydromassage tub (404.4 and 680.72). However, you can locate switches next to the tub or shower space. On a side note, switches must be located at least 5 feet from pools, spas, and hot tubs [680.22(C)] (Figure below).

Q. I've heard that unused wires must be removed unless they're tagged for future use. My plant has lots of unused conduit runs with the old feed wires taped up in the panel. Does the Code say these runs should be removed? It's starting to look like a spider web in some spots.

A. The NEC doesn't require unused power and lighting conductors to be removed, except in temporary electrical power and lighting installations (527.3) and in cellular and underfloor raceways [372.13, 374.7, and 390.7].

However, it does require you to remove accessible portions of abandoned low-voltage and limited-energy cables. According to Chapters 7 and 8, abandoned cables don't terminate at equipment and aren't identified for future use.

Q. Is it illegal to affix a permanent lock on an electrical panel? I want to be able to quickly shut off breakers or the whole panel in case of an emergency.

A. The NEC doesn't prohibit a permanent lock on a panelboard or disconnect switch. This means that a switch could be locked open or locked closed. I realize that in some cases either of these two conditions could be a hazard, but this issue isn't within the scope of the NEC.

Q. I designed a chiller plant based on a shop drawing that listed the following: rated-load current (RLC) at 147A, minimum circuit ampere at 184A, and maximum overcurrent protection at 250A. I specified a 250A circuit breaker with 4/0 AWG conductors (184A×1.25=230A). However, a couple engineers in our office say that 3/0 AWG, rated 200A at 75°C, could be used because you don't multiply the minimum circuit ampere rating by 1.25. They also say if I use 3/0 AWG, then the circuit protection device must be sized not greater than 200A. Are they right?

A. Almost. It's true that the minimum circuit conductor size is based on the “minimum circuit ampacity” as identified on the equipment nameplate, which is 184A. This is because the rated minimum circuit ampacity for the motor-compressor already includes 125% of the motor-compressor RLC (440.32) (147A×1.25=184A). Table 310.16 identifies 3/0 AWG conductor at 75°C as having an ampacity of 200A, which is acceptable for this minimum circuit ampacity.

The branch-circuit conductors, control apparatus, and motors in circuits that supply hermetic refrigerant motor-compressors must be protected against overcurrent due to short circuits and grounds by a protective device with a rating or setting that doesn't exceed 175% of the motor-compressor rated-load amperes (RLA) (440.22) (147A×1.75=257A). So a 250A circuit breaker is fine to protect the 3/0 AWG conductor for hermetic refrigerant motor-compressors.

Sec. 440.22 does permit overcurrent protection to be as large as 225% of the motor-compressor RLA if the 175% rating isn't sufficient to carry the starting current of the motor. The nameplate on the a/c equipment considers this factor.

Q. What is the maximum conduit run length for rigid nonmetallic conduit, before a junction or pull box is required?

A. The NEC only limits conduit run length by prohibiting splices in raceways [300.13(A)] and limiting bending radius to no more than 360° (352.26).

Send your questions to mike@mikeholt.com