Machine Design

Switches and the NEC

Most wiring in buildings is installed and maintained by electricians rather than by engineers.

Nevertheless, installers may not necessarily be qualified. And poorly configured wiring can cause problems in the engineering lab which can range from ground loops and conducted interference to an unsafe work environment.

It pays for even non-electricians to be familiar with basic installation practices spelled out in the National Electrical Code (NEC) so they can notice obvious infractions. Gross difficulties should serve as a wake-up call for potential problems with the rest of the electrical system

Wall switches and circuit breakers are likely the part of the building electrical system with which technical personnel have the most direct contact. The NEC article that covers switch connections is Article 404. The most basic prescription of 404 is that switches make or break the hot lead and not the grounded or neutral conductor. It also dictates wire color for hot, neutral, and ground conductors: black for hot, white or gray for neutral, and green or a bare wire for ground.

Article 404 also sets a limit on how full a switch enclosure can be. Splices and taps inside the enclosure can take up no more than 75% of the wiring space at any cross section. Wiring at any cross section can take up no more than 40% of the enclosure space. And there are important provisions for enclosures containing more than one switching device. The Code says there can't be more than 300 V measured between any point on adjacent devices mounted in the same box. It dictates use of an isolating barrier in this case.

The Code addresses switch accessibility as well. This part pertains mostly to circuit breakers and breakers used as switches. The center of the operating handle grip at its highest position must be no more than 6 ft 7 in. above the floor or working platform. Higher installations are OK if they are next to the equipment they supply and accessible by portable means (like, say, a ladder). But there is no minimum height requirement though local codes may be stricter.

Article 404 spells out important provisions for grounding. There must be a means to ground a metal faceplate, even if the faceplate isn't metal. (Someone may install one later.) The Code considers a switch grounded to an effective ground-fault current path if it mounts to a metal box with metal screws.

Also OK is mounting via metal screws to a nonmetallic box having integral means for grounding. Ditto for a ground conductor or bonding jumper connected to an equipment grounding termination on the switch.

There are numerous other provisions in Article 404 that cover such topics as the mounting of switch boxes, faceplates, and other installation issues. Many public libraries carry copies of the NEC for those interested in delving further into Code compliance.

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