A metal building is a strong and versatile enclosure. In many cases these structures can be erected by the landowner in a few weekends. Wiring these structures safely and efficiently can be performed once the enclosure is sealed from all outside weather. Local regulations must be consulted concerning the correct installation of the electrical system so the utility can run service to the shop building. All electrical wiring should be run inside electrical metallic tubing (EMT), also called conduit.
The metal frame must be electrically grounded together. This means that all the metal parts of the building should be conductive. The corners of the building can be tied to the earth by running a separate ground rod at each corner of the building. This will ensure a direct path to ground for any lightening strikes. Some electrical equipment can be damaged if encountered by lightening when the metal building has no direct path to the earth. All wires should be run in metal conduit; metal boxes should also be used. A separate green grounding wire should also be run inside the metal conduit to ensure a direct path to the grounding system in case of device failure.
Separate colored wires should be used to designate different circuits coming from the main panel box. Green wires are used to designate the earth grounding system wire. White wires are used to identify the common or neutral wire of all electrical systems. Single-phase hot wires can use the two colors of black and blue for identification. Low voltage, 240 VAC (volts alternating current), 3-phase circuits can use black, red and blue as the voltage-carrying conductor colors. High voltage, 480 VAC, 3-phase wiring can use the BOY color scheme of brown, orange and yellow.
All lighting should be run on its own circuit and not electrically tied to any devices or receptacles. For the most part on a 120 VAC lighting circuit you can connect up to 2400 watts of lighting to a 20-ampere circuit breaker. This is equivalent to 24 100-watt light bulbs. Welders, table saws and other large motor devices should be connected into their own circuit and circuit breaker. These devices should not be shared on the circuit if they might be used at the same time. Receptacles can be shared on a circuit breaker if the outlets are spaced apart and the devices connected into them are not going to be operated simultaneously. Use the correct gauge wires for the shop device load. Use only stranded wire. Stranded wire will carry more power than a single rigid wire. Use only 12 gauge and above for all conductor sizing regardless of the load. 12-gauge wire is rated for 20 amperes, 10-gauge wire will carry 30 amperes, an 8-gauge wire is rated for 40 amperes, and a 6-gauge in most cases will handle 50 amperes of power. Consult the National Electrical Code for higher ampere ratings.
- National Electrical Code 1987; National Fire protection Association Publishers; 1987
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