National Electrical Code Articles and Information

National Electrical Code Top Ten Tips: Article 547, Agricultural Buildings

1. Article 547 takes up about three full pages, making it among the shortest of NEC articles. The main reason for its brevity it the agricultural buildings it covers are subject to the first four Chapters of the NEC and it merely modifies or adds to those requirements. Article 110 is especially important in this regard.
    
2. Despite being one of the shortest articles in the NEC, Article 547 has one of the longest statements of scope. That's because the main concern is ignition from either:
   (A) Excessive dust and dust with water [547.1(A)], or
   (B) Corrosive atmosphere [547.1(B)].
    
3. The equipotential plane [547.2] is an important concept in agricultural buildings. It's a bonding system that eliminates various differences of potential. Don't confuse this with "ground" which means a connection to the earth [100]. When voltage is zero (equipotential), current will also be zero (Ohm's Law). Thus a properly constructed bonding system eliminates the dangerous flow of current between metallic objects and thus eliminates spark hazard in environments with ignitible gases. Of course, the voltage differerence is not exactly zero (because the conductors have some small amount of resistance), but for practical purposes it is. You can get a large resistance and thus a significant and dangerous difference in potential across a bad connection; this is why making connections correctly is vital.
    
4. You are limited on permissible wiring methods. You can use Types UF, NMC, copper SE cables, jacketed Type MC cable, RNC, LFNC, or other cables or raceways suitable for the location. And you must use approved termination fittings [547.5(A)]. For areas with excessive dust or dust with water, you can use the wiring methods of Article 502, Part II.
    
5. Where an equipment grounding conductor is installed underground within a location falling under the scope of Article 547, it shall be insulated [547.5(F)].
    
6. All 125V, single-phase, 15A and 20A receptacles must be GFCI-protected if installed outdoors, in reas having an equipotential plane, in damp or wet locations, or in dirt confinement areas for livestock [547.5(G)].
    
7. You must install a site-isolating device at the distribution point where two or more structures are supplied from the distribution point [547.9(A)(1)]. At the site-isolating device, you must connect the system grounded conductor (e.g., neutral) to a grounding electrode system via a grounding electrode conductor [547.9(A)(5)].
    
8. Equipotential planes indoors. They must be installed in confinement areas with concrete floors where the metallic equipment is located, if said equipment may become energized and is accessible to livestock [547.10(A)(1)].
    
9. Equipotential planes outdoors. They must be installed in concrete slabs where metallic equipment is located, if said equipment is energized and is accessible to livestock [547.10(A)(2)].
    
10. Equipotential planes must be connected to the electrical grounding system, via a solid copper bonding conductor that is at least 8AWG [547.10(B)]. It would not hurt to run dual connections (from different attachment points) in case of mechanical breakage or an electrical event such as induced current from lightning. While this will add a little cost, it will be a good insurance against even greater losses incurred due to the flow of undesired current. Possible outcomes of such flow include catastrophic burning of the buidling due to igniting gases, reduced output from stressed animals, and even the deaths of individual animals