Larry Jacobson, Minnesota Extension Engineer and Doug Reinemann, Wisconsin Extension Engineer
In the past, some localities exempted agricultural facilities from application of the National Electric Code (NEC). Minnesota and Wisconsin have both adopted the NEC as the minimum requirement for electrical installations in agricultural facilities. As a result, regulations in both states require that new agricultural facilities be wired in accordance with the NEC. The NEC is the reference standard used to resolve disputes on electrical wiring and equipment issues.
Chapters 1 to 4 of the NEC list the basic rules that apply to all electric wiring. Article 547 in chapter 5 lists additional rules or exceptions for electrical installations in agricultural buildings.
Since the NEC is updated every three years, it is important that individuals involved in designing and building agricultural buildings are aware of the changes made in the code. For 1999, the changes can be described in three general areas: clarifying wiring methods (section 547-4), clearly stating allowable methods of providing electrical service to agricultural buildings (547-8), and revising the requirements for installing equipotential planes (547-9).
Section 547-4 was rewritten to require an equipment grounding conductor on all wiring methods covered in this section, as well as in the physical-protection section. It also more clearly states the recommended wiring materials to be used in agricultural buildings&emdash;namely wire types UF, NMC, and copper SE.
The section on electrical services (547-8) was also revised and now requires a main disconnect at the distribution point where more than one service originates. This section specifies three methods of supplying services to agricultural buildings. It also allows for a four-wire feeder without all of the rules of the old four-wire system where a disconnect was required only at the distribution point (see section 250-32). Also, the rules for outside feeder circuits were changed and will be Part B of Article 225.
Possibly the most important change for builders and equipment suppliers was the more clearly stated section 547-9 that contains the equipotential-plane requirements. An equipotential plane is required for livestock (excluding poultry) facilities except where there is no electric service to the building nor metal equipment accessible to livestock tha t is likely to become energized. An equipotential plane is defined as an area accessible to livestock where wire mesh or other conductive elements embedded in concrete are bonded to all metal structures and fixed, nonelectrical metal equipment that may be energized, and are connected to the electrical grounding system to prevent a voltage difference from developing within the plane. All general purpose (15 & 20 amps) electrical receptacles in an equipotential plane shall have ground fault circuit interrupter (GFCI) protection. Slatted floors that are supported by structures that are part of an equipotential plane do not need to be bonded. Finally, a voltage gradient at the edge of an equipotential plane may not be required for situations in which livestock are moved into or out of a building through a loading chute. Wisconsin has adopted an exception to the requirement for equipotential planes for new livestock facilities that have floors that do not contain conductive elements (reinforcing steel).
For more detailed information on this subject, see ASAE paper No.983008, “Agricultural Wiring Changes in the 1999 National Electrical Code,” by LaVerne Stetson and Barry Bauman, and 1998 ASAE Standards, 45th ed. EP473 “Equipotential Plane in Animal Containment Areas,” ASAE, St. Joseph, MI 49085.
This page is part of the Biosystems and Agricultural Engineering Department web at http://www.bae.umn.edu/