Growing up in Northern Minnesota, I worked on my grandfather’s 30-cow dairy farm. In the early 1970s, Grandpa decided to upgrade his milking operation and build a new barn. It was a huge undertaking, as the new barn included an automated milking system and a bulk tank that allowed for the milk to be picked up by the local milk truck.
In comparison, today’s milking operations are nothing short of amazing. There are dairy operations that accommodate 10,000 cows. These barns have 24-7 robotic milking systems, automated feeding systems, sophisticated ventilation systems, and on-site veterinary facilities for herd management (Photo 1) ― something my grandfather would never have thought possible.
Since its introduction in the 1978 edition of the NEC, Art. 547 has morphed and changed to keep pace with ever-changing technology (Photo 2). Because agricultural buildings are considered a Chapter 5 “special occupancy,” it is important to note that the “general wiring methods” found in Chapters 1-4 of the NEC apply unless modified or supplemented in Art. 547 (Sec. 90.3).
Let’s review some of the notable changes that took place over the years in reference to this Code Article.
For the 1987 NEC, the Correlating Committee created an Ad Hoc Subcommittee to address “Electrical Grounding of Agricultural Buildings.” The committee introduced many grounding requirements that still exist today. The changes included the requirement for copper equipment grounding conductors (EGCs) on all circuits as well as the introduction of the term “equipotential plane” (EP) to define the area used to reduce the potential effects of stray voltage and mandate that the conductive elements be connected to the grounding system. In addition, underground installations now required EGCs to be insulated or covered copper. A new 4-wire system requirement was added to address when an overcurrent device was not provided at the distribution point.
In the 1996 NEC, new language mandated conductive elements (wire mesh) be embedded in a concrete floor to form the EP. There was also a new requirement for a voltage gradient at entrances and exits for all new installations.
A distribution point disconnect requirement was added during the 1999 NEC revision cycle. Other changes adopted in the 1999 NEC included requirement of 3- or 4-wire distribution systems, depending on where the overcurrent protection device was located, and voltage gradient(s) at entrances and exits were revised to more permissive language. In addition, an exception for the EP was added for the slatted floor installations supported by a structure that is part of the EP grounding system.
The 2002 NEC introduced GFCI protection requirements for the 125V,15A or 20A general use receptacles in outdoor areas, wet and damp locations, and where an EP was installed. Language deleted the requirements for voltage gradients at entrances and exits. The Code text clarified the language around the requirement for the premises site isolation disconnect. In addition, rules were added to recognize that the utility provided disconnect could be used as the site isolation device. Also, the EP bonding requirements were adjusted to include only areas where conductive elements were “likely to become energized.”
With release of the 2005 NEC, we now had a definition for the term “site isolation device.” The new verbiage limited the site isolation device to a pole-top disconnect, as stated in Sec. 547.9. Underground installations (i.e., ground-mounted disconnects) were simply referred to as the distribution point. The clarification mandated that the feeders and branch circuits, when extended from the distribution point, required an overcurrent protection device. Remember, during this Code cycle, the site isolation switch was not required to have overcurrent protection, which is still the case today.
New language was included in the 2008 NEC to require an additional GFCI protected 125V, 15A, or 20A receptacle within 3 ft of where an accessible non-protected GFCI receptacle was located. Furthermore, luminaires were required to have a suitable guard where exposed to physical damage.
With release of the 2011 NEC, CMP-19 (now CMP-7) deleted the language for GFCI-protected receptacle within 3 ft of the accessible, non-protected GFCI receptacle. However, this Code cycle introduced a labeling requirement for all agricultural site distribution points and mandated that the 8 AWG EP bonding conductor be solid.
The 2014 NEC revised the definition of “equipotential plane” and new provisions allowed for the separate covered or insulated underground EGC to be either copper or aluminum.
The 2017 NEC deleted the term “covered” in Sec. 547.5(F), which meant the separate underground EGCs were now only required to be “insulated.”
With the recent release of the 2020 NEC, GFCI protection is now required for other agricultural-related structures [Sec. 210.8(B)]. However, in outdoor wet and damp locations, dirt confinement areas, and where an EP is installed, only the 125V, 15A and 20A receptacle(s) require GFCI protection. The distribution point required by Sec. 547.9 clarifies that the disconnect is only required for buildings or structures that confine livestock. In addition, the revised text allows non-agricultural buildings or structures to be fed with existing 3-wire feeders if they meet the requirements of Sec. 250.32(B)(1), Exception No. 1.
As we look to the future, we need to remember that the general conditions surrounding agricultural environments are still very much the same today as they were when I was a kid working on my grandfather’s dairy farm. These areas are harsh, damp, wet, and corrosive. The requirements set forth in Art. 547 are meant to ensure equipment longevity ― especially when it comes to grounding and bonding components ― and protection of livestock and the individuals working in these facilities.
Dean Hunter is the chief electrical inspector for the Minnesota Department of Labor and Industry. He currently serves as an IAEI representative on the NEC CMP-7 (formally CMP-19) and is a member of the UL Electrical Council. He also participates as a technical committee member on NFPA 225, 501 and 501A.
Inspector Intel articles are provided by the International Association of Electrical Inspectors (IAEI), www.iaei.org, a membership-driven, non-profit association headquartered in Richardson, Texas, that promotes electrical safety throughout the industry by providing education, certification of inspectors, advocacy, partnerships, and expert leadership in electrical codes and standards.
