11.4 Agricultural Buildings, Mobile Homes, Marinas & RV Parks

11.4 Agricultural Buildings, Mobile Homes, Marinas & RV Parks

Alabama has poultry houses, cattle operations, manufactured-home communities, coastal marinas, and campgrounds, and a journeyman working outside metropolitan Birmingham will meet all four. Prov allocates six questions to Special Occupancies and Equipment, and those questions are drawn from all of NEC Chapter 5, not only from hazardous locations, health care, and pools. Each of the four articles below has a small number of signature rules that carry the questions.

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Agricultural Buildings (Article 547)

Article 547 applies to agricultural buildings or parts of buildings where excessive dust (poultry and livestock confinement, feed handling) or a corrosive atmosphere (manure, litter, and the vapors they produce, plus periodic washdown) exists.

Wiring Methods

Because ammonia-laden, humid air destroys ordinary steel and standard enclosures, 547.5 requires wiring methods and enclosures suited to the environment: nonmetallic raceways such as PVC and liquidtight flexible nonmetallic conduit, Type UF cable, nonmetallic boxes, and fittings listed for damp, wet, or corrosive locations. All boxes and fittings must be weatherproof, and luminaires must be watertight and guarded where exposed to physical damage.

The Equipotential Plane (NEC 547.10)

This is the defining rule of the article and the reason it exists. Stray voltage as small as one or two volts between a wet concrete floor and a metal waterer or milking parlor stanchion is enough to discourage a dairy cow from drinking or to make hogs refuse a feeder, because livestock are far more sensitive to potential gradients than people are, and they stand on wet concrete with four widely spaced contact points.

• An equipotential plane must be installed in concrete floors of livestock confinement areas where metallic equipment is accessible to animals and likely to become energized, and in the areas around such buildings where the animals stand.
• The plane consists of conductive elements, typically the floor's reinforcing steel or a bonded wire grid, embedded in or under the concrete.
• The plane must be bonded to the electrical grounding system using a solid copper conductor not smaller than 8 AWG, with connections that are listed and suited to burial in concrete or earth.

Site-Isolating Device (NEC 547.9)

Where a farm has more than one building or structure supplied from a common distribution point, a site-isolating device must be installed at that distribution point to disconnect all ungrounded conductors from the supply to the whole site. It must be pole-mounted or mounted where accessible, be capable of being locked open, and be marked to identify it as the site-isolating device.

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Mobile Homes and Manufactured Homes (Article 550)

Supply to the Home

• The supply is a feeder, not a service, and it must consist of four continuous insulated color-coded conductors: two ungrounded conductors, one insulated grounded (neutral) conductor, and one equipment grounding conductor.
• The rating of the supply cord or permanently installed feeder is normally 50 amperes minimum for a single manufactured home, and the calculated load determines whether a larger feeder is required.
• The grounded conductor is kept separate from the equipment grounding conductor and from the metal chassis throughout the home. The neutral bar in the distribution panelboard inside the home floats. Bonding the neutral to the frame would put normal return current on the chassis and on any metal skirting, steps, or plumbing that touches it.

Service Equipment Location (NEC 550.32(A))

The service equipment for a mobile home must be located adjacent to the home, in sight from and not more than 30 ft (9 m) from the exterior wall of the home it serves. A disconnecting means may alternatively be located in the home where it is a manufactured home installed on a permanent foundation with the service conductors installed as a service.

Receptacles and GFCI (NEC 550.13)

Receptacle placement inside a manufactured home follows the general dwelling spacing rules, with GFCI protection required at kitchen countertop receptacles, bathrooms, receptacles within the specified distance of sinks, outdoor receptacles, and receptacles serving the exterior of the unit.

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Recreational Vehicle Parks (Articles 551 and 552)

Article 551 covers recreational vehicles and the parks that serve them; Article 552 covers park trailers, which are larger units intended for seasonal placement.

Site Supply Equipment (NEC 551.71)

Every RV site supplied with electricity must have at least one 20-ampere, 125-volt receptacle. Beyond that minimum, the Code sets percentages of the total sites that must be equipped with larger receptacles:

• A defined majority of sites must have 30-ampere, 125-volt receptacles, and
• A defined minimum percentage must have 50-ampere, 125/250-volt receptacles.

Read the current percentages out of 551.71 on exam day; what the exam tests reliably is that 20 A is the floor at every site and that the receptacle types are the three standard shore-power configurations.

GFCI and Load Calculation

• All 125-volt, single-phase, 15- and 20-ampere receptacles provided at RV sites must be GFCI protected. The 30-ampere and 50-ampere shore power receptacles are not required to be GFCI protected.
• Feeder and service load calculations for the park use the demand factors in Table 551.73(A), which fall sharply as the number of sites increases because the sites are not occupied or loaded simultaneously.

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Marinas, Boatyards, and Docking Facilities (Article 555)

Salt water, immersion, and the presence of swimmers make Article 555 one of the strictest in Chapter 5.

Electrical Datum Plane

All electrical equipment must be located at or above the electrical datum plane, a reference elevation defined relative to the design flood level, the highest tide, or the highest water level for the body of water involved. Electrical connections, receptacles, and enclosures below that plane are prohibited because they will eventually be submerged.

Ground-Fault Protection and Electric Shock Drowning

Alternating current leaking from a boat or a dock into the water produces the same voltage gradient described in the swimming pool section, and it produces the same result: electric shock drowning, in which a swimmer is paralyzed by a few milliamperes and drowns silently. Article 555 addresses it with two layers:

• Shore power receptacles supplying boats must have individual ground-fault protection set to open at 30 milliamperes or less.
• Upstream feeders and branch circuits serving the docking facility require ground-fault protection at a coarser setting so that a single boat's leakage does not black out the entire marina.

Note the threshold carefully: 30 mA, not the 4 to 6 mA of a Class A GFCI. The higher setting is a deliberate engineering compromise, because boats with normal wet insulation leak enough current to trip a 5 mA device constantly. Thirty milliamperes is low enough to interrupt a lethal gradient and high enough to hold under normal marine leakage.

Signage and Testing

Permanent safety signs warning of electric shock hazard and prohibiting swimming must be posted at the facility, and the facility must have a documented means of determining leakage current, so that a boat putting current into the water can be found and disconnected.