Damp, wet and corrosive locations, cords and temporary wiring

Water, chemicals and rough handling are the enemies of electrical installations. Water reduces the resistance of the human body and of insulation, corrodes terminals and enclosures, and provides paths for leakage current; corrosive vapors eat through steel boxes and conduit; and flexible cords, which are meant to be moved, are easily crushed, cut and overloaded. This lesson covers how the code classifies locations by moisture and corrosion and what it requires in each, the proper and improper uses of flexible cords, and the rules for temporary wiring on construction sites, where all of these hazards meet.

In the National Electrical Code, damp and wet locations are defined in Article 100 and addressed in each equipment article (for example 314.15 for boxes, 406.9 for receptacles, 410.10 for luminaires), flexible cords in Article 400, and temporary installations in Article 590.

Dry, damp and wet locations

The code classifies locations by their exposure to moisture:

• Dry locations are not normally subject to dampness or wetness, though they may be temporarily damp, such as a building under construction.
• Damp locations are protected from weather but subject to moderate moisture: partially protected locations under canopies and roofed porches, basements, some barns and cold-storage warehouses.
• Wet locations are subject to saturation with water or other liquids: underground installations, locations in concrete slabs in contact with earth, areas exposed to weather, and areas washed down with hoses.

Equipment must be identified for the location: boxes, receptacles, luminaires, enclosures and wiring methods carry markings such as "suitable for damp locations" or "suitable for wet locations". Inside a raceway in a wet location, including underground and above ground outdoors, conductors are considered to be in a wet location and must be rated for it, because condensation and water ingress are inevitable.

Receptacles outdoors

Outdoor receptacles need protection matched to how they are used. In wet locations, receptacles where a plug may be left in for a long time (such as for holiday lights or a pool pump) need "in-use" (extra-duty bubble) covers that keep the receptacle weatherproof with the plug inserted; flat flip covers protect only when nothing is plugged in. Outdoor receptacles must be weather-resistant types and, in most cases, GFCI-protected (Lesson 9 and ELEC-130).

The NEC (406.9(B)) requires 15 A and 20 A, 125 V and 250 V receptacles in wet locations to have enclosures that are weatherproof whether or not a plug is inserted, marked "extra duty", and to be listed weather-resistant types; GFCI requirements are in 210.8.

Corrosive locations

Corrosive environments include agricultural buildings with manure and fertilizer vapors, swimming pool equipment rooms with chlorine, wastewater plants, chemical plants and coastal sites with salt spray. Ordinary galvanized steel boxes and conduit can corrode through in a few years, exposing conductors and breaking bonding paths. Solutions include:

• Non-metallic wiring methods: rigid PVC conduit, non-metallic boxes, and cables with corrosion-resistant jackets.
• Corrosion-resistant metals: PVC-coated rigid steel, stainless steel and aluminum where suitable.
• Enclosures rated type 4X (Lesson 8).
• Equipment located outside the corrosive area where possible, for example panelboards in a separate clean room.
• Special attention to bonding, because corroded metal raceways cannot be relied on as bonding paths; separate bonding conductors are used instead.

Flexible cords and cables

Flexible cords are made for equipment that is moved or needs flexibility: portable appliances, pendants, connections to equipment that vibrates or must be removed for service. They are identified by type letters indicating their construction and use, such as SJOW (junior service, oil-resistant jacket, weather-resistant) or SOOW (hard service, oil-resistant jacket and insulation, weather-resistant). Hard-service cords (S) suit rough use; junior-service cords (SJ) lighter use.

Because cords are easily damaged, the code limits their use. Flexible cords must not be used:

• As a substitute for the fixed wiring of a building.
• Run through holes in walls, ceilings or floors, or through doorways and windows.
• Concealed behind walls, ceilings or floors, or above suspended ceilings.
• Attached to building surfaces.
• Where subject to physical damage.

The NEC (400.10 and 400.12) lists the permitted and prohibited uses of flexible cords, and Table 400.5(A)(1) gives their ampacities.

Temporary wiring

Construction sites, events and emergencies need power for a limited time. Temporary wiring rules relax some permanent requirements (for example, allowing cables to be run on the surface) but strengthen others, because sites are wet, wiring is exposed and tools are abused:

• Time limits: temporary installations are permitted only for the duration of construction, remodeling, repair, testing or emergencies, or for limited periods for decorative lighting and events.
• GFCI protection for receptacles used by personnel, typically all 125 V, 15 A to 30 A receptacles on construction sites, including those on portable generators.
• Protection from damage: cords and cables kept off the ground where vehicles pass, supported and protected at sharp edges.
• Overcurrent protection and disconnecting means as for permanent wiring, with all wiring removed promptly when no longer needed.

In the US, temporary installations are covered by NEC Article 590, which requires GFCI protection for all 125 V, single-phase, 15 A, 20 A and 30 A receptacle outlets used by personnel (590.6), and OSHA (29 CFR 1926.404) requires ground fault protection on construction sites.

Pools, hot tubs and fountains

Water, bare skin and electricity together lower the current that can cause harm, so pools, hot tubs, spas and fountains have their own sections of the code. Their rules are detailed and change often, so an installer works directly from the current requirements; the principles behind them are consistent:

• Ground-fault protection for pool pump motors, underwater luminaires above low voltage, and receptacles near the water.
• Distances: receptacles, switches and luminaires must be set back minimum distances from the inside wall of a pool or tub, or be specially protected.
• Equipotential bonding: all metal parts within a set distance, including the pool steel, ladders, pump motors and a copper grid or the reinforcing steel in the deck, are bonded together with a solid copper conductor so that there is no voltage difference between anything a swimmer can touch. This is not grounding to clear faults, but making everything the same potential.
• Underwater luminaires supplied through listed transformers at low voltage, or protected by GFCI, in listed wet-niche or dry-niche housings.
• Overhead conductors kept well clear of the water and diving structures.

In the NEC, pools, spas, hot tubs and fountains are covered in Article 680, including equipotential bonding in 680.26.