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Equipotential bonding has long been a requirement in Article 680, ensuring that both step potential and touch potential are kept as close to zero as possible, thereby minimizing any stray voltage (voltage gradients) and the accompanying stray current in the immediate area of the pool.
Voltage gradients and stray currents can cause disorientation, drowning or potential lethal shocks where the body resistance has been lowered by immersion in chemically treated water. By ensuring a zero potential difference between all conductive surfaces in the pool and the surrounding area, a person will not be adversely affected by stray electrical currents.
In conventional swimming pool construction, steel rebar is used as reinforcement within the concrete. The rebar is installed in a grid pattern by crossing lengths of the steel from side to side and lengthwise in the pool before the concrete is poured.
At each point where the rebar crosses, the rebar is tied together by steel tie wire. This steel rebar cage provides an equipotential bonding grid to which all metal parts in the pool and any metal parts in the area surrounding the pool can be attached using a solid copper conductor, not smaller than 8 AWG (American Wire Gauge).
Since steel rebar installed in concrete may have a tendency to rust and corrode, especially when exposed to chemically treated pool water, the steel rebar is encapsulated within a nonconductive compound, such as epoxy, to decrease deterioration of the rebar and extend the life of the pool. This epoxy-coated rebar is installed in exactly the same way as regular steel rebar but the resulting grid or cage is nonconductive and cannot be used as the bonding grid connection for metal in and around the pool.
Like epoxy-coated-rebar pools, fiberglass and plastic-liner pools also do not have an effective conductive-bonding grid, since rebar is not used in their construction. These pools must rely on an alternative method to achieve an equipotential bond for the pool.
In the 2002 NEC and previous editions of the NEC, there were four methods to provide a bonding grid for a pool.
These included using (1) the structural reinforcing steel of a concrete pool where the reinforcing rods are bonded together by the usual steel tie wires; (2) the wall of a bolted or welded metal pool; (3) a solid copper conductor, insulated, covered or bare, not smaller than No. 8 AWG; or (4) a rigid-metal conduit or intermediate-metal conduit of brass or other identified corrosion-resistant metal conduit.
The most common method of creating a bonding grid for a pool with epoxy-coated rebar, a fiberglass pool or one with a pool liner was to use the solid 8 AWG or larger copper conductor to bond all the metal in and around the pool together into an interconnected grid.
This 8 AWG or larger bonding conductor was installed from one metal part to another, providing a connection to ensure an equipotential bond.
In the 2005 NEC, Section 680.26(C) was amended to provide three methods of establishing an equipotential bonding grid. The first two methods-(1) the structural reinforcing steel of a concrete pool where the reinforcing rods are bonded together by the usual steel tie wires and (2) the wall of a bolted or welded metal pool-remain the same as in the 2002 NEC.
The third method is new in the 2005 NEC and is to be used as a bonding grid where a steel-rebar grid is not available and there is not a metal pool wall.
This alternative-bonding grid shall be constructed of minimum 8 AWG bare, solid-copper conductors arranged in a 12-by-12-inch network of conductors with a tolerance of four inches, shall be installed in a uniformly spaced perpendicular grid pattern, and shall cover the contour of the inside of the pool and extend horizontally into the pool decking by a minimum of three feet.
These conductors must bond to each other at all points of crossing and connections must be made in accordance with the requirements in Section 250.8, such as exothermic welding, listed pressure connectors, listed clamps or other listed means. The final requirement is to secure the below-grade bonding grid within or under the pool and deck.
This new bonding-grid requirement will have a profound effect on the pool industry wherever a pool is constructed using a method other than steel rebar and concrete. EC
ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or at [email protected].