Article 680 and Equipotential Bonding Grids
21 Nov 2007
By: Joseph Kempker -
Article 680 has really taken the pool and electrical contracting businesses by surprise with the enforcement of the bonding grid around water areas. This bonding grid is designed to create an equipotential medium that will assist in displacing stray voltages that may arise. By definition, the meaning of equipotential is creating equal potentials at all points within the installed system. According to the 2005 National Electrical Code, and Article 680, this equipotential bonding grid can be created in many different ways and by many different methods. We will discuss these equipotential bonding grid methods later in this educational article.
However, what are stray voltages? The term stray voltage has been used for the past 40 years to describe a special case of voltage developed on the grounded neutral of an electrical system1. I use the term electrical system because this stray voltage could come from the utility company or any number of different electrical installations. If this stray voltage reaches sufficient levels, animals or humans being exposed to grounded devices may receive a mild electrical shock that can cause a behavioral response1. This behavioral response usually comes in the form of a tingling sensation or in worst-case scenarios an electrical shock that reaches the “let go” threshold value of 10 ma or higher. Stray Voltage is classified as a low frequency form of conductive Electro-Magnetic Interference (EMI)2.
In most buildings, stray voltage is not a problem because the levels are generally below the perception level of humans and usually there is no sensitive electronic equipment, which can be affected by these wandering voltages2. However, we also know that these electronic pieces of equipment (i.e. computers, ballast, etc.) are a source of currents flowing back through the neutral from harmonic affects. Harmonics are the frequency components of voltage, current and power. With linear loads, these three components generally follow the motion of a sine wave. In non-linear systems (electronic), these components fall out of alignment and create these harmonic affects that produce excess currents flowing back on the neutral.
In order to understand stray voltages an individual has to resort back to a simple formula known as Ohm’s Law. Ohm’s Law states that a conductor carrying a current of 1-ampere, between two points, with a potential difference of 1-volt has a resistance of 1-ohm3. This of course gives us the relationship (V = IR). With this in mind a stray voltage is the extraneous voltage that is generated on grounded surfaces when current flows through the resistance of a ground path2. As individuals have learned over the year’s stray voltage is primarily caused by excessive current traveling back on the neutral conductors. This excessive current is normally transferred to the ground via the ground wire and grounding electrode connection and only travels through a connected object if it is at a different electrical potential.
What we are really talking about is a low impedance source of voltage being generated by excessive current traveling back on the neutral conductors. Impedance (Z) is the overall resistance of a circuit to alternating current (AC). Impedance takes into consideration the resistance, capacitance, and inductance within an electrical circuit and is measured in ohms4. These three components can be laid out in a Phasor diagram in order to show their vector relationship. Impedance is calculated using the formula (V = IZ). This formula is similar to Ohm’s Law except that the ampere (I) value is being replaced by the (Z) impedance value. Impedance is expressed as one volt per ampere or one ohm4.
Now that you have a general understanding of stray voltages and their origin, let’s go back and take a look at Article 680 of the 2005 National Electrical Code. Article 680 addresses the issues surrounding the installation of permanently installed pools, storable pools, spas and hot tubs, pools and tubs for therapeutic use, and hydromassage bathtubs. Part one of Article 680 provides the general requirements and definitions associated with the above types of installations. Part 2 takes us to permanently installed pools and the debated and confusion about Article 680.26 that deals with equipotential bonding and bonding grid systems. This confusion arises because other articles within 680 refer individuals back to part two, which is permanently installed pools.
For instance, part four in the general requirements (680.40) directs an individual to comply with the provisions of parts one and four of this article (680). If an individual looks at outdoor installations of spas or hot tubs (680.42) they are now required to meet the provisions of parts one and two of this article, except as permitted in 680.42(A) and 680.42(B), that otherwise apply to pools installed outdoors. Article 680.43 (indoor spa and hot tub installations) also requires an individual to comply with the provisions of parts one and two of this article except as modified by this section. Fountains that have water common to a pool, and permanently installed therapeutic pools, are also required to meet the provisions of part two of this article.
The main issue and confusion with part two of Article 680 is the requirements for equipotential bonding and the bonding grid system. Article 680.26(A) informs an individual that the equipotential bonding by this section shall be installed to eliminate voltage gradients in the pool area prescribed5. This article section proceeds to inform an individual of the parts that have to be bonded together as specified in Articles 680.26(B)(1) through (B)(5). The confusion now arises with the requirements of Article 680.26(C). This article spells out and describes the requirements for creating an equipotential bonding grid system and states “The equipotential common bonding grid shall extend under paved walking surfaces for 1 m (3 ft) horizontally beyond the inside walls of the pool and shall be permitted to be any of the following”5. Let us take a further look at these permitted methods and requirements of Article 680.26(C).
Article 680.26(C) further informs an individual that the parts specified in part (B) have to be connected to an equipotential bonding grid with a #8 cooper, insulated, covered, or bare conductor. It also allows an individual to use rigid metal conduit of brass or other identified corrosive-resistant metal conduit for this connection. Part (C) also discusses the different methods of connections that are permitted by the National Electrical Code. The following are the three permitted methods associated with constructing the common equipotential bonding grid:
1. Structural Reinforcing Steel. The structural reinforcing steel of a concrete pool where the reinforcing rods are bonded together by the usual steel tie wires or the equivalent
2. Bolted or Welded Metal Pools. The walls of a bolted or welded metal pool
3. Alternative Means. This system shall be permitted to be constructed as specified in (a) through (b)
a. Materials and Connections. The grid shall be constructed of minimum #8 bare solid copper conductors. Conductors shall be bonded to each other at all points of crossing. Connections shall be made as required by 680.26(D).
b. Grid Structure. The equipotential bonding grid shall cover the contour of the pool and the pool deck extending 1 m (3 ft) horizontially from the inside walls of the pool. The equipotential bonding grid shall be arranged in 300 mm (12 in.) by 300 mm (12 in.) network of conductors in a uniformly spaced perpendicular grid pattern with tolerance of 100 mm (4 in.).
c. Securing. The below-grade grid shall be secured within or under the pool and deck media.
As an individual looks at the whole picture associated with this equipotential bonding grid, we have to start to put the requirements together in order to meet the minimum standards of the National Electrical Code. From Article 680.26(C) an individual knows that they have three options to create this equipotential bonding grid system. They also know that whichever equipotential bonding grid system they employ it will have to extend a minimum of three feet horizontally from the inside walls of the pool and under paved walking surfaces. However, what if I do not have any paved walking surfaces. I should have a situation that does not require an equipotential bonding grid system to be installed horizontally 3-foot from the inside walls of the pool.
If method one is used an individual has to extend the reinforcing steel from the contour of the pool to a horizontal distance three feet from the inside wall of the pool. This will allow the equipotential bonding grid to meet the requirements of incorporating the paved walking surfaces within its boundary. If method two (typical pool construction) is employed an individual will have to connect to the walls of a bolted or welded metal pool by a permitted method described in Article 680.26(C). However, an individual will still have to extend 3-feet horizontally from the inside walls of the pool. This is another debated and confusion area within Article 680.26 and is clarified in the 2008 NEC.
The issue is what conductive material does an individual use to construct the equipotetnial bonding grid in these typical pool installations that is required to extend 3 feet horizontally from the inside walls of the pool. If I use the alternative means describe in part three, the bonding grid system is permitted to be constructed as spelled out in (a) through (c). This would mean that I am using a minimum #8 bare solid copper conductor to make my grid and that I have to construct my grid as per (b) and secure it as per (c). This is where the debate comes in. Does an electrical contractor have to use rebar for this 3-foot extension area or can they use wire mesh. What about a #8 equipotential bonding grid ring? The goal is to equal out all the points so that there is no point at a difference within the pool or water area.
As I have mentioned, the 2008 National Electrical Code is further clarifying the explanation of the equipotential bonding grid. At this point the 2005 National Electrical Code has issued a Tentative Interim Amendment (TIA) (05-02 Log No. 821) to handle the problems and concerns associated with Article 680.26(C). This (TIA) will be the new terminology and installation requirement for the equipotetntial bonding grid system. As I have always stated, the wording was incorrect in the 2005 NEC in relationship to the term paved-surface. This is now an exception to Article 680.26(C) that provides the explanation and installation requirements for the 3 foot horizontal equipotential bonding grid under these types of surfaces.
The exception to Article 680.26(C) states “The equipotential bonding grid shall not be required to be installed under the bottom or vertically along the walls of the vinyl lined polymer walls, fiberglass composite, or other pools constructed of nonconductive materials. Any metal parts of the pool, including metal structural supports, shall be bonded in accordance 680.26(B). For the purposes of this section, poured concrete, pneumatically applied (sprayed) concrete, and concrete block, with painted or plastered coatings, shall be considered conductive materials.” There is also some additional wording that has been added to 680.26(C)(1) that states “Where deck reinforcing steel is not an integral part of the pool, the deck reinforcing steel shall be bonded to other parts of the bonding grid using a minimum #8 solid copper conductor. Connection shall be made as per 680.26(D).”
New additions to Article 680 are Article 680.26(B)(1)(a) that looks at the requirements of unencapsilated structural reinforcing steel and the fact that it shall be bonded together by steel ties wires or the equivalent.Where structural reinforcing steel is encapsulated in a nonconductive compound, a copper conductor grid shall be installed in accordance with 680.26(B)(1)(b). Article 680.26(B)(1)(b) also states that the copper conductor grid shall be constructed of a minimum #8 solid copper conductors bonded to each other at crossing points. It also requires the grid to conform to the contours of the pool shell and pool deck and shall be arranged in a (12 inch x 12 inch) network pattern with a 4-inch tolerance for spacing.
Article 680.26(B)(2)(b) addresses the perimeter surfaces of the pool area and requires at least one #8 solid copper conductor to be secured within or under these surfaces (18 – 24) inches measured horizontally from the inside walls of the pool. If this conductor is installed beneath the final grade material, the bonding conductor is required to be buried (6 – 8) inches below the subgrade. This article also permits the bonding grid to be a single #8 solid copper conductor, wire mesh (547.10(B)), or rebar in the concrete. Article 680.26(C) has added a new twist for bonding the pool water. An intentional bond of a minimum conductive surface area of (9 in2) shall be installed in contact with the pool water.
I hope that this article has brought you up to speed on Article 680 of the National Electrical Code. The equipotential bonding grid has been clarified in the 2008 NEC and it appears that the wording is now correct when talking about, and dealing with, these bonding issues related to permanently installed swimming pools. Please keep in mind that spas and hot tubs, whether installed indoors or outdoors, have to meet the requirements of part 2 of Article 680. Fountains that have water common to a pool, and permanently installed therapeutic pools, are also required to meet the provisions of part two of this article. With this in mind, let us move onto the next great debatable item that will surface within the National Electrical Code and that will keep our minds wheels turning.
1 Midwest Rural Energy Council. (2006, January 1). Equipotential planes for stray voltage reduction (A self-help guide). Washington, DC: Midwest Rural Energy Council.
2 Bass Associates. (1998). What causes stray voltages. Minneapolis, MN: John Bass.
3 Dodge, J., Haber-Schaim, U., & Walter, J. (1986). Physics (6th ed.). Lexington, MA: D.C. Heath and Company. (Original work published 1971)
4 Sears, F., Young, H., & Zemansky, M. (1982). University physics - part 2 (6th ed.). Menlo Park, CA: Addison-Wesley. (Original work published 1949)
5 Early, M., Sargent, J., Sheehan, J., & Caloggero, J. (Eds.). (2005). NEC 2005 national electrical code. Quincy, MA: National Fire Protection Association.
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