All references are based on the 2014 edition of the NEC.
At first glance, the bonding requirements in Sec. 680.74 seem fairly straightforward. However, upon further thought and interpretation, they may not be so clear. The literal wording states, “Both metal-piping systems and grounded metal parts in contact with the circulating water shall be bonded together using a solid copper bonding jumper, insulated, covered, or bare, not smaller than 8 AWG.” But if you think about it for a moment, you may come to the conclusion that the hot and cold water pipes supplying the water to the hydro-tub are really not “in contact with the circulating water.” Most textbooks will show a No. 8 bonding jumper connected to those water pipes. Is this bonding unnecessary? Well, the literal meaning of the words could be interpreted that way, since those pipes are generally NOT in contact with the water circulating in the tub. However, I believe the intent of the Code is to bond the metal water pipes supplying the hydro-tub as well as any metal pipes or other grounded parts that may be “in contact with the circulating water.”
This requirement was first proposed for the 1999 NEC. Prior to 1999, hydro-tub bonding was not specifically required by the NEC. The substantiation provided with the proposal stated that a woman using the hydro-tub received a shock when she touched the faucet. The shock apparently was caused by a difference of potential between the water in the tub and the metal piping of the faucet. The proposal was originally rejected by the Code Making Panel, but the idea was later accepted in some form and eventually became a requirement in the 1999 edition of the NEC. Other proposals since then have tried to clarify this issue, but the wording still seems to cause some confusion.
It’s logical and makes perfect sense to bond the hot and cold water pipes supplying the hydro-tub if a difference of potential could develop between the equipment ground of the circuit supplying the hydro-tub equipment and the water pipe supply lines. Making contact with conductive parts at different potentials could be an extreme shock hazard for a person who is completely soaked with water. So while the literal wording may not require it, I believe the intent is to bond those water supply pipes, too.
