Why Bonding?

I’ve been in the electrical trade for 33 years. I’ve been an apprentice, journeyman, master, electrical inspector, and now the Training Director at the IEC Gulf Coast Chapter in Houston, Texas.

One common denominator throughout the years is the misinterpretation, misapplication, or the complete disregard of Article 250 and proper bonding.

When I was an electrical inspector, there was an incident where the groundskeeper for an apartment complex was killed. What did he do that was so careless? He took a break and leaned up against a small air conditioning (AC) condensing unit that was outside one of the apartment units. After investigation, it was noted that whoever installed the wiring to the condensing unit didn’t think the green wire was important enough to connect. There was a ground-fault on the AC condensing unit, and because the equipment grounding conductor wasn’t installed, the fault-current had no place to go until someone touched it. The groundskeeper lost his life, and his family has been forever changed due to one little green wire.

Table 250.122 of the National Electrical Code® (NEC) tells how to size the equipment grounding conductor. In the scenario given above, if the over current protective device for the AC condensing unit was rated at 30 amps, all that would have been needed would have been a 10awg copper conductor about 6 feet long.

How would using the equipment grounding conductor and properly bonding the metal case of the condensing unit to the disconnecting means have possibly saved the groundskeeper’s life?

First, let’s look at Article 250. 4 (A) (1) – Electrical System Grounding (emphasis on grounding). “Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.” Notice that nothing was said about the grounding providing a fault-current path between electrical equipment or facilitating the proper operation of the over current protective device (breaker or fuse). The electrical system that the AC condensing unit was connected to was grounded at the service but the proper bonding was neglected. So let’s look at Article 250.4 (3), (4), (5).

“Bonding of Electrical Equipment – Normally noncurrent-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.”

“(4) Bonding of Electrically Conductive Materials and Other Equipment– Normally noncurrent- carrying electrically conductive materials that are likely to become energized shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.” In the scenario given, after the electrical service was properly grounded via ground rod, ground plate, building steel, etc., proper bonding needed to be maintained from the electrical service all the way to the AC condensing unit via metallic conduit and/or an equipment grounding conductor.

“(5) Effective Ground-Fault Current Path – Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a low-impedance circuit facilitating the operation of the over current device…It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground-fault may occur to the electrical source. The earth shall not be considered as an effective ground-fault current path.” This tells us simply that bonding, if done properly, will facilitate the operation of the fuse or breaker.

In summary, i