The term “Bonding” is defined in Article 100 of the National Electrical Code (NEC) as the permanent joining of metallic parts to form an electrically conductive path that will ensure electrical continuity and the capacity to conduct safely any current likely to be imposed.

The bond between two metal parts may be a direct metal-to-metal connection or a conductor that provides permanent joining of the two parts. Whatever means is used to provide the conductive path, the bond must have the capacity to safely conduct any likely fault current. Sizing the main bonding jumper to handle fault current is a fairly simple process.

There are many different bonding jumpers installed in an electrical system, but there is only one main bonding jumper, and it is located at the electrical service. It is defined in Article 100 as the connection between the grounded circuit conductor and the equipment grounding conductor at the service.

For a grounded electrical system, Section 250-28 requires an unspliced main bonding jumper to connect the equipment grounding conductor(s) and service disconnect enclosure to the grounded conductor within the service enclosure for each service disconnect. If the service consists of more than one service disconnect within a listed assembly, the main bonding jumper must bond the grounded conductor(s) to the assembly enclosure.

Since the main bonding jumper is a critical path for a ground fault, it must be copper or other corrosion-resistant material and must not be spliced. The bonding jumper can be a wire, screw, bus, or other suitable conductor.

The purpose of the main bonding jumper is to provide a path for a ground fault back to the source of supply to the service. The path for the fault current would be from the point of the fault back through the equipment grounding system to the service disconnect enclosure, and through the equipment bonding jumper to the equipment ground bar and then to the grounded conductor of the system. The grounded conductor of the system may also serve as the neutral for some applications and may carry neutral currents as well as transmit any fault current back to the utility company transformer or other source of power.

Depending on the impedance of the system, fault current levels can often be hundreds of thousands of amps relative to the normal current in the circuit. For this reason, the size of the bonding jumpers on the supply side of any overcurrent protective device and the grounded service conductor connected back to the source of supply must be sized large enough to conduct this potentially high fault current. Since the main bonding jumper is connected in series with the service grounded conductor and is subject to the same amount of fault current, it must be sized similarly to the grounded conductor.

Section 250-24(b)(1) requires the grounded conductor brought to service equipment to be sized not smaller than the required grounding electrode conductor specified in Table 250-66, but it is not required to be larger than the largest ungrounded service-entrance phase conductor. Where the service-entrance phase conductors are larger than 1,100kcmil copper or 1,750kcmil aluminum, the grounded conductor must not be smaller than 121/2 percent of the area of the largest service entrance phase conductor.

Likewise, Section 250-28(d) requires the main bonding jumper to be sized not smaller than the sizes shown in Table 250-66 for grounding electrode conductors. Where the service entrance phase conductors are larger than 1,100kcmil copper or 1,750kcmil aluminum, the bonding jumper must have an area not less than 121/2 percent of the cross-sectional area of the largest phase conductor.

For example, assuming four paralleled 500kcmil copper conductors per phase, the overall circular mill area of the combined phase conductors would be 4 x 500kcmil, or 2,000kcmil. The minimum size of the main bonding jumper would be 0.125 x 2,000 kcmil, or at least 250kcmil.

Occasionally, an installation will involve the use of a material for the main bonding jumper that differs from the material of the phase conductors. In such instances, the minimum size of the bonding jumper is to be determined on the basis of the use of phase conductors made of the same material as the bonding jumper and equivalent ampacity of the installed conductors.

For example, assume the phase conductors are 500kcmil aluminum, which are permitted to carry 310 amperes. If the main bonding jumper is copper, then the size of the main bonding jumper would have to be based on the size of a copper conductor that is permitted to carry 310 amp. A 350kcmil copper conductor, using the 75 degrees C column from Table 310-16, would be an equivalent copper conductor. The main bonding jumper would then have to be at least No. 2 copper based on Table 250-66.

By complying with these few rules, the main bonding jumper will provide a low- impedance path capable of safely carrying sufficient ground fault current to cause the overcurrent protective device to open the supply to the faulted circuit.

ODE is staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or mark.c.ode@us.ul.com.