Most power quality and safety issues in electrical installations arise from misapplication of the grounding and bonding requirements of Art. 250. One common problem is installers ground where they should bond.
While the NEC provides clear descriptions of grounding and bonding in Art. 100, the words are often misused in the various articles. Typically, the error involves saying “grounding” instead of “bonding.” This error is even in nomenclature such as “equipment grounding conductor.” You should not be grounding your load side equipment. You should be bonding it.
Bonding is a means of providing electrical continuity between metallic objects. Simple definition, right? What’s not always simple is correctly applying the NEC requirements, some of which changed with the 2011 revision. That will be the focus of this article. Many of these changes were for the sake of clarity.
Bond all metal raceways and enclosures that contain (or support) service conductors [250.92]. Interestingly, the NEC requires raceways and enclosures that contain feeder or branch conductors to connect to the circuit “equipment grounding conductor” [250.86], which is actually a bonding conductor [Art. 100].
If a panel knockout is oversized, concentric, or eccentric, or uses reducing washers, bond around that opening. Use a bonding jumper, not a standard locknut (Fig. 1).
Fig. 1. If a panel knockout is oversized, concentric, or eccentric - or uses reducing washers - use a bonding jumper, not a standard locknut.
The NEC gives you the choice of four methods for ensuring electrical continuity at service equipment, service raceways, and service conductor enclosures [250.92(B):
Fig. 2. An SSBJ isn't required within nonmetallic conduit because the service neutral conductor serves as the effective ground-fault current path.
This last method needs more discussion. To bond one end of the service raceway to the service neutral conductor, you must use a listed bonding wedge or bushing with a bonding jumper. Size it per Table 250.66, based on the area of the largest ungrounded service conductors within the raceway [250.102(C)].
When a metal raceway containing service conductors terminates to an enclosure without a ringed knockout, you can use a bonding-type locknut. Bonding one end of a service raceway to the service neutral provides the low-impedance fault current path to the source (Fig. 3).
Fig. 3. Bonding one end of a service raceway to the service neutral provides the low-impedence fault current path to the source.
You can’t have “separate grounds” between communications systems and your service. You must provide an external intersystem bonding terminal (for connecting communications systems bonding conductors at service equipment) [250.94]. For structures supplied by a feeder, do this at the metering equipment enclosure and disconnecting means (Fig. 4).
Fig. 4. For structures supplied by a feeder, you must provide an external intersystem bonding terminal at the metering equipment enclosure and disconnecting means.
The resulting termination must:
The 2011 revision helps distinguish between the rules for bonding jumpers upstream from an overcurrent device versus bonding jumpers downstream from an overcurrent device.
The NEC now clarifies that bonding jumpers on the load side of an overcurrent device must comply with all of Sec. 250.122, not just Table 250.122. It also:
Equipment bonding jumpers must:
Supply-side bonding jumpers:
Size bonding jumpers on the load side of feeder and branch circuit overcurrent devices per 250.122, based on the rating of the circuit overcurrent device. The equipment bonding jumper doesn’t have to be larger than the largest ungrounded circuit conductors [250.122(A)].
If you use a single equipment bonding jumper to bond two or more raceways, size it per 250.122, based on the rating of the largest circuit overcurrent device.
You can install equipment bonding jumpers, bonding jumpers, or bonding conductors inside or outside of a raceway.
Metal-piping systems, such as sprinkler, gas, or air, that are likely to become energized must be bonded to the electrical system. This bonding prevents a difference of potential that can produce flashover and ignition.
The equipment grounding conductor (for the circuit that’s likely to energize the piping) can serve as the bonding means [250.104]. Via an Informational Note, the NEC now alerts the reader that the National Fuel Gas Code, NFPA 54, Sec. 7.13 contains further information about bonding gas piping.
If it’s likely to become energized, exposed structural metal that forms a metal building frame must be bonded to the one of the following:
Size the bonding jumper per Table 250.66, based on the area of the ungrounded supply conductors. The bonding jumper must be copper if within 18 in. of the earth [250.64(A)], securely fastened to the surface on which it’s carried [250.64(B)] and adequately protected if exposed to physical damage [250.64(B)]. All points of attachment must be accessible, except as permitted in 250.68(A).
You must bond a separately derived system (SDS) to either the:
You must bond the SDS to exposed structural metal (interconnected to form the building frame), unless the structural frame serves as the grounding electrode [250.52(A)(2)] for the SDS.
In all three of the above cases:
Previous NEC revisions required you to bond structural metal (if likely to become energized) to the service equipment enclosure. But what about a structure supplied by a feeder or branch circuit?
The 2011 revision makes it clear that you must bond the structural metal (if likely to become energized) to the disconnecting means of the structure, regardless of the type of circuit feeding the premises.
Figure 250.1 lays out Art. 250 into three informational blocks (plus a fourth that’s off to the side). We’ve now addressed bonding, which is off in a block by itself. But contrary to what 250.1 may indicate, the other blocks are not grounding pure-plays. In our next issue, we’ll see where grounding and bonding collide.