Picture an electrician installing a load center (technically, a panelboard) on a 120/240V AC system at a location in the building remote from the service location. The electrician finds the bonding screw in an envelope, notes printed on the envelope say something about grounding, and the electrician thinks, “Grounding is good, therefore more grounding is better,” and installs the screw. This screw is intended to be the main bonding jumper, bonding the neutral conductor to the enclosure at a service disconnect location.
Our fictional electrician has committed one of the most frequent violations of NEC requirements. 250.24(A)(5): Load-Side Grounding Connections, which says, “A grounding connection shall not be made to any grounded circuit conductor on the load side of the service disconnecting means except as otherwise permitted in this article.” Included in the “otherwise permitted” are separately derived systems, separate buildings and existing ranges and dryers.
Had this been a service panelboard, this connection is required to be installed in service disconnect enclosures by 250.28, but installation is prohibited at panelboards downstream from the service, by 250.24(A)(5) and 250.142(B).
Bonding the enclosure to the neutral conductor at a remote panelboard places the neutral conductor and the equipment grounding path (enclosures, metal raceways, equipment grounding conductors) in parallel from that point back to the service equipment. The normal neutral current will divide, some flowing through the neutral and some flowing on the equipment grounding path. The equipment grounding path is unpredictable; metal conduits may be strapped to steel building frame members, and this current could be flowing in many paths. Electrical connections between elements of these paths could be so poor that arcs and sparks could be produced, and nearby flammable materials ignited.
Wherever current flows, there is voltage drop. Therefore, this parallel path to the neutral conductor may consist of exposed metal and will have a voltage on it. Even though it may be small, it can shock and surprise persons contacting any place in the path. Should the neutral conductor accidentally become open, a fairly frequent cause of trouble, all of the neutral current would flow through the equipment grounding path, resulting in a high impedance, and greater fire hazard.
One of the permitted connections to a neutral conductor beyond the service is in an existing branch circuit to a range, dryer, counter-mounted cooking unit or wall-mounted oven. In these cases, the frame of the appliance is grounded by connection to the neutral conductor. New branch circuits must provide an equipment grounding means separate from the neutral.
When installing an old range on a new branch circuit, the three-wire cord and cap must be exchanged for a four-wire cord and cap, and the jumper between the neutral terminal and the frame of the appliance removed. When installing a new range on an existing branch circuit, a three-wire cord and cap must be used, and the jumper must be installed between the neutral terminal and the frame of the appliance.
Another location where the neutral may be used for grounding of equipment downstream from the service is in a separate building, but only sometimes.
• There must be no equipment grounding conductor run to the second building
• There can be no conducting metallic paths such as gas piping, water piping or metal conduit connected to the grounding systems in both buildings
• There can be no ground fault protection installed at the service location in the first building
If these three criteria are met, then the neutral at the second building can be treated as it would be at a service—connected to a grounding electrode and by means of a main bonding jumper connected to the equipment grounding means.
It is preferred that the neutral at the second building be insulated from the disconnect enclosure, and an equipment grounding conductor be run with the feeder for the grounding of equipment in the second building.
The other condition where a neutral conductor is connected to an equipment grounding conductor is for the secondary of a transformer establishing a separately derived system. As required by 250.30, a bonding jumper must connect the secondary neutral to the equipment grounding conductors and that the neutral be connected to a grounding electrode conductor, just as for a service.
Just because the bonding screw is supplied with a load center does not mean you have to use it. If the equipment is labeled as “service equipment,” then the neutral will be bonded to the enclosure at the factory. Where the bonding screw is furnished, it means the equipment could be used as service equipment with the bonding screw installed, or as a panel downstream from the service without the bonding screw. EC
SCHWAN is an electrical Code consultant in Hayward, Calif. He can be reached at firstname.lastname@example.org.