Why Do We Bond at the Service Panel and Not a Subpanel?

An important question was raised by one of our viewers. Why do we bond at the Service Panel and not at the sub panels? A very valid and super important question that Dustin brings answers to in the latest episode of Electrician U.

So, in a typical scenario, we would bring our ungrounded (hots), grounded (neutral) and grounding (ground) conductors into our service. This is at the point that we would bond the grounded and the grounding conductors together. From there, all of the conductors leave and can enter another panel where they are kept separated with no bonding. The primary reason for that grounding conductor is to set an alternate path back to the voltage’s source (breaker or fuse etc.) in case something happens, so there is another path back for the breaker to trip. Without that alternate path, the chances for someone to get shocked are considerably higher.

One of the reasons we separate the conductors past the Service Panel has to do with Objectionable Current. Simply stated, it is current that is going multiple directions at the same time; those directions we DON’T want it to go in! normally, that current leaves the source on the ungrounded wire, travels through the source, and returns to the source on either the grounded conductor or the other ungrounded conductor at the equipment. Objectionable current would be if a wire came off and touched something it wasn’t supposed to and sent the current somewhere else. The neutral conductor is expected to carry current; ground wires are not meant to carry them on anything other than a fault condition. If we didn’t bond the neutral and ground together at some point, we potentially could have objectionable current flowing on BOTH the neutral and ground at the same time and trying to return to source creating a considerable amount of havoc along the way!

If there was an event, say a ground fault where one of the ungrounded conductors touched the case of a dryer, the grounding conductor would carry that objectionable current momentarily back to the service panel where it is bonded to the neutral thereby, completing the circuit. In fact, since the load itself was taken out of the equation (by touching the case and bypassing the load itself) the breaker sees SO MUCH current that it does exactly what it is supposed to do and will trip. If that grounding wire was not there and the complete circuit was not made, there would be potential for current to be on ALL the metal downstream from the service point and someone could get severely injured.

So, wouldn’t extra bonding be good by bonding in each panel location? If we were to bond in EVERY panel, then we would leave MULTIPLE paths that the current COULD take on its way back to source to complete the loop, and we would be defeating the purpose of trying to direct the current back to its source. In addition, if the neutral and ground were bonded in EVERY panel then under the same circumstances, we would be sending potential to EVERYTHING plugged into ALL the circuits. So, we BOND it at the one spot only so we can keep the current corralled along the way WE want it to go and not energize everything along the way!

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