Grounding and Bonding for Detached Buildings

 

Bonding of electrical remote or sub panels in detached buildings

Bonding is required with the exception of outbuildings containing only one 120 volt branch circuit.   Electric systems at detached buildings require bonding and  a separate grounding electrode system (GES), which is commonly a grounding electrode (ground rod). See attachment for wiring illustration. One illustrates a 3 wire feeder and one illustrates a 4 wire feed.

Service Panel

When inspecting the service panel (main breaker/fuse box) you will need to determine that it is the first disconnect in the system downstream from the meter. There are two common exceptions.

The first would be in rural systems where the meter and a service panel / disconnect are located on a pole and not the house or shed and are more than ten feet apart.  Second the meter and inside breaker panel may be separated by a considerable distance (usually 10 feet or more) with a disconnect located directly below or close to the meter.

The second is most often found in duplexes and apartments but may also be the result of adding a garage or room addition and moving the meter but not the breaker box.  In this case a disconnect is located near the meter. Make sure the service wires to the remote are in conduit.

Remote or Sub Panels

In both of the above cases what most people would refer to as the main breaker panel is actually a “remote panel” (please use remote not “sub panel”).  If the wires feeding the remote panel are H H N G it is a 4 wire feed and neutrals must be isolated and grounds separated and bonded to the panel body.

If it is a three wire feed H H N no ground from the supplying panel (this includes metallic conduit) then it is a 3 wire system.  A three wire remote panel is treated the same as a service panel with grounds and neutrals allowed on the neutral buss.  (Otherwise there is no connection to ground).  In this case the neutral bus containing both neutrals and ground wires would be bonded to the panel body.  We recommend upgrading a three wire system to a 4 wire system for safety.

To be clear ground rods do not provide ground fault clearance for the breakers.  The earth ground will only allow about 4.8 amps to flow to ground at 120 v and this will only energize the ground wire but will not trip a breaker.  The ground rod is to help dissipate electricity overloads from lightning strikes and power surges.

Review this power point presentation by Paul Abernathy for a more in depth review

https://www.nachi.org/groundingbonding2007.htm

Drawings courtesy of
Joe Funderburk, CMI

Certified Master Inspector

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