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IdahoBob 01-28-2007 09:07 PM

Workshop Panel Grounding/Bonding
I need your advice concerning the panelboard in my detached workshop building. It is powered from a 100A 2 pole breaker in the main panel and run underground using triplexed, direct burial, USE wire (XPLE insulation, 2#2, hot, & 1#4 Neutral, aluminum conductors) wire (No conduit). This is a 3 wire circuit with no grounding conductor and was probably installed about 1985 (based on the date on the wire). The workshop panel has a 100A main breaker for a disconnect and a #4 bare copper ground wire from the panel ground bar that disappears into the floor slab. It appears that when originally installed, the neutrals and ground were bonded together but later some one separated them and removed the neutral bonding screw ( the screw was found laying in the bottom of the panel and several circuit grounds were spliced to reach the ground bar).

The building contains copper water (above and blow grade) piping, above ground steel gas piping, telephone and cable TV. None of these are currently grounded in this building but that could easily be fixed. It is not feasible to add a new grounding conductor from the main panel because the feeder is all under either a concrete driveway or the floor slab of the workshop building itself.

Should I bond the grounds to the neutral bus and panelboard enclosure or leave them separated?

Should I bond the water and gas piping to the ground bar in the panel?

Stubbie 01-28-2007 10:57 PM

The 3 wire feeder with no equipment grounding conductor ran with the circuit conductors requires bonding of the neutral and ground in the sub-panel. Reinstall the green screw. See NEC 250.32(B)(2).

One concern is that if the water pipes, phone , gas, and data is ran from the main dwelling to this detached structure code requires that a 4 wire feeder be installed.

Yes, you would bond the metal piping systems. Your local code may not allow bonding of the gas pipes so check this locally. See NEC 250.104(A)(3) and (B).


IdahoBob 01-29-2007 10:23 PM

Thanks for the quick reply. Do you know when the language about requiring a 4 wire feed when there is water or gas piping or Tel etc was first added to the NEC? This is a 1985 installation and I want to at least meet the 1982 NEC since I can't find a way to get that ground wire out there.

Stubbie 01-29-2007 11:13 PM

I dont know when it first came into effect. But it is in the 1993 code cycle.

You also need a grounding rod installed at the detached structure.


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