workshop wiring

[todd]

I recently built a new detached shop (the kind of shop that my wife parks her van in). The electric service to the shop is fed from the house via 240V 50A breaker. When I had the electrical inspected, the inspector noted that since I had more than one circuit in the shop subpanel, the subpanel needed its own ground rod. The electrian came back and put the ground rod in and connected the wire from the ground rod to the neutral bus. The inspector came back again and said that the wire from the ground rod needed to be connected to the panel case instead of the neutral. I figured I could take care of that quickly myself, but now I have a question. There is a ground from the main panel that currently connects to the panel case. Do I leave that in place when I connect the wire from the new ground rod? In case a pic would help, here it is before I moved the new ground wire off the neutral bus.

bottom left conduit is the service from the house with two hots, a neutral, and the ground wire. The conduit to the right of that is the wire that connects to the ground rod (the wire is black, but is marked as ground with green tape). I moved this wire to the ground connection in the bottom middle of the box, but I don't know what to do with the ground wire that is there now that connects back to the main panel.

todd

[Doug Miller]

Yes.

OK.... here's what you need to do: Install a separate grounding bus bar (maybe ten bucks at the Borg). This will bolt directly to the panel chassis; no need for a jumper.Then connect both the grounding wire from the main panel, and the wire to your ground rod, to that bus bar. Done.

The panel probably already has a place ready-made for attaching such a bar. Look for two or three raised bosses on the inside, either at the right or left side (possibly both) with a tapped hole in the center of each one. Those are your attachment points.

Alternatively, you could remove the bonding bar that connects the two bus bars shown in the photo, and use the bar on the left as your grounding bus. Connect the wires as above, and add a jumper to the panel chassis as well.

[Bewildered]

That box looks almost exactly the same as a GE box I just bought to, also, put a 50a subpanel in my shop; though you are further along than I am. My box has a ground bus attached to the two back holes in the upper right. All grounds, including the ones from your panel and grounding rod go to that. If you don't have one, you have to buy one. Actually, I don't see your circuit grounds, but maybe they just don't show up in the picture. I paid $32 at Lowes for my box with 6 breakers and the grounding bus installed. How did you do?

[Charley]

By code the Neutral is only permitted to be connected to ground at one point and that one point should be in the main electrical service panel. Now the code reuuires that the sub panel cabinet be connected to ground via the ground wire from the main panel and also via a #6 or larger copper wire to the ground rod. It wouldn't hurt to also connect a #6 ground wire from the panel ground to a clamp on any nearby metal water pipe, if one exists. Telephone, cable TV, and other electrical services should also be ground connected together, at least at one point on the property.

Before the code required that a sub panel in an out building had to be grounded via a separate ground rod, a 60' pine tree next to my shop was struck by lightning (3' away). It was such a large strike that the top 16' of this pine tree was blown off and fell to the ground. The lightning also jumped from the tree trunk into the metal face of the door and then up from the top corners of the door through the 1/2" plywood sofitting above the door into the armored electrical cabling above the door (blew 1"X6" slots through the plywood). Although the electrical panel in the shop was grounded via the service cable from the shop to the main panel in the house, at the time I didn't have a separate ground rod installed at the shop, as it wasn't required by code. The ground connection via the electrical cable to the house aparently wasn't sufficient to carry the lightning load because the lightning jumped between the ground and neutral in the sub panel in the shop and went back to the main panel in the house that way too. This resulted in a large burn mark in the sub panel and burns between neutral and ground in many of the electrical outlet boxes both inside the shop and the house.

Four days later, while I was just sitting in the shop looking at some plans on my workbench, the receptacle directly behind my radial arm saw suddenly threw sparks and set fire to the wall around it. Nothing electrical in the shop was running at the time, except for the overhead lights, which are on a separate circuit. It's a good thing that I was there. I was able to quickly shut off the power and use an extinguisher to put out the fire on/in the wall. If I hadn't been sitting there when this happened I would have lost my whole shop.

After this incident I replaced many of the outlets in my shop and in my house, personally inspected all of them, and added a ground rod to my shop panel wiring. I now frequently take the time (at least once every spring) to check to be sure that my phone wiring, my cable TV wiring, and my house electrical panels are all ground connected to each other and to the ground rods. When I make this check I remove, clean, and replace any connection that appears not to be perfect.

I have always had a significant respect for lightning, but this incident enhanced this respect significantly.