Grounding question for subpanel

mn1247

First, let me thank everyone for all the excellent advice on my new barn panel. It's an electrical masterpiece!

I have one last question regarding grounding. To review the situation, I've got a new subpanel in my barn that connects to the main service panel in the house via three aluminum 1-0 wires enclosed in metal conduit. The conduit runs underground, between the house and barn, inside PVC. There are phone and coax lines inside the PVC (alongside the conduit) At the barn, there is a single grounding rod connected to the ground bar inside the new panel.

What's the best way to ground this? Should I bond ground and neutral? Should I keep or remove the barn's grounding rod? Should I try to use the metal conduit as a fourth conductor (ground) back to the house?

Thanks for your help
Eric

brric

Are you saying the metallic conduit is inside pvc?

Code05

You have metal conduit inside of PVC? Explain, please.

AllanJ

Bonded at the subpanel: The ground rod. The conduit. (The subpanel itself). Use a #6 copper wire to the ground rod.

The neutral is kept separate from the grounded objects out at the subpanel.

Note that the conduit with the power feed runs alongside the PVC with the telephone etc. wires.

In the (main) house the conduit should have been attached to the main panel sufficiently to be bonded to the latter.

__________________
The disadvantages of crab apple trees. In summer, the apples are too sour to pick and eat. In winter the birds come and leave dropping all over the place.

mn1247

Thanks. I didn't install the underground connections (they came with the house), but here's what's there...

Everything runs inside PVC (or some sort of plastic conduit). Within the PVC are a coax, a phone line, and a metal conduit. Inside the metal conduit are the three 1/0 aluminum (insulated) wires.

The ends of the metal conduit are affixed to the sides of the two panels (house and barn), but only in the sense that the conduit itself is screwed onto the panels with large nuts. The panels are gray and appear to be coated, so I'm questioning whether there is good electrical contact. There are no other connections bonding the conduit to the ground bars of the panels.

Clear as mud?

Eric

Code05

Yep. What type of metal conduit is this? Our answers are dependent.

AllanJ

One style of nut used to hold conduit and other things to the panel has teeth along the rim. These dig into the gray paint on the panel and provide the bonding.

Unlike screws that hold wires in place these nuts are meant to be tightened with lots of force, although not so much as to strip the threads or rip the panel off the wall.

__________________
The disadvantages of crab apple trees. In summer, the apples are too sour to pick and eat. In winter the birds come and leave dropping all over the place.

mn1247

The conduit is flexible metal (looks like aluminum, with a raised spiral running along the outside. Sort-of screw-like). It's not solid metal pipe.

I'll look for the "teeth" on the nuts... not sure there were any. Is that the only option to bond the conduit?

Thanks
Eric

brric

This "flexible metal" conduit is buried in the ground? Is it pvc coated? Is it continuous between the panels?

Code05

Houston, we have a problem.

Code05

It is in the PVC pipe with the phone and water.

AC, MC, or FMC this ain't good. He can see metal, it is not PVC coated MC.

brric

I think it's unlikely but I was wondering if it is LFMC.

mn1247

One more try....

Picture a big gray platic pipe running underground from house to barn. Open this and find flexible metal conduit, with coax and phone lines alongside. Open the flexible metal conduit and find three 1/0 insulated Al wires.

Definitely no water.

The conduit looks like it's just aluminum; no coating. It looks like this...

http://www.hiwtc.com/products/fmc-fl...791-145911.htm

As I said, I bought the house this way. I'm just swapping the panel in the barn.

Is it wrong?

Eric

brric

FMC is not listed for use in wet locations. Underground is a wet location.

AllanJ

Nuts and fittings without teeth can also provide a bond, although whether or not they do is harder to determine. If the gray paint is scraped off where other metal parts are attached then the likelihood of (successful) bonding is greater.

A conduit formed from a spiral with edges that physically lock together to achieve the cylindrical shape (water can soak in) is usually not an acceptable equipment grounding conductor. An additional conductor which can be an aluminum strip, running through a flexible conduit, makes the conduit into an acceptable EGC when the strip is bonded at the ends in an approved fashion that does not in all cases require wire nutting with other ground wires.)

__________________
The disadvantages of crab apple trees. In summer, the apples are too sour to pick and eat. In winter the birds come and leave dropping all over the place.