Underground Wire Short

Underground Wire Short | Here's the section of wire that caused my problem. You can see how the dead short caused a small fire underground burning away the plastic wire insulation. Underground wire, in my opinion, should be placed in conduit. That said, read what Mike Cyr, one of my newsletter subscribers, sent to me below. Copyright 2019 Tim Carter

Underground Wire Short - Conduit and Sand Are the Answers

I didn't build the house I'm living in. Each month I discover something new where the builder and his subcontractors took a shortcut.

A few months ago I had an underground wire short. This wire extends from the corner of my house up to the base of an oak tree. From there it split off to a spotlight and then to a post lamp at the end of the driveway.

Was the Wire Rated For Underground Burial?

Yes, the wire was rated for direct burial.

Was the Wire Put in Conduit to Protect It?

There was one piece of 3/4-inch PVC conduit that ran under the blacktop driveway. On either side of the conduit, the wire was buried in the rocky soil without a conduit.

What Caused the Underground Wire Short?

Frost movement in my soil caused the wire short. It's important to realize I live in New Hampshire and the ground freezes each winter. It then thaws in the spring.

The freezing and thawing cause movement in the soil. Water in the soil expands by 9 percent in volume when it turns to ice. This movement wore away the insulation on the wire where it exited the end of a piece of PVC conduit.

Would the Short Have Happened if the Wire was in Conduit?

Maybe. In my case, you can see in the photo below what was the primary cause of the short. The wire exited the conduit and made a sharp 90-degree bend. That bend in the wire contributed the friction along the edge of the conduit.

The conduit is at the top of the photo. You can see the original underground gray wire in the trench. Look at the closeup photo below to see the wire make the sharp 90-degree bend. Copyright 2019 Tim Carter

The red arrow points to the exact location of the underground wire short before I pulled the wire out of the ground. May the electrician who put in this wire have black flies around his face for months on end every year. Copyright 2019 Tim Carter

I feel conduit is a great idea because it protects underground wires from gardening shovels and spades.

I cut the wire and made an above-ground splice in a waterproof code-approved junction box. I installed all new wire from this junction box to the oak tree on the other side of the driveway. This wire was placed inside new PVC NEC-approved conduit. I connected my new conduit to that which existed under the driveway.

Here's where I made the splice connecting a new underground cable to the old buried one. The conduit on the left travels up under the drive to the oak tree. The conduit on the right has the original wire that extends back to the corner of the garage. Copyright 2019 Tim Carter

Is There Another Way to Prevent an Underground Wire Short?

Mike Cyr is one of my newsletter subscribers. He's a great man that I hope to meet one day. He routinely responds to my comments in my newsletter and had this to say when I bemoaned my shorted wire in the spring of 2019:

"I have a friend who just retired as a master electrician and has done a lot of my work and we have buried some wires underground over the years.

He is against burying wire in conduit, and his reasoning is this: He claims that frost up here in northern Maine can go deep and can bring rocks up out of the ground and lift and stretch the conduit and actually break a joint or splice in the conduit, thus breaking the wire or opening a small gap in the conduit enough to fill with water and freeze the next freeze-thaw cycle and break the wire.

When he and I buried wire many times and many years ago, he would use direct burial wire which is designed for underground and we would put about 1-2 feet of sand under the wire and then make the trench 1 to 2 ft. wide and zig-zag the wire in the trench thus allowing the wire some slack to move with the frost. We would then bury it with sand or crusher dust.

He told me he has never had a wire break doing it this way but has had many come apart in the conduits. This is just some food for thought."

Mike's electrician friend is right. Frost does push rocks up through the soil. Just ask any New England farmer! I don't dispute anything Mike's electrician said and protecting the wire with lots of sand is an excellent idea.

If you're going to bury your wire the way Mike's electrician does it and you're passing through a garden area, be sure to bury the wire quite deep so no shovel, spade, or roto-tiller can cut the wire.

What Does Wire Buried in Conduit Look Like?

It looks like victory. Can you see how this would prevent the wire from rubbing against anything? Sure, in extreme conditions as described by Mike you could get a failure. But if you live where the soil doesn't freeze, the conduit is the only way to go.

This is 3/4-inch PVC electrical conduit. The 90-degree fitting at the tree will connect to an additional piece of conduit that extends up into a new 4x4 waterproof box. The conduit is in a shallow trench about 6 inches deep. Be sure to follow the National Electric Code when installing yours. Note that local inspectors can modify some codes. Copyright 2019 Tim Carter

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