Cleaning Up After Doofus: Repairing A Large Damaged Electrical Cable

By Bruce W. Maki, Editor

Oops, Mistakes Happen:

How on Earth does a person manage to cut through a cable? All too easily. I believe that the most common cause is drilling holes and making cuts through studs and floor joists. And sometimes the wrong wire gets cut during remodeling work.  I'd rather not talk about the cause of my little fiasco... what's important is to know that:

• Everyone makes mistakes, even professionals.
• Circuit breakers really do protect you from yourself.
• Fix your mistakes.
• Learn something from it.
• Don't do it again.

Suffice to say that somehow this electrical cable managed to get a big hole burned in it. This cannot simply be wrapped with electrical tape, because one or more of the conductors shorted. I know, I was there. My face was only about 8 inches away when it sparked. I think I "saw stars" for a week after.

Had I been so fortunate as to catch this insulation burn-through before the wires shorted, I might have been able to fix this without having to cut out the damaged section. Once the wires short, the momentary surge of current (which can be as high as 10,000 Amperes) surely melts the wires at the point of arcing.

The easy fix would have been to:

• Turn off the power at the circuit breaker.
• Cut away the cable jacket around the burn.
• Separate the conductors.
• Inspect them for damaged insulation.
• Thoroughly wrap the damaged insulation with electrical tape, twice.
• Replace the cut-away cable jacket by wrapping the cable with electrical tape. Twice.

But the easy fix was not possible, having burned at least one conductor all the way through.

This type of repair is straight-forward:

1. Cut out the damaged section.
2. Obtain a short piece of the same size cable, plus two junction boxes and enough wire nuts.
3. Install each cut end in a junction box.
4. Connect the two cut ends with the new piece of cable, making all connections inside the junction boxes.

Step 1: Cut Out The Damaged Section

Before Any Wires Are Cut:

• First I made sure the power was off. The breaker had tripped, but I moved it all the way to the "OFF" position.
•  To be sure, I checked for voltage with my test meter (only after I verified that the tester was working properly).

 

After ensuring that the power was off, I cut out the burned section of cable. (I wanted to save it as a souvenir of my handiwork. Maybe I'll have it mounted on a plaque.) Then I stripped back the cable jacket (the black outer wrapper), and I stripped a bit of insulation from the end of each conductor.

The wire being repaired here powers an electric range, so the cable is quite large. This is 8-3G cable, which means:

• The conductors are American Wire Gauge (AWG) number 8. 
• There are 3 conductors, the white, the black, and the red coated wires.
• There is also a Ground wire (the bare copper wire).

Step 2: Obtain Materials.

I used a pair of 4" square metal junction boxes. There was no way 8-3G cable would fit inside the smaller boxes.  The National Electrical Code specifies limits on how many wires (and what sizes) may be connected in each size of junction box. To be safe, I always use the biggest box possible. There is a danger of overheating if too many wires are packed into a small box.

I also bought a short piece (about 2 feet) of 8-3G cable, and four 3/4" cable clamps. The cable clamps must be big enough for the cable. Bringing along a sample of the old cable is always a good idea.  I already had a supply of wire nuts.

Steps 3 & 4: Make Connections

I connected each end of the old cable to a metal junction box. This required removing a pair of "knock-outs" and installing a 3/4" cable clamp in each hole. At the first junction box, I stripped the end of the new cable (the one with the white cable jacket) and laid out the wires for connecting.

The size of wire nut was crucial here. This large wire required a large wire nut.

On the back of the box is a table listing all of the various wire size combinations that can be connected with that wire nut. These big blue wire nuts are capable of connecting 2 or 3 conductors of #8 stranded wire.

Making The Connections:

I made the large strands of wire "fan" out, and then placed two conductors side-by side. Then I used a pair of pliers to twist the strands together. This makes the strands inter-lock fairly well.

I installed a wire nut. These larger nuts take quite a bit of twisting force to install. Then I pulled on each wire to make sure the joint was tight.

This was repeated for all of the wires, at both junction boxes. But, I added a short "pig-tail" to the ground wire connections, so the metal box could be connected to ground.

Note how a bare ground wire is wrapped under a screw. This connects the box to ground. At this point, the metal boxes were attached to the floor joists above, using sheet metal screws. I never use nails, they don't seem to hold well.

Why is grounding the box so important?

• If one of the hot wires (black or red) ever has its wire nut fall off, the wire could also touch the metal box.
• If the box is grounded, that event will cause the breaker to trip (and keep tripping after being reset).
• If the box was not grounded, that event would cause an open circuit, and the box would have a voltage applied to it. Touching the box would give a person a potentially dangerous shock.

What could make a wire nut come loose? Vibration. The cable being moved around or wiggled by anybody doing work on the house. Kids playing around. This is an unlikely scenario, I'll admit, but it is possible. And the electrical code tries to cover all possibilities.

All the connected wires were packed into the box. This large wire is difficult to flex.

I installed a cover plate over each box.

Finally, I fastened the cable to the house structure with large cable staples. There are also cable straps available for this purpose.

Then I turned the power back on, and everything worked fine. Whew!

Okay, okay, okay... if you really must know how I burned through this cable, I'll explain how.

Copyright © 2000, 2005  HammerZone.com

Written May 31, 2000
Revised January 7, 2005