We make sure the power is
turned off, cut out the damaged section of cable, and splice a
new piece of cable in between the cut ends.
Bruce W. Maki,
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,
- 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:
- Cut out the damaged section.
- Obtain a short piece of the same size cable, plus two junction
boxes and enough wire nuts.
- Install each cut end in a junction box.
- 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
Then I stripped back the cable jacket (the black outer
wrapper), and I stripped a bit of insulation from the end of
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
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
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
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
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
Then I turned the power back on, and everything worked fine.
Okay, okay, okay... if you really must know how I burned
through this cable, I'll explain how.
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What's New Project
- Cordless Drill/Driver
- Wire Stripper
- Wire Cutters
- 4" Square Deep Metal
Junction Boxes (2)
- Cover Plates (2)
- 3/4" Cable Clamps (4)
- Pan Head Screws
- Ground Screws