Twelve Common Wiring Problems

Most electrical problems are caused by poor planning, poor design, cheap and improper material, and easy-to-avoid mistakes. Fortunately, forewarned is forearmed. Save yourself a ton of costly repairs and no small amount of aggravation by avoiding these 12 wiring problems.

Two notes before we begin

  • Electricity is dangerous. If you don’t know what you are doing then skip this article and call a pro
  • Make sure you have all the tools

1. Overloaded service panel.

This nuisance typically begins when the house is built. Anxious to cut costs, homeowners accept a low bid that specifies a small, cheap main service panel that may be 100 percent full the day it’s installed. As years go by, additional circuits are added to breakers that may already have a heavy load—and breakers start kicking.

If you’re about to build or remodel, avoid this problem by having all prospective contractors bid on the same panel—a large, well-designed, high-quality one. For 200-amp service, demand a 40/40 panel, which holds the largest number of circuits allowed: 40. The main cutoff switch should have a large handle that you can get several fingers on, so it’s easy to turn on and off. Look for neutral and grounding bus bars (“buses,” for short) that run the full vertical length of the mounted breakers. It’s best when they’re mounted next to the breakers, not off to the side next to the knockouts.

2. Improperly wiring subpanels.

Even the pros screw up this one. A subpanel is any panel downstream from the main service panel, even if the first panel is a cutoff panel—an outside disconnect required by code in some areas—mounted at the base of the electrical meter. The neutral and ground wires must be kept separate from each other in the subpanel. The neutral bus must be on plastic standoffs, plastic feet that electrically isolate the bus and wiring from the metal panel. The ground bus must be attached to the panel’s metal frame. If the ground and neutral wires get crossed, neutral current can flow to everything attached to the grounding circuit, including water pipes and even ductwork (which is normally in contact with water lines).

three-way switches

3. Improperly wiring three-way switches.

The rules are simple: connect the hot wire (normally black) from the incoming power cable to the COM, or common, terminal of the first three-way switch. Then connect the neutral (white wire) from the same incoming cable to the neutral (white wire) of the load, or outgoing power cable. Connect the black load wire to the common terminal of the opposite three-way switch. Finally, connect the travelers from one switch to the travelers of the other: these are the “leftover” screws on each three-way switch. Because there’s no polarity, it doesn’t matter which remaining wires you attach to which screws.

4. Tapping a 14-gauge circuit onto a 12-gauge circuit.

(Note that 14-gauge wire is smaller than 12-gauge.) This error normally happens when the installer looks for a cheap way to add a few lights to an existing appliance circuit. The problem is that once you add 14-gauge cable into a 12-gauge circuit, the thinner wire must be connected to a 15-amp circuit breaker—not the 20-amp breaker that 12-gauge normally takes. This means you wasted all the extra money you spent on all the 12-gauge cable. Or, if you leave the section of 14-gauge wire connected to a 20-amp breaker, you risk the wire overheating and possibly starting a fire.

5. Wiring stapled wrong.

This is probably the most common amateur error. Cable staples should be snug, not tight. Staple a cable too tight, and you may well short circuit all the individual wires within. Also, position cable so that the staples fasten down the cable’s flat side, not its edge. Staple the cable every 2 to 3 feet and within 8 inches of an outlet box.

6. Leaving too much-sheathed cable within the outlet box.

Another common amateur mistake. All that extra sheathing takes up needed space in the box. To get the right amount, feed the cable into the box and then cut it so that no more than 2 or 3 inches of cable protrudes from the front of the box. Then remove all but 1/2 inch of the cable sheathing, and be sure that section is held down by the box’s cable clamp. (This last detail is important: cable clamps should secure still-sheathed cable, not individual wires.)

7. Using an outlet box that’s too small.

Almost everybody—pro and amateur—make this mistake. Receptacles, switches, GFCIs, wires and wire connectors all take up space. Stuffing everything into a tiny box may pinch, break and short out wires. When mounting boxes within walls, always use the deepest box available, which is 3 1/2 inches from front to back. Never use the common 1 1/2-inch-deep metal “handy box” for anything—there’s not enough volume.

8. Installing a back wired receptacle or switch.

Some receptacles and switches allow you to make connections by pushing stripped wire-ends into holes in the devices’ backs. Back wiring seems easier than attaching wires to screw terminals, but back wired connections just aren’t as strong. The problem is that the flat metal piece that secures the wire in a backwired device starts to fatigue once it’s depressed, and that leads to a loose connection—or a complete loss of contact. Because of this, backwired devices are prohibited from use with 12-gauge circuits. If you ever lose a string of outlets that were working perfectly one minute before, chances are backwiring was the cause.

9. Forgetting to bring in the feeder or power cable.

The nightmare of all installers—and it happens all the time. A circuit of switches and receptacles is wired perfectly, but there’s no power source feeding in. To avoid this, the pros wire their outlet boxes the same way every time. Always reserve the upper-left knockout in the outlet box for the feeder cable. This way, you only have to glance at the box to see if you brought in power.

10. Accidentally mixing the cable gauges.

There’s no good reason for this to happen but it often does, especially on big jobs with lots of half-used coils of 14- and 12-gauge cable lying around. Since manufacturers rarely stamp the gauge on the cable sheath in a conspicuous manner, the gauges get mixed. Pros avoid this common pitfall by spray-painting the sides of all coils of a particular gauge while they’re still rolled tightly together.

GFCI receptacles

11. Wiring GFCI receptacles incorrectly

GFCI receptacles are easy to wire improperly, and people can get hurt—so heads up on this one. Connect the incoming power cable to the terminals marked LINE on the back of the GFCI receptacle; connect the outgoing cable—which feeds power to the next outlet downstream—to the GFCI terminals marked LOAD. The words LINE and LOAD are often not as conspicuous as they should be, so look closely. The second common problem occurs when neutral wires get confused with hot wires. There’s only one solution. Take your time and double-check your work: people’s lives may depend on wiring GFCIs properly.

12. Overfusing a 60-amp fuse panel.

This error can burn down your house. Old 60-amp service panels simply can’t support enough circuits to power a house. Consequently, when circuits become overloaded and fuses blow, too many homeowners compound the problem by increasing the amperage of the fuses till the circuits stop blowing—30-amp fuses usually do the trick. Trouble is, these circuits are invariably wired with 14- and 12-gauge wire and should only take 15- and 20-amp fuses, respectively. Increasing the fuse size to 30 amps is a fool’s cure; sooner or later, such circuits will overheat and perhaps start a fire. There’s really only one safe solution: replace the old fuse panel with a new circuit breaker panel and add several new circuits.

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