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9 Common Wiring Mistakes

Updated on May 1, 2008

Home Wiring

This article can be viewed in full in the November 2007, edition 190 of Fine Homebuilding. "9 Common Wiring Mistakes and Code Violations" was originally published by Joseph Fratello.

"Whether to save time or money, lots of non-electricians do electrical work. This is especially true during the rough-in phase of new construction: drilling holes, running wire, and nailing up boxes. Remodelers take on tasks as seemingly mundane as installing a new light fixture.Before tackling electrical work, you should be aware of a few things.

"First check with your local or state building department to see what licenses or permits are required. The National Electrical Code dictates minimum requirements for safe electrical installation, but local bbuilding authorities often impose their own codes. Second, in some areas, homeowners are not allowed to perform electrical work unless they are licensed or certified electricians. It's a good idea to check with your local or state building authorities before doing any work. In many instances, a homeowner can hire a licensed electrician who can pull a permit and supervise any rough wiring that the homeowner might do. Potential liabilities are involved, but various resources clearly explain the correct methods that make electrical work code-complaint and safe. If you're ready to go to work, keep reading. The following is a list of mistakes to avoid during electrical installations."

9 Common Wiring Mistakes

1. Protecting wiring from nails and screws.

Problem: There must be a 1-1/4 inch clearance from the edge of a wood-framing member to any wire to keep drywall screws and long trim nails from puncturing the insulation and causing a short.

Solution: Wiring passing through holes closer than 1-1/3 inch to the framing face must be protected with nail plates. Several runs of wiring can be corralled with inexpensive Cable Stackers, which maintain the distance mandated by code.

2. Don't mix line-voltage and low-voltage wires.

Problem: Parallel runs of line- and low- voltage wires cause interference in electronics and/or communications, such as TVs and telephones. Also, any uninsulated contact between low- and line- voltage wires in a box can damage equipment or cause a fire.

Solution: Maintain a minimum of 6 inches between parallel runs, and you won't have to shout over the phone. Don't bring low voltage and line voltage together in the same box. Instead, use separate boxes or a box that has an approved divider.

3. Don't stuff too many wires into a switch or outlet box.

Problem: Overcrowded boxes can overheat, cause insulation melt, and potentially cause a fire.

Solution: If the box is too small, use a larger box and a plate known as a plaster (or mud) ring.

4. Use a splice box when installing a new fixture to old wire.

Problem: Because of compatibility issues related to safe operating temperatures, new fixtures can overload an older wiring system and cause a fire if improperly installed.

Solution: A splice box and a minimum of 3 feet of new wiring should connect a new light fixture to a circuit wired before 1987. It's the preferable alternative to rewiring the entire circuit, Here's a good way to determine the wiring's age: Insulation jackes made after 1987 are stamped with the date of manufacture; those made prior to 1987 have no date.

5. Don't use wire runs as a clothesline.

Problem:Wires are commonly stapled across the undersides of floor joists, where the wires are often used to support hanging objects.

Solution: When running wire through a floor system, drill properly sized holes, or use a running board that's at least a 1x4. Large runs of wire can be organized with raceways, a system of plastic clips, that support the wires. Removable covers look tidy.

6. Don't crowd holes with too many wires.

Problem:Running too many wires through the same drilled hole can cause friction burns on the insulation jacket as the wire is pulled. Damage is often not visible and could cause a fire.

Solution: Check the NEC Handbook to determine the correct number of wires for a specific size hole.

Switch Box

Looking closely, you can see the date stamped on the wire, meaning that it was made after 1987. (Left, outter side of wire)
Looking closely, you can see the date stamped on the wire, meaning that it was made after 1987. (Left, outter side of wire)

7. Make sure recessed lights don't become fire hazards.

Problem: Unless the fixture is rated for insulation contact (IC), there must be 3 inches of space between the fixture and any insulation.

Solution: If you retrofit non-IC recessed fixtures, secure the insulation so that it cannot spring back and contact the light after it's installed.

8. Don't disable a smoke detector with bad placement.

Problem: A common mistake is to install a smoke or carbon-monoxide detector to close too an HVAC duct.

Solution:The increased circulation around the duct can dilute air quality and fool the detector. Make sure there's at least 36 inches of space between the duct and the detector.

9. Don't bury splice boxes. It's dangerous.

Problem:Splice boxes installed behind drywall are impossible to find and service. Problems such as short circuits might go undetected and cause a fire inside the wall.

Solution:Find a spot where the splice box is accessible and still not obvious. In kitchens, try mounting a box slightly above the upper cabinets. Unless the room is enormous, you will never see it. If the cabinets go to the ceiling, mount a box in the back of the cabinet; the only part you will see is a flat blank plate.

Fine Homebuilding

Fine Homebuilding Magazine

The Fine Homebuilding magazine shows in-depth techniques for various home projects. You'll find hands-on construction tips that are more than fully explained for easy comprehension.

The projects range from building foundations to from carpentry and interior finishing to system installations (whether plumbing, heating, or air).

Each issue contains in-depth reviews about tools, techniques, and materials that you may need for fine homebuilding projects.

You'll find information from various departments covering builder's tips, remodeling ideas, and answers to reader's questions.

This is a magazine that I love to peruse on my days off to get ideas for my next project. I have learned wood staining tips that I used to build my corner desk, and I've found tool reviews that I've used when looking for better tools to effectively build various projects around my home.


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      3 years ago

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      3 years ago

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