Wisconsin Historical Society

Guide or Instruction

Does Your Historic Building Need New Electrical Wiring?

Does Your Historic Building Need New Electrical Wiring? | Wisconsin Historical Society

Before you can determine if your historic house or building needs new electrical wiring, you should learn a little bit about the original electrical wiring system in your structure. You should conduct a thorough inspection of the electrical system in your historic structure in any of the following situations:

  • You’ve just moved into a historic house or building
  • Your historic house or building is suddenly exhibiting electrical problems, such as breaker switches that frequently trip off
  • You are considering the purchase of a specific historic house or building

Here is a step-by-step inspection that will help you to know the basic safety of your electrical wiring.

Step 1: Locate Your Electrical Service Panel

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Dane County. A modern 200 amp breaker panel with 20 amp breaker switches. Source: WHS - Historic Preservation - Public History.

Start your inspection by finding the electrical service breaker panel or panels in your house or building. Be aware that some historic structures have more than one panel, particularly if the building has more than one tenant. The breaker panel is usually in the basement. Sometimes each tenant's panel will be on the level the tenant occupies.

Open the door of the panel to check the amperage shown on the nose of the main shutoff switch. Most houses need at least 200 amps, or 200 amps per tenant in a building.

Step 2: Observe the Visible Wires

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Properly installed wiring is contained within conduit and junction boxes. Labeling can be helpful for future owners. Source: Bob Yapp

Observe the electrical wires running throughout your basement and attic. Do they look neatly done or haphazardly installed? Historic houses may have wiring that was not done correctly, or a mixture of old and new wiring. You may see illegal wires stapled to the bottom edge of basement joists, the top of attic joists or just hanging loosely. Any haphazardly installed wiring is a sign that your house or building needs rewiring.

If your wiring looks neat and tidy, your house or building may have been rewired in the recent past. If you see evidence of haphazard wiring, call an electrician for a more thorough inspection. A professional electrician will have the ability to test the wires running through your wall and ceiling cavities that you can't see.

Step 3: Identify Knob and Tube Wiring

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La Crosse County. Knob and tube wiring as seen in this photo should be replaced as it can be a fire hazard. Source: WHS - Historic Preservation - Public History.

As you are inspecting the electrical wires in your house or building, you should determine if your structure has any antique wiring known as knob and tube wiring. Knob and tube wiring consists of two wires running parallel to each other and threaded through a hole in the joist. A porcelain tube inserted into the hole prevents the wire from touching the wood. When these wires are bent to make a turn, they are wrapped around a porcelain knob nailed to the side of the joist. Since knob and tube wiring has no grounding wire, it can be a fire hazard. Make a note if you find knob and tube wiring during your inspection, because it will have to be replaced.

Step 4: Inventory Your Electrical Outlets

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Dane County. A GFCI outlet is required when outlets are located near water sources such as sinks. Source: WHS - Historic Preservation - Public History.

Look around all the rooms of your house or building and record the number of electrical outlets in each room. If you are planning a substantial rehabilitation to your structure, you might be required by building codes to install a three-pronged, grounded outlet every six feet. A licensed electrician can help you determine if you must meet this requirement.

Look at the outlets within six feet of every sink or water-supplied fixture. These outlets should be ground fault circuit interrupter (GFCI) outlets. A GFCI outlet always has a push-in button for testing and another button that acts like a breaker switch. A GFCI outlet, and any others properly wired to it, will trip and prevent an electrical short if there is a hazardous water-related grounding situation.

Step 5: Inspect Your Ceiling Light Fixture

EnlargeKnob and tube wiring

Milwaukee County. As seen in this photo, the ceiling light fixture contains only two ungrounded wires which is unsafe and does not meet current code. Source: WHS - Historic Preservation - Public History.

Choose a ceiling light fixture to observe. Carefully remove the two screws that hold most light fixtures to the ceiling. Look at the wires that are connected to the fixture, but do not touch them.

If you see only two old wires connected to the fixture, the wires are most likely ungrounded knob and tube wires. This is an indication that most of the ceiling light fixtures in your house or building will need to be rewired.

The information presented here is not intended to provide comprehensive technical advice or instructions on solving historic preservation issues. Any information contained or referenced is meant to provide a basic understanding of historic preservation practices. Read full disclaimer.