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Types of Electrical Systems in Historic Buildings | Wisconsin Historical Society

General Information

Types of Electrical Systems in Historic Buildings

Types of Electrical Systems in Historic Buildings | Wisconsin Historical Society

It can be challenging to install and operate a new electrical system in a historic structure. If your historic house or building needs new electrical wiring, be sure to hire a professional electrician who understands historic structures. The electrician who rewires your house or building needs to have experience threading electrical wires through original plaster walls and the ability to remove old wiring. Check to make certain the electrician you hire is a professional licensed by the city where you live.

You might find it easier to address problems with the electrical system in your historic house or building if you know a little history about electrical wiring and service in Wisconsin.

Original Electrical Wiring

The electrical system in your historic house or building may have been installed long ago. The date when electricity was first installed in Wisconsin homes varies greatly. For example, a few homes in Appleton had electricity as early as 1882. Virtually all urban houses had electricity (or could be wired for electrical service) by the 1920s. But some rural areas did not become fully electrified until the late 1930s.

EnlargeKnob and tube wiring

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

The first electrical wiring in most historic houses and buildings in Wisconsin was knob and tube wiring. Knob and tube wiring is a two-wire system with no third, grounding wire like modern electrical systems. One wire is “hot,” or active, and the other wire is neutral. The two copper wires run parallel but separate from each other through the floor and ceiling joists as well as stud wall cavities. The wires are threaded through porcelain tubes placed in holes drilled through the joists and studs to insulate the wires from the combustible wood. In places where a wire needs to bend in a new direction or run along a wood surface, the wire is wrapped around a porcelain knob that also prevents the wire from touching the wood.

Because knob and tube wiring has no third grounding wire, this system presents an electrocution hazard. In addition, some original copper knob and tube wires were coated with cotton cloth that was soaked in a flammable asphalt solution. The industry soon switched to rubber coatings, but some original knob and tube wiring could still have this flammable coating. As the original wires age, the coatings become brittle and the wires become exposed and dangerous.

Original Electrical Service

EnlargeEarly fuse box

Milwaukee County. An example of an old 60 amp, screw-in fuse panel found in a historic house. Source: WHS - State Historic Preservation Office.

The electrical service transmitted to early houses usually provided 30, 40, 50 or 60 amps of power. These services did not have the 220-volt capabilities needed today to run modern electric dryers, stoves and central air conditioning. Early service panels had different types of fuses. The most common fuse was a glass screw-in type with a brass or copper filament that would melt if there was an electrical short. The melted filament would, in turn, cut off electricity to the circuit. These types of fuses and their service panels still exist in some houses.  

Today, the minimum code requirement for electrical service to your home is 100 amps with 220 volts. Because of the high power demand of today’s lifestyles, you might find it necessary to have 200 amps for your house or building. Some homes with larger electrical demands may even have a 400-amp service provided to the service panel.

Modern electrical service panels have breaker switches. Like the old screw-in fuses, each switch is rated for 15, 20, 30, 40, and even 50 amps. The amperage of the breaker switch depends on the power demand of the appliance being run and the corresponding wire size it needs: The more power needed, the larger the diameter of wire and the larger the amperage of the breaker. If too much power is being drawn from a circuit or there is an electrical short, the breaker switch trips off instead of allowing the wire to overheat and cause a fire.

CAUTION: Don’t make the common mistake of buying fuses with a higher amp rating than the circuit demands. This is a prescription for fire. The size of the wire running to the outlet might be rated for only 15 amps. If you use a 20-or 25-amp fuse to prevent the breaker from tripping, it could cause the wire to overheat or allow the short to happen before the breaker fuse trips, possibly starting a fire.

Grounded Electrical Systems

EnlargeGFCI outlet

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

Like lightning, electricity seeks the ground. All legal electrical wiring today has a copper ground wire running through every cable. These ground wires are all tied together and attached to a steel rod driven into the ground. Three-pronged, grounded outlets provide a third hole for the ground prong on a plug.

Some older grounded systems are connected to a water pipe instead of a steel rod, but this is not legal or safe. If you come in contact with a live wire or shorted appliance, you could become the vehicle that directs the electricity to the ground. This is how many people get electrocuted by house wiring.