Wiring and Grounding in Older Homes

The wiring inside most older houses wasn’t designed to handle the electrical needs we have today. Resist the urge to perform workarounds when doing electrical work inside your home.

Inside the walls

If your electrical system is original to your pre-1940s house, you might have what's known as "knob and tube" wiring - single-insulated copper wires that run through porcelain tubes in wall joists, supported by porcelain knobs between joists.

Why is knob and tube wiring a problem?

The single layer of insulation - brittle rubber or even fraying cotton - wasn't designed to last. If it hasn't completely disintegrated, it could be eaten through by rodents. This leaves the energized copper wiring exposed.
Covering it with fiberglass batts or other thermal insulation is a significant fire hazard because it doesn't allow heat in the copper wiring to dissipate.
It wasn't designed to support the electrical loads modern homes demand. In many cases, homeowners resort to "over-fusing" the circuits to prevent blown fuses.
The system isn't grounded (see below).

In fact, many insurance companies won't cover older homes with knob-and-tube wiring - even if it's been inspected.

If you're not sure what kind of wiring is inside your walls - or if you're considering buying an older home - call a certified electrician to inspect the entire electrical system.

If you're planning a home improvement project, shut of the power at the service panel, and use extreme caution when removing outlets, drilling holes or removing wall boards.

Plugging in your stuff

Grounding is another common issue in older homes. Modern homes are required to have a ground connection to ensure that in the event of a short circuit, current will flow through the ground system and trip a breaker or blow a fuse instead of back through the electrical appliance.

However, many older homes have ungrounded outlets - only two slots instead of three - and this can be a problem with all the three-pronged cords that fill your home.

It's tempting to just work around the problem, but keep in mind the consequences can be dangerous.

Never cut off the third grounding prong on an appliance plug.
Don't try to install a grounded outlet in an ungrounded receptacle.
Use "cheater" adapter plugs only on a temporary basis. When using these devices, make sure the small metal circle on the bottom contacts the screw in the middle of the outlet.

If you've invested in a computer, a high-definition TV or other sensitive electronics, also keep in mind that power surges carry a greater risk with older, ungrounded wiring. To protect your equipment, talk to your electrician about wiring upgrades and surge protection.