knob and tube wiring
Illustration: Ian Warpole
Knob and tube wiring may be covered with building insulation, which causes overheating
Old Wiring: Is It Safe?

Today's standard household wiring is a plastic-sheathed, insulated three-wire cable, universally known by the trade name Romex. But the vintage copper wiring in many older houses works just as well as the new stuff, as long as it's in good condition and hasn't been altered in a way that violates code. Here are some wiring systems you'll find in older homes.

Knob and Tube
The earliest residential wiring system has a cloth-covered hot wire and a neutral wire, which run parallel about a foot apart. Ceramic knobs anchor the wires to the house framing; ceramic tubes are used where wires cross or penetrate framing.

Caveats: Cannot be grounded or spliced into a grounded circuit. Its soldered connections may melt if too much current flows through them. Rewire or disconnect any circuits covered with building insulation; it causes this wiring to overheat.

armored cable
Illustration: Ian Warpole
Armored cable insulation should be checked every 5 years or so
Armored Cable (Bx)
The successor to knob and tube. A flexible steel sheath covers hot and neutral wires, which are insulated with cloth-covered rubber. The sheath provides a ground, so grounded receptacles are easy to retrofit.

Caveats: Sheath must be anchored securely to a metal outlet box. Check condition of insulation every five years or so; it degrades over time, as shown above, or if too much current is allowed to flow through the circuit.

two-wire plastic sheathed cable
Illustration: Ian Warpole
Grounded receptacles cannot be retrofitted to two-wire plastic-sheathed cables
Two-Wire Plastic-Sheathed Cable

An early PVC-insulated (Romex) wire.

Caveats: Plastic is easily damaged. Grounded receptacles cannot be retrofitted to this wire.

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