Identifying Old Wiring
The oldest type of wiring system found in homes is called knob-and-tube, named for the insulating knobs and tubes that are used to run the wiring along and through the house framing. Knob-and-tube wiring was run as individual wires—one black hot wire and one white neutral wire—throughout the home. The ceramic insulators keep the wires from touching each other and from touching the wood and other combustible materials. To make connections and wire splices, electricians soldered the wires, then wrapped them with a rubber electrical tape called friction tape. Splices typically were not made in junction boxes, like they are today. Given the age of these wirings systems (most date back to before 1940), it’s usually difficult to identify the hot and neutral wires because both are essentially black with dirt and dust. The insulation on the neutral wires may also be a dark color with a white line or tracer, rather than all white.
No Ground
Being a two-wire system, knob-and-tube wiring does not have a ground system for safety. This doesn’t necessarily make the wiring unsafe to use, but it does rule out an important safety feature found on modern wiring systems. It also means there’s no ground to protect appliances and sensitive electronics, leaving them vulnerable to damage from power surges. It’s not feasible to add a ground to knob-and-tube wiring, so if you need a true ground for any circuits in your home, you’ll have to replace the wiring. Knob and tube systems can be fitted with GFCI receptacles to improve their safety, provided the wires are connected with the proper polarity (hot wire to hot terminal, neutral wire to neutral terminal). GFCI receptacles do not create a true grounding pathway, but they sense ground faults and shut off power.
Can You Keep Old Wiring?
According to the National Electrical Code (NEC) and most local codes that follow the NEC, existing knob-and-tube wiring can remain in use in a house. It may also be legal to add extensions to knob-and-tube systems, provided the proper materials and techniques are used. Historic homes may be granted special permission for various restoration work on knob-and-tube wiring systems. In situations where knob-and-tube wiring needs work, it’s possible to splice the old wiring with new non-metallic (NM) cable, using junction boxes to protect all connections. However, this and any other work done on knob-and-tube wiring must adhere to local code requirements.
Common Problems to Look For
Knob-and-tube wiring becomes dangerous when the wire insulation has worn away, when installation or alteration practices were improper, or when it’s covered with building insulation, which can cause the wiring to overheat and potentially start a fire. Here are some typical problems with old wiring that are possible hazards and may indicate the wiring should be replaced:
Cracked, missing, or damaged wire insulation: Exposed parts of the metal wire is a very common issue and wires with damaged insulation should be addressed by a pro.Wiring surrounded by building insulation of any type: Be very careful when insulating an attic to make sure you are not surrounding active knob-and-tube running in joist cavities wiring with loose fill or batt insulation.Exposed splices: Exposed splices (not in an electrical box) that are wrapped with modern plastic electrical tape usually indicates the wires are not soldered when they should be.Fuses in the fuse box that are larger than 15 or 20 amps: Old wiring wasn’t sized for today’s large appliances, which may need 30-, 40-, or 50-amp service. A 30-amp or larger fuse used in a knob-and-tube system indicates an “overfused” circuit, which is highly dangerous. As a general rule, old wiring circuits should be fused for no more than 15 amps.
Replacing Two-Wire (Two-Slot) Receptacles
Since knob-and-tube wiring does not have a grounding pathway, they are typically wired with two-slot receptacles, which do not have the round grounding slot. When you are replacing a damaged receptacle, it’s a very common mistake for people to replace a two-slot receptacle with a three-slot grounded receptacle, which is the standard in most installations. However, this is a serious error, since the presence of the three-slot receptacle implies a grounding system that is not present with knob-and-tube wiring. The NEC allows two solutions: replacing the old two-slot receptacle with a new two-slot receptacle; or, replacing the receptacle with a GFCI receptacle, that is labeled “No Equipment Ground” to identify the fact that it has no grounding pathway. This solution will make it possible to use modern three-prong plugs.
