How to Identify Wiring in an Old House

Last updated August 12, 2024
The older the house, the greater the risk of fires, electric shocks or other problems. These hazards are fueled by aging wiring and electrical systems that were not built to modern safety codes.
Old house wiring identification can reveal whether a home’s wiring system is hazardous and if it needs to be repaired or replaced. This guide reviews the basics of old house wiring identification and how to find the hot wire in an old house.
Difficulty:
Intermediate
Duration:
Under 2 hours
Table of Contents
Old House Wiring Types
Old House Wiring Colors
How to Read Wire Labels
Which Wire Is Hot in Old Wiring?
Tips for Old House Wiring
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Old House Wiring Types

Homes more than 30 years old can have obsolete wiring systems or deteriorated wires that need to be replaced. Old house wiring identification can involve checking the type of wiring system, the wire colors and labels and other methods for identifying hot wires.
Residential electrical wiring has evolved over more than a century, but you can still find remnants of antiquated wiring in some old homes. These are the most common old house wiring types as well as the standard modern types.
- Knob and tube (1880s-1940s): The first type of common U.S. residential wiring involved a black hot wire and a white neutral wire covered in rubberized cloth fabric and attached to porcelain fixtures. The original systems lacked grounding and are considered functionally obsolete today. With ground wires added, knob and tube wiring can still pass current electrical codes. However, it is difficult to insure.
- Flexible armored cable (1898-present): Also called Greenfield or BX and primarily used in the 1920s-1940s, this system involved wire housed in flexible metal with no separate ground wire. Some communities have electrical codes that still require it.
- NMC (1930s-present): Known as the first-generation of sheathed cable, this type used cloth or fabric sheathing to insulate hot and neutral wires. Cloth sheathing is safe when in good condition, but its presence indicates an old wiring system.
- NM (non-metallic) cable (1950-present): Also called Romex, this system replaced cloth sheathing with plastic and has become the most common type of electrical wire in modern homes.
- Metal electrical conduit (1950-present): Metal tube enclosure containing multiple conducting wires.
- Aluminum wire (1960s-1970): Aluminum wiring was briefly popular as an alternative to copper but fell out of use over fire safety concerns.
- Plastic electrical conduit (1990-present): Plastic tube enclosure containing multiple conducting wires that has largely replaced metal conduit.
Prior to 1970, many homes were built with 60-amp service for smaller electrical loads. These can be inadequate for the demands of modern devices, and contemporary homes usually have 200-amp service.
Electrical fluctuations in older wiring can damage modern appliances or devices. Use surge protectors to divert excess electricity to a grounding wire.
Checking the circuit breaker box or fuse panel offers a quick indicator of the age of the home’s electrical system.
- 30-amp fuse panels: Common in homes before 1950
- 60-amp fuse panels: Common in homes between 1950-1965
- Circuit breaker panels: Gradually replaced fuse panels beginning in the 1960s
Old House Wiring Colors

Old house wiring colors can be the most direct indication of whether a wire carries a live electric current or is neutral.
These are the standard house wiring colors:
- Black: hot wire
- Red: secondary hot wire, often used with larger appliances
- White: neutral wire
- Green: ground wire
In 2002 the electrical industry introduced cable color coding based on size and amperage. These are the color standards for cables:
- Black: 8- or 6-gauge wire, 45 or 60 amp circuits
- Orange: 10-gauge wire, 30-amp circuit
- Yellow: 12-gauge wire, 20-amp circuit
- White: 14-gauge wire, 15-amp circuit
- Gray: Underground feeder (UF) cable
Cable sheath coloring is not required by the National Electrical Code, so cable colors in some homes may not match the standard.
Pro Tip: The American Wire Gauge (AWG) system rates larger wires with smaller numbers, so a 10-gauge wire is thicker than a 14-gauge wire.
How to Read Wire Labels

Many types of insulated wire and cable have labels on the sides. The numbers and letters indicate such topics as insulation type, maximum voltage rating and the wire size and material.
These common letters on wire labels have the following meanings:
- G: Ground (also W/G or WITH GROUND)
- T: Thermoplastic, fire-resistant insulation
- H: Heat-resistant up to 167 degrees Fahrenheit
- HH: Highly heat-resistant up to 194 degrees Fahrenheit
- W: Wet or suitable for wet and damp locations
- X: Insulation made of a flame-retardant synthetic polymer
- N: Nylon-coating for resistance to gasoline and oil
- AL: Aluminum wire (also identified by ALUM or ALUMINIUM)
- CU: Copper wire
Which Wire Is Hot in Old Wiring?

One of the biggest challenges with old wiring is discoloration. The original system in an old home may have paired black wires with white wires. Over time, the wires can become so worn or covered with dust that you may not be able to tell them apart by looking at them.
In addition, some houses, even newer ones, may have the wrong-colored wires carrying current due to faulty installation or rewiring. Always test the wires in an old house with a multimeter to prevent shocks and other hazards.
- Always wear protective rubber gloves when dealing with electrical currents.
- If wiring ends are exposed, such as when installing a new light, cap the ends with wire connectors for safety.
- Set the multimeter to measure voltage.
- Turn on the circuit breaker.
- Place the multimeter’s red probe on the wire you believe to be live and the black probe on the neutral wire.
- If the multimeter display reads between 110-120 volts, the wire is live. If the display reads zero, the wire contains no electric current.
- Turn off circuit breaker.
- Wrap hot wire with black electrical tape.
- Wrap the neutral wire with white electrical tape.
Tips for Old House Wiring

Consider the following tips during old house wiring inspection or identification:
- Electricians updating a house can sometimes find examples of “handyman tampering,” or electrical systems with unconventional splices or other unusual modifications. If you find one example of an unusual splice or inappropriate wiring, expect there to be more.
- Check the electrical panel for rust or other signs of moisture. Make sure all circuit breakers are the same, as mismatched circuit breakers in the same box can cause issues.
- Make sure the ground wire is the correct size for the house.
- Check for cracked, damaged or missing wire insulation or any exposure of the metal wire.
- Check for unprotected wire runs through ductwork and other inappropriate areas.
- Exposed splices wrapped in plastic electrical tape may need soldering.
- Do not leave old wiring in place. Even if it is disconnected and the home has a new wiring system, remnants of the old wire can lead to mistakes with future wiring jobs.
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