Old House Wiring Inspection & Repair: Electrical Grounding, Knob & Tube, & Electrical Wiring in Older Buildings InspectAPedia® -
How to inspect & repair or improve electrical grounding in older homes
How to detect & repair improper electrical outlet polarity.
How to determine what voltage and amperage service a home
How to recognize knob and tube electrical wiring and unsafe "extension cord" wiring
How to evaluate the number of electrical circuits in an older home
Guide to Electrical Hazards in Buildings: inspection, detection, & repair advice
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This article answers basic questions about assessing the electrical service, capacity, wiring type, condition, and safety in older homes. Readers of this article should also see ELECTRICAL DEFINITIONS. This website provides information about a variety of electrical hazards in buildings, with articles focused on the inspection, detection, and reporting of electrical hazards and on proper electrical repair methods for unsafe
electrical conditions.
Older homes often have electrical receptacles and fixtures that are ungrounded, and many local codes do not require that they be rewired so they're grounded. Still, grounding is worth adding to your system because it adds protection against electrical shock. Grounding provides a third path for electricity to travel along, so if there is a leak of any sort, it will flow into the earth rather than into the body of a person who touches a defective fixture, appliance, or tool.
An electrical system is grounded with a grounding rod driven at least 8 feet into the ground outside the house or by connecting to a cold water pipe. Each individual branch circuit must be grounded as well, either with a separate wire that leads to the neutral bar of the service panel or with metal sheathing that runs without a break from each outlet to the panel. (In theory, electrical outlets can be grounded individually, but this is impractical.)
Absence of good electrical grounding at older homes
Do Older Homes Have Proper & Safe Electrical Ground Wiring?
Often an older building has poor or no working local electrical ground, relying instead on the incoming neutral wire from the electrical service.
Or the building's main electrical ground may have relied on connection to a metal water pipe connected to a well; we've found building ground wires connected to a metal water pipe which used to run out of the building and into earth (possibly a pretty effective ground) but where the metal piping exiting the building had been replaced with a newer plastic water line between the well and the building. In other words the local ground was completely ineffective.
Modern electrical grounding at residential properties requires use of one or more grounding electrodes connected by an un-spliced wire between the electrode and the ground and neutral bus in the main electrical panel.
Bare aluminum electrical ground wires are sometimes found to have corroded entirely through where the wire touched a damp foundation wall. We also find that the ground wire between the electrical panel and a building water pipe or grounding electrode has become separated, loose, spliced, or lost entirely, as shown in our photo.
Ungrounded, un polarized electrical circuits in older homes
If your outlets have two slots that are the same size, then they are neither polarized nor grounded. This leaves you with no protection against shocks from defective fixtures or appliances using that outlet. At the very least, you need to install polarized outlets. You cannot and should not install grounded electrical outlets on circuits where no ground path is actually present (such as knob and tube wiring). To provide a grounded outlet where no ground is present is dangerous.
Some locations in your house- especially where the outlet and/or appliances may become wet- require ground-fault circuit-interrupter (GFCI) receptacles. Older, ungrounded circuits usually are protected by polarization, which is less effective than grounding but better than nothing. Grounded and polarized receptacles work only if they are wired correctly.
An older home may have electrical service that is inadequate or even unsafe. It can be confusing, as well. If you are unsure about your home’s wiring, have a professional check it out.
Some older homes have only 120-volt electrical service. The electrical cable bringing electricity to the building provides two wires - one live or "hot" (rather than two) and a neutral entering the house.
This means you will not be able to have any 240-volt circuits or large appliances.
Our photo shows a single hot wire and a single neutral wire which is grounded by the utility company somewhere upstream from this home.
See AMPS VOLTS DETERMINATION for a detailed procedure on determining the whether your building is served by 120V or 240V.
Amperage Available at Older Homes
Modern electrical service provides at least 100 amps of power, which is enough to power, a medium-size house with average number of appliances. A house built in the 1950s or before may only have 30-amp service (the circuit box will have only two fuses) or 60-amp service (four fuses - see our photo). With so few circuits, the number of appliances you can run will be limited.
Even if a building has had additional electrical circuits installed, thus improving the distribution of electrical power in the home, if the main electrical panel has not been up-graded it is possible that it is too small for the current usage in the building.
If your building has been wired correctly, the circuit breakers or fuses should protect the building from a fire due to overloaded circuits, and what will remain is an inconvenience: having to replace fuses or re-set circuit breakers.
If the building wiring is incorrect, damaged, or obsolete, the combination of those conditions with insufficient incoming building amperage may increase the risk of fire.
See AMPS VOLTS DETERMINATION for a detailed procedure on determining the ampacity available at a building.
Some Potential Electrical Problems in Older Home
Here are a few things to consider when inspecting the electrical system in an older home.
Warning: this list of electrical wiring defects and safety concerns in older homes is incomplete. Contact Us to suggest corrections, changes, or to add additional items.
Knob-and-tube wiring in older homes
Knob and tube electrical of wiring has been installed in homes from the 1920s right up into the 1970's in some jurisdictions.
Knob and tube electrical wiring may be functional in a home and it was in its original concept a safe wiring method, separating the two conductors in air (see our photo at left) and using durable ceramic insulating knobs and tubes to mount the wire.
Knob and tube electrical wiring may not need to be replaced, but it certainly deserves careful inspection and possibly replacement or repair, because knob and tube systems lack an electrical ground (less safe), may have damaged insulation (less safe), or may have been improperly modified or extended (unsafe).
Please see Knob and Tube Electrical Wiring for a detailed discussion of the identification, inspection, and repair of this electrical wiring system.
Sources of electrical shorts and wiring faults in older homes
Loose taped wires, old wire damaged because it’s exposed, and multiple wires slipping off a single terminal screw may seem like minor problems, but are not.
Insufficient number of electrical circuits in older homes
As modern homes use more appliances and more electricity than folks did even twenty years ago, if the number of circuits in a home has not been increased it's likely that the home's electrical circuits are overloaded.
Too often in an older home we find that the electrical circuits have been "extended" by someone who has no idea about safe electrical wiring. People use lamp cord (Zip cord) or extension cords as permanent circuits, sometimes even routing such wires through walls and ceilings - a fire hazard, and certainly not in compliance with electrical codes.
At an inspection where we found lamp cord run through a wall to add wall-mounted lighting, a by standing real estate agent later asked the tenant to simply "cut and remove the wiring" - leading to a serious electrical shock event. It would have been a better idea to hire a licensed electrician.
Too often in an older home, especially one using a fuse panel, occupants are tempted to simply screw in a higher-amperage fuse to stop fuses from 'blowing". Over-fusing is dangerous: it risks setting the building on fire. Be sure that the amperage rating of your fuses or circuit breakers matches the wiring of each circuit:
A 15-amp fuse or circuit breaker is what's needed on a 15-Amp #14-gauge copper electrical wire.
A 20-amp fuse or circuit breaker can be used on a 20-Amp #12-gauge copper electrical wire.
Warnings about Polarized Electrical Plugs, Receptacles, and Lighting Fixtures
Reversed polarity shock hazards: "Polarity" in an electrical receptacle and on the device that plugs into or connects to it means that we're making sure that we connect the "hot" or "live" side of the electrical circuit to the connection point in the appliance or device that was intended to be "hot" or "live".
Carson Dunlop's sketches show why it's important to respect polarity when connecting an electrical receptacle, a lamp or any other appliance. In short, reversed polarity on a light fixture means it's easier to receive a dangerous electrical shock by touching the shell of the bulb socket or even the side of the bulb itself while screwing in a new light bulb.
Reversed polarity device burn-up or fire hazards: Never clip or file down the prongs on a grounded or polarized plug in order to force it to fit into an older electrical receptacle. The risk is that your plug will be installed with reversed polarity - connecting the "hot" side of the electrical circuit to the normally neutral-wired side of the appliance.
We've found appliances (a coffee maker) that simply burned up when connected in this fashion. Even though power was "off" according to the coffee maker "on-off" switch, feeding live voltage to the wrong side of the coffee make's circuit board led to a component burn-up and failure of the appliance.
Do not attempt to work on your electrical wiring, switches, or outlets unless you are properly trained and equipped to do so. Electrical components in a building can easily cause an electrical shock, burn, or even death.
Even when a hot line switch is off, one terminal on the switch is still connected to the power source. Before doing any work on the switch, the power source must be turned off by setting a circuit breaker to OFF or removing a fuse. See SAFETY FOR ELECTRICAL INSPECTORS and Electrical Wiring Books & Guides
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Carson Dunlop, Associates, 120 Carlton Street Suite 407, Toronto ON M5A 4K2Toronto. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Thanks to our reader Ed (anon) for assistance with technical page editing 11/26/2009.
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