Evaluating Your Home's Wiring System

 Is knob and tube wiring adequate for modern living? Is your home's electrical service panel adequate? Here's a guide to inspecting older wiring.

Editor's note: This story is adapted from the U.S. Department of Housing and Urban Development's Residential Rehabilitation Inspection Guide, 2000.

Found fairly often in houses built before 1930, "knob and tube" wiring should be carefully inspected to failed insulation and other safety issues. If has adequate capacity and is otherwise safe, it is still legal in many places.

Electrical systems for small residential buildings are usually simple in concept and layout. Primary components are the:

In unaltered buildings built since about 1940, the electrical system is likely to be intact and safe, although it may not provide the capacity required for the planned reuse of the building. Electrical capacity can be easily increased by bringing additional capacity in from the street and adding a larger panel board between the service entry and the existing panel. Existing circuits can continue to use the existing panel and new circuits can be fed through the new panel.

The electrical systems of small residential buildings built prior to about1940 may require overhaul or replacement, depending on rehabilitation plans and the condition of the electrical system. Parts of these older systems may function very adequately and they can often be retained if the rehabilitation isn't extensive and the load-carrying capacity is adequate.

A thorough and informed assessment of the electrical system will determine the extent to which it can be reused. This assessment should be conducted only by a qualified electrician who is experienced in residential electrical work. When universal design is a part of a rehabilitation, consult the Housing and Urban Development publication Residential Remodeling and Universal Design for detailed information about electrical devices.

To determine the capacity (measured in amperes) of the building's existing electrical service at the main panel-board, check the following:

The building's service capacity is the lowest of the above three figures.

Once the service ampacity has been determined, compare it to the estimated ampacity the building will require after rehabilitation. If the estimated ampacity exceeds the existing ampacity, the building's electrical service will need upgrading. The method for estimating required ampacity is found in the National Electrical Code, Article 220.

Similarly, the service capacity of each branch circuit can be determined by checking the markings on each branch circuit conductor. If no markings can be found, a plastic wire gauge may be used to measure the wire size (with the power disconnected), although an experienced person can often determine the size by eye. Find the ampere rating of the conductor, either by its markings or wire size, in the National Electrical Code, Table 310-16, or applicable local code.

The safety standards for the following assessment procedures are generally based on the requirements of the National Electrical Code.

Typical service equipment

Inspect for the following conditions in the electrical service between the street and the main panel board:

The main panel board is the distribution center for electric service within the building and protects the house wiring from overloads. Inspect the panel board as follows:

Typical electrical service entry and main panel board for a single family residence. This type of grounding applies only if the water pipe is metal. If the water pipe is plastic, a separate driven ground rod is required.

• Test. An electrical ground (resistance-to-ground) test may be used to determine whether the electrical system is well grounded to the earth. The test requires the use of an ohm-meter and should be performed by an electrician. 
• Over-current protection. Check the rating of the fuse or circuit breaker for each branch circuit. The amperage of the fuse or circuit breaker should not exceed the capacity of the wiring in the branch circuit it protects. Most household circuits use #14 copper wire, which should have 15 amp protection. There may be one or more circuits with #12 copper wire, which should have 20 amp protection. Large appliances, such as electric water heaters and central air conditioners, may require 30 amp service, which is normally supplied by #10 copper wire. If there is an electric range, it would require a 40 or 50 amp service with #6 copper wire. Central air conditioning equipment will have an over-current protection requirement on the nameplate. Aluminum wire must be one size larger than copper wire in each case (e.g., #14 to #12), but it should not be used for 15 and 20 amp circuits. Make sure that no circuit has a fuse or circuit breaker with a higher ampere rating than its wiring is designed to carry. Air conditioners and other equipment with motors may have circuit breakers up to 175 percent ampacity of the conductor rating to allow for starting current. Look near the panel board for an inordinate number of new or blown fuses, or breakers taped in the "on" position. Be suspicious of 20 or 25 amp fuses on household lighting circuits. These are signs of frequent overloads and inadequate electrical service. Other indications of overloading are the odor of burned insulation, evidence of melted insulation, discolored copper contact points in the fuse holders, and warm fuses or circuit breakers. 
• Test: Flip all circuit breakers on and off manually to make sure they are in good operating condition. A commercially available circuit breaker and resistance tester, which can simulate an overload condition, can be used to test each breaker. Such a test should be performed by an electrician. Note that this test is not recommended for computers, VCR's, clocks, and many similar devices. 
• Many older residential buildings have more than one panel-board or fused devices. Check that all supplementary over-current devices are located in metal boxes and that they are not in the vicinity of easily ignitable materials. All panel boards must have covers. It should be possible to turn off all electrical power to a dwelling from a single location. 

Branch Circuits

The oldest types of residential wiring systems are seldom encountered today. They include open wires on metal cleats, wiring laid directly in plaster, and wiring in wooden molding. These systems proved quite hazardous.

The oldest wiring system that may still be acceptable, and one still found fairly often in houses built before 1930, is "knob and tube." This system utilizes porcelain insulators (knobs) for running wires through unobstructed spaces, and porcelain tubes for running wires through building components such as studs and joists.

Note whether knob and tube wiring splices are mechanically twisted, soldered, and taped, as required. Knob and tube wiring should be replaced during rehabilitation; but if it is properly installed, needs no modification, has adequate capacity, is properly grounded, has no failed insulation, and is otherwise in good condition, it can be an acceptable wiring system and is still legal in some localities.

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Check with local building code officials. Also check the terms and conditions of the home insurance policy in force to see if knob and tube wiring is excluded. In San Diego, knob and tube wiring does not meet code requirements. It is very difficult to purchase homeowners insurance for properties containing knob and tube wiring. The greatest problem with such wiring is its insulation, which turns dry and brittle with age and often falls off on contact, leaving the wire exposed. Insulation that can be seen to have failed also will likely have failed where wiring is concealed. If any failed insulation is observed, the knob and tube wiring should be replaced.

Other approved wire types include:

• NM (non-metallic) cable, often called by the trade name "Romex," a plastic covered-cable for use in dry locations (older NM cable may be cloth covered). 
• NMC, similar to NM but rated for damp locations. 
• UF (underground feeder), a plastic-covered waterproof cable for use underground. 
• AC (armored cable), also called BX, a flexible metal-covered cable. 
• MC (metal-clad cable), a flexible metal-covered cable with a green insulated ground conductor. 
• EMT (electrical metallic tubing), also called "thin wall," a metal conduit through which the wires are run in areas where maximum protection is required. 

In the picture at right, the armored cable and junction box are in good shape and can be reused, even if the lighting fixture is relocated.

Check branch circuits for the following:

• Marking. The function of each branch circuit should be clearly and legibly marked at its disconnect, fuse, circuit breaker, or on the directory on the panel board. 
• Connected loads. Trace branch circuit conductors to determine that their connected load does not exceed their rating (e.g., a 30 amp clothes dryer connected to a 20 amp circuit). Generally speaking, each dwelling unit should have two to four 15 amp circuits for lighting and convenience outlets; two 20 amp circuits for appliances in the kitchen, dining, and laundry areas; and separate circuits of appropriate ampacity for large appliances such as dryers, ranges, disposals, dishwashers, and water heaters. 
• Check the size and length of all branch circuit wiring against the requirements of the local electrical code. Buildings built before 1980 may be considered to have an inadequate number of circuits because present day codes require a separate laundry circuit and a separate circuit for the bathroom receptacle. For air conditioning units, many local codes will allow one wire size smaller than called for in the disconnect. 
• Test: A voltmeter may be used to measure voltage drop due to excessive branch circuit length, poor wiring connections, or undersized wire. Measurements must be made under a connected load. This test should be performed by an electrician. 
• Grounding. It is best that all circuits be grounded to the panel board, but this was not required by the National Electrical Code prior to 1965. Do not assume that circuits in metal cable are grounded without testing each outlet. Also, do not assume that three-prong plug convenience outlets are connected to ground. Remove each one to observe the presence of a connected ground wire. Check to see whether GFI (ground fault interruption) type receptacles have been installed in laundries, kitchens, and bathrooms, and test their operation. These types of receptacles were not required before 1990, but are easily installed as replacements. 
• Test: Commercially available circuit analyzers can be used for checking the following circuit conditions: open ground, open hot, open neutral, hot/ground reversed, hot/neutral reversed. Operation of these analyzers varies by manufacturer. 
• Condition and safety. Check that all wire types and equipment are installed properly in accordance with good practice. Check the conductors' exposure to possible damage or abrasion. Look for proper fastening, clearance, and frayed or damaged insulation. Make certain that all wire splices are made in work boxes and that all boxes for splices and switches have cover plates. Check all exterior receptacles to make sure they are of the waterproof type. 
• Test: A meg ohm test may be used for detecting deteriorated insulation. It requires a Megger tester and operates at high voltage. With the electrical service disconnected, branch circuits should read at least one meg ohm to ground. If lights or appliances are connected to the circuit, readings should be at least 500,000 ohms. This test should be performed by an electrician. 
• A visual inspection of insulation on accessible circuits will usually determine whether additional tests should be performed by an electrician. Look for unprotected wire runs through ducts and other inappropriate areas. Inspect for evidence of" handyman tampering" (e.g., unconventional splices), and if found in one location, expect it to be more widespread. Check for surface-mounted lamp cord extension wiring. It is dangerous and must be removed. It is best to remove all unused wiring or wiring that will be abandoned during rehabilitation work to avoid future confusion or misuse. 
• Aluminum wire. Aluminum wire was used in residential buildings primarily during the 1960s and early 1970s. Inspect with local code requirements in mind. Be sure that aluminum wire is attached only to approved devices (marked "CO-ALR" or "ICU-AL") or to approved connectors. Problems with aluminum wiring occur at connections, so feel all cover plates for heat, smell for a distinctive odor in the vicinity of outlets and switches, and look for sparks and arcing in switches or outlets and for flickering lights. Also check for the presence of an oxide inhibitor on all aluminum wire connections. All such conditions should be corrected. Aluminum wire should not be used on 15 and 20 amp circuits. Whenever possible, aluminum wire and its devices should be replaced with copper wire and devices appropriate for copper. If aluminum wiring is not replaced, it must be frequently inspected and maintained. 
• Smoke Detectors. Check to see if buildings have functioning smoke detectors. Detectors should be wired to a power source, and also should contain a battery. Most likely, buildings built before 1970 will not have detectors, but they should be added. 

 

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