Taking a New Look At Old Residential Wiring

Safety and economic concerns lead the way for most electrical jobs on older buildings.

By David Herres

Electrical workers see many old residential buildings and are asked to evaluate the wiring in these aging structures. Such inspections are performed at varying levels of intensity.

Safety is the primary concern but efficiency and economy are also important. The National Electrical Code addresses hazards to life or property but not efficiency. It is best to begin with the Code and if a wiring installation is found to be compliant, only then is it appropriate to look at efficiency and economy.

NEC is revised every three years and each revision incorporates numerous changes. It is not expected that every building be immediately brought up to code. In order to be in compliance, an electrical installation must be in compliance at the time of the installation. Nevertheless, it is common to do certain types of optional upgrades.

For instance, if a house is being sold and subject to an inspection, the lending organization will want to see GFCIs installed in all currently required locations. GFCI and arc fault protection are great safety features and can be installed without running new branch circuits so these upgrades are highly appropriate.

Getting started

To evaluate a residential electrical installation, it is necessary to ascertain the scope of the inquiry. You have to look at the history of the electrical installation, the purpose of the inspection and the wishes of the owner, lending organization or insurance company.

If it is a routine sale, to check the status of the structure or a situation with no history of water inundation or fire, you will probably be asked to just look it over to see if it is safe. At the other extreme, if there has been prolonged flooding or other issues with the building, the interior finish probably will be removed in which case a total rewiring is in order.

A fundamental NEC requirement is that all wiring terminations be accessible. A junction box concealed behind building finish (a so-called "blind box") is not allowed. Therefore, it is possible to inspect all splices and terminations without tearing out finish walls or ceilings.

If there has been brief, limited water exposure, a good procedure would be to detach all light fixtures and hardwired equipment, check for any sign of corrosion, retorque terminal screws, twist off wirenuts, make sure the metal is shiny, and retighten. Receptacles, switches and similar devices can receive the same scrutiny as well as junction boxes and enclosures.

Before and after the job, it is possible to measure circuit parameters using a loop impedance meter. This instrument gives excellent load and no load evaluations so that you can spot trouble early, repair, retest and provide documentation. Some models are available in the $600 neighborhood and will be worthwhile down the road. Alternately, an inexpensive circuit analyzer can reveal wiring errors.

Doing the job

Any electrical inspection will involve removing the entrance panel cover. Begin in an information-gathering mode and look for design errors. Are the service entrance conductors sized properly with correct overcurrent protection? Aluminum entrance cable terminations must be installed with corrosion inhibitor. Also, aluminum tends to creep with time so that connections can loosen. If so, a hot spot will develop leading to more corrosion and heat until the connection opens up, seen by the occupants as a power failure, or combustible material outside the enclosure ignites.

Good preventive maintenance includes verifying the presence of a corrosion inhibitor and retorqueing the lugs. The same procedure is recommended at the meter socket and anywhere aluminum terminations are found.

Check the weatherhead. It should be well secured to the building and in good condition so that water cannot get inside the service entrance cable jacket, travel into the meter and cause corrosion at these high current terminations. If the concentric service cable enters the meter socket enclosure at the top, it must be well sealed so that water cannot infiltrate.

Breakers clip or bolt onto the bus bars. With the cover off, gently wiggle the breakers, especially the main, from side to side while under load. If a frying sound or flickering of lights is noted, pitting and corrosion have set in and will quickly get worse. Beware of cleaning the metal with any sort of sandpaper as abrasive particles can imbed in the metal making local hot spots. If a busbar is damaged, it is not necessary to buy a whole new entrance panel, just get a busbar kit based on the serial number inside the box.

While the cover is off the box, it is a good time to verify that none of the branch circuits are overfused. Also, it is important that multiwire branch circuits have been grouped properly with ungrounded conductors connected to opposite legs so that current on the neutral subtracts, as opposed to adding.

Another important area to check is grounding. NEC Article 250 covers permitted ground electrodes with minimum spacing, and sizing for ground rod conductors, etc. An important requirement sometimes neglected is bonding to metal water pipe and structural elements. This should be done with bare or insulated copper wire with no splices and with UL-listed clamps. Sizes are given in NEC Table 250.66.

Other systems such as telephone, satellite, CATV and broadband Internet access, should be grounded in accord with NEC Chapter 8, Communications Systems, and all grounds interconnected to prevent dangerous voltage differentials.

Economic concerns

Generally the NEC does not get involved in efficiency and economy, so for guidance in these areas we must look elsewhere.

Undersized service, feeder and branch circuit conductors cause voltage drop. Any amount greater than 5 percent at the far end of a branch circuit is considered unacceptable. Lights will dim when another load is switched on. At reduced voltage, AC motors run slower than rated speed and more of the electrical energy is converted to heat. They start hard under a load or fail to start at all and experience a shorter service life.

NEC contains guidance for sizing out a service based on the square footage of various occupancies together with a listing of the electrical equipment to be installed. But this calculation, while highly accurate, does not account for future expansion. It is expensive in material, labor and downtime to put in a new service just because some new equipment is added.

Extra optional feeders and branch circuits can be included in the original installation or an upgrade. American homeowners are great gadget lovers and as the years go by it is likely that a given residence will undergo expanded electrical usage. It is less expensive to create extra capacity as part of the initial installation. In addition, many homeowners may desire auxiliary power in case of outages. NEC requirements are in Article 702, Optional Standby Systems.

If an optional standby generator is installed, a transfer switch is necessary to ensure that a dangerous backfeed situation is not created. If the secondary of the utility transformer becomes energized, it actually becomes a step up transformer and high voltage appears on the overhead lines exposing workers to an unanticipated hazard. A transfer switch prevents this dangerous situation from occurring by connecting the load to either the normal utility source or the standby power source but never both simultaneously.

If there has been brief, limited water exposure, a good procedure would be to detach all light fixtures and hardwired equipment, check for any sign of corrosion, retorque terminal screws, twist off wirenuts, make sure the metal is shiny, and retighten. Receptacles, switches and similar devices can receive the same scrutiny as well as junction boxes and enclosures.

Before and after the job, it is possible to measure circuit parameters using a loop impedance meter. This instrument gives excellent load and no load evaluations so that you can spot trouble early, repair, retest and provide documentation. Some models are available in the $600 neighborhood and will be worthwhile down the road. Alternately, an inexpensive circuit analyzer can reveal wiring errors.