How Much You Can Keep?

Basically, you can retain any and all safe wiring and devices. An electrical system will do what it’s designed to do and will do so reliably until you extend a circuit or the system beyond its design. You should remove or replace the following:

• Any corroded or damaged wiring
• Wiring that has not been installed properly (such as wire running along the bottom edge of an exposed joist rather than through holes drilled in the joist)
• Devices or fixtures that render a circuit unsafe
• Conductors that are the wrong gauge for their circuits
• Any cable rated for interior use but installed outdoors You might be getting more than you bargained for if your system has been repeatedly altered over the years.

If additions have been made based on convenience (“Hey, look! I found a couple of wires.”) rather than logic, you could end up with an odd variety of lights, receptacles, and other fixtures all over your house. It might be best to run a new circuit or two to cover all these miscellaneous runs if you’re not already rewiring the whole house.

Mixing Old and New Wiring

You can connect new cable or wire to existing cable or wire if you follow these rules:

• All connections must be made inside an electrical box. The one exception is knob-and-tube wiring, which is the only electrical system that can be spliced in the wall. To do so, you must solder the conductors or use a mechanical splice such as a split bolt; a wire nut is not sufficient.
• Junction boxes must be kept accessible. They cannot be covered up.
• Don’t change the wire gauge. (New wire has to match existing wire.)
• Be careful not to overextend the circuit by adding more loads than it’s designed to handle.

Knob-and-tube wiring doesn’t lend itself to easy identification of the hot and neutral conductors. You might have a junction box or device box packed with wires. Identify the hot lead the same way you would identify it in a less-crowded box:

1. Identify the circuit and turn off the power at the panel.
2. Check the connections with a voltage tester to confirm that the power is off.
3. Carefully remove any tape or wire nuts from the connected wires (those running back toward the panel).
4. Mark the wires so you know which ones were connected to each other.
5. Separate the wire ends so they’re not touching each other or the sides of the (metal) box.
6. Turn the power on at the service panel and test the wires one at a time until the hot lead lights the bulb on the tester.

Mark this as the hot line conductor. Messy junction boxes often indicate that a device or fixture was added without a lot of consideration as to its effect on the circuit. Your best bet is to confirm whether the circuit can or cannot safely support the addition and then deal with it appropriately. It might be that all you’re looking at is an unkempt, but safe, series of connections.

Messing Around With Old Wiring

The easiest residential wiring to work with (short of metal conduit, which you’ll never find in a typical home) is grounded NMB copper cable. It’s newer than knob-and-tube wiring, the insulation is tough plastic, and you have none of the safety dilemmas that you have with aluminum wiring. If your electrical system was recently installed and inspected, it should be simple to trace circuits and calculate loads as you plan your additions and changes.
Regardless of the age of your system, you’ll have to tally your amperage usage for the total system as well as for each individual circuit affected by your work. Simply looking inside your service panel and counting the breakers or looking for an empty space to install a breaker isn’t enough. Wiring or changing your system generally means doing some damage to your house.
The larger the access holes and openings for boxes, the more repairs you’ll have to do. Your working goal should be to keep the repairs and patching to a minimum. As you can see, there’s more to electrical work than simply deciding to add a receptacle and running some wire.