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.