ossible combinations of switch positions: two with the light on and two with the light off.
In the simple situation of two switches and a single switched load this system offers no advantage, and in fact has the disadvantage of requiring four wires (including neutral) between "ends" of the installation, compared to three wires in the standard system.
However, in the unusual case in which a switched load is wanted at both ends (e.g. illuminating a long hallway), and an unswitched load (e.g. receptacle) is wanted at both ends as well, this system's four wires saves one wire compared to the standard system, which would require five wires (two travelers, one neutral, one unswitched-hot, one switched-hot) to serve all these loads. However, in this application the system cannot be extended (e.g. with "four way" switches) to offer more than two switch locations.
The Carter system was a method of wiring 3-way switches in the era of early knob-and-tube wiring. This now-obsolete wiring method has been prohibited by the National Electrical Code since 1923, even in new knob-and-tube installations which are still permitted under certain circumstances. This wiring system may still be encountered in older "grandfathered" electrical installations, however.
In the Carter system, the incoming live (energized) and neutral wires were connected to the traveler screws of both 3-way switches, and the lamp was connected between the common screws of the two switches. If both switches were flipped to hot or both were flipped to neutral, the light would remain off; but if they were switched to opposite positions, the light would illuminate. The advantage of this method was that it used just one wire to the light from each switch, having a hot and neutral in both switches.
The major problem with this method is that in one of the four switch combinations the socket around the bulb is electrified at both of its terminals even though the bulb itself is not lit. As the shell may be energized, even with the light switched off, this poses a risk of electrical shock when changing the bulb. This method is therefore prohibited in modern building wiring.
More than two locations
For more than two locations, two of the interconnecting wires must be passed through an intermediate switch, wired to swap or transpose the pair. Any number of intermediate switches can be inserted, allowing for any number of locations. This requires two wires along the sequence of switches.
Using three switches, there are eight possible combinations of switch positions: four with the light on and four with the light off. Note that these diagrams also use the American electrical wiring names.
As mentioned above, the above circuit can be extended by using multiple 4-way switches between the 3-way switches to extend switching ability to any number of locations.
Switches built to North American standards identify the terminals by color-coding. The common is often colored black and the pair of traveler connections often colored gold. There is no standard for indicating the terminals on 4-way switches, so they may need to be checked with a meter or a continuity tester to deduce the internal contacts.
Most electricians know these simple guidelines when wiring multiway switching.
- The lightbox always needs a neutral.
- Either one switchbox or the lightbox has hot and neutral from the mains.
- The common terminal of one 3-way switch goes to a hot from the mains, and the common terminal from the other 3-way switch goes to the light.
- The traveler terminals connect to each other at each switchbox.
- Usually a 3-conductor cable is connected between the multiway switches. A pair of conductors for travelers, and the remaining conductor for either neutral or hot conductor. This is dependent on which box the mains are in.
- Only a 2-conductor cable is needed between the light and one of the switches if the switches are connected directly.
- 3-conductor cables are needed between the light and each switch if the switches are not connected directly via another path.
Low voltage relay switching
System