<P> 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 . </P> <P> 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 USA 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 . </P> <P> 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 . </P> <P> 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 . </P>

How many 4-way switches are required to control a light from five locations