Carter wiring system: A prohibited Multiway switching

Carter wiring is a 3-way or multi-way wiring method used during old Knob-and-Tube (K&T) installations, a practice that has been prohibited by the National Electrical Code (NEC) since the early ’90s.

In Carter wiring, both the phase and neutral lines are connected to the switch. One throw of the two Single-Pole Double-Throw (SPDT) switches is connected to the phase line, while the other throws are connected to the neutral line. The load or bulb is then connected between the common poles of the two SPDT switches.


The light will be OFF when both switches are in the same position, meaning both of the bulb terminals have either phase or neutral connections. When the switches are flipped to the opposite direction, the terminals receive opposite polarity, and the light turns ON.

In contrast to another multi-way switching system, the two-socket terminals do not have fixed phase and neutral points. They change to either phase or neutral depending on the position of the switch.

The Carter wiring connection is similar to the H-bridge arrangement used in DC circuits, which are used to alternate the polarity across the load.

When the switch position is flipped to the phase on both switches, even though the bulb does not light up, the socket may be charged at both of its terminals. Thus, in this wiring system, we can’t simply replace a bulb by just turning off the light, as we cannot predict whether the socket is de-energized or not. During maintenance, complete isolation of the supply line cannot be guaranteed by switching it off, because we can’t be sure it is in a neutral position at both poles. Therefore, it is necessary to either turn off the main switch or use a tester to ensure an open terminal.

Due to safety concerns and the high risk of electric shock, Carter wiring is no longer recommended.

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