I believe the change from three to four wires started with the 1996 NEC.

In the three-wire system there are (at least) two possible problems.

1. There is relatively high resistance in the neutral line. To the extent that the neutral is carrying significant current (think the 115V drum motor in a dryer) there will be a voltage on the appliance frame equal to the voltage drop in the neutral line.

2. There is an open in the neutral line. If one of the 115V devices is turned on you could have close to 115V on the appliance frame. This situation is why the neutrals on receptacle outlets in split circuits are required to be pigtailed and not just daisy-chained through the device.

That said, the NEC folks took their sweet time to finally disallow 3-wire circuits, I guess there were few problems in real life.

If the circuit were run before the 1996 NEC took effect I would make a comment, but say that upgrading is not required. If the circuit war run when the new rule was in force I would have a problem with that.

- BOB