Originally Posted by
Tim Moreira
What difference does it make if you have a 3 or 4 wire dryer if both of the wires go back to the *Service Equipment* panel and are bonded together there anyway.
Now if the go back to a panel that is not *Service* then I would agree.
Am I looking at this correctly or not???
There are two
possible problems, which I mentioned in an earlier post.
Originally Posted by
Bob Mayer
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
In case 1. the problem is that the "ground" is also the neutral, there is current flowing through it, and the voltage at the "ground" point on the appliance frame is not the ground potential at the service because of voltage drop. With four conductors this is not a problem, because there is no current flowing through the ground conductor. The voltage here would probably be small, but real.
In case 2. we violate your assumption "if both of the wires go back to the *Service Equipment* panel". If there is a break in the neutral circuit, There could be almost 115V on the appliance frame. This is a similar problem to losing the neutral on a split circuit or on the service except here the neutral, with the far from zero voltage, is on the appliance frame. (A friend called me out for an electrical problem last autumn, weird voltages all over. The neutral was open at the transformer.)
- BOB