Originally Posted By: phinsperger This post was automatically imported from our archived forum.
jmyers wrote:
BTW...if you disconnect the green wire on that three wire 240 volt appliance, it won't work. Unless, of course, it is being grounded by some other means, like the earth, a metal pipe, etc.....
Why would a 240V appliance not function without being grounded? Doesn't all the amps flow though the two hots only?
Originally Posted By: jpope This post was automatically imported from our archived forum.
jmyers wrote:
120 volt never has four wires, always three.
That's what I said. . .
jmyers wrote:
BTW...if you disconnect the green wire on that three wire 240 volt appliance, it won't work. Unless, of course, it is being grounded by some other means, like the earth, a metal pipe, etc.....
Now wait a minute. If this is true, aren't we essentially violating NEC 408.20 which requires neutrals to be isolated from the grounds at all points down stream of the service?
That's why I questioned this in the first place. I assumed that the FMC was allowed to act as the GC and that the "green wires" should have actually been isolated and white (neutral).
There is the potential of a parallel neutral path with this set-up, is there not?
I give up. I'm going back to working graveyard at 7 Eleven.
-- Jeff Pope
JPI Home Inspection Service
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Originally Posted By: Greg Fretwell This post was automatically imported from our archived forum.
OK I will try to answer the questions.
Sherman set the "way-back" machine to 1993.
250.60 said the frames of dryers and ranges may be connected to the "grounded" conductor. That means the 3d wire in a 3 wire dryer circuit will get connected to the neutral bus.
As a second way to look at this, you would not want to intentionally put current on the grounding conductor.
BTW you could only use an uninsulated conductor in type SE if this circuit started at the service disconnect. If it was insulated you could come from a sub panel <in a poorly worded article>. A lot of inspectors/local codes said it had to be insulated either way.
That is why you will see 10-3/wg going to dryers a lot of times, even if it had a 3 prong receptacle.
As for why an 120/240 3w appliance wired wouldn't work <correctly> if the grounded wire was disconnected, you would lose the return path for the 120v loads. On the other hand if they followed the usual installation instructions for a dryer in those days and connected a separate ground wire from the frame to a cold water pipe there would still be a grounded path, assuming the bonding was good.
Originally Posted By: jmyers This post was automatically imported from our archived forum.
Greg,
Are you saying that a 240 volt appliance without any 120 volt components will work without the third wire?
Nice explanation....
Jeff P,
FMC? On the exterior? Are you sure it is not liquidtite? I believe Joe T posted something here a while back which stated that FMC was not allowed to be used as a GC. Now you are allowed to use the MC and AC style cables with the bonding strap.
Originally Posted By: Joey D’Adamo This post was automatically imported from our archived forum.
jmyers wrote:
...a 240 volt appliance without any 120 volt components will work without the third wire?
I personally can't imagine why it wouldn't. I wish I had single phase power in my apartment to show you what happens to a multimeter across the two hots. It reads 240 (or so). But its exactly the same as two hots in three phase. You get voltage. Nothing else is needed.
What you have in single phase power is a center-tapped transformer. Shown in what is probably the world's worst ever diagram:
Ground is completely extraneous to this. Except that the center tap is bonded to ground to keep it at 0 potential compared with the ground. This is where it gets its name neutral from.
Originally Posted By: jmyers This post was automatically imported from our archived forum.
Joey,
You are absolutely correct, it will work assuming it is truly a 240 volt unit although it is mighty dangerous to do so. I certainly would not suggest touching it! 
Jeff P,
I don't know about your part of the woods but typically around here they use liquidtite on the outside including attachment to the unit. liquidity would not create a parallel neutral, FMC (or greenfield if you are going by the trade name) would.
Greg has passed some good information along to everyone that is reading this thread and hopefully no one will have to use it!
BTW....I don't see anything wrong with the last one you posted either.
I guess this would be a good time for me to admit to my mistakes.....While I always have known the grounding wire should be attached to the neutral bar, I never called the un-insulated wire, or the lack of floating that bar in the disconnect. Oops!
Originally Posted By: Joey D’Adamo This post was automatically imported from our archived forum.
jmyers wrote:
Joey,
You are absolutely correct, it will work assuming it is truly a 240 volt unit although it is mighty dangerous to do so. I certainly would not suggest touching it! 
OK now i'm soooo confused.
If I have 2 phase wires and together they are 240 v or 208 v or 120 v on a delta power supply, or whatever.... point is... two phase wires with voltage... then the return path is not through the ground unless a wire is loose and touching the chassis. So grounding the chassis would be just a safety measure the same as with any other device. In an old 3-wire stove, the chassis is bonded to neutral, but only 120v loads in the stove are actually using that as a return path.
Right?? I really can't see it any other way. I'll hafta call my GF's dad who's a sparky to explain this to me, but I really think I have it right.
