220 neutral wire question

This is perhaps a safety question. Most of the disconnects to 220v appliances are two hots plus ground (I have an air conditioning compressor, a well pump, and an accessory heater wired this way) with no neutral wire. The neutral isn't "necessary" as each hot is 180 degrees out of phase negating the need for a separate neutral.
But doesn't this mean if there is a short the appliance case AND the separate disconnect box (if metal) are BOTH electrified since their grounds are connected and there is rarely if ever a separate ground-to-earth at the appliance?
If so is this why 220v household clothes dryers are now four wire (hot hot neutral ground) or is there something in the dryers that needs 120v and thus needs the neutral for the 120v circuit?
If the disconnect were to a subpanel that is a different issue since the subpanel would require a distinct neutral (old school was to combo neutral and ground at the downstream panel but I think code frowns on that now).
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snipped-for-privacy@verizon.net wrote:

Right so far...

Well, yes, but that's no different from a 120V appliance that develops a hot-to-case short, except that the voltage is higher.

The latter. In a typical electric dryer, only the heating elements are 240V. The motor and timer are 120V. Likewise in an electric range: the elements are 240V, and the control circuits are 120V.

If that ever was permitted by Code, it must have been over 20 years ago. I'm quite sure that the 1984 Code prohibited it.
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Doug Miller (alphageek-at-milmac-dot-com)
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Doug Miller wrote:

Incorrect, the voltage is still 120V. The two hot legs of the 240V circuit are 240V relative to each other, but each is only 120V relative to the ground or neutral which are bonded together at the service entrance.
Pete C.

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Sorry, you're right of course. Thanks for catching that.
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If there was a short to ground, you could conceivably get 240v if you somehow also contacted the other hot. Not too likely, but not impossible.
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subpanel would require a distinct neutral (old school was to combo neutral and ground at the downstream panel but I think code frowns on that now.<<<<

I'm looking in one of my older subpanels and it appears to me that there are separate yet bonded together neutral and ground bars. Older but under 20 years old I'm sure. In other words the line input from the main panel is separated (three wire plus ground) but on the load side at the subpanel it looks like ground and neutral are the same. The subpanel goes have its own earth ground.
I'm thinking what I describe above would not meet today's code?
main panel subpanel hot----------------hot hot----------------hot neutral-----------neutral + ground ground----------(to above) | | | | earth earth
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snipped-for-privacy@verizon.net wrote:

That's correct -- it does not meet current Code. Moreover -- if it's less than 20 years old -- it did not meet Code at the time of installation either.

Remove the jumper connecting neutral and ground in the subpanel.
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Doug Miller (alphageek at milmac dot com)
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snipped-for-privacy@verizon.net wrote:

Unless there is a failure of the ground wire the short to the appliance case be a short to ground and the breaker will kill the circuit and both legs will be cut if it was properly connected to begin with.

They are providing for a neutral for 120V needs, like a timer.

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Joseph Meehan

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Joseph Meehan wrote:

Exactly what I was thinking. How people can be saying that the appliance and the disconnect case will both be hot at 240V is beyond me.

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If the ground was broken and there was a short, then there would be 120v at the box connected to the broken ground; further assuming nothing had good enough contact to ground to trip the breaker. It is not the type of thing anyone should lose sleep over, but it is possible.
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ground at: http://www.selfhelpandmore.com/homewiringusa/2002/definitions/termground/index.htm
general at: http://www.faqs.org/faqs/electrical-wiring/part1 /
see floating neutral at: http://www.codecheck.com/pdf/electrical/ccwsamplep39elec.pdf
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