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
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
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.<<<<>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.<
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
neutral-----------neutral + ground
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.
I don't think anyone was saying there would be 240v, just possibly 120v.
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
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