Doug Miller wrote:
In article .com, wrote:
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.
Right so far...
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?
Well, yes, but that's no different from a 120V appliance that develops a
hot-to-case short, except that the voltage is higher.
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.
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?
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 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).
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.
--
Regards,
Doug Miller (alphageek-at-milmac-dot-com)
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