2 wire plus ground or 3 wire plus ground is dependent on the NEMA
configuration of the outlet to be fed. To be specific, the NEMA receptacle
and outlet chart I have in my office here calls out the following:
2 Pole, 3 Wire, Grounded - 250V rating: This outlet for 15A is NEMA 6-15R -
it has two blades for the hots and one for the ground. No neutral. There
is also a 20 amp version NEMA 6-20R, again two blades for the hots and one
for the ground. One can tell the differentiation between the 15A and 20A by
noting the the hot blades are perpendicular to one another on the 20A
3 Pole, 4 Wire, Grounded - 125/250V rating: The outlet for 20A is NEMA
14-20R - it has 4 blades, two for the hots, one for the neutral and one for
the ground. This outlet is what we use now for electric dryers, ranges,
etc. The NEC has required this change in the 1996 code from the other style
outlet with just the ground. Apparently this was warrented to add the
neutral for electronic controls to have 120V at the unit. There are 30A and
50A versions of this configuration, NEMA 14-30R and NEMA 14-50R respectivly.
3 Pole, 4 Wire, Grounded - 250V rating: The outlet for 30A is NEMA 15-30R -
it has 4 blades, three for the hots, and one for the ground. No neutral.
Used for motor feeds, etc. We refer to this outlet as 3 Phase, not normally
found in residential applications. There are 50 and 60A versions as well.
There are NEMA versions for ungrounded 3 pole 3 wire 125/250V NEMA 10-20R,
and 4 pole, 4 wire 3 Phase or 120/208V, NEMA 18-20R, but I have yet to use
one of these in any application.
Hope this helps, just FYI. Talk to your local electrician or supply house,
they have the charts and would most likely let you look at one. The web is
probably a good source as well for NEMA configurations.
73 KM0T - Mike King PE
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