Stupid AC wiring question (220-ish Volts...)

Brent Hilpert hilpert at
Thu Feb 5 22:45:16 CST 2015

On 2015-Feb-05, at 8:16 PM, Josh Dersch wrote:
> On 2/5/2015 8:00 PM, Brent Hilpert wrote:
>> On 2015-Feb-05, at 7:28 PM, Josh Dersch wrote:
>>> And, let's face it, I'm spoiled and I demand instant gratification and I'd like to know as soon as possible if this machine is a basket case or not.
>>> So:  since all this stuff is in the basement, I'm just about 15 feet away from the dryer, which at first glance runs off an outlet that meets my needs.  I even have a NEMA 10-30p plug here that I could wire up to the existing power cable for the computer.  But looking into it I have doubts that it's actually that simple; in particular since this house was built well before 1996 and so the outlet is not grounded; there's a neutral lug and two hot lugs (I assume two 120V A/C lines out of phase?) and I'm guessing that might not sit well with the power supply in this computer.
>> Yes, the two hots are 120VAC to neutral each, 180 degrees out of phase, giving 240 between the two hots.
>> Yes, you should be able to use those two hots to power the target machine, as long as the 240 input lines for the target are isolated from ground. Normally this will be the case, I just mention it in case there is some strange circumstance where one side of the target input is connected to ground. Measure the R between chassis and the two line inputs, it should be very high R on both.
> Yes, there's high resistance between both line inputs and chassis ground, higher than my DVM can measure.
>> Is the existing plug/line input to the the target 3-wire (2 hot + GND) or 4-wire (2 hot + neutral + GND)?
>> And have you confirmed that one wire IS GND/chassis? (it could conceivably be 2 hot + neutral).
> The existing plug is a 3-wire affair, white/black for hot and green ground using a NEMA 6-20P plug.  I  have confirmed that ground is wired to the chassis (with the DVM.)
> So is it as simple as wiring the white/black wires up to the HOT terminals on the NEMA connector and leaving the ground disconnected (from the "netural" pole on the NEMA... or should that be connected as well)?  Anything else I should watch out for?

If you are convinced it's wired for 240 then yes, you should be able to wire the hots as you say.
You MUST somehow GND the chassis however, either:
	- confirm the dryer receptacle neutral is adequately tied to ground, or
	- if the dryer receptacle is wired with armored/BX cable that may be the ground, or
	- waterpipe confirmed as adequate ground, or
	- connect to a confirmed ground on a normal 120V outlet, although there is a warning involved in such a scenario,
	   if the ground conductor is much smaller than the hot conductor, a fault can blow the ground conductor open,
	   at which point you have no ground and a live chassis. In theory the circuit breaker should open before the ground blows, but that's theory.

Alternatively, if you'd like to examine rewiring the machine for 120, we could look at that, via photos, etc.
(First question would be what are the target power/VA requirements, to consider whether it's worthwhile on 120).

More information about the cctech mailing list