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

Brent Hilpert hilpert at
Thu Feb 5 22:00:17 CST 2015

On 2015-Feb-05, at 7:28 PM, Josh Dersch wrote:

> I picked up a "large" machine* (for certain definitions of large) that's currently configured for 220-240V; my house at the moment lacks 220V receptacles.  I was already planning to get an electrician out here to put in some beefier wiring at some point (I have a couple of machines that draw 15-20A off of 120V that I'd like to be able to run) but I wasn't yet planning on doing 220V unless I have to.
> (It looks like in theory it's possible to reconfigure the supply for 120V but I lack the docs to do so with any level of confidence...)
> I don't know if this machine works, and I'd rather not invest in 220V wiring quite yet unless it does.  

In theory, if you have two 20A 120V circuits installed, you could also have 220V 20A from the same circuit(s) at the additional expense of just a 220V socket.
However, local codes may have something to say about such an arrangement.

> 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.

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).

> But then, I'm a rank amateur when it comes to house wiring and A/C and power supplies and all of that so I thought I'd ask the cctalk collective whether this can be made to work or if I should simply wait for a professional to take care of it...
> Thanks as always,
> Josh
> * An AMT DAP 610, if you must ask.  It's an array processor from the late 80s, with 64x64 1-bit processors.  If the machine doesn't run I'm pretty much SOL for spare parts, schematics, service manuals, or anything beyond customer-level documentation (which I've recently scanned, btw, if anyone's curious...)

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