I don't know much about 240 V single phase power so please bear with me! I am confused about the wiring. Here is the situation.

In my outlet there are 3 wires, black, red and blue. I used my Fluke multimeter to check the voltages in between these lines. To my understanding, I should have L1 and L2 which are two signals 180 degrees apart, and a neutral.

I checked around with the multi meter until I found a voltage of 240 V and figured these were the L1 and L2 lines (red and black) and the third one (blue) must be the neutral, since the voltage between the third one and any of the other 2 was only 120.

So using just these two lines, I wired up my device (which is a DIY single phase to 3 phase converter for my milling machine) and it works just fine. But then I realized that the machine isn't grounded. For 120 V service, I am used to having a ground wire that I can hook up to the chassis for safety. So I go into the distribution box that the outlet is connected to and I see bare wire, which I presume is ground. So just to be sure, I shut off all of the power and did a continuity check between this bare wire and each wire in the outlet and I got continuity with the red wire! I don't understand this at all since the red and black were my L1 and L2 lines!

Can anyone explain why this is the case? I am pretty sure I CANNOT use the neutral line as a chassis ground and I want this machine to be grounded. Any help would be appreciated!

EDIT: In regards to wire color code. I am located in central PA.

  • 2
    Probably a good idea to mention where you are in the world so additional comments can be made on the relevant wiring code.
    – replete
    Mar 29 '17 at 0:32
  • Good point! Done!
    – user41178
    Mar 29 '17 at 0:50
  • Blue is a neutral on equipment mfg for CE listing (overseas) Being a licensed electrician here in the U.S. it was difficult to get used to equipment that was built in other countries that used Blue as the neutral. It is possible to see a low impedance to ground / neutral through devices like transformers and lamps that look to be a short to DC (meters use DC to measure resistance).
    – Ed Beal
    Mar 29 '17 at 13:23
  • What kind of outlet and plug are you using? If you don't know, post a picture. This may help: en.wikipedia.org/wiki/NEMA_connector
    – mkeith
    Mar 29 '17 at 15:26
  • @EdBeal, it sounds like the OP is saying there is a blue wire in the outlet. Have you seen blue wire used for neutral in a residential installation in the US? Would that satisfy code? To the OP, is the blue wire actually in the outlet in the wall?
    – mkeith
    Mar 29 '17 at 15:31

Red, black and blue are the classic colors for 3-phase power (other than wild-leg). The color combination does not specify voltage; that's what voltmeters are for.

An important factor is the lack of neutral. In the U.S. neutrals must be white or gray. It is illegal to "tape" or "paint" another wire color to be neutral, unless the wire is 4 AWG or larger. Measure the voltage between each wire and ground/earth. You should get a significant voltage if the wires are not wrong-colored.

How well do you really know the wiring here? Are you sure you don't already have 3-phase or maybe someone beat you to it and installed a phase converter somewhere? Maybe that has since been removed? No kidding, I have a 240V 3-phase converter at a 120-240 service panel, that panel's transformer is fed from 3-phase service which is literally 20 feet away from it. The things people do!

Now, the red wire being grounded is just wrong. However this could occur if the grounding system is not proper, or if the neutral-ground bond has failed. (And I have one of those in a panel right now.) All it takes is one ground-fault to "lift the ground" to hot potential (or more accurately, lower the hot to grounding potential, lifting the neutral away from ground.)

  • Maybe the wire is really green, but has faded or discolored in such a way that the OP thinks it is blue. This would explain everything, including the voltages reported by the OP. The OP said red-to-black is 240 and both red and black are 120 to the "blue" wire.
    – mkeith
    Mar 29 '17 at 15:48
  • He also says red and ground are the same potential. That's super wrong. Mar 29 '17 at 15:52
  • Nope. Not at the same potential. He said they showed continuity when the main power was disconnected. That makes sense. If the DC resistance of the load is small enough, it will be recognized as continuity by the Ohm-meter.
    – mkeith
    Mar 29 '17 at 15:54
  • Thanks for the comments guys!So I went back, turned on the power, and measured the voltage between each wire and the ground. For two of the wires, I got 120 volts each (red and black) and the blue one I got pretty much zero(so this is prob neutral,which makes sense with the way themachine is wired). When I initially did the continuity check, I got like 4 ohms.Is it possible that it simply is a low impedance path to ground as you suggestedBecause there WAS a potential difference between the ground and the red and black wires when the power was on.... Can I use this ground to ground my chasis?
    – user41178
    Mar 30 '17 at 1:18
  • @user41178 not at 4 ohms you can't. Worst case, run a proper ground, the rules now give you a lot of latitude to do that. Are you in metal conduit? That is the ground. What's happening at the other end of that cable run? Mkeith thinks the blue wire is a ground wire that's faded, green wires will also look blue under sodium lights. Try lighting it up with a cell phone and see if it still looks blue. Mar 30 '17 at 1:26

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