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I'm getting some unexpected readings when testing a circuit for my wall mounted AC&heat. I'm on standard USA 120V/240V split-phase/three-wire AC power; the specific circuit is 240V.

here's a picture of my wiring terminal:

wiring terminal

United States Wiring

When comparing black(phase 1) to red(phase 2) I get the expected 240V. When I compare black or red to ground, I get the expected 120V. When I compare red or black to white(neutral) the value jumps around and does not settle within any recognizable/readable range. It also jumps around the same way when comparing neutral to ground. Why is this? I am using a basic digital multimeter(Mastech MS8268).

I want to wire in some extra fans with a fan speed controller, for a filter-box I am building. The controller is only rated to 120V. So I was thinking I would wire it between phase 1 and neutral, although after getting my readings, I am concerned. Is there a problem? Do I need to use a different circuit? I assume the unit was installed correctly, and it has been running fine, although it gets clogged with dust...

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    \$\begingroup\$ Neutral may not be connected thus floating. \$\endgroup\$ – user16307 Apr 27 at 15:56
  • \$\begingroup\$ What does the multimeter show between neutral and ground? \$\endgroup\$ – user16307 Apr 27 at 16:00
  • \$\begingroup\$ Can you switch of the fuse and measure the resistance between ground and neutral? Maybe there is a connection issue in the neutral and all the current ist currently returned via ground? \$\endgroup\$ – Christian B. Apr 27 at 16:17
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    \$\begingroup\$ are you sure that the white is a neutral? could it be a command wire used to communicate with the compessor? \$\endgroup\$ – Jasen Apr 28 at 3:38
  • \$\begingroup\$ OK, so I opened up the breaker. It looks like there is no neutral connected. I only have phase-1, phase-2(checked 247V with multimeter) and ground. I traced all leads to a 12/2 bundle leaving the box. This circuit also powers an outdoor condenser. I suppose somewhere along the line they switched to 12/3 wire. It's kind of troublesome that they did not use the same wire for the whole circuit. ...so I guess I have a disconnected neutral at all my wall units... is there a problem? \$\endgroup\$ – kipbits Apr 28 at 14:07
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After searching the manual, I can confirm /u/jasen's suspicion of a communication wire, thus explaining the floating value. I probably should have checked the manual first, but I suspect this is similar for all brands.

https://portal.fujitsugeneral.com/files/catalog/files/HFI%20Install%20Guide8.pdf

outdoor unit

http://www.fujitsuklime.com/wf-doc/fujitsu-klima-uredjaj-zidni-inverter-asyg09leca-installation-manual.pdf

wall wiring

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  • \$\begingroup\$ You can "accept" your own answer to indicate that the question has been answered. \$\endgroup\$ – Transistor Apr 28 at 14:49
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There is something wrong with your Neutral wire and this is dangerous. You must track down the path of the neutral and fix it before you do anything else. Never use the ground wire for any power return to the breaker box, this is a deadly mistake. The Neutral and ground are to be shorted together at the breaker box but they serve different purposes. All the current must be in the Neutral (although in this 240V split-phase it may be small with most of the current in the Red and Black wires). The ground wire's only other connection is to the metal housing of the fans and provides safety. If the field coil in a fan motor was to become shorted to the motor housing this would prevent someone from being electrocuted. If you were to use the ground wire for your fan power you could cause someone to die. Don't do it.

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