I need to connect two devices on the same leg so they can communicate. Basically I am trying to send my internet to my remote garage over the power line. I need to have them both on the same leg in order for it to work. I have a 220v 100A line from my house to the garage. I need to plug one device in my computer room outlet and one in my garage in an outlet wired to the same leg as the one in the house. How can I easily identify which is which?

  • \$\begingroup\$ While this might also fit on other sites, it's also a perfect opportunity to apply EE principles to a real world example. \$\endgroup\$ – Chris Stratton Aug 14 '14 at 16:22
  • \$\begingroup\$ How remote is the garage? I guess it's pretty remote if Cat 6 or WiFi are not applicable. \$\endgroup\$ – ntoskrnl Aug 14 '14 at 19:17

This is assuming North American 120:120 wiring.

Suppose you plug an extension cord into the outlet, then run it back to the spot in the house. Also suppose you have a voltmeter rated for safe mains operation and know how to use it (if that's not true, get assistance from someone (friend, neighbor, licensed electrician, whatever) who does know how to do these things safely.

Identify the 'hot' wire on each pair (should be 120VAC relative to earth). Measure between the two hot wires. If they're on the same leg, the voltage will be very low. If they are on opposite legs, it will be more like 240VAC.

  • \$\begingroup\$ Much better option than powering down the entire house! I know little-to-nothing about transformer configurations and house wiring, is what I suggested above plausible? \$\endgroup\$ – sherrellbc Aug 14 '14 at 13:43
  • \$\begingroup\$ @sherrellbc There would still be a bunch of loads connected every which way between the two lines and neutral (eg. A/C unit between two hots), so the resistance might be fairly low but it should be do-able. I don't like the idea of an ohmmeter on the mains, for obvious reasons. \$\endgroup\$ – Spehro Pefhany Aug 14 '14 at 13:52
  • \$\begingroup\$ If you disconnect mains what is the risk of using an ohmmeter? Also, what would the loads between the other phases and neutral have to do with your measurements on another phase? Are you implying that the measurements resistance would be too low to measure perhaps? \$\endgroup\$ – sherrellbc Aug 14 '14 at 14:02
  • \$\begingroup\$ @sherrellbc No risk if the power is locked out, but not everyone knows how to lock out the power and it might be far away (100A power to a garage implies a massive house to me!). It might also be too low to measure reliably with a cheap ohmmeter (a stalled 50A A/C unit might be < 1 ohm). \$\endgroup\$ – Spehro Pefhany Aug 14 '14 at 14:11
  • \$\begingroup\$ If I understand Spehro correctly the two outlets would need to be in close proximity in order to measure the voltage across the two legs. In my case they are 350ft apart. One outlet is in my house the other is in my garage 350 feet away. Am I missing something simple here? \$\endgroup\$ – Tom Prevost Aug 15 '14 at 13:49

Easiest w/ simple tools: Short it out and see what breaker trips. Bad Idea, don't do it. I have done it and it'll freak you out. It is safer to shut off circuits one by one until a light turns off or a loud radio shuts off.

Easiest w/ tools: Fox/hound device.

Or, just run a capacitor across the 2 legs so that the signal can cross. They sell Plug-In Phase Couplers that do just that plug in to the oven or dryer outlet (then the dryer or oven plug into them.)


I do not know a great deal about wirings in homes, but if the outlet is indeed fed from one of the single-phase sources in your garage then the first thing that comes to mind is a continuity test. Be sure to disconnect mains and test it with a probe first since these voltage ranges can be deadly.

Once you an absolutely sure mains is disconnected then you can go into your computer room and short the neutral and power together. Then go to your garage and use your meter to test for any continuity between the given leg and neutral.

This is of course just a suggestion and is not something I have done before. Please comment for better or worse.


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