I found three wires in a box: 1 blue 10 AWG, 1 blue 10 AWG and one white 12 AWG. There is a 125 volt reading across the two blue wires and a 125 volt reading across one of the blue wires and the white wire. There is no voltage across the other blue wire and the white wire. When I shut off one 20 amp breaker the voltages disappear. Which wire is neutral and which is hot? I thought the wires might be connected to a double breaker but not so. Note: I am in Mexico and wire color conventions are not always adhered to.
The only way you could read the voltages you see is if there is no load on the wires and you're reading capacitive pickup. Likely the other end of the two wires is at a switch, or this end was a switch. (Are you using a voltmeter or something like a neon tester -- likely if you used a voltmeter you'd see like 70 volts for one of the "hot" readings and 30 volts for one of the "no voltage" combos.)
JCorry wrote:There is a 125 volt reading across the two blue wires and a 125 volt reading across one of the blue wires and the white wire. There is no voltage across the other blue wire and the white wire.
It sounds like one blue is hot and the white is hot and the other blue wire is a neutral. The blue wire that has voltage with the white wire sounds like it's a grounded conductor (IE a ground or a neutral) . If you have no voltage between the other blue wire and the white wire, that's likely because they're both connected to the same busbar in the panel (assuming 240v single phase service like I would see in the US).
You can check the neutral and hot wires with an extension cord or a piece of wire. If you connect this external wire to a known ground (you may have to go back to the panel to do this) you can test the voltage between the your confirmed neutral and the other wires. If you find that two wires have 125 volts of potential and the third wire has no potential (0 volts), the 0 volt wire is a neutral or ground.