If I Have Two Black Electrical Wires, How Can I Tell Which Don a pair of protective rubber gloves before testing to find out which black wire is hot. You may Set your multimeter to measure voltage. Voltage is the pressure of electricity flowing through a wire, Place the prong of the multimeter's red wire on the bare metal on the end of one of the black wires. Place the prong of the multimeter's black wire on the bare metal on the end of a white wire, then See all full list on hunker What happens when two hot wires are connected - answers wwwers››Modification of Old Electrical WorkWiring to the circuit breakers is 220 volts. The circuit breaker box has 2 110 Volt lines. If you connect two black lines together from one side. Doing your own telephone wiring Repairpersons have sometimes remedied this by running the one phone line across the black and yellow wires rather than replacing the cabling. If this has happened, you won't be able to run a second line thru the four-strand wire. (This is uncommon, but it is a gotcha to be aware of). Four-strand wire supports up to two phone lines.