How to tell which wire is positive and negative when both are black. Black wires usually signify something hot.
However, if you find a wire that has two black stripes on it and you want to rewire your circuit, it’s essential that you determine which black wire is live or active before working with the circuit.
The easiest and safest way to do this is through the use of an amp meter or specifically a multimeter.
How to tell which wire is positive and negative when both are black
If a device that has multiple wires ends up with both black wires but one of them also has either a red or white stripe down the side of it.
You must use the wire without any stripes attached to it because it will in fact cause your device to power itself off and what you want is for your device to power on.
Put your multimeter on voltage measurement
The selector switch on your multimeter will read zero volts if you are holding the probe leads in your hands and you then close it.
When the switch is connected to a device or should be closed or open, it will read AC voltage, depending on how you configured the dial settings prior to measurement. Do not use an analog multimeter to test polarity.
Plug the wires into the multimeter
Now that the alligator clips are connected to each set of wires, it’s time to connect them all together.
In this particular circuit, unlike some others, you can connect “either side” of one wire lead with any side of another wire lead.
That is, for this circuit you can clip either end of a wire and attach it to however many other ends there are on one lead which in this case would be 3 connections.
Check that the black lead is still plugged into the COM port on the multimeter (the volt/DCV symbol) and that now the red lead is plugged into the port marked with the “V” symbol.
Check out the reading
When measuring voltage, it’s important to first determine if you are checking a positive or negative wire.
To attempt this, make sure your multimeter is set to AC volts with the needle pointing down, and then hold the red lead (positive) between the thick and thin sections of any of your leads.
Next, locate a nearby power source such as an outlet or simply touch one side of your black lead (negative) to the other side of one of your wires.
If you hear a humming noise from your multimeter, you know that the wire is attached to a grounded piece as a whole (you can remove it now).
If no sound is coming from your meter, hold one end of any wired device in one hand.
Make sure the reading is positive
If your multimeter has any reading that’s a positive number, like 9.2 for example, the leads are connected correctly.
This means that the wire which is hooked to the red lead is positive and the wire which is hooked to the black lead is negative.
If your multimeter has a negative reading, then you have reversed your leads and if you follow this mistake you may damage your wires or cause faulty readings due to inconsistent voltage and resistance in our circuit.
Make sure the reading is negative
Connect the red lead to the wire to which the black lead was just connected and vice versa. Make sure the positive reading is 9.2 when you turn them over.
This means that you may have a faulty multimeter and will need to return it to the store to be checked or purchase a replacement tool in order to continue using it.
How to tell which wire is positive and negative when both are black
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