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For a low-voltage DC circuit, check the device’s + and − markings or wiring diagram first. If those are missing, a correctly set digital multimeter can show which of two conductors is at higher potential. Wire color alone is not proof. Household AC wiring is different: its conductors are hot/live, neutral and equipment ground—not positive and negative. If unidentified wires may be connected to mains power, stop and contact a qualified electrician.
First identify the kind of circuit
Positive and negative describe polarity in a direct-current (DC) circuit, such as a battery circuit or many low-voltage power supplies. Alternating-current (AC) wiring is described by conductor function: hot or live, neutral, and equipment ground. These terms are not interchangeable, and a DC color convention cannot identify household AC conductors.
How to identify polarity in a low-voltage DC circuit
Check markings and documentation
Start at the power source and the device terminals. Look for + and − marks, labels on the supply, or the manufacturer’s wiring diagram. A stripe, tracer, or other cable marking can help you distinguish conductors, but it does not establish polarity unless the equipment documentation says what it means.
Verify with a digital multimeter
Only use this method on a known low-voltage DC circuit, with a meter and leads rated for the circuit and environment. Follow the instrument manual and inspect the leads before measuring.
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- Set the meter to DC voltage (often marked
V⎓or with a solid line over a dashed line). - Put the black lead in
COMand the red lead in the voltage input, usually markedVorVΩ. Do not leave the red lead in the current-measurement jack. - Place one probe on each conductor. A positive reading means the conductor touched by the red probe is at higher potential than the one touched by the black probe.
- If the display shows a minus sign, the red probe is on the lower-potential side relative to the black probe. Swap the probes to confirm the sign if needed.
The meter’s sign tells you the relationship between the probe points. It does not certify that the circuit is safe, that the conductors are undamaged, or that they are suitable for a particular device.
How household AC wiring differs
Do not identify household AC conductors as positive and negative. Their roles are hot/live, neutral, and equipment ground. In the US workplace wiring context described by OSHA, white or natural gray identifies a grounded conductor, while green, green with yellow stripes, or bare is used for equipment grounding. These are context-specific requirements, not a universal color code for DC wiring or a guarantee about every existing installation.
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OSHA’s US workplace rule states: “No grounded conductor may be attached to any terminal or lead so as to reverse designated polarity.” See OSHA 29 CFR 1910.304. Other wiring contexts and jurisdictions can have different requirements. If you cannot establish a conductor’s identity or whether it is energized, do not experiment with it; contact a qualified electrician.
Quick Recap
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Safety limits when wires are unidentified
- Do not use wire color, a low-voltage DC method, or a cable detector as proof that an unknown household conductor is safe to touch.
- A cable detector may fail to detect a wire carrying only a small current, according to the UK Health and Safety Executive’s guidance on work near electricity.
- Live measurements require an instrument and leads rated for the expected voltage and environment, in good condition. Fluke’s guidance on live-measurement safety and test-lead checks is intended as measurement guidance, not a do-it-yourself method for making uncertain household wiring safe.
- If the wires may be connected to mains power and their identity is uncertain, stop and have a qualified electrician assess them.
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