To find the hot wire with a multimeter, you set the dial to AC voltage, touch one probe to a known ground or neutral point, and touch the other probe to the wire in question. A reading near 120 volts means that wire is hot. A reading of zero or close to zero means it’s neutral or ground.
What You Need Before Testing
Any digital multimeter with an AC voltage setting will work. Look for the symbol that resembles a V with a wavy line (~) above it. This is the AC voltage mode, which is what household wiring uses. If your multimeter has multiple voltage ranges, select the one above 120 volts (typically a 200V or 600V range). Many modern auto-ranging meters handle this automatically.
Plug the black probe into the port labeled COM (common). Plug the red probe into the port labeled V or VΩ. Do not plug the red probe into the port labeled A, which is for measuring current and can create a dangerous short circuit if used while testing voltage.
Identifying Wires by Color
In U.S. residential wiring, colors follow a standard pattern. Black is the most common hot wire. Red is used as a second hot wire in circuits like those for 240-volt appliances or three-way switches. White is neutral, and bare copper or green is ground. In three-phase systems, blue can also be a hot wire.
Canadian wiring follows the same general scheme for single-phase circuits: black or red for hot, white for neutral, and green or green-yellow for ground.
Color codes are helpful, but they’re not a guarantee. Previous homeowners or electricians may have wired things incorrectly, which is exactly why testing with a multimeter matters. Never trust color alone.
How to Test a Wire Step by Step
With your multimeter set to AC voltage and the probes in the correct ports, you’re ready to test. The basic principle is simple: voltage is always measured between two points. You need a reference point (neutral or ground) and the wire you want to identify.
If you’re testing at an outlet: Insert the black probe into the round hole (ground) or the taller vertical slot (neutral). Insert the red probe into the shorter vertical slot. A reading between 115V and 125V confirms the shorter slot is connected to the hot wire. Most readings land right around 118 to 120 volts.
If you’re testing loose wires in a box: First, identify your ground wire (bare copper or green). Touch the black probe to that ground wire, then touch the red probe to each remaining wire one at a time. The wire that produces a reading near 120 volts is hot. A wire that reads zero volts or just a few volts is neutral. If you get a small voltage between neutral and ground (a few volts), that’s normal, especially when appliances are drawing power on the circuit.
If you suspect two hot wires: In a 240-volt circuit, you may find two wires that each read around 120 volts when tested against ground. Testing those two wires against each other will produce a reading near 240 volts, confirming they’re both hot on opposite phases.
What the Numbers Mean
The key readings to understand are straightforward. Hot to neutral should read approximately 120 volts. Hot to ground should also read close to 120 volts, and it’s often slightly higher than hot to neutral by a volt or two. Neutral to ground should read very low, typically under 2 volts when loads are connected to the circuit, and essentially zero with no load.
If you test a wire against ground and get 120 volts, that wire is hot. If you get zero, it’s neutral. If every wire reads zero against every other wire, the circuit is likely off at the breaker.
A reading that’s significantly lower than expected (say, 90 volts or 60 volts) can indicate a loose connection, a shared neutral problem, or a failing component somewhere in the circuit. That’s worth investigating further.
Testing Without a Known Reference Point
Sometimes you don’t have an obvious ground or neutral to use as a reference. In that case, you can touch the black probe to a known grounded metal surface, like a grounded metal electrical box, a metal water pipe, or the bare ground wire from another circuit. Then touch the red probe to the mystery wire. If you get roughly 120 volts, the wire is hot.
This method depends on having a reliable ground connection at your reference point. A metal box that isn’t properly grounded won’t give you accurate results. If you’re uncertain about the box’s grounding, test it first by measuring between the box and a wire you know is hot (from another working outlet, for example).
Staying Safe During Live Testing
You’re working with live electricity, so the risk of shock is real. At minimum, wear shoes rated for electrical work (marked “EH” on the sole) and safety glasses with side shields. Rubber-insulated gloves rated for electrical work add an important layer of protection. Avoid synthetic clothing like polyester or nylon, which can melt onto skin during an arc flash. Natural fiber clothing like cotton is the safer choice.
Hold the multimeter probes by their insulated handles only. Keep your fingers behind the finger guards on the probe tips. Use one hand whenever possible, keeping the other hand at your side or in your pocket. This reduces the chance of current traveling across your chest if something goes wrong.
Before you test, verify that your multimeter itself is working correctly by testing a known live outlet first. A faulty meter that reads zero on a live wire could lead you to grab a hot conductor. Also check that your probe leads aren’t cracked or damaged, since exposed probe wiring can conduct electricity to your hand.
If you’re doing any work on the wires after identifying them (splicing, connecting to a device), turn off the circuit at the breaker first, then re-test to confirm it’s dead before touching anything with your hands.