Turn power off, separate wires, restore power, use a tester: the always-hot feed is line; the wire that carries power onward is the load.
Safety and prep
Work only if you feel comfortable and know how to shut off a breaker. Use a non-contact voltage tester first, then a meter for confirmation. Turn power off before moving wires, and cap any loose conductor you are not testing. Take a photo of the box before you touch anything so you can put it back if needed. Mark each cable with tape as you learn what it does. If you are in North America, you can read the NEC free access portal to see definitions of line, load, and grounding terms.
Tools that make the job easy
- Non-contact voltage tester for a quick live check
- Two-lead multimeter for voltage and continuity
- Insulated screwdriver and needle-nose pliers
- Assorted wirenuts and a roll of electrical tape
- Labels or painter’s tape for tagging line and load
New to meters? Read how to use a multimeter for probe basics and ranges.
Common wire clues at a glance
| Conductor or mark | Typical look | Usual role |
|---|---|---|
| Black or red insulated | Solid color, no stripes | Hot feed or switched hot |
| White insulated | Plain white | Neutral, unless re-marked as hot |
| White re-marked | White with black or red tape/ink | Hot in a switch loop or cable run |
| Green or bare | Green jacket or bare copper | Grounding conductor |
| Screws labeled LINE | Often bottom of GFCI | Incoming hot and neutral |
| Screws labeled LOAD | Often top of GFCI | Outgoing hot and neutral to downstream |
Colors and labels help, yet they do not prove function. Always test. Older work and repairs can reuse colors.
Identifying the load wire step-by-step
- Kill the power. Switch off the breaker. Verify with a non-contact tester at the device box.
- Pull the device forward. Keep wires attached for your first look. Note any labels like LINE and LOAD.
- Map the cables. Count how many outer sheaths enter the box. One cable usually feeds a single device. Two or more often means one feed plus one or more loads.
- Label neutrals and grounds. Neutrals are white and usually tied together with a wirenut. Grounds are green or bare and bonded to the box or device yoke.
- Isolate the hots. Remove the hot wires from the device and cap the ends separately so nothing touches.
- Power on to test. Turn the breaker on. Use your meter set to AC volts. Touch one probe to a known neutral bundle and the other to each capped hot.
- Find the line feed. The conductor that reads system voltage to neutral (for many homes ~120 V) with the device removed is your line. It stays hot regardless of the switch position.
- Find the load. The conductor that was on the other device screw and does not read full voltage on its own is the load. When you reconnect the device and toggle it, this wire energizes only when the device sends power onward.
- Confirm downstream. Leave the device out, tie line to suspected load with a wirenut, power on, and see if downstream outlets or lights come alive. Power off, untie, and wire the device correctly.
- Label for later. Wrap a small tag or tape flag on the conductor and write “LOAD.” Do the same for “LINE.”
How to tell line vs load on a switch
Single-pole switches have two brass screws and a green ground screw. One brass screw connects to the incoming hot (line). The other goes out to the fixture (load). Three-way and four-way setups add travelers and can confuse any quick guess, so testing wins every time.
Clues without a tester
Look for a feed-through. If two or more cables enter the box and one cable heads to the light, the constant hot from the feed cable is the line. The conductor that rises in the same sheath as the neutral going to the fixture is the load. On remodels the white in a two-wire switch loop may be re-marked and used as a hot down to the switch; the colored conductor back up is the load to the light.
With a multimeter
Set the meter to AC volts. With power on and the switch disconnected, check each conductor to the neutral bundle. The one that reads full voltage all the time is the line. Recheck with the switch back in the circuit: line shows full voltage on the feed side; load shows voltage only when the switch is on.
Identify a load wire in a light fixture box
At the ceiling you will see two or three cables. One brings power in, one goes to the switch, and sometimes one continues power onward. The load to the lamp is the conductor that leaves the switch loop and ties to the fixture lead. The neutral from the feed ties to the fixture neutral. If the white in the loop is re-marked, treat it as hot and not as neutral.
Switch loop basics
In older work, power reached the ceiling first, then dropped to the switch on a two-wire cable. The white was used to carry hot down to the switch and should be marked with black tape. The colored conductor brought switched hot back up to the lamp. In that setup the load to the lamp is the colored return from the switch.
Two-cable and three-cable boxes
Two cables: one feed, one switch loop or one feed, one load to another room. Three cables: feed, switch loop, and onward load. Separate the hots, power on, and test each to the neutral bundle. Only the feed will show voltage without any ties. Tie feed to one candidate at a time to learn which branch carries power forward; that paired branch is a load.
Identify a load wire on GFCI and smart controls
GFCI receptacles and many smart switches have distinct terminals. LINE takes the incoming hot and neutral. LOAD feeds downstream outlets while adding protection or control. The device body usually prints the words next to the screws. If you reverse them, the device may still power itself, yet downstream outlets lose protection or the smart control will not control the circuit you expect. Leviton’s overview of line and load on GFCI shows photos of the terminals and labeling.
Smart switches and dimmers
Most smart wall controls want a neutral, a line hot, a load lead to the fixture, and ground. Some older houses lack a neutral in the switch box. If your control needs a neutral, it will say so in the manual. Confirm your neutral by finding the white bundle tied together in the back of the box, not a lone white on a switch screw. If your control has leads instead of screws, the lead tagged “Line” goes to the always-hot feed; the one tagged “Load” goes to the wire that runs to the fixture.
Why line vs load matters
On a GFCI, only conductors landed on the LOAD pair receive downstream protection. Feed a bathroom light or a second outlet from the LINE pair and those devices will not be protected. On a smart control, swapping line and load often leaves the control powered but unable to switch the lamp. Timers and motion sensors behave the same way. Getting line and load right means protection trips when needed and controls act on the circuit you want.
Troubleshooting signals and quick fixes
| Symptom | Likely cause | What to try |
|---|---|---|
| GFCI sets but downstream dead | Line and load reversed | Move feed to LINE, branch to LOAD |
| Breaker trips on switch on | Load tied to neutral or ground | Separate bundles, find stray tie, correct it |
| Outlet face live but no protection | Using LINE only | Move downstream branch to LOAD |
| Light stays on | Switch bypassed | Move switch leg off feed pigtail and onto switch |
| Meter reads low voltage | Backfeed through lamps or testers | Use a real load or a two-lead meter for confirmation |
Color clues and why they mislead
Color hints save time, yet they are not a guarantee. Repairs and older habits can place a white on a hot screw. Tape marks help but can fall off. In North America black and red are common for hots, white for neutral, green or bare for ground. In many IEC regions brown is live, blue is neutral, and green-yellow is protective. Mix-ups happen on remodels and in junctions you cannot see. Treat color as a clue, not an answer, and let the meter decide.
Test methods that work
Non-contact tester
Good for a quick yes/no on the presence of hot. It will chirp near a live conductor. It can also chirp from induced voltage near bundles. Use it as a first pass, then step up to a meter.
Two-lead multimeter
Set to AC volts for live testing. Place the black probe on neutral or ground, red probe on the wire in question. Full system voltage marks a line or a switched load that is on. Zero or a small ghost reading suggests a load that is off or an induced reading. For continuity and resistance tests, power must be off and the conductor must be isolated from the rest of the circuit. The Fluke guide linked above shows probe placement and ranges with photos.
Proving neutral
A neutral is not just a white wire. Find the bundle of whites tied together, often with a pigtail to a device. Test from the suspected neutral to a known hot. Full voltage tells you that white is neutral. No voltage means you may be holding a re-marked hot.
Before you close the box: quick checklist
- All conductors landed under the correct screw or wirenut
- Copper fully under the clamp with no nicks
- No bare copper touching a neighboring terminal
- Grounds tied together and bonded to the yoke
- Neutral bundle intact and tight
- Device ears snug to the box with no pinched insulation
- Faceplate installed and sits flat
- Labels on line and load for the next service
Mistakes to avoid
- Guessing based on color alone
- Leaving power on while moving conductors
- Failing to isolate wires before voltage tests
- Using the ground as a working neutral
- Stuffing two wires under one screw where the device is rated for one
- Hiding wirenuts deep in the box with no slack for service
- Skipping labels after you figure things out
Quick reference: where the load wire usually lands
- Single-pole switch: The load lands on one brass screw; the line lands on the other brass screw; ground on green.
- Three-way switch: The common screw takes line on one switch and load to the fixture on the other; travelers ride the two matched screws.
- GFCI receptacle: Downstream branch hot and neutral land on the LOAD pair; incoming hot and neutral land on the LINE pair.
- Smart switch or dimmer: Lead or terminal marked Load runs to the fixture; Line ties to the always-hot feed; Neutral to the neutral bundle.
- Combination switch/outlet: The tab decides if the device shares a feed. Load may land on a switched side; check the diagram on the body.
- Ceiling light box: The load from the switch ties to the fixture hot lead; the neutral bundle ties to the fixture neutral lead.
Work slowly and carefully, test twice, and label as you go. A calm method finds the load wire every time and leaves the box tidy and neat for the next person who opens it.