A load wire is the conductor that carries switched or downstream power from a control device (like a switch or GFCI) to the fixture or subsequent outlets it protects. In standard US 120V NM-B wall cables, the load wire is typically black (the exact same color as the incoming line wire), but on device pigtails and terminals, manufacturers use specific colors like red or blue, or mark the terminals with yellow tape to identify the load side.
If you are staring into a junction box trying to figure out which wire goes where, the most critical thing to understand is that 'load' and 'line' describe function, not strictly color. Assuming the wire color in the wall always dictates its function is the most common mistake DIYers make when upgrading to smart switches or GFCI receptacles.
The Direct Answer: Line vs. Load Wire Colors
To wire a device correctly, you have to look at the colors in two different contexts: the raw cable inside the wall, and the pigtails/terminals on the device itself.
| Context | Line (Power Source) | Load (Downstream/Switched) | Neutral | Ground |
|---|---|---|---|---|
| Standard NM-B Wall Cable | Black | Black (or Red in 3-way) | White | Bare / Green |
| GFCI Receptacle Terminals | LINE screws (no tape) | LOAD screws (Yellow tape) | Silver screws | Green screw |
| Smart Switch Pigtails | Black wire | Red or Blue wire | White wire | Green wire |
| 3-Way Switch Travelers | Common (Black screw) | Travelers (Brass screws) | White (in ceiling box) | Bare / Green |
As outlined by the National Electrical Code (NEC), the grounded (neutral) conductor must be white or gray, and the equipment grounding conductor must be bare or green. However, the NEC does not mandate a specific color for the ungrounded (hot) load wire in a standard switch loop, which is why both your line and load in a standard single-pole switch box are usually black.
What the Load Wire Actually Changes in a Circuit
Connecting the correct wire to the load terminal dictates what gets controlled or protected. Reversing line and load on a standard single-pole switch just means the switch works backward (which is physically impossible to notice). But reversing them on a GFCI or a multi-location smart switch changes the safety and functionality of the entire circuit.
Worked Numeric Example: The GFCI Protection Zone
Imagine a 15A bathroom circuit. The GFCI receptacle receives 15A from the breaker on its LINE terminals. The LOAD terminals feed a downstream vanity light (drawing 2A) and an exhaust fan (drawing 1.5A). The total continuous load on the downstream side is 3.5A.
If you mistakenly wire the 15A breaker source to the LOAD terminals, and the 3.5A vanity/fan feed to the LINE terminals, the GFCI will still reset and power the bathroom. However, the downstream vanity and fan now have zero ground-fault protection. If a moisture-induced 6mA leakage fault occurs at the vanity light, the GFCI's internal sensor won't see it, because the fault current is returning through the LINE neutral, bypassing the sensor coil. According to OSHA electrical safety guidelines, failing to provide intended GFCI protection in wet areas is a severe shock hazard.
Where You Meet This in Practice
You will encounter the line vs. load distinction in three primary residential scenarios:
1. Standard Single-Pole Switch Loops
In a traditional switch loop, power goes to the light fixture first, and a 2-wire NM-B cable drops down to the switch. Both the black and white wires in that drop cable are 'hot' (the white brings line power down, the black takes load power up). Neither is a true neutral. Both are functioning as ungrounded conductors. The load wire here is the black wire returning to the fixture.
2. GFCI Daisy Chains
When protecting downstream outlets (like kitchen counter receptacles), you must use the LOAD terminals. Modern GFCIs from manufacturers like Leviton and Eaton ship with a strip of yellow tape covering the LOAD screws. This is a deliberate physical barrier to force you to pause and verify which wires are the downstream load before peeling the tape and terminating them.
3. Smart Switches and Dimmers
This is where color coding actually matters on the device side. If you are installing a Lutron Caseta dimmer, the black pigtail is line, the blue pigtail is load, and the white is neutral. If you are installing a Leviton Decora Smart switch, the red pigtail is typically the load. Always check the specific manufacturer's wiring diagram, as mixing up the red/blue load wire with the black line wire will instantly fry the internal triac or microcontroller on the smart switch.
Common Confusions and How to Test
The biggest trap for DIYers is opening a switch box, seeing two black wires, and guessing which is line and which is load. If you guess wrong on a smart switch, it won't turn on. Here is how to definitively test them using a digital multimeter set to AC Voltage (200V or 600V range):
- Isolate the wires: Ensure the breaker is ON for testing, but keep all bare wire ends separated and safely away from the grounded metal box.
- Test Wire A to Ground: Touch the black probe to the bare ground wire (or metal box) and the red probe to the first black wire. If it reads ~120V, this is your LINE (hot) wire.
- Test Wire B to Ground: Move the red probe to the second black wire. If it reads 0V (or a very low ghost voltage of 2-5V), this is your LOAD (switched leg) wire, assuming the light bulb is installed and the circuit is complete.
- Cap and Label: Turn the breaker back OFF and wrap a piece of black electrical tape around the LINE wire so you don't lose track of it while wiring the new device.
Load Wire FAQ
What happens if you reverse line and load on a GFCI?
If you reverse line and load on a GFCI receptacle, the GFCI will still power itself and reset normally. However, any downstream outlets wired to the load side of the circuit will lose ground-fault protection. Furthermore, the 'Test' button on the GFCI face will likely fail to trip the receptacle, giving you a false sense of security. Always use a plug-in GFCI tester to verify downstream protection after installation.
Is the load wire always the red wire in a 3-way switch?
No. In a 3-way switch setup, the red and black wires connected to the brass-colored screws are called 'travelers.' They carry the hot current back and forth between the two switches depending on their toggle positions. The actual load wire (the switched hot going to the light fixture) is connected to the black 'common' screw on the second 3-way switch. That common wire could be black, red, or even a re-identified white wire depending on how the cable was routed.
Can I use a white wire as a load wire?
Yes, but it must be properly re-identified. In older switch loops, the white wire in a 2-wire NM-B cable was often used to bring line power down to the switch, while the black wire acted as the load wire returning to the fixture. Per NEC Article 200.7(C)(2), if a white wire is used as an ungrounded (hot) conductor, it must be permanently re-identified with black or red electrical tape, or paint, at both ends to warn future electricians that it is carrying line voltage, not acting as a neutral.
