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Line vs. load wiring: what’s the difference? LINE is the incoming, energized side of a device, carrying power from the electrical panel or another upstream source; LOAD is the outgoing side, carrying power to downstream outlets or equipment. On GFCI and AFCI devices, the two terminal groups serve different functions and must not be interchanged.
The distinction is simple, but identifying the correct cable is a safety-critical task. The device markings, circuit tracing, suitable voltage testing, manufacturer instructions, and local requirements matter more than wire color or whether a terminal is located above or below another terminal.
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- LINE wiring brings power from the electrical panel or another upstream source to a device.
- LOAD wiring carries power from the device to downstream receptacles, equipment, or other wiring.
- On a GFCI or AFCI, mixing up LINE and LOAD can prevent the device from resetting and can defeat the intended downstream protection.
- Wire color, terminal position, and cable location alone do not reliably identify LINE and LOAD.
- Power must be turned off and the absence of voltage verified with suitable test equipment before conductors are handled.
- Manufacturer instructions and locally adopted electrical requirements control the permitted installation method.
What is the difference between line and load wiring?
Line vs. load wiring: what’s the difference? LINE is the incoming, energized side of a device, carrying power from the panel or another upstream source; LOAD is the outgoing side, carrying power to downstream outlets or equipment. On protective devices such as GFCI and AFCI receptacles, the two terminal groups serve different functions and must not be interchanged.
Leviton describes the distinction for GFCI devices this way: “The line wires bring power from your home’s electrical panel to the GFCI, while the load wires carry power from the GFCI to other outlets downstream.” Read the manufacturer’s GFCI explanation of LINE and LOAD for the device-specific context.
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| Term | Electrical direction | Typical connection | Function on a protective device |
|---|---|---|---|
| LINE | Source to device | Cable from the electrical panel or upstream source | Supplies the GFCI, AFCI, or other device |
| LOAD | Device to downstream wiring | Cable continuing to other receptacles or equipment | Can carry the device’s intended downstream protection when the installation supports it |
How does the source-to-device-to-load arrangement work?
The simplest model is source → device → downstream load. The electrical panel or another upstream source supplies the LINE side. The device receives that power, and the LOAD side supplies anything connected farther along the circuit.
Electrical panel or upstream source
|
LINE side
|
GFCI device
|
LOAD side
|
Downstream receptacles or equipmentThe diagram shows electrical direction, not a universal terminal layout. A particular product may place LINE terminals above, below, or beside LOAD terminals. Terminal position, conductor color, and the physical location of a cable in the box are not substitutes for the device markings, circuit tracing, and installation instructions.
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Which wires go to LINE and which go to LOAD?
The cable energized from the panel or upstream source belongs on the marked LINE terminals. A cable that continues to downstream receptacles or equipment belongs on the marked LOAD terminals when the device is designed to supply that downstream circuit.
On a GFCI receptacle, the incoming hot and neutral normally connect to the marked LINE terminals. A downstream cable may connect to the marked LOAD terminals when the installation is intended to extend the GFCI’s protection to downstream outlets or equipment. A LOAD terminal is not merely an extra connection point; the device and circuit must be correctly configured for feed-through protection.
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For an AFCI device, the same source-versus-downstream principle applies. Leviton’s AFCI installation guidance instructs the installer to identify LINE and LOAD, connect the incoming cable to LINE, connect the downstream cable to LOAD, and test the result.
Does the LOAD side protect downstream outlets?
The LOAD side can protect downstream outlets or equipment when a compatible GFCI or AFCI device is correctly installed and the downstream conductors are connected to the designated LOAD terminals. Connecting a cable to LOAD by itself does not guarantee protection; the product, circuit, wiring arrangement, and installation must all be correct.
For a GFCI, downstream outlets supplied through the device can receive the intended ground-fault protection. The device’s instructions determine which terminals provide that function and how the installation must be tested. Leviton’s GFCI product and support information explains the role of GFCI receptacles and their LINE and LOAD connections.
If downstream protection is not intended, the installation may use a different wiring arrangement. Do not copy a GFCI diagram onto an ordinary receptacle, AFCI, dual-function device, or circuit breaker without checking that product’s diagram and instructions.
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What happens if LINE and LOAD are reversed?
Reversing LINE and LOAD is not a harmless labeling mistake. A GFCI may fail to reset, may fail to supply power, or may not provide the intended protection to downstream wiring. Leviton specifically identifies LINE/LOAD reversal as a cause of GFCI reset problems in its official troubleshooting material.
An AFCI can show similar symptoms. Connecting the incoming LINE conductors to LOAD terminals can prevent an AFCI from resetting and can prevent it from supplying power. A protective device that appears installed but will not test or reset correctly should not be bypassed or left in service while the wiring is uncertain.
| Situation | Likely result | Correct response |
|---|---|---|
| Incoming cable connected to LINE and downstream cable connected to LOAD | Normal intended arrangement, if compatible with the device and circuit | Follow the product diagram and complete the required test |
| LINE and LOAD mixed up on a GFCI | Device may not reset or may not provide intended downstream protection | Turn off power, verify de-energization, and correct the wiring using the instructions |
| LINE and LOAD mixed up on an AFCI | Device may not reset or supply power | Stop troubleshooting while energized and use the manufacturer’s installation procedure |
| Unknown cable function | Terminal assignment cannot be safely assumed | Trace and test the circuit, or use a qualified electrician |
How do you identify the line wire?
Identify LINE by determining which cable is energized from the panel or upstream source, not by guessing from color or terminal position. In a replacement, the old device’s markings may provide a clue, but an earlier installation could be incorrect or undocumented, so the old connection should not be treated as conclusive proof.
- Turn off the circuit at the breaker or disconnect before opening the device box.
- Separate and secure conductors so they cannot accidentally contact one another or the box.
- Verify the absence of voltage with suitable test equipment and an appropriate procedure for the circuit and task.
- Determine which cable is connected to the source and which cable continues downstream.
- Match each cable to the device’s clearly marked LINE or LOAD terminals and follow the supplied wiring diagram.
- Restore power only when the installation is complete, then perform the manufacturer-required test.
A non-contact voltage tester can be useful as a preliminary screening tool, but a single non-contact test is not a complete electrical-safety verification for every circuit or task. OSHA guidance emphasizes de-energizing equipment and verifying that voltage is absent with suitable test equipment; OSHA’s electrical de-energization and verification directive provides the safety framework. Use equipment and procedures appropriate to the circuit, and do not rely only on a breaker handle, wire color, or non-contact tester.
Do line and load wire colors matter?
Wire colors can help identify conductor roles, but wire color alone does not establish whether a cable is LINE or LOAD. LINE and LOAD describe the cable’s electrical function in relation to the device. A black conductor is not automatically LINE, and the top terminals are not automatically LINE.
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Correct identification requires the device labels, the circuit path, testing, and the manufacturer’s instructions. Conductor insulation color also does not eliminate other concerns such as shared neutrals, altered wiring, incompatible conductors, or a circuit that is supplied from more than one source.
What is the difference between ordinary receptacle, GFCI, and AFCI wiring?
The LINE/LOAD distinction is most consequential on protective devices, although the exact wiring rules depend on the product and circuit.
| Device | How LINE and LOAD apply | What requires special care |
|---|---|---|
| Ordinary receptacle | A downstream cable may continue the branch circuit through the receptacle | Follow the receptacle’s terminal markings and instructions; do not assume a GFCI arrangement applies |
| GFCI receptacle | LINE supplies the GFCI; LOAD can supply downstream wiring intended to receive GFCI protection | Reversal can prevent reset and defeat the intended downstream protection arrangement |
| AFCI device | LINE supplies the AFCI; LOAD connects to downstream conductors where the product specifies | Reversal can prevent reset or prevent the device from supplying power |
| Dual-function or breaker-based device | Terminal functions are product-specific | Use the exact breaker or device diagram and installation instructions |
When should you stop and call an electrician?
Stop work and use a qualified electrician when the circuit path or device compatibility is unclear. A documented, straightforward replacement is different from an installation involving unknown or altered wiring.
- More than one circuit or possible source is present in the box.
- The circuit has a shared neutral, unusual voltage, or an arrangement you cannot identify.
- The conductors are aluminum, damaged, overheated, too short, or otherwise incompatible with the device.
- The box is damaged, overcrowded, lacks required grounding provisions, or cannot safely contain the connections.
- The protective device will not reset or test correctly after the wiring is verified.
- You cannot positively identify the source cable and downstream cable.
- You do not have suitable test equipment or the training to verify de-energization.
A GFCI receptacle with LINE and LOAD terminals may be an appropriate replacement product only when its rating, wiring method, conductor compatibility, and instructions match the circuit. A product description does not replace the supplied instructions, local requirements, or professional assessment.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How do local electrical codes affect LINE and LOAD wiring?
Local code, the adopted NEC edition, manufacturer instructions, and inspection requirements may affect the permitted wiring method. In the United States, NFPA 70—the National Electrical Code—is a principal electrical installation standard, but the NEC is not automatically a nationwide law in identical form.
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The Electrical Safety Foundation International’s explanation of the NEC notes that state and local jurisdictions adopt and enforce electrical requirements. The applicable rules therefore depend on geography, the adopted edition, local amendments, the building type, and the specific installation. Do not rely on a generic online diagram when inspection or permitting requirements apply.
A practical decision checklist
- Is the device marked LINE and LOAD? Treat those markings as controlling.
- Which cable comes from the source? That cable is the LINE cable.
- Which cable continues downstream? That cable is the LOAD cable when the device is intended to feed downstream wiring.
- Is downstream protection intended? Confirm that the device and wiring diagram support it.
- Has power been verified absent? Turn off the circuit and use suitable test equipment before touching conductors.
- Is anything unusual? Stop for shared neutrals, multiple circuits, aluminum wiring, damaged boxes, unknown paths, or unclear voltage.
- Has the device been tested? Complete the manufacturer’s test procedure before considering the installation complete.
Frequently Asked Questions
What is the difference between line and load wires?
LINE wiring is the incoming cable from the electrical panel or another upstream source. LOAD wiring is the cable leaving the device to supply downstream receptacles, equipment, or other wiring. On a GFCI or AFCI, the source cable normally connects to LINE and the downstream cable connects to LOAD when the device instructions specify that arrangement.
How do I identify the line wire?
The line wire is identified by tracing and testing the cable connected to the upstream source, not by assuming that a black conductor or top terminal is LINE. Turn off the circuit, verify the absence of voltage with suitable test equipment, and follow the device instructions; use a qualified electrician if the circuit path is unclear.
What happens if LINE and LOAD are reversed?
Reversing LINE and LOAD on a GFCI or AFCI can prevent the device from resetting or supplying power and can defeat the intended protection for downstream wiring. Turn off power, verify de-energization, and correct the connections according to the exact manufacturer wiring diagram.
Does the LOAD side protect downstream outlets?
A LOAD connection can extend a compatible GFCI or AFCI device’s intended protection to downstream outlets or equipment when the device, circuit, and installation are correct. A cable connected to LOAD does not guarantee protection by itself, so the manufacturer’s instructions and required test must be followed.
The Bottom Line
LINE is the incoming power side; LOAD is the downstream side. On GFCI and AFCI devices, connect the source cable to LINE and the downstream cable to LOAD only when the product instructions call for that arrangement. Reversed connections can prevent reset and defeat intended protection. Turn off power, verify the absence of voltage, follow the exact device diagram, and use a qualified electrician when the circuit is unclear.
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