Ligne ou charge
Line is the side of a device that the supply comes in on; load is the side that goes out to whatever comes next. On a breaker, a contactor, a GFCI or a timer the terminals are marked, and the words tell you which way the power flows through the device.
Why the device cares
For a plain switch it hardly matters — current passes either way. For a GFCI it is the whole device: the monitoring electronics sit on the line side, and the load terminals protect everything downstream. Swap them and the receptacle can still light a tester while its own face goes unprotected. Contactors and overloads mark L1/L2/L3 in and T1/T2/T3 out for the same reason: the protection is built to look one way.
How to find which is which
Before anything is connected, the marking on the device decides it. In a box full of existing wires, prove it with a meter: turn the circuit off, separate the cables, turn it back on and measure — the cable that is live with everything apart is the line. Then off again before you touch anything.
One worked, so the idea sticks
Finding the line cable in a GFCI box
- Two cables are in the box. Power off; separate them so neither touches the other or the device.
- Power on. Measure hot to neutral on cable 1: 0 V. On cable 2: 120 V (a practice reading).
- Cable 2 is the line — it is live with nothing else connected. Cable 1 goes downstream: it lands on the load terminals.
- Power off, land them, power on, press Test and Reset, and check a downstream receptacle goes dead on Test.
Never land both cables on the line side to “make it work” — every receptacle downstream loses its protection.
Questions fréquentes
What happens if line and load are swapped on a GFCI?
Most newer GFCIs will not reset when wired backwards. Older ones may appear to work while leaving their own face unprotected. Either way it is wrong, and the fix is to swap the two cables.
Is “line” the same as “hot”?
No. Line and load are sides of a device; hot and neutral are the conductors. The line side of a GFCI has both a hot and a neutral terminal, and so does the load side.
Do breakers have a line and load side?
Yes: the bus is the line side and the terminal screw is the load side. Some breakers are marked for which way they may be fed, which matters when one is back-fed.
Voir aussi
- GFCI — and the swapped fault
- Contactor wiring
- Sous tension, neutre, mise à la terre
- Utiliser un multimètre
- Glossary: line vs load
- Back to the glossary
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