sparkytools

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Conducteur commuté

A switched hot is the hot after it has been through a switch: live when the switch is on, dead when it is off. In a lighting circuit it is the conductor that runs from the switch to the fixture. It does the same job as the hot; it is just under control.

Why the switch breaks the hot

A switch always goes in the hot, never the neutral. With the hot broken, everything past the switch — the socket, the fixture’s wiring — is at no voltage to ground when the light is off. Put the switch in the neutral instead and the light still turns off, but the socket stays live the whole time, waiting for someone to change a bulb.

In a switch loop

When the power reaches the light first, a two-conductor cable goes down to the switch and back: one conductor carries the hot down, the other brings the switched hot back up. There is no neutral in the switch box at all. How the conductor used for the hot is re-identified is a code rule — the idea is simply that the loop carries hot down and switched hot back.

One worked, so the idea sticks

Reading it with a meter

  1. A single-pole switch in a box, cover off, circuit on. Meter to a known ground.
  2. Line terminal: 120 V to ground with the switch on or off (a practice reading). That is the hot.
  3. Other terminal: 120 V with the switch on, 0 V with it off. That is the switched hot.
  4. If neither terminal ever reads 0 V to ground, the switch is breaking something other than the hot — find out why before you close it up.

Meter safety first: rated meter, proven on a known source before and after.

Questions fréquentes

Is the switched hot the same colour as the hot?

Often, but not always — and a colour is never proof. Measure it. The colour rules for switched conductors are in your code book.

Does a switched hot need a neutral with it?

The fixture needs a neutral to complete the loop, and many smart switches need one in the switch box too. A plain switch does not — it only breaks the hot.

What is a switched receptacle?

A receptacle, or one half of a split receptacle, fed from a switched hot, so a wall switch turns a lamp plugged into it on and off.

Voir aussi

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