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I am upgrading a lot of light switches in my home to Z-Wave smart switches. It is easy when it is a simple switch, but a 3-way, 4-way or n-way switch circuit is trickier. When doing so, I need to know where the hot "line" wire is and where the most downstream "load" wire is.

I am able to accomplish this but it is time consuming. My current approach is as follows:

  1. Shut power off at the breaker for the circuit
  2. Pick a switch and disconnect it
  3. Turn power back on at the breaker
  4. Probe the wires at the disconnected switch. If I find a hot wire, then go flip a switch elsewhere in the circuit and probe again. If the hot wire remains hot, I have found the most upstream "line" wire. If not, I know the most upstream "line" wire isn't at this switch. [REPEAT FROM STEP 1 with a new switch]
  5. Once the "line" wire is identified, I can then use a similar technique to identify the wire that powers the lights.

This takes a really long time, especially when other people (and children) are in the house and you have to ensure nobody else is in danger of touching an exposed wire.

I can reliably and consistently identify the line & load wires using this method, but I am really hoping someone has a hack or shortcut to quickly accomplish this same goal.

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    In an n-way switch complex, there are only two places that always-hot can possibly be, and you can spot it instantly without turning on the power. It really helps if you understand n-way switches. Do you? – Harper - Reinstate Monica Dec 4 '17 at 3:21
  • Well, I have a moderate understanding of them. It is my understanding that the "hot" will need to be on a 3-way switch and not one of the 4-way or relay switches... is that what you mean? You are still left with determining which of the 3-way switches has the "hot" wire... does the same logic apply to the "load"? That is to say, only one of the 3-way switches can power the "load"? – dissolved Dec 4 '17 at 3:34
  • No, that's just the stuff you do know. There's more to it. Pause to round out your knowledge... particularly, learn all about messengers and how they are neither hot nor line nor load. – Harper - Reinstate Monica Dec 4 '17 at 3:35
  • Are messengers synonymous with travelers? – dissolved Dec 4 '17 at 3:38
  • Yes messengers synonymous with travelers. – Michael Karas Dec 4 '17 at 14:41
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When you addressing two 3-ways and one 4-way. Remember that one side of the common on the three way is the line side and the other common is the load side or switch leg. The two wires that connect between the 3-ways switches are carriers. If you have a 4-way it simply flips the carriers, two carriers in two carriers out. So, no there should never be a configuration where a line side "hot" leg will be connected to the 4-way. Although it may pass through the box.

Other than that the only other way to save time is to have a helper turn the breakers on and off to save you some steps, or employ a circuit tracer to trace out the circuits. These may be well and fine for someone who is doing a large number of circuits but usually isn't cost effective for a one off DIYer.

Good luck

  • Right, I understand that the 4-way switches won't have line or load, but only travelers. But I believe one of the 3-way switches can have both line & load, as shown here on the "Light Center" diagram. – dissolved Dec 4 '17 at 17:24
  • @dissolved Actually they don't, take time to trace out the wiring. A line and a load wire may be in the same junction box depending on where the line (source) conductor comes in and where it is going next, but the schematic for making a multiple 3-way and 4-ways work is always the same. Line coming in on one 3-way and load going out on the other. – Retired Master Electrician Dec 4 '17 at 17:38
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Every 3-way switch has two brass screws, and the two traveler wires always go on those. They also have one black screw whose name is common, and either your always-hot or your switched-hot wires will go on those.

Every 4-way screw has two brass screws for two travelers inward, and two black screws for two travelers outward. Every wire on a 4-way is a traveler.

This means in an entire 3/4-way complex of any size, every wire is a traveler, except two. Those are always at 3-ways, always on the ends, always on black screws, and are always either always-hot or switched-hot.


If you're fond of not losing your mind, I have a suggestion. I work in conduit with individual wires in my choice of color. I use 2 yellow wires for travelers*. Travelers are interchangeable, and there is no need to distingish them from each other, though it sure helps to distinguish them from other wires. Two identical odd-color wires running together are quite distinctive. If you're not in conduit, you are stuck with black white red for everything, but you can mark them with colored electrical tape. They sell 5-packs of colored electrical tape for $3 in a lot of stores. I'm fond of yellow for travelers, green** and blue for alt-travelers if two separate sets are nearby, red for switched-hot, black for always-hot and the law already requires neutral be white. (But white can be anything).

* If I have several 3-way circuits in close proximity, then I'll use purple, blue, etc. for the others, again in pairs, it's quite distinctive.)

** Small wires cannot be remarked with green tape to be a ground. So it changes nothing; the wire is still a hot.

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