UA 1st year, plumbing, so this is pure curiosity. The electrician on our job (commercial bathrooms in a building in Montclair, long corridor outside them) was explaining the corridor lights at lunch. 3-way at each end, I followed that from the other thread on here. Then he said there's a 4-way in the middle and it "flips the travelers" and he lost me. Flips them how? Why can't you just put a third 3-way in the middle?
Electrical Contractor · 22 years · Monmouth County, NJ
Mentor✓ Accepted answer
Honestly it's simpler than it sounds. Both travelers come into the 4-way, and it either passes them straight through or crosses them over, so whichever way the other switches sit, flipping it changes which traveler is live at the far end. A 3-way can't do that swap, which is why the middle one has to be a 4-way. Draw it: two 3-ways on the ends, the 4-way in the middle, and trace it with your finger in every position.
Electrical inspector (former contractor) · 26 years · Ocean County, NJ
Mentor
One thing to add for when you see these in the field: check the 4-way's instructions for which screws are the pair in and which are the pair out. They aren't always laid out the same, and mixing them up gives you exactly the "works sometimes" complaint.