Chicago & Carter 3 ways?

[TheFinalBoss]

Replacing switches in an early 1900s house with knob-and-tube, I found two sets of 3-way switches (top and bottom of stairs) controlling a ceiling fan and a livingroom light. Oddly, traveler terminals were jumped together for each light's switches, and common terminals were also jumped. Wiring the new switches identically caused the breaker to trip. I'm familiar with illegal three-way methods (Chicago and Carter ways) but stumped by these jumpers across different loads. I'm a bit new to all this and Im kinda struggling to understand this crazy setup before we rewire to code. Even my journeyman is baffled. I'm very curious about the logic behind this unusual wiring.

 

[Megalodon]

Replacing switches in an early 1900s house with knob-and-tube, I found two sets of 3-way switches (top and bottom of stairs) controlling a ceiling fan and a livingroom light. Oddly, traveler terminals were jumped together for each light's switches, and common terminals were also jumped. Wiring the new switches identically caused the breaker to trip. I'm familiar with illegal three-way methods (Chicago and Carter ways) but stumped by these jumpers across different loads. I'm a bit new to all this and Im kinda struggling to understand this crazy setup before we rewire to code. Even my journeyman is baffled. I'm very curious about the logic behind this unusual wiring.

Could be that this uses old Carter system logic? I guess my tip is just to mark all the wires, kill power, then trace each one. You can't trust anything until it's verified. Best of luck to you!

 

[TheFinalBoss]

Could be that this uses old Carter system logic? I guess my tip is just to mark all the wires, kill power, then trace each one. You can't trust anything until it's verified. Best of luck to you!

Thanks for the tip, will tattoo this on my mind.

 

[Ground3dGuy]

That old wiring with jumpers? It's a trick to cut down on wires back in the day, but it's risky now and can trip modern breakers. Map everything out before you touch it and rewire it with proper 3-wire setups for safety.

Fixing the whole circuit or just patching this part?

 

[PGB1]

We encounter Carter & Chicago in early 1900's industrial facilities often.
Below is a quick sketch which may help explain how Carter works.

In the sketch, you will notice that, depending on switch position, the load either has two line going to it (lamp off), two natural going to it (lamp off) or one of each (lamp on)

The scary part its when someone rearranges the overcurrent devices in the load center or panel board and ends putting the line side for the switches on opposite phases. But, it does not last long because the fuse opens or the breaker trips.

Note that Chicago is essentially similar. And, for DC buildings, instead of switching line & neutral, it would switch positive and negative.

If you like old timey wiring stuff, check out a book by David Shapiro called "Old Electrical Wiring". Or pick up the oldest Audel's book on wiring that you can find in the vintage book store or on line. Lots of creative stuff!

Hopefully you will have sufficient conductors present for an easy upgrade to modernity!
Paul