I have a circuit with 2 15 amp breakers. They don’t act, individually they both have to be shut off to shut off the circuits. Am I looking for an outlet with two devices? Example switch and outlet? The breakers used to break individually, but something happened in the circuit to join the breaker, I have replaced the breaker but the problem is still there. I think it maybe somewhere in the house joining the two circuits, but how do I troubleshoot it. I flipped the two circuits and found all the outlets and lights that are on those circuits, but I thought one outlet might be common to both circuits and shorting them together. It is a house in the state of Washington.
Assuming this is in the USA, 120V system, there are generally two possibilities:
- 240V circuit
A dryer, range, air conditioner or other large appliance will use a 240V circuit, which will be implemented in the breaker panel as a pair of breakers - one on each "leg". However, that is normally for 30A or more. I can't think of any reasons to bother with 240V that wouldn't also use more power. So 15A x 2 is much more likely to be...
- MultiWire Branch Circuit MWBC
This uses two breakers, each controlling a single hot wire and sharing a neutral. The breakers need to be tied together for common shutoff. Because US wiring does not normally have breakers on the neutral wire, without common shutoff you could turn off the breaker on one circuit in order to work on it but if there is any usage of the second circuit then there would be current flowing through the shared neutral wire, which would be a major safety hazard.
Where are MWBCs?
They could be anywhere. A typical use is to have one hot power the top receptacle and the other hot power the bottom receptacle in a series of duplex receptacles. But really they could be anywhere (including receptacles, lights, etc.) and, from the perspective of the person using the receptacle (plugging in a toaster or radio or vacuum cleaner or whatever) it looks and functions no differently than an ordinary circuit.
You're talking about two separate functions: common trip and common maintenance shutoff.
Mains power has a neutral wire, and one or more "hot" poles. Most outlets and lights tap between neutral and a pole. Some high-power devices like tap pole to pole because they work better on higher voltage (to be more precise, less current so lower voltage drop.) Some devices use multiple poles and neutral also. As it works out, with up to 3 poles, the neutral will not carry more than the busiest pole.
Our bailiwick in the DIY stack is residential. North American power always provides two poles, arranged opposite (180 degrees) so their voltages add up. Europe/Asia usually supplies 1 pole but can supply 2 or all 3 if the need is there.
When a device takes 2 or more poles, both hots have common maintenance shutoff, and here's why: even if the multi-pole circuit serves all line-neutral loads, a maintainer might plug a radio into one outlet and flip off breakers until the radio stops, and deem the circuit safe. Then get nailed by the neutral or the other hot.
In some cases common trip is also required for safety. Common trip mechanisms are not the obvious handle-tie; they use internal mechanisms within the breaker. Accessory "Handle Ties" can be obtained that will allow ganging individual breakers for common maintenance shutoff. These do not guarantee common trip.
When you see a 2-pole breaker, you are dealing with (in North America)
- a 240V-only appliance like an air conditioner or water heater;
- a 240V/120V appliance like a dryer or range/oven (120V for the oven light)
- a "multi-wire branch circuit" where all the loads are hot-neutral, they just share neutral to save wire.
What is the circuit? If you have for example a power system where you derive your voltage across two legs and not a leg to neutral it could be the case where you fuse both legs to ensure no current flows in the event of a fault. This example im providing is common practice on ships since the power is a delta system and is derived across two legs of the transformer. However in such a system there isnt any neutral but the power inductively couples the ships hull. In this case it was common to read 60VAC from any leg of the transformer to the hull of the ship. So if there was a fault and only one leg popped, there is a chance someone can get electrocuted from the low leg of the device being powered, hence the need to provide a breaker on both legs. Hope this helps!