Current on a 12 gauge wire

[GregbkH]

Less chance for problems if every circuit has it's own neutral. No shared neutral circuits. BTW, almost all circuits are designed to run at 80% max current. 15 amp breaker = 12 amps, 20 amp breaker = 16 amps.

 

[Deere Dude]

Less chance for problems if every circuit has it's own neutral. No shared neutral circuits. BTW, almost all circuits are designed to run at 80% max current. 15 amp breaker = 12 amps, 20 amp breaker = 16 amps.

I wouldn't let them do it in my house either. Down the road you are going to need to decipher this system for an addition or a change. I like to keep it simple and not something that needs a crack electrician to figure out and legitimize. Remember KISS. Keep it stupid simple.

 

[LD1]

Im confused as well. Like others said, TWO 12/3 wires, using a MWBC (multi-wire-branch-circuit) is a total of 4 circuits. And 4 single pole 20a breakers (or two double pole). If this is indeed the case, you should have plenty of power for the appliances, just dont put two of the 15a appliances on the same circuit. Dishwasher on one circuit, Water heater on another, outlet on a 3rd, and a spare (maybe a second outlet).

How far is the run? You never mentioned this. But 12ga may be too small for the amperage if the run is too long.

And you CAN use a GFI recep. on a MWBC. Its done all the time. Good way to save wire if one knows what they are doing.

Really, the only issue I have with what you describe is running romex in conduit. Thats against code as romex is not supposed to be in conduit.

 

[mbohuntr]

I wouldn't do it with 2 heavy load circuits, I've seen lights use it, but those aren't lights... Shared neutral circuits pose other hazards as well for the person working on them..

 

[KYErik]

I've never heard of this but it does make sense. So, with a standard, double breaker (that would normally be used to feed a 220v appliance like a clothes dryer). If it is used in this manner (two 110 volt circuits) and just one of the breakers trips because of an overload, it will have enough "force" to flip the other breaker switch/open the other circuit?

 

[woody]

I've never heard of this but it does make sense. So, with a standard, double breaker (that would normally be used to feed a 220v appliance like a clothes dryer). If it is used in this manner (two 110 volt circuits) and just one of the breakers trips because of an overload, it will have enough "force" to flip the other breaker switch/open the other circuit?

That's the idea, you are correct

 

[LD1]

I wouldn't do it with 2 heavy load circuits, I've seen lights use it, but those aren't lights... Shared neutral circuits pose other hazards as well for the person working on them..

Whats it matter if the circuit is heavily loaded or not? Electricity is hazardous if you dont know what you are doing. Thus why only qualified people should mess with it. A competent electrician would have no issues with a MWBC. ITs basically the same way the power enters your house. Two lines at 120v each (240v line to line), with a shared neutral. Thats all thats being done on a MWBC. The only "hazard" is you can have the potential between two of the lines to be 240v. So if you take a switch or recep. out of a workbox and touch the wrong two wires, you have a 240v shock instead of a 120v shock. But again, no different than working in the breaker panel.

I've never heard of this but it does make sense. So, with a standard, double breaker (that would normally be used to feed a 220v appliance like a clothes dryer). If it is used in this manner (two 110 volt circuits) and just one of the breakers trips because of an overload, it will have enough "force" to flip the other breaker switch/open the other circuit?

Thats correct. But single pole breakers usually have a hole in the switch where a pin can be inserted and accomplish the same thing. Some also use a plastic thing that clips over the switches.

If the MWBC is wired properly, there is no risk to the circuit or appliances of only one leg trips. The risk is to the person working on said circuit. IF they dont realize its a MWBC, and assume the circuit is de-energized because the breaker is off, they can be shocked by the neurtal because the other "leg" of the MWBC is still hot.

 

[downsizingnow48]

Curious how much money such a circuit might save. That's the reason for doing it am I right? Or is there improved function of some kind? Don't see why a KISS circuit couldn't be used why the complexity.

 

[aczlan]

Curious how much money such a circuit might save. That's the reason for doing it am I right? Or is there improved function of some kind? Don't see why a KISS circuit couldn't be used why the complexity.

It lets you easily put two circuits in a box, also one 12/3 is cheaper than two 12/2 wires.
It's also common in conduit work where you run separate cables as you only have to pull 4 wires, not 5 or 6.

Aaron Z

 

[ultrarunner]

Wire Fill is less so no need to up size conduit, material cost is less, less fill in the breaker box, less neutral on the neutral buss, fewer knockout in the panel box.

It really is quite common here...BUT the individual breakers must be full size and tied together.

The down side is a faulty appliance will cause a otherwise good circuit to trip since breakers are tied together.

This happened when a Dishwasher and Disposal were wired this way... disposal had a problem that also tripped the circuit to the refrigerator... homeowner simply thought the disposal was not working and not a clue power was cut off to the refrigerator.

At the home my brother bought... the panel was full and someone had installed a space saver tandem HALF SIZE breaker so both circuits flowed back on the same neutral AT THE SAME TIME.