Current on a 12 gauge wire

[3Ts]

We are building a home in the country and there are no building inspections or code requirements in this area. I've told the builder that we want to at least meet all the code requirements even though there will not be any official inspections. So, here's the question: There are (2) romex 12/3 wire with ground running thru conduit (with water pipes) to the kitchen island. Supposedly there are (2) single pole 20 amp breakers tied into these wires somehow. I suspect the black wire is tied to one breaker and the red is tied to the other breaker. The primary current loads are:
- Dishwasher - calls for it's own 15 amp breaker
- 2 gallon hot water heater under the sink - probably a 1500 watt heating element (~12 amps)
- small appliance outlets - about 1500 watts for an electric skillet or waffle iron.
1st, I don't see how (2) 20 amp circuits can carry that load. I talked to the electrician and he says "not to worry, it's fine". (I've seen 12 gauge wire on a 30 amp breaker around here and I know that is NOT "fine".) It seems to me that the electrician is using a single 12 gauge neutral to carry the current from (2) 20 amp hot wires and the electrician tells me that 12 gauge can do that. :shocked: Nor do I understand how he's going to provide for the 3 circuits with 2 circuit breakers. I suppose that if we sequence the usage "properly" everything will work.

Am I behind the times? or am I missing something? I'm getting ready to tell the builder to stop work until this is fixed, but don't want to do that unless I'm right. Any thoughts?

 

[buckeyefarmer]

Each circuit needs its own neutral.

 

[KennyG]

This is "Edison" circuit wiring with a shared neutral. A lot of people frown on this today but it is legal in most code jurisdictions. It is usually required that the two circuits have a "bonded" breaker so you can't trip only one circuit. This avoids have a neutral current on a tripped circuit. You don't have to worry about the load on neutral because the circuits are out of phase. If you have exactly equal loads on both circuits, they will balance and there will be no neutral current.

 

[ultrarunner]

Actually come across this all the time with older construction.... have not checked recently but the last time I did it was OK with the city as long as the circuit breakers were tied together.

You mentioned 30amp... by convention here 20 is the limit amperage wise.

It would make sense to add more circuits... what if a toaster oven or skillet gets plugged in in addition to the dishwasher, water heater, disposal???, etc...

 

[ray66v]

As mentioned it's a shared neutral circuit.

If it's wired properly, each hot is on a different leg of your electrical panel. If the breakers are side by side, they are on different legs of the panel.

As mentioned, both handles should be tied together. This was often not done with these. This prevents someone who is working on the wiring, from being shocked by the neutral wire, when working on one of the circuits.

With ac current, the neutral wire is only hot half the time, as the current cycles. If the second hot is on the other leg of the panel, the neutral wire can supply power the half of the time it's not being used by the other leg.

Done properly, this works fine, and is fairly common.

However, since you can't share the neutral on GFI, or arc-fault circuits, use of a shred neutral circuit is restricted today.

 

[Oldoak]

This is new to me. Apparently it is acceptable, but certainly not preferable, 'least not for me. And if the dishwasher calls for it's own 15 amp breaker, it should definitely not be on a 20 amp circuit.

Appears the electrician is going the cheapest route.

As mentioned, both handles should be tied together. This was often not done with these. This prevents someone who is working on the wiring, from being shocked by the neutral wire, when working on one of the circuits.

Wouldn't using a double pole 20 amp breaker give you the same protection? Then there wouldn't a need to tie two separate breaker handles together.

 

[ultrarunner]

A full size double pole 20 is ideal... as long as you are not using the space saver or half size breakers.

The key is each breaker cannot be on the same side hot leg of a 220v panel...

 

[3Ts]

Thanks. So it sounds like the electrician is running a 20 amp, 240 v circuit to the island and splitting the legs there. I'll verify that he's using a full size 2 pole breaker. However, There are still 3 things that draw 1500 watts (12.5 amps), so he's got to be putting 2 of them on the same leg for a total of 25 amps on a 20 amp leg. That's a 125% load and according to the circuit breaker trip curves http://http://static.schneider-electric.us/docs/Circuit%20Protection/Molded%20Case%20Circuit%20Breakers/0100-400%20A%20Frame%20FA-LA/FA-FC-FH/0600DB0105.pdf (page 2) , that is in the manufacturing tolerance window of 2 to 3 minutes up to 2 or 3 hours before it trips. It does not seem like a good idea to me, but I can now see why he says it will work - he's hoping one of those 1500 watt items will turn off before it trips the breaker.

 

[buckeyefarmer]

If he's running (2) of the 12/3 wires you are good number of circuit wise, but I wouldn't let them do it in my house.

 

[woody]

We are building a home in the country and there are no building inspections or code requirements in this area. I've told the builder that we want to at least meet all the code requirements even though there will not be any official inspections. So, here's the question: There are (2) romex 12/3 wire with ground running thru conduit (with water pipes) to the kitchen island. Supposedly there are (2) single pole 20 amp breakers tied into these wires somehow. I suspect the black wire is tied to one breaker and the red is tied to the other breaker. The primary current loads are:
- Dishwasher - calls for it's own 15 amp breaker
- 2 gallon hot water heater under the sink - probably a 1500 watt heating element (~12 amps)
- small appliance outlets - about 1500 watts for an electric skillet or waffle iron.
1st, I don't see how (2) 20 amp circuits can carry that load. I talked to the electrician and he says "not to worry, it's fine". (I've seen 12 gauge wire on a 30 amp breaker around here and I know that is NOT "fine".) It seems to me that the electrician is using a single 12 gauge neutral to carry the current from (2) 20 amp hot wires and the electrician tells me that 12 gauge can do that. :shocked: Nor do I understand how he's going to provide for the 3 circuits with 2 circuit breakers. I suppose that if we sequence the usage "properly" everything will work.

Am I behind the times? or am I missing something? I'm getting ready to tell the builder to stop work until this is fixed, but don't want to do that unless I'm right. Any thoughts?

you said there are two romex 12/3 wire, am I reading that wrong because if that means there are two cables with 12/3 w/ground wires he could possibly run four circuits or maybe I miss understood. but on the other hand if it is on a kitchen island it should be GFI circuits and not Edison anyway.

 

[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.