A question for the electrician out there.
I’ve got a 12/3 gauge wire sharing a common ground across two 20 amp CBs. Black to one CB red to another, the CBs are independent (not tied together). Black goes to a different outlet than red.
I think this is a fire waiting to happen because the common is being expected to carry 40 amp. Am I correct?
Replies
"I've got a 12/3 gauge wire sharing a common ground across two 20 amp CBs. Black to one CB red to another, the CBs are independent (not tied together). Black goes to a different outlet than red.
I think this is a fire waiting to happen because the common is being expected to carry 40 amp. Am I correct?"
First of all the ground wire never carries any current except in a fault condition. And in a fault it can carry well more than 40 amps before the breaker trips.
But I think that you mean the Grounded Conductor, more commonly called the Neutral.
If the two breakers are installed on different legs, as they should be, of the 240 then the neutral only carries the difference current.
This is called a multiwire circuit.
It is OK as long as it properly installed. But it does offer opertunities to be missused such as connect to breakers on the same leg.
Yes, I am referring to the common (white) wire and not the ground (bare). The other two lines (red/blk) are on the same leg. So I guess I'll have to put one on the other leg.
Thanks.
>>The other two lines (red/blk) are on the same leg.
Are you sure?
In most (all?) load center boxes, the legs alternate as you go down each bank of breakers
Sort of like:
Leg1 .... Leg1
Leg2 .... Leg2
Leg1 .... Leg1
Leg2 .... Leg2
Etc.
NOTE: Corrected per IBEW's message #8
Edited 6/6/2004 8:39 am ET by Bob Walker
Bob,
Aren't you misleading him? Yes,as you go down the panel the breakers are on alternate legs,but breakers across from each other are on the same buss.
1-2
3-4
5-6
7-8
1 and 2 are black,3 and 4 red,5 and 6 black, 7 and 8 red.
Barry
Thanks for the correction; I don't know what I was thinking of when I posted that.
I've corrected the original message, as noted.
I see that while I was typing my response, others were posted. Make very sure you you know what you've got before switching anything around. Look at the hot bus bars before you change anything and see how they connect to the breakers. You can also check with a meter or a Wiggy connected from the screw on one breaker to the screw of the other. If the voltage is zero the breakers are on the same phase (and are unsafely connected). If the voltage is 240, they are on opposite phases and are connected correctly.
Thanks, everyone, for the clarification on why this should work, providing the CBs are on seperate legs. I have a much better understanding now.
Will verify that each CB is on a seperate leg the first thing tomorrow morning. Too late to do this tonight.
Well, everything is kosher. each CB is on a different leg. Thanks everyone for the explanations, advice and reasoning beind it.
Edited 6/7/2004 4:16 pm ET by Mike
Perhaps you mean the two circuits are sharing a common neutral? If the circuits are wired correctly, the red is attached to one breaker and is the hot for one circuit, the black is attached to the other breaker and is the hot for the other circuit. The white is the neutral, is attached to the neutral bus bar in the breaker panel and serves both circuits, and the bare wire is the ground, is connected to the ground bus in the panel, and also serves both circuits. This arrangement is common, and is safe so long as the two breakers are on opposite phases.
In your panel, you should have two hots and one neutral coming from the utility. One hot feeds one hot bus in the panel, connecting to half of the breakers, and the other hot feeds the other hot bus, feeding the other breakers. Generally panels are arranged so that the busses alternate, so that any two breakers installed adjacent to one another (that is, one immediately above the other if the breakers are stacked vertically) are on opposite phases. So, any two breakers that are adjacent can be wired in the manner I think you meant.
The reason this is fine, in a simplified way, is that the neutral from the power company is at the same potential as the ground, that is zero volts, and one hot is 120 volts "above" ground potential while the other is 120 volts "below". If you connect between either hot and the neutral you have 120 volts, which is how most circuits are set up. If you connect between the two hots, you have 240 volts difference, which is how you get 240 volts for, say, an electric water heater.
If you have two 20 amp breakers wired on opposite phases sharing a common neutral, and one is drawing 15 amps and the other is drawing 10 amps, the 15 amps and the 10 amps are sharing the common neutral, but are flowing in "opposite directions" and partially cancel, so the actual current through the neutral is 5 amps. If both are drawing the exact same amps, there is no current at all in the neutral.
If both circuits are (incorrectly) wired to the same phase, the currents don't subtract, they add, so 15 amps in each would produce 30 in the neutral, which would then be overloaded, resulting in the situation you fear.