Multiwire amperage question

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sparkync

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I went on a service call today about a breaker tripping. Lady had a microwave and toaster oven that had tripped the breaker. I took the panel cover off and amped the circuit she thought it was. I had her to run the toaster oven and microwave so I could see how many amps it was pulling. It was pulling 14.3 amps on a 20 amp breaker. Obviously it wouldn't take much to trip it. Refrigerator and washer was on the circuit also. Again, it's obvious why the breaker would trip, but that's not my question. My question is, that when I turned the breaker off to check the terminal connections on the outlet, I found out the circuit I had amped, was not the right one even though I did see the amperage go down before I took my amp meter off the wire, after the microwave had timed out. When I found the breaker that did control it, it was the "red" line of a multiwire circuit (12/3). I know I had my meter on the black wire of the 12/3, the first time. After checking the red wire, it had the same amperage as the black wire had. I checked the black wire again, and had no amperage. I AM getting old and am retired, but I do know what I did. Is there any theoretical explanation for this, maybe a loose neutral in the circuit, or am I just
"loosing it"? She has been living there 25 years, and this is the only time she can remember that the breaker tripped.
For my "sanity sake", hope someone has an answer:)
Thanks
 
There is some missing info...
Did only one of those breakers trip, or both?
Are the red and black on the same leg or opposite?

I'm guessing you didn't force an overload to trip it and find out more?
 
sparkync said:
I went on a service call today about a breaker tripping. Lady had a microwave and toaster oven that had tripped the breaker. I took the panel cover off and amped the circuit she thought it was. I had her to run the toaster oven and microwave so I could see how many amps it was pulling. It was pulling 14.3 amps on a 20 amp breaker. Obviously it wouldn't take much to trip it. Refrigerator and washer was on the circuit also. Again, it's obvious why the breaker would trip, but that's not my question. My question is, that when I turned the breaker off to check the terminal connections on the outlet, I found out the circuit I had amped, was not the right one even though I did see the amperage go down before I took my amp meter off the wire, after the microwave had timed out. When I found the breaker that did control it, it was the "red" line of a multiwire circuit (12/3). I know I had my meter on the black wire of the 12/3, the first time. After checking the red wire, it had the same amperage as the black wire had. I checked the black wire again, and had no amperage. I AM getting old and am retired, but I do know what I did. Is there any theoretical explanation for this, maybe a loose neutral in the circuit, or am I just
"loosing it"? She has been living there 25 years, and this is the only time she can remember that the breaker tripped.
For my "sanity sake", hope someone has an answer:)
Thanks
Click to expand...
Those are both short time loads and they could total at least 25 amps without causing the breaker to trip.
 
It would help if you also look at the current on the neutral of the 12/3 as you make your measurements.
From what was said, it sounds like the red and black wires might be fed with the same phase and connected together dowstream, maybe at one of the outlets. But more info is needed. For example, a simple table could be made showing the currents on the R, B, W wires at the two breakers as you switch them between different on/off states.

To be really thorough, you could also put a clamp meter around all three of the wires (R, B, W) from the 12/3 to make verify that it always reads zero. This is to make sure that conductors from other circuits are not involved.
 
Can you verify that turning off ONE of those breakers actually turned off power to the microwave ?
 
Thanks for the replies. 1st of all, it was a house built probably in the 60's. I'm sure that was before the requirement for "tie handles" on multiwire circuits. No, both breakers did not trip together as far as I know, from what she said. Yes, I did verify the right breaker that controlled the oven and microwave. Yes, toaster oven, microwave, washer outlet, and refrigerator was on the same circuit. No, I did not amp the neutral circuit. I did take the reading with the toaster oven and microwave running. Refrigerator was on. Don't know if the compressor was kicked in or not. No, I did not run the wash machine while doing the test. She said it tripped while only the toaster oven and microwave was running. There was a shelf about 6" in front of the panel. I had to get on a step stool and bend over to get to the panel:( I don't see an explanation for my original question. Guess it will go into the file of "unexplainables" for me. I told her if it did it again, we'd have to run another circuit to the toaster oven. It is the only one that is feasible to get a wire to. I will have to run conduit from an outside panel then through the brick into the back of the outlet. The ceiling over the subpanel where the circuit is coming from, has no attic space, plus it is in her garage where the roof is slanted. If it happens again, I will try to amp the neutral while I'm there. Thanks for the replies.
 
You can turn one circuit into three running just one new cable. Run a 12-3 to the existing box, cap off the cable feeding power into that box. That leaves the original circuit feeding the upstream receptacles.

Supply the receptacle in this box with one hot, say the black wire, and connect the red to feed the remaining downstream receptacles. The white would be pigtailed to this and the downstream receptacles.
 
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