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05-06-2011, 08:20 AM   #1
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2 hots 1 neutral ?

In a recent post I learned you could not use a tandem 20A breaker for 10-3 UF because the one neutral wire would be overloaded. I am confused about this. They said the problem would be eliminated if I used 2 single 20A breakers which would split the hots over 2 seperate poles. I understand that part but there still is just one neutral wire. Would I pigtail this to two seperate neutral bars? Please help me understand the what to do and why of this issue at the 200A house panel and the small panel I will use at the shed. Thank you.

05-06-2011, 08:32 AM   #2
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When you split the hot across the two legs the neutral only caries the difference in current. Example one hot pulls 10 amps and the other pulls 5 amps the neutral only carries 5 amps.
If both hots are on the same leg, which they are with tandems, the neutral carries the sum of the currents. In the above example the neutral would be carrying 15 amps.

 05-06-2011, 08:34 AM #3 Lic Elect/Inspector/CPO   Join Date: Apr 2011 Location: NJ Posts: 369 Rewards Points: 250 The service has two conductors and bus bars are 180 degrees out of “phase” and can be understood by visualizing two waves running together where the trough of one wave is opposed by the crest of the other wave. Thus these waves are able to travel on the same wire without creating any problem (as in the top half of the picture below). How the waves of a muti-wire circuit work Here is where the trouble can start—when the two circuits share the SAME bus bars (as in the bottom half of the picture above). The neutral wire can now be required to carry the possible full load of both circuits on the same wave length—-30 amps—-and the wire is only rated for 15 amps. Insulation starts to melt from overheating and the risk of an electrical fire increases when this happens. To make it easier to wire breakers in the electrical panel the bus bars are configured like fingers such that every other breaker space is a different bus bar. Most electrical panels have breakers that increase the capacity of the panel by having two breakers in each of these spaces—-this is usually where the mix-up occurs and the two hot conductors end up on the same
 05-06-2011, 10:02 AM #4 Member   Join Date: Dec 2010 Location: Pass Christian, MS Posts: 66 Rewards Points: 75 Thanks. I understand now. Can I use the tandem out at the shop panel (I plan on purchasing a small 4 or 6 space box)? I only plan on needing 12 amps (fridge and lights) on one circuit and 15 amps (small tool or small AC) on the other.
05-06-2011, 12:39 PM   #5
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Quote:
 Originally Posted by Joe Willie Thanks. I understand now. Can I use the tandem out at the shop panel (I plan on purchasing a small 4 or 6 space box)? I only plan on needing 12 amps (fridge and lights) on one circuit and 15 amps (small tool or small AC) on the other.
It isn't the wire size, it is how the wiring is connected. You can use tandem breakers as long as each circuit leaving the panel has only a hot and a neutral.

 05-06-2011, 12:43 PM #6 Member   Join Date: Dec 2010 Location: Pass Christian, MS Posts: 66 Rewards Points: 75 Thanks.
 05-06-2011, 04:16 PM #7 walt1122   Join Date: Aug 2007 Posts: 293 Rewards Points: 334 instead of the tandem can't he just use a double throw breaker and insure that there are two seperate hot legs. If this is to supply the shed? shouldn't this be treated as a sub panel? How far away is the shed? What will it power? thanks Walt
 05-06-2011, 05:50 PM #8 Member   Join Date: Nov 2007 Location: Nashua, NH, USA Posts: 7,906 Rewards Points: 1,418 Since you are running a 10-3 cable, you can use a double 30 amp breaker at the source (either a main panel or a subpanel) but you will need at subpanel at the destination end (garage, shed) If you feed the cable with a 15 or 20 amp breaker, you don't need a subpanel at the other end; just a box with receptacles will suffice. If you already bought the tandem breaker (single wide, occupies one slot), you can use it in a subpanel at the destination end of your 10-3 cable provided that the breaker is compatible with the make and model of subpanel. It would feed circuits wired with 12-2 cable. You could also use it back at the main panel for other circuits wired with 12-2 cable, in order to free up a slot in the panel for something else. Usually there is just one neutral bus (bar) in a panel and the neutral from the 10-3 cable goes there and ithere is no problem connecting the red and black wires to the double wide (2-slot) double breaker. If there are two neutral busses (not ground busses) they are interconnected and you connect the neutral wire to just one, usually the closest. __________________ The good conscientious technician or serviceperson will carry extra oils and lubricants in case the new pump did not come with oil or the oil was accidentally spilled, so the service call can be completed without an extra visit. Last edited by AllanJ; 05-06-2011 at 06:00 PM.
05-06-2011, 06:13 PM   #9
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Quote:
 Originally Posted by AllanJ Since you are running a 10-3 cable, you can use a double 30 amp breaker at the source (either a main panel or a subpanel) but you will need at subpanel at the destination end (garage, shed) If you feed the cable with a 15 or 20 amp breaker, you don't need a subpanel at the other end; just a box with receptacles will suffice.
He will also need a disconnect for this MWBC, a pair of single pole switches will do.
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05-06-2011, 10:13 PM   #10
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Quote:
 Originally Posted by rjniles He will also need a disconnect for this MWBC, a pair of single pole switches will do.
Not all area will approve it and I know few spot in Wisconsin I have dealt with it before they will required double pole switch for MWBC.

Merci,
Marc

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