Can I use a 14-3 Romex for two different electrical circuits on a 15A QT/tandem breaker? Or should I use one 14-2 for each?

My mistake, apologize.

Post edited: the QT breaker is a space saver. My panel is 30 space 40 circuits. So I intend to have two different circuits on one space. I will use a 1 pole Siemens Q1515 with no clip. Here is the link Tandem breaker

So turning off individual circuit.

  • Yeah, we call those "double-stuff" breakers because it's cramming two singles into one space. They are indeed 1 pole, and it's the same pole for both sides, which means use of MWBC (14/3) would be a complete disaster. – Harper - Reinstate Monica Feb 22 '19 at 23:25
  • Thanks Harper. It is clarified now, see my below comment about....electrical terminology......"double-stuff" .....that's some stuff :). So, using let's say the "double-stuff" as a Q1520: the 15 side will have 14-2 and the 20 side will have either another 14-2 or 12-2. Correct? – cadobe Feb 23 '19 at 1:09
  • Yeah, if that existed. I'm not used to seeing tandems with different values on each side , but nothing prevents them from existing. – Harper - Reinstate Monica Feb 23 '19 at 1:37
  • Here is the one.....superbreakers.net/tandem-circuit-breakers/… – cadobe Feb 23 '19 at 14:22
  • Yup, that's one of them. Very cool. The 20 side must have 12-2 wire; you cannot put #14 on a 20A breaker, it is too small and will overheat. – Harper - Reinstate Monica Feb 23 '19 at 14:29

No No No!

First, that trick with the 14/3 wire and shared neutrals, is actually one circuit. It is called a multi-wire branch circuit or MWBC.

MWBCs have a number of special rules. Top of the hit parade is you can't use a Tandem or double-stuff breaker!

  • The two hots must have 240V voltage between them (be on opposite poles). If so, the neutral wire will carry the difference of currents. (if one is 11A and the other is 13A, the neutral carries 2A). A double-stuff/duplex has both circuits on the same pole, and the neutral current would stack (24A) instead of cancel, overloading the neutral.

  • Common maintenance shutoff -- the handles must be tied; shutting off one subcircuit must also shut the other one off. Either by using a 2-pole 240V breaker (same as you would for a dryer) or a factory approved handle tie between two single breakers. They don't make handle ties for double-stuffs, so if you respect this rule, the above rule takes care of itself.

  • Must pigtail neutrals -- you can't have it where removing, say, a receptacle interrupts neutral for the other half-circuit.

These types of circuits have fallen out of favor because they make life very complicated when you bring GFCI or AFCI into the picture.

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With two adjacent regular breakers, you can share the neutral, a multiwire branch circuit. Adjacent breakers are on different legs, so the neutral only carries the difference in current between the two hots due to a cancellation effect.

With a tandem breaker, both breakers are on the same leg / line, there is no cancellation effect on the neutral, so you can't share the neutral - you'd be doubling the load on the neutral conductor!

So yes you must use two 14/2 cables rather than a single 14/3.

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  • That make sense. The think I do not understand is "tandem". Tandem means together right? So where is the difference between the two breakers? lowes.com/pd/Siemens-Qt-15-Amp-2-Pole-Tandem-Circuit-Breaker/… and lowes.com/pd/Siemens-Qt-15-Amp-2-Pole-Tandem-Circuit-Breaker/… besides the price? The last one is labeled as Q1515NC which means no clip. Visualizing outside, there is no clip in between the switches tying them together. But both are the same. Any clue on this? – cadobe Feb 22 '19 at 2:22
  • The "no-clip" breaker is made for panels made before 1967. After 1968, CTL panels came out, which will only accept a tandem in certain positions. They started making them this way to prevent you from putting too many tandems in a panel. – batsplatsterson Feb 22 '19 at 9:05
  • Got it. Thanks a lot for clarifications. It is confusing at a first glance and for someone not being familiar with electrical terminology, special if you Google it, there are so many trades talking different. Anyhow, NC stands for no clip. In other hand, the tandem ones can have or cannot have the common trip, am I right? (I found that the NC stands for no common trip also) – cadobe Feb 22 '19 at 19:31
  • @cabode - I don't think I've seen any tandem breakers with common trip, and I can't think of reason to use. I know what you mean, I find the electrical manufacturers web sites and reference materials surprisingly difficult to wade through. – batsplatsterson Feb 22 '19 at 22:29

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