4

If I feed a duplex receptacle from two separate branch circuits, I'm supposed to remove the jumper tabs between the receptacles. But what would happen if I didn't remove the tabs?

  • Why would someone want to do this in the first place? Would make a lot more sense to put in two side-by-side outlets -- tho' as an audiophile I'm not a fan of mixing branches in any one place due to the likelihood of phase mismatch & resultant line noise. – Carl Witthoft May 26 '16 at 18:53
  • @CarlWitthoft split receptacles allow you to use more things at once, without tripping the breaker. With a split receptacle, I can draw 15 amps from the top and bottom receptacle at the same time – Tester101 May 26 '16 at 19:02
7

With a 120/240V single split-phase system, there are two possible outcomes.

Separate legs

If the branch circuits feeding the device are on separate legs of the service, then the tab will be creating a direct short-circuit between the legs.

Service legs direct short-circuit

This will cause a high current through the circuits, which should trip one of the breakers fairly quickly.

Same leg

If the receptacle is fed by branch circuits on the same leg of the service, the danger is a bit more subtle. Initially everything works fine, no sparks, no tripped breakers. In fact, this situation can go unnoticed indefinitely, and never cause a problem.

The problem only occurs, if you try to draw more current than the individual branch circuit ratings. In the example below, the branch circuit protection for each circuit is 15 amperes.

Split receptacle same service leg.

If you put a load on the circuits, you'll find that the current will take all available paths. This means that the current will actually be divided between the circuits, so that each will see half the current that's flowing through the load. The yellow highlighting below, depicts the electricity's path through the circuits.

Electrical flow through the circuit

Still, there's no problem here. As long as the load does not draw more than the circuit rating, everything chugs along just fine.

The effects of this mistake only become evident, if the load draws more than the circuit rating. If the load in our example draws 30 amperes, you'll start to see the problem.

Load drawing over branch circuit rating

If you measure the current flow at various points along the circuit, the problem becomes obvious. Each branch circuit only sees 15 amperes, so neither breaker should trip. However, the receptacle will have to handle the full load current, which is well beyond the rated current. The receptacle will heat up and fail, potentially starting a fire as it does.

  • Just to be clear -- does breaking the tab in this case mean that each half of the receptacle will be able to draw only 15A and thus stay within its rating? Is the 15A rating on a receptacle for the total draw on both halves or for each half independently? – A. I. Breveleri May 26 '16 at 17:50
  • @A.I.Breveleri - Breaking the tab means that each half will be protected by exactly one 15 amp breaker. This way, an excess load will trip the breaker. – DoxyLover May 26 '16 at 17:54
  • @DoxyLover: Yes that's the obvious part. What I'm asking is whether the receptacle can be driven beyond its rating by loading each half to 15A, for a total of 30A (evenly divided between halves) for the entire fixture. – A. I. Breveleri May 26 '16 at 17:58
  • @A.I.Breveleri Yes, each half can handle 15 amperes, and the whole device can handle 15 amperes. – Tester101 May 26 '16 at 18:00
  • @A.I.Breveleri see this answer for more detail. – Tester101 May 26 '16 at 19:15
5

It's worse than tester101 says. Each circuit may not see half, there may be an imbalance of currents between the redundant paths. (Especially if one has a problem, such as being completely broken). Now how are the wires protected from overload? The hots have rather nice breakers on them, but the neutrals are not breaker protected! Nothing will detect an overload there.

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The opposite is an even bigger problem - suppose current flow does balance. One circuit has other outlets, and those are overloaded to 27A. They draw through this outlet, drawing a relatively balanced 14A from one breaker and 13A from the other. Whoops!

enter image description here

2

There are two main hazards.

Overloads can occur, as detailed in other answers. Neutral wires and the loads themselves can easily be overloaded, causing all kinds of problems - even a fire.

The other hazard is that turning off the breaker for a circuit will not have the intended / expected effect of de-energized the loads and receptacles on that circuit. This could be dangerous, and will be confusing, for the person trying to turn off that circuit to do maintenance or etc., or for situations where the breaker is used as a switch to turn things off when not in use.

  • 1
    Having on separate breakers same leg the hazard of being shocked because one is still on is probably the largest true hazard.+ – Ed Beal Oct 12 '18 at 13:13
0

If you add two live 15 amp lines to one outlet you will blow the breakers if not the main.

  • Not always -- if they're on the same leg, it won't trip any breakers, just create other problems (with parallel paths) – ThreePhaseEel Oct 11 '18 at 22:44
  • @ThreePhaseEel - I'm surprised nobody's mentioned that if they're on the same phase but different breakers, switching off just one of the breakers is now not enough to de-energize that circuit. – Damien_The_Unbeliever Oct 12 '18 at 5:37
  • This answer would be better than most for the simple reason this it what DIY folks do when replacing outlets on MWBC's all the other thrash about being on the same leg is just that the true current carrying ability of the wire is well over 200% greater than the code required OCPD. + – Ed Beal Oct 12 '18 at 12:21
  • Not only is that not exact, it would create safety issues (over-current and de-energizing). – Jeffrey Oct 12 '18 at 18:09

protected by ThreePhaseEel Oct 12 '18 at 11:40

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