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So Ive been seeing a lot of this lately. 2 wires on a three pole breaker. Whether they used phases a,b or a,c how big of a deal is this really? Im getting tired of finding a two pole breaker to change them.
 

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It is perfectly compliant
 

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The only problem is panel space and cost. There is no requirement that all of the poles of a breaker be used and each pole has its own trip unit so they will work fine.
 

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It's done a lot around here. We're not about to buy, let alone stock, 2 pole I-line breakers.
 

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Actually... you have to read the fine print in some cases.

"World Class" (the euphemism for "made for the Europeans") breakers designed to IEC standards will sometimes require that you "loop back" one pole through the un-used pole so that current is flowing in all 3. IEC designed 3 pole breakers will often have a "differential trip" design to their bi-metal thermal sensors that will bias the trip threshold in the event of a phase loss. This is because they are allowed to use the breakers as 3 phase motor protection devices by adjusting the bi-metal trips just like an overload relay. So if you do not loop back to have current flowing though all 3 legs, the breaker will trip on LOWER current than if not.

By "loop back" I mean that Line 1 goes to terminal L1, out of terminal T1 to the load just like normal. But Line 2 goes to terminal L2, then T2 loops back to terminal L3, and the Line 2 load circuit connects to terminal T3.

You are not likely to see these breakers in load centers and panel boards any time soon, so don't panic. But for stand alone breakers, it's becoming very common.
 
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