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I want to install a wall mount mini-split a/c (115v20amp). The panel is 50+ feet away and routing a new (12/2) line will be extremely difficult. However, when the house was built an extra 14/3 line was installed for future basement finishing.

Can I use it with a 20amp breaker if I tie line 1 (red) and 2 (black) together, and use it as 12/2? ... or is this just wishful thinking!

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That's "paralleling". You can't do it.

Using multiple wires to a single load is never permitted, except in industrial applications with VERY large wires, AND with special equipment on the supply side which resolves the safety issues therein, AND with very special and picayune installation requirements. It is totally infeasible in a residence.

Also, you forgot all about neutral

This may be news, but neutral is also a wire. Neutral can't carry any more current than hot. So in your scheme, your solitary #14 neutral will carry 20A, which is no-go obviously.

There's one more thing about neutral. You notice it doesn't have a circuit breaker. The only thing that prevents neutral from overloading is monogamy - neutrals only return current from their partner hot. Thus, the breaker on their partner hot protects the neutral. What if a neutral is promiscuous and returns current from two hots? It overloads, and the breakers will do nothing to prevent this!

But wait. I'm sure I've heard of a way to get more power out of /3!

That's correct. That is where you put 240V across the two hots (red and black), so they are on opposite poles. In that case you can attach 120V loads between a hot and neutral, and neutral only handles the difference in current - all other current flows from hot1 to hot2. Hot1 can supply 15A of 120V loads, and Hot2 can supply 15A of 120V loads. **But no more than 15A of 120V, ** so this does not help this here situation at all.

But you can also ignore the neutral, and use red and black to supply 240V at 15A to an appliance. It has to be a 240V appliance, obviously. That is your solution here. Tell your supplier you need the unit in 240V, where it will only draw 10A. That will easily be supplied by your 14/3.

  • Notice: If you get a 240v unit make sure the red & black wires go to a double breaker with the two handles tied together. This would have 120v from red to neutral, 120v from black to neutral. With red and black on different phases in the breaker panel this gives 240v between them. If you don't have 240v between these two hot wires you'll need something different. – Eric Simpson Jul 2 at 2:24
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No

You can't combine wires to get more capacity. If one of the wires broke or disconnected you would end up with a problem that would not be protected by the breaker properly.

However, even if that were allowed, which it isn't, you have another problem: the neutral wire needs to carry 20A as well, and you don't have a "spare" there.

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