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I stand corrected, 240v not 220v so I hope my question now makes sense.

I have 2 30amp breakers connected together. I believe this gives me 240v. I plan to connect this circuit to a 240v irrigation pump. It looks like the original wire from the main circuit is 12 gauge. The pump will be around 160' from the breaker. can I use 10 gauge wire from the back of the house to the pump or must I use 12 gauge? The distance from the back of the house to the pump is around 100' I would like to mention that the wire will go from the house to a relay box then to the pump. The relay is going to be connected to the irrigation timer and the water pump. We live in Melbourne, FL.

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  • Welcome to DIYSE. FYI, it's 120v and 240v. 220 is an obsolete half-century-old spec that no longer applies. Not entirely relevant, but it cleared up some confusion for me when I learned.
    – isherwood
    Jul 17, 2019 at 20:38
  • It was 110/220 when mains electric power was first sold to the masses in the 1910s.
    – Harper - Reinstate Monica
    Jul 17, 2019 at 21:43

2 Answers 2

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10 gauge is larger than 12 gauge. If you meant "can I use the smaller one coz it's cheaper", the answer to that is pretty universally no, but you don't need us to tell you that.

The distance might be a factor that might call for going with a larger wire (that would be #8 or #6). But if your pump is recommending the 30A breaker, its draw is certainly somewhat less due to the 125% derating requirement -- its draw is probably 19-23A. (still, too much for #12 wire). You don't need to care about voltage drop at the 30A breaker trip. You need to worry about it for actual working current of worst-case 23A. At that particular distance and #10 wire, your voltage drop will certainly be around 3%, which will even make the wire salesman happy.

Wire salesmen want you to be around 3% voltage drop on any wiring run. But Code doesn't require anything like that, so they're just saying it to push wire. Code gets concerned when drop gets around 8%. I bet this is a homerun, so all the voltage drop will be in this cable. This drop may be something like 3.17%, and voltage drop calculators online will say "that is over 3% therefore unacceptable" but that's silly.

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As the gauge number goes up, the size goes DOWN. You cannot use 12ga wire on a 30A breaker, 10ga is the minimum size. Whether or not you can use 10ga wire for something that is 160' away is another issue relating to what's called "voltage drop" and is caused by the natural resistance of the wire based on the actual load at the other end. How you combat voltage drop is to increase the wire size relative to the load. Since you did not post any information on the irrigation pump, we cannot help you further.

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