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# What is the formula?

## What is the formula?

#1
08-13-11, 07:21 AM
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Join Date: Sep 2006
Posts: 58
What is the formula?

Does anyone know the formula to determine what size wire to use based on the minimum circuit ampacity for an a.c.?

#2
08-13-11, 08:42 AM
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#3
08-13-11, 09:41 AM
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Join Date: Sep 2006
Posts: 58
yes, this is basic wire sizing. but if I install an air conditioner with a max breaker size of 30amps but the minimum circuit ampacity is under 20 from what I understand I can run 12 gauge. I'm not sure what the formula is for this calculation. But yes, in most situations 12ga for 20 amp, 10 ga for 30amp etc.

#4
08-13-11, 09:58 AM
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Join Date: Oct 2009
Location: Texas
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Copper prices can vary from one week to the next. You might monitor this to save some money.

Undersized wire can result in a drop in voltage when the unit runs. This can cause a higher amperage with inductive loads like a compressor.

Be safe.

#5
08-13-11, 10:22 AM
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Join Date: Mar 2006
Location: Wet side of Washington state.
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There is no formula, you need to take into account the full length of the cable from the supply (normally the Service panel) to the A/C unit. If this length is less than about 150 feet no calculation for voltage drop is necessary, just size the wire to the minimum circuit ampacity.

I suspect that where you are confused is that normally #14 wire is limited to 15 amperes, #12 limited to 20 amperes and #10 is limited to 30 amperes. While these are the correct maximum ampacities they are for general use and NOT specific branch circuits that are primarily a motor load. Because motors take a high inrush of current for starting it is acceptable to size the wire by the RUNNING current and size the fuse or circuit breaker for the starting current. That is why the minimum circuit ampacity wire size seems too low for the size of the fuse or circuit breaker.

If your minimum branch circuit ampacity is greater than 15 but less than 20 you would use #12 wire. If the minimum branch circuit ampacity required is between 20 and 30 you would use #10 wire. If the run from the power source is longer than about 150 feet you need to make a voltage drop calculation and follow that for wire sizing.