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Old 01-02-2011, 11:17 AM   #1
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Low Voltage calculations

I want to double check my calculations for low voltage lighting. I Have a 15 amp circuit, with some outdoor 110v floods on it. With everything on, it draws 7.4 amps. I would like to T off the junction box with a line to a transformer to run some 12v deck lighting. I want to make sure I am under my max amps of 15.

The deck lighting I want to run, as I said is 12v. The fixtures are as follows:

15 x 11 watt fixtures
12 x 7 watt fixtures.

How many amps will I draw in addition to the 7.4A already on this 15A circuit? I calculated about 2.5 amps in addition to my 7.4, which puts me at about 10A on a 15A circuit.



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Old 01-02-2011, 01:01 PM   #2
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You are adding a total of 249 watts of lighting or 20.75 amps at 12 volts. Assuming the lighting transformer is only 80% efficient (very worst case), that's about 2.6 amps at 120 VAC.

7.4 (existing load) + 2.6 (added) = 10 amps total


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Old 03-31-2011, 03:46 PM   #3
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Another way to calculate

I take the 12V wattage and divide by 120 to get the amps the low voltage lighting is using. 249 watts / 120 is a little over 2 amps.
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