very small *why shouldn't I already know this* transformer calculation

[royta]

OK, this pertains to a low voltage landscaping transformer.

Output voltage is 12V
Output watts is 300W
Output amps would calculate to be be 25A

Input voltage is 120V

It's just a 10:1 transformer, so the input amps would be 2.5A, right?

 

[iwire]

300 W / 120 V = 2.5 A

 

[haskindm]

That is correct. Watts is watts. Divide 300 watts by 12 volts = 25 amps output. Divide 300 watts by 120 volts - 2.5 amps input. There is some loss in the transformer, but for our calculation purposes we can assume that a transformer is 100% efficient.

 

[LarryFine]

Keep in mind that the actual input current will be 1/10 the actual output load current, not the rated maximum.

 

[royta]

Keep in mind that the actual input current will be 1/10 the actual output load current, not the rated maximum.

Right. Thanks. Despite the fact that 300 watts happens to be a transformer size for many manufacturers, it really just was a random arbitrary number that I picked. It was easy to calculate with too.

The distances from the transformers to the fixtures are VERY long. On the furthest zone, the first fixture is at about 250 feet and the last fixture is about 350 feet. I'd have to break that zone into a bunch of 60W sections and run runs of LV to each section. Big money in copper. It will be better to run line voltage along the fence line and move the transformer closer to the load. However, I need to figure out the voltage drop for the line voltage too.

 

[stickboy1375]

Right. Thanks. Despite the fact that 300 watts happens to be a transformer size for many manufacturers, it really just was a random arbitrary number that I picked. It was easy to calculate with too.

The distances from the transformers to the fixtures are VERY long. On the furthest zone, the first fixture is at about 250 feet and the last fixture is about 350 feet. I'd have to break that zone into a bunch of 60W sections and run runs of LV to each section. Big money in copper. It will be better to run line voltage along the fence line and move the transformer closer to the load. However, I need to figure out the voltage drop for the line voltage too.

I would highly recommend moving the transformers closer....

Voltage Drop Formula
Length of run in feet _________
X (multiply)
Number of watts on run _________
Total = _________
? (divide)
cable size constant _________
=
Voltage drop _________

Cable size constants
12 gauge 7500
10 gauge 11920
8 gauge 18960

 

[LarryFine]

It will be better to run line voltage along the fence line and move the transformer closer to the load. However, I need to figure out the voltage drop for the line voltage too.

For a given conductor size, it would be 1/10 the voltage drop. You'd have less drop with 120v on #14 than 12v on #10.