figuring load

[fisherelectric]

How many amps would a 50 watt load at 12 volts put on the line side of a 120v/12v transformer?

 

[charlie b]

50 watts divided by 12 volts equals 4.17 amps.

 

[fisherelectric]

So would that be .42a on the 120volt side?

 

[al hildenbrand]

Then you need to allow for the turns ratio. If the line side is 120 V, then:

( 12V ? 120V ) x 4.17 A = 0.417 A

 

[al hildenbrand]

So would that be .42a on the 120volt side?

Yes. . . .

 

[Alex Aragon]

If you really need to "split hairs":

Transformers are not 100% efficient.
They often lose 10 to 15% of the power delivered to the loads.

So (50w @ 12v) from 50w x 1.15= 57.5w / 120v = .48a

They also will have a parasitic loss even with no loads connected. (The bigger the transformer the greater the loss.) If it is warm to the touch or makes any vibration that is "watts out the window".

 

[infinity]

If the transformer was 100% efficient you could just divide 50 watts by 120 volts to find the current.

50/120 = .41