# 120v/12v x former amperage ?'s



## JBrzoz00 (Jan 11, 2011)

120 volts feeding a 12v 50 watt transformer with a 50 watt lamp. The low side will be pulling 4.16 amps, 50/12=4.16. What is the formula to find what the transformer pulls on the 120v side?


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## 480sparky (Feb 1, 2009)

Pretty much a 10:1 ratio.... so the amperage will be close to the same ration.


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## JBrzoz00 (Jan 11, 2011)

Thanks, Lady at a clothing store wants some track light with a good bit of 50 wat MR-16s. I think I'm going to use 2 circuit 120v track and the transformers that go between the fixture and track. I want the option of her able to put a lot of lights on without over loading it.


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## 480sparky (Feb 1, 2009)

JBrzoz00 said:


> Thanks, Lady at a clothing store wants some track light with a good bit of 50 wat MR-16s. I think I'm going to use 2 circuit 120v track and the transformers that go between the fixture and track. I want the option of her able to put a lot of lights on without over loading it.



I'd ask for a lighting layout... these people tend to go overboard with loading lighting circuits.

Can the heads work with 2-circuit tracks? Are they adjustable between circuits, or are there two head models available?


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## JBrzoz00 (Jan 11, 2011)

There is no real layout. Seems to be the way things are going now a days. With the amount of changes I do real prints are worthless anymore. 

The heads are all the same on the ones ive used. To change from cir 1 to cir 2 the tab on the part of the fixture that goes in the track is pulled up or down to contact the energized metal in the track.


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## sparks1up (May 5, 2010)

An MR16-500 watt is only about 5-watts on the line side so unless you are using a dimmer you can load up several tracks without overloading any circuit. 

If you are using a dimmer you are limited to the wattage of the dimmer calculated at the 12-volt wattage of 50-watts per fixture. And I believe electronic low voltage dimmers are maximum 400-watts.


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## Seven-Delta-FortyOne (Mar 5, 2011)

sparks1up said:


> An MR16-500 watt is only about 5-watts on the line side so unless you are using a dimmer you can load up several tracks without overloading any circuit.
> 
> If you are using a dimmer you are limited to the wattage of the dimmer calculated at the 12-volt wattage of 50-watts per fixture. And I believe electronic low voltage dimmers are maximum 400-watts.


You're actually mistaken on that point. Wattage is a measure of power used. It is the same regardless of voltage (minus any transformer losses, of course). Amperage changes proportionally to voltage and resistance. One way to figure this is 50 watts devided by 120 volts = .416 amps.


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## sparks1up (May 5, 2010)

A.T.C. said:


> You're actually mistaken on that point. Wattage is a measure of power used. It is the same regardless of voltage (minus any transformer losses, of course). Amperage changes proportionally to voltage and resistance. One way to figure this is 50 watts devided by 120 volts = .416 amps.


Not sure which point you mean? Are you saying that 50-watts at 12 volts is still 50-watts at 120-volts? Because if that is so, there are a lot of POCO's that are going to be very disappointed!


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## Seven-Delta-FortyOne (Mar 5, 2011)

I apologize. Sometimes I don't say things clearly when I'm typing. Here is what I mean. The power company in my area generates power at 4160 volts and steps it up to 110,000 volts for the grid. Now 1 amp at 110,000 volts is 110,000 watts. Discounting line loss for the time being, when the 110,000 volts reaches the residential service area, and is stepped down to 240 volts, we now have 458.33 amps, but still 110,000 watts of power (IxE=P). This is oversimplified because there is line loss, along with different formulas for 3-phase power. Now in your house, if you plug in a 50watt 12 volt transformer, it will draw 4.16 amps at 12 volts, but it will draw .416 amps at 120 volts, but still 50 watts.


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## Tinstaafl (Jan 6, 2008)

sparks1up said:


> Are you saying that 50-watts at 12 volts is still 50-watts at 120-volts?


That's exactly what he's saying. Pick up any basic electricity book and find the section which discusses the equation P=IE.

Your trade is listed as Electrical. What sort of electrical work do you do?


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## GettingBy (Aug 19, 2010)

Measure it.

This info is hard to find on the Net.
Cell phone charger supplies are about 25% efficient IIRC.
The 9 W 'frmr that feeds my answering machine is only 40% efficient according to the nameplate data and that's pretty lousy.
Hosfelt.com shows a 27 W wall adaptor that is 71% efficient.

For 50 W out this would take 50 x 1/0.71 = 70 W in, so you should pull slightly less than 70 W because a larger 'frmr takes advantage of Economy of Scale, and more than 50 W.

Unless it is a switch-mode 12 V, 50 W supply!


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## sparks1up (May 5, 2010)

You are right I am talking watts and thinking amps. Still what I was trying to convey is that he can run a lot more lights on a single 120-volt 20-amp circuit at 12-volts 50-watts than he can at 120-volt 50-watt.


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## Seven-Delta-FortyOne (Mar 5, 2011)

sparks1up said:


> You are right I am talking watts and thinking amps. Still what I was trying to convey is that he can run a lot more lights on a single 120-volt 20-amp circuit at 12-volts 50-watts than he can at 120-volt 50-watt.


No, he cannot. 120 volts at 20 amps gives you 2400 watts. 2400 watts is all you get, whether it's 12 volt, 120 volt, or 12,000 volt. The amperage will change, not watts. Watts is a measure of POWER CONSUMED. IE=P. Delmars Standard Textbook of Electricity is an excellent introduction to Electrical Theory. I highly recommend it to anyone seriously considering working in this trade.


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