# How much AC will I need for my 12v DC transformer?

I feel like my question probably has a simple answer, but I don't deal with the back end of electricity often, so I want to make sure I do things right.

I am building a series of LED RGB light boxes for my wife for a trade show background. They'll need a total of 850 watts separated semi-evenly across five LED decoders. This is the decoder I am using (http://www.amazon.com/gp/product/B00PQASR9K?psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00).

I am planning to buy five 12v 30a transformers, which I believe will be plenty sufficient for this load. This is the transformer I'm planning to get (http://www.amazon.com/Singpad-Supply-Regulated-Transformer-360Watt/dp/B00N2PRMWY/ref=sr_1_1?ie=UTF8&qid=1433898154&sr=8-1&keywords=Singpad+new+style+led+power+supply).

My question is this. How much AC power will I need? Is it a 1 to 1 ratio (so the AC would also need to be sufficient to handle 850 watts)? What phase do I need?

My options for electrical power that I can purchase for the trade show are: 1) 20 amps single phase 120v 2) 30 amps three phase 208v

There are a couple of additional options with more amps, but I think one of these should be sufficient. Which will I need?

Also, I've never hooked up a transformer of this kind, so bonus points if you can point me to an online tutorial.

Thanks! Nick

• The equation for calculating the required current at the input of the transformer is (Vin * Iin) * e = Vout * Iout with e being the efficiency (I would use 0.6 for a defensive calculation). Jun 10, 2015 at 5:59
• There is no such thing as a "DC transformer". Feb 14, 2017 at 15:26

First, a bit of nomenclature. What you have are not transformers. They are DC power supplies. Sorry, but I'm picky about terms.

Second, power supplies like you are buying are usually about 90% efficient, so you'll need about 850/.9, or 950 watts.

Third, your power supplies take 120 volts in, so you will need 120 volt AC power for them. It's true that you can get this from 208 3-phase, but at your level of knowledge that's not a good idea.

So. Will 20 amps at 120 volts do what you want? The power available is 120 x 20, or 2400 watts, more than twice what you need.

Sounds good to me.

Connecting your supplies to AC is best done by connecting a standard power cord to the input of each supply. Start by looking at the data sheet which comes with the supplies. Then take one of your supplies to an electrician, and ask him to show you how to connect it. You should be able to do the others by yourself, but make sure you pay attention to the color coding on the wires. Finally, buy an AC power strip, plug it into the wall socket the show people point you at, then plug the power supplies into the strip. You should be all set, and the power strip will have a master switch which will allow you to turn all your boxes on or off.

• I have doubts about a 12V model, let alone an el-cheapo being 90% efficient. Highest DELTA Medical 12V I remember is 89%. Their 24V does do 92% I believe. But since the budget requirement is <50% than available I'm not going to complain too much :-) Jun 10, 2015 at 3:28
• For a cheap non-certified PSU, expect 75% efficient with a 0.7 PF. Basically double the output power to get input VA. May 29, 2017 at 8:35