 This is going to be a walkthrough of a step-up auto transformer. What we're going to be doing is we're going to take this voltage here and we're going to step it up over to this load and we're going to see what happens with the current at this load and then the current at the primary itself and we'll talk about the current across the common winding. The first thing we have to do is you need to figure out what the voltage is over here. Now it's kind of the opposite of a step-down auto transformer where we multiplied the voltage by the percentage to get the voltage over at the load. This time we're going to divide it. We're going to take 208 volts and divide it by 57% or .57 to get the voltage at the 29 ohm load. That works out to be 365 volts at the load. Next step is to get the current at the load and just like before when we had the step-down auto transformer we're going to take the voltage divided by the ohmic value of the load to get the current at the load. That works out to be 12.6 amps. So we've now determined the voltage at the load. We have the current at the load. Our next step is to be figuring out what the power being dissipated at this load is. Let's do that by taking 365 volts and multiplying it by 12.6 amps. That works out to be 4,600 watts. So we've determined the voltage, we've determined the current and we've determined the power being dissipated at the resistor. Our next step is to figure out what our primary current is over here. Now remember just like every transformer, VA in is VA out or power in is power out. So if this load is dissipating 4,600 watts then this side the source has to give 4,600 watts. So let's take that over to our voltage here. So now we have 4,600 watts at the source. We take 4,600 watts and divide that by 208 volts. We get a current of 22.1 amps. Now the only thing left to figure out now is the current that is going across this winding here, a common winding, right up to this point right there. Now let's see what happens. We're going to move the 21 amps down to here to this side. We're going to move the 12.6 amps down to this side. And just like when we talked about with the step down auto transformer I have 22.1 amps on this side of the node and 12.6 amps on this side of the node. It doesn't just suddenly magically lose that current, that 10-ish amps. It has to drop it across something. It's going to be dropping it across this winding, the common winding. So that's going to work out to be 9.5 amps. And there you have it, we've worked out everything. Again, kind of easy, kind of fun. We get the voltage, we divide it by the percentage to get the voltage at the load. We take the voltage at the load divided by the load to get the current. We take the voltage and the current to get the power. Power in is power out. So we take this power over to this side. This side divided by 208 gives us this current. And then this current minus this current gives us the current across the common winding. That is a step up auto transformer.