 In previous videos, we've talked about how to determine the percent impedance. In this case, we've got two transformers here with a percent impedance of 2.5%. In this video, we're going to learn how to take that percent impedance and calculate what the fault current is for each transformer, as well as the fault current for the entire bank. Now, as I've always said, the first thing we want to do with our ratings here is calculate out what the rated current is. So I have a 480 to 120 volt transformer, 10 KVA. So what that means is each winding is going to be able to handle 10 KVA. 480 is on the H side because that is the higher voltage, which means this side is 120 volts. So when we're determining fault current, we are going to be basing it off of the secondary side because it is the secondary that draws the current, which means that the secondary side controls the primary current. So oftentimes, you'll hear us say that the secondary is the source of current for a transformer. Therefore, we're going to use that current to determine fault. So 10,000 divided by 120 volts gives me 83 amps for this transformer right here. Using the same idea on the second one, I'm going to take 5 KVA. Watch for that. So each winding can handle 5 KVA. 5,000 divided by 120 volts gives us 42 amps. Next up, we're going to take these currents and use the percent impedance to calculate fault current. So we're going to see here that the formula for fault current is fault current is taking your I rated divided by your percent impedance. That's just a fancy way the cool kids write it this way, percent Z or percent impedance. So again, it is off of the rated current, not the actual current, because what will happen is if we get into a short circuit situation, it doesn't care what the actual current being used is because it's in the fault condition. So we use the actual highest current that it is rated for, which is that 83 amps that we calculated. So the same thing, 42 amps on this side. So therefore, we're going to take the 83 amps and divide it by 2.5%, which is 0.025. Don't take 83 and divide it by 2.5. Take 83 and divide it by 0.025, which means that the available fault current on the secondary, guy through or somebody through a wrench across the whinies here would be 3,320 amps. We do the same thing on the other transformer. So I'm going to take 42 amps, divide that by 0.025, because that's the percent impedance. And I end up with an available fault current for this transformer of 1,680 amps available to us if somebody went ahead and threw a wrench across this transformer. If I needed to calculate out what the total fault current was, say someone came along and threw a bar or shorted it out the secondary bus, the whole bus, all you have to do is take your current because they're being paralleled of 3,320 plus 1680 amps and you end up with 5,000 amps. And so there you go. You've got your fault currents figured out. You figured out the rated current for each secondary transformer, divided it by the percent impedance and you've got your fault currents. To get your secondary bus fault current, you just add the two of them up. There's another quick trick to double check your work for the secondary bus. What we can do is we can add the KVA's here because we've got a 10 KVA transformer in parallel with a 5 KVA transformer. And we can figure out what the total KVA is. In this case, it's easy enough. It's 15 KVA. We can divide that by the voltage to get the actual secondary bus rated current. And we can take that bus rated current and divide it by the percent impedance. Let me walk it through with you. So right now I took that 15,000 divided by 120 volts and I end up with 125 amps. That means at full rated current, both transformers on the secondary bus will draw a total of 125 amps. We take 125 amps and we divide that by percent impedance of 2.5%. Remember when you're paralleling transformers, the third rule is that you have to have the same percent impedance. This is one of the reasons why we're going to take 125 amps divided by 0.025. And we're going to get an answer of, boom, 5000 amps. So it's the same thing. You can go ahead and double check your work. Again, you can calculate the percent impedance for each transformer and then add them up together to get the secondary bus fault current. Or you can just go ahead and add up your KVA's divided by the rated voltage. Get your current. Current divided by percent impedance will give you the secondary bus.