 This is going to be a walkthrough of instrument transformers, their purpose and how we can use them in a wattmeter to determine what the actual line power is. Now this walkthrough is going to be on a purely resistive circuit. I'll be doing another circuit that has inductive properties in another video. Now here's your scenario. What we have here is we have a potential transformer, a PT, that is rated at 15 kV to 120 volts, and we have a CT, a current transformer over here that is rated 300 amps to 5 amps. Now, PTs or potential transformers always have a ratio of something to 120 or 110 or 115, whereas CTs, current transformers, always have a ratio of something to 5 amps. So that way you know that on the secondary side, which is this side, that if I was reading 5 amps, I'd be reading 300 amps on the top side. Over on this side, if I was reading 120 volts on this voltmeter, that would mean that I'm seeing 15 kilovolts on my primary side. Now what we have here are our transformer ratios, 15 kV to 120, 300 to 5 amps. We can't really work with those transformer ratios. We have to go down to a mathematical ratio. So what we're going to do is we're going to take 15,000 to 120, and we're going to divide it into each other to get a ratio of something to one, and we're going to do the same thing over here, and we're going to get our mathematical ratios. So here we have the potential transformer as a ratio of 125 to 1, and the current transformer has a ratio of 60 to 1. Now we have our ratios that we can use to determine what our multiplier is, which is this guy right there. Now what a multiplier is, is it allows us to take whatever we read on the wattmeter, and we can use the wattmeter reading and the multiplier to figure out what our actual true line power is up top here. Because we can't just throw a wattmeter up here. Because sometimes you could have megawatts, and these wattmeters are not built to be able to handle megawatts. You'd end up experiencing what I've said in my last video, kablazelflam. What we have then is our multiplier. We're going to take 125 times 60 to get our power multiplier. And that gives us a multiplier of 7,500. There's no unit behind that. It's just an actual multiplier. Now for a wattmeter reading, what we've got here is I've got a line voltage of 14.5 kV, and over here I've got four amps on my secondary side, so I don't need to worry about that. But I do need to figure out what my voltage is going to be on this side here. So I can take this voltage and divide it by 125 and get my wattmeter reading. So by taking 14,500 volts divided by 125, I get 116 volts there. So now at my wattmeter, I have 116 volts applied. I have four amps going through it. I end up with my wattmeter reading of 464 watts. But that's not my line true power. Don't be thinking that this number here is the true power up here. That is just what my wattmeter itself is reading, using the Pt secondary side and the Ct secondary side, that four amps there. If I want to figure out what my circuit true power is, it becomes very easy. There's two ways I could do it. I could take this voltage and then I can take this current, step it up and get this current and go this voltage that is this current to get my line true power. Or even easier, what I can do is take this power here that I'm reading on the wattmeter and use my multiplier, so I can go 464 watts times 7,500 and I would get my circuit true power on the primary side. That ends up being 3.48 megawatts and that is our true power on our line side. Therefore, again, just like every other transformer video and issue that we've come across so far, they're not as complicated as they seem at first. Again, it's all about breaking it down to ratios. I determine what this ratio was mathematically. I determine what this mathematical ratio was. This ratio times this ratio gave me this multiplier. I took this voltage, I divided it by my mathematical ratio, I got my secondary PT voltage, I take that voltage and I multiplied it by the secondary CT current and I got my wattmeter reading and I took my wattmeter times my multiplier and I got my actual circuit true power. In upcoming video, I'll show what happens when we throw a power factor in there. Again, not as tricky as you would think, but something you've got to watch out for.