How to determine a transformer's power

I've been asked several times how to determine the power rating of an unmarked 50 Hz transformer, so I'll try to explain and show a couple of examples.

There are quite a few ways to determine the power rating of a 50 Hz transformer. I'll list just a few.

1. Markings.
Sometimes you can find an explicit power rating on a transformer, though it may not be obvious at first glance.
This is a very basic option, of course, but it's worth looking first.


2. Core size and power rating.
There are tables listing the power ratings of specific cores. However, cores were made in a wide variety of shapes and sizes, and their manufacturing quality also varied, so the tables aren't always accurate.
And they aren't always easy to find quickly. Still, you can use tables from datasheets for standardized transformers as a rough guide.

3. Standardized transformers.
A huge number of standardized transformers were made during the Soviet era and afterward. You can identify them by markings that start with ТПП, ТН, or ТА.
ТА transformers are less common, while ТПП and ТН models are quite widespread.


For example, let's take a ТПП270 transformer.


Find the datasheet for this series and look up our transformer. It will list the voltages, currents, and power rating.
I've uploaded the datasheet to the documentation section as a PDF file. You can also check the core dimensions there and estimate the power rating from its size by comparing it with yours. If your transformer is a little larger, you can scale the rating accordingly, since a transformer's power rating is directly proportional to its size.


The markings on the ТН61 transformer are almost invisible, but they're there :)


There's a separate datasheet for it, which I've also uploaded to my blog.


Sometimes a transformer has markings, but it's impossible to find anything useful based on them. Unfortunately, datasheets for these transformers are very rare.


4. Calculating power from wire diameter.
If there's no other information, you can estimate the power rating from the diameter of the winding wires.
You can measure the primary winding, but sometimes it's inaccessible.


In that case, measure the diameter of the secondary winding wire.
In this example, the diameter is 1.5 mm.
The rest is simple: first, calculate the wire's cross-sectional area.
Divide 1.5 by 2 to get 0.75, the radius.
Multiply 0.75 by 0.75, then multiply the result by 3.14 (pi). The wire's cross-sectional area is 1.76 mm².

The current density is generally taken to be 2.5 A per 1 mm². In our case, multiply 1.76 by 2.5 to get 4.4 A.
We know the transformer is rated for an output voltage of 12 V—or, if we don't know that, we can measure it with a multimeter. Multiply 4.4 by 12 to get 52.8 W.
The label says 60 W, but transformers are often wound with undersized wire these days, so overall the numbers are close enough.


Sometimes a transformer label lists not only the number of turns in the windings, but also the wire diameter. But take that with a grain of salt, since labels can be wrong.



In this example, I first found a section of wire I could measure, then lifted it slightly so I could get the calipers in.


After measuring it, I found that the wire diameter was 0.25 mm, not 0.355 mm.
Let's try the calculation method I described above.
0.25/2=0.125
0.125x0.125x3.14=0.05 mm²
0.05=2.5=0.122 A
0.122x220 (winding voltage) = 26.84 W.

The method described above is also great when there are several secondary windings and measuring each one is inconvenient.


5. Reverse calculation.
In some cases, you can use transformer design software. These programs have fairly extensive core databases, and can also calculate custom core dimensions based on measurements you can take.
I use the Trans50Hz program.


First, select the core type. mostly these are toroidal, EI laminated, and EI strip-core types.


From left to right: toroidal, EI strip-core, EI laminated.
In my example, I’ll measure an EI strip-core transformer, but the same method can be used to determine the power rating of other types of transformers.


Step 1: measure the width of the side leg of the core.


Enter the measured value in the program.


Step 2: core width.


Enter this value in the program as well.


Step 3: window width.
There are two ways to do this. If you can access the window, simply measure it.


If you can’t access it, measure the overall width, then subtract four times the value from Step 1 and divide the remainder by 2.
For example, if the overall width is 80 mm and the value from Step 1 was 10 mm, subtract 40 from 80. That leaves 40; divide by 2 to get 20, which is the window width.


Enter the value.


Step 4: window length.
This is essentially the length of the bobbin that holds the wire, and it can often be measured without any trouble.


Enter this value too.


Then click the Calculate button.


We get an error message.


The reason is that the program is initially set up to calculate the power of a high-power transformer.
Find the highlighted field and change its value so that the power (voltage multiplied by current) does not exceed our estimated core power rating.
You can enter any values here, even 1 V and 1 A; it doesn’t matter. I set it to 5 V.


Click Calculate again, and we get the result we’re looking for. In this case, the program calculated that our core’s power rating is 27.88 W..
The result is roughly in line with the calculation based on wire diameter, which gave me 26.84 W, so the method works well enough.


5. Measuring the maximum temperature.
Conventional (iron-core) transformers should not heat up above 60 degrees during operation, and this can also be used when estimating their power rating.
There are exceptions, though. For example, a UPS transformer can have a high power rating despite its modest size because it only operates for short periods and will shut down before it overheats. In this case, for example, its power rating might be 600 W, but only 400 W for continuous operation.
There are also Chinese manufacturers who sometimes use undersized transformers in cheap adapters. They get hot as ovens, which is not normal; the actual power rating of a transformer can often be 1.2–1.5 times lower than advertised.

To measure the power using the method above, use any load—light bulbs, resistors, and so on. You can also use an electronic load, but in that case connect it through a bridge rectifier with a filter capacitor.
Wait about an hour. If the temperature hasn’t exceeded 60, increase the load. I think the rest of the procedure is clear.
There is one small caveat: the transformer’s temperature can vary noticeably depending on whether it has an enclosure and how large it is. Still, this method gives a very accurate result. The only downside is that the test takes a long time.

I’ve used transformers like these very rarely over the past 10–15 years, so they’re lying somewhere on the back shelves on my balcony. While looking for one, I came across some rather interesting indicators, the IN-13. I bought them for a level indicator in an amplifier, but eventually abandoned the project. Now I’ve found them again and am wondering what I could make with them. If you have any interesting ideas or suggestions, I’ll try to build something and show the process in a review.


That’s all. As a bonus, here’s a video on determining a transformer’s power rating from its dimensions.