Did you measure the output with a load connected or with no load?

With load. It's 250v.

The transformer voltage will sag a little with load. To get the rated voltage at full load the open-circuit voltage will be higher than rated. What current was your load drawing when you measured 250 V?

about 0.4 Ampere. I tried a KillAWatt meter. It's 245 volts. So you mean to say when you use the full rated ampere, the voltage would be 240v?

Are you sure you've correctly wired primary connections? It looks like they can be set up to trim different inputs to 240V.

Yes. The primary is wired 240v. The secondary is wired 240v. It's a high quality medical class isolation transformer. It even has dielectric shielding which is not common in normal toroid. Specs at catalog.triadmagnetics.com/Asset/VPM240-2080.pdf So it should be more accurate. It's really true that with full load, the voltage can sag a little? Is this only true for toroid transformer or also the traditional shell/core type too? What principle can make voltage sag at full load, any reference? Thanks.

Voltage drop under load is unavoidable due both winding resistance and stray inductance. This second effect is usually lower in toroidal transformers but in any case cannot be zero. I have no handy reference to the background theory but it's indeed the very basic of electrical engineering, if you Google you'll be overwhelmed

what is the measured primary voltage? what kind of instrument you use. It's very unlikely that a transformer would output higher voltage, it all matters about number of turns. But if they failed to count, you can check it by swaping the primary and secondary. See the difference.

measured primary voltage is exactly 240v. Output at no load to 0.5A (so far) is 245v to 250v. I guess when load hits 2.08A that output would become 240v as one comment above emphasized. Didn't you guy expect it?

What @Marko said: Try to put the input voltage on the secondary windings, and measure the primary ones. If you get lower voltage, the amount of windings is off. If you get higher voltage, it should drop when under (more) load.

Why does it matter? Are you depending upon the mains supply to be exact for something or are you feeding a switcher?

I put more load equivalent to 410va (out of 500va maximum) or about 1.7A (out of the 2.08A maximum rating), the out voltage drops to 236v. So carloc and Transistor were right the voltage sags with load (I didn't swap the primary and secondary). Well. I guess if your project loads are low and buy high capacity isolation transformer, you will have larger voltage so must buy only the capacity suited to the load, isn't it? Anyway is it related to the "Voltage Regulation: 4.2% TYP from full load to no load" in the spec at the url above?

@Jtl Consider that when a large amount of current is drawn from the output of the transformer, the resistance of the windings will have a voltage drop equal to \$I_{output} \cdot R_{windings}\$. Since the transformer has no control system to stabilize the output voltage, it must have a higher voltage output under no load (open circuit). Nominal voltage output appears when nominal power/current is drawn.

So it's related to its spec "Voltage Regulation: 4.2% TYP from full load to no load". What does 4.2% mean?

@Jtl, it means 4.2% of the the voltage. So about 10 V. It sounds like your transformer is in spec.

Check the tolerances of your loads as well. Often things that work at 220/240 will be rated higher, sometimes for 277/347V. Anything rated for 240VAC will handle some amount of supply error, although I'm not at all sure whether 250VAC would be too much. Some loads are more current than voltage sensitive.That said, it will probably assist any reply if you list your intended loads and ratings.

Everyone's comments have been directed towards checking which of the "common culprit" misunderstandings you have. Load regulation, voltage drop/sag, internal resistance, checking of connections, reversed voltage check, are all people helping with more or less the same thing. If you take a course or read a textbook, early on you'll find a discussion of "ideal" components and the difference between them and what can be expected in real life. In real life, pretty much everything has resistance (save superconductors), parasitic inductance(save self cancelling things like a mobius resistor)

I bought 2 of them. One will power a fridge with these specs imageshack.com/a/img922/7803/q2q6YZ.jpg assuming the load is just 200w and worse case scenario if the output is still 250v. Would the fridge take the load?

and parasitic capacitance. Stuff like this tends to be assumed knowledge among professionals and often gets missed in online self learning, so if you're really interested in ee, working through a basic textbook or two might be extremely useful. Also note that if a transformer has a nameplate rating of 240V, that may be it's measured output voltage at no load or full load depending on manufacturer's rating and intent of use.

That fridge is only rated for 230V, so you should be aiming to supply it with 220V, although it may survive 240V or even 250V. The most potentially voltage sensitive thing I see is the motor starting cap, but someone else here might spot something else. You may wish to check what voltages and currents are available to you if your transformer is reversed, as replacing the fridge is probably what you're trying to avoid.

Are all toroid transformer reversible in the primary and secondary orientation? Are there no toroids where you can't reverse the primary and secondary? The model has dielectric shielding that is grounded. Won't it be sensitive to primary and secondary orientation. I forgot my electrical lessons more than a decade ago and just reviewing some I missed.