# What happens to the transformer output if the rated primary voltage is lower than actual reading from the wall socket?

If my transformer is rated 220VAC on the primary and my wall socket is reading 240VAC:

1. Do I get a "free" increase in secondary voltage?
2. If yes to no.1, by how much? Is this the correct formula? (1 - (220/240)) x secondary VAC
• Mains power varies. That's fundamental. It not only varies in the subsystems but it varies within a home, too. A transformer name-plate rated for 220 VAC on the primary will almost certainly be fine at 240 VAC, measured. And yes, the secondary voltage will vary with the primary supply voltage. Your formula isn't the one I'd propose. But it will produce a value that is close enough in this case. Commented Nov 6, 2023 at 5:52
• A few decades ago, home appliances (i.e. wall sockets plug able) used to be rated 220V. Today they are rated 240V because it's more realistic given that voltage can fluctuate from 210 to 250V and still be considered as normal. Myself I still kept the old habit of saying "220V". Yet, everything rated 220V should normally support voltages up to 280V as 240V rated do. Commented Nov 6, 2023 at 9:42
• One more thing to consider with "copper&steel" transformers is mains frequency: The increase in volt×seconds going from 60 Hz to 50 Hz is bigger than going from 220 V to 240 V. Commented Nov 6, 2023 at 22:05

1. Yes

2. No, if you are sending 240V into 220V transformer, that's a ratio of 240/220 or 1.09 higher.

Your formula calculated how much less the voltage is if you feed 220V into a 240V transformer.

But depending on where you live, mains voltage may be defined to be around 230V anyway with some tolerance up and down, especially if you live in a country that has formally changed from 220V or 240V to 230V.

Permissible mains voltage variation is ± 10 % in the European Union, -6% to + 10% in Australia, the UK and India and ± 5 % in the USA and Canada.

• (In the EU, it was 230 V -10% to +6% from 1987 to 2009. It is ±10% since.) Commented Nov 6, 2023 at 8:06
• @greybeard - Hi, I presume you intended to say 'in the EU -6% to + 10%' from 1987 to 2009 and ±10% since. Thank you very much for the information. My answer has been edited accordingly. Commented Nov 6, 2023 at 10:06
• @greybeard - Got it! Thank you. Commented Nov 7, 2023 at 2:44