Is it ok to plug in my iMac via a few extension cables?

And I'm going to disagree with your post! 😁

There are only two worries. One is voltage drop. Assuming that the only item on the extension cords is the iMac, the voltage drop will be less than one volt. That's assuming skinny #18 wire and a whopping 10 amps. Extension cords are commonly #18 and occasionally #16. I can't locate any figures regarding current draw but I can't imagine it to be anywhere near 10 amps.

The other worry is someone accidentally kicking one of the cords and disconnecting it. That's really no different than loss of power from the power company or an overload elsewhere on the circuit causing a blown fuse or circuit breaker trip.

Honestly, I'd be more worried about the toaster because of its high current draw. I've learned never to use the toaster when I have Christmas lights strung about.

An electrician would probably tell you it's a bad idea. Not so much because it will hurt the computer but because there are some dangers inherent in using extension cords.

http://www.esfi.org/resource/extension-cord-safety-tips-478

Of course, I live in a house that's over a hundred years old. Extension cords are a fact of my life. I've used one with my Macs for the last twenty years with no problems. So, the short answer is I agree with CountryGirl56.

Well, I'm going to disagree here - I think it's a really bad idea; one heavy duty cable maybe, but you said "a few". Having a lamp or toaster on an extension is one thing, but an electronic device is too sensitive in my humble opinion. Besides, where is your UPS coming into that equasion. Apple had an article that recommends plugging the adapter for laptops directly into the power source - I don't see why a desktop computer should be different (can't find that right now).

An I agree with both of the above.

If you live in a 100 year old house the way some of us do you may wish to consider getting your computer onto its own circuit that is not heavily used. I know when we used to plug the vacuum cleaner into the upstairs circuit (all except the remodeled bathroom on a single circuit) lights would dim. The same when the laser printer was running. I put the laser printer on an extension cord from the bathroom circuit and we plug the vacuum in downstairs and have a long extension cord when we are using it upstairs.

Fortunately, of late years, I have a laptop. So, I can unplug it during the day.

Our big problem is the kitchen. We used to have a coffee pot and a toaster that couldn't share the same circuit and be turned on at the same time.

Shouldn't be a problem.

You should be fine, as long as you choose the proper rating and length of extension cord.

Personally I would use a heave duty cord that is just a little longer than what is needed.

see > How to Select the Right Electrical Extension Cord