Im confused with the formula-

Watts / volts = amps

Im running a wire from my one room to fuse box. Its going to be #10/2 wire on a 30 amp breaker. The equipment can be powered by 120v or 240v by simply changing the powercord, which has 3 prongs for the 240v. My equipment is 3000 watts and im almost possitive its 33 amps total. The equipment will run for about 8 hrs a day. My question is with the formula. Since im going with a 240v will it bring the total amps to 12.5 instead of the 33amps. 3000w/240v = 12.5amps. and then my 240v 30 amp breaker will be fine or do i have to go with 240v 8/2 gauge wire 40 amp breaker. thank you so much for your time and input.

And you would have to do more than just change the power cord.

3000W/240V=12.5A is correct. If it will run for more than 3 hours, then you apply a 1.25 factor bringing the amperage to 15.6. This could be wired using 12ga and a 20A breaker.

doesn't that make a difference also?

Thanks for the quick response. The distance from the room to fuse box is approx 50 ft of wire. I bought 10/2 gauge wire and a double 30amp breaker. The equipment is 3 1000 watt Quantum ballast(H.I.D Lighting). The equipment if im not mistaken will run for 8hrs a day. And with these ballasts there is a optional cord for 120v and 240v im going with the 240v both plugs are included for either 120v or 240v. Im still getting told that i have to run 10/2 and others are saying 10/3. I just want to make sure i install the right gauge wire so my electrician can just hook it up at box. I will return the breaker and 10/2 wire if need be.

If these are straight 240V lights (and they should be) you need 10/2 with ground not 10/3.
I'm almost afraid to ask what you might be doing with that kind of lighting in a residential setting.
Of course, if it is not residential, then you should not be doing any wiring there unless you are licensed, and if you were, you wouldn't be here.

im just gonna go on the safe side and go with 8/2 with ground black wire white wire and bare ground on 40 amp breaker 240v. thanks for all your responses and hopefully this keeps my wife busy for a few hrs a day.