So with the whole 240/120 thing you’re talking about phases. These are basically electrical currents that function like wavelengths, operating at specific frequencies. The amperage basically says how much punch that phase/frequency can have.

If you put two twenty Amp circuits on the same neutral. The electricity will pass through both motors, and come back to the panel. The panel will read that those breakers are putting out more than 20A and trip the breakers.

BUT

If you put those breakers on different phases, the wavelengths alternate. Instead of really high peaks and valleys, there are more frequently occurring similar ones. So the breakers will read with in limits, and function. This is called sharing a neutral

What your guy is trying to do, is save a lot of work, and some money on material. Instead of running two wires, he’s trying to share neutrals.

As a 25 year construction worker, many of which have dealt with electricity. Two full years as an electrician.

Running one 240 circuit might sound intimidating. But it’s an efficient way to run a lot of juice to one location. All of your large appliances are ran on a 240, if they’re electric.

I can’t exactly explain to you why, but most efficient electric motors are 240. I agree with his initial recommendation. Without being on site, or having exact measurements, I cannot say that the cfm is uncalled for. I was going off of your 1300sqft est on the attic.

Just know that the things you are looking for are appropriate air exchange. Not too much, not too little.

Edit

With 240, it’s more about the amperage. Basically he’s talking about still running two 20A circuts, but in one wire. Level two car charges are like 48A 240, and level three is like 100A 240.

If you have an electric stove, that’s 50A 240

Also, is this supposed to replace the air in the house, and attic? Because 3000cfm is akin to a wind tunnel. That would replace the air in an attic and house in about a minute. Without proper air exchange points, that actually sounds problematic to seals.

Depends on the cfm (cubic feet per minute) of the fan. This will tell you how much it’s working to replace the air in your house.

Also depends on your current circuit, and your circuit current, lol. I’m willing to bet that the current circuit is 20amp, so I don’t know why he’s trying to replace that. I’m willing to bet the fan is also using almost all of the available circuit. You need to find out what the draw is on the fan for that. A 20amp circuit can handle 2400 watts. But due to code limiting draw at 70%, you can only put 1680w on there legally. That’s for saftey(fire) reasons.