My wood furnace has a 3-prong cord for the blower, which tore when I was moving it. The blower is a 1/2hp 110V blower. I think it said 3.2 amp, I am not sure. Can I hard wire it directly to my circuit breaker using 12-2 wire? Also, which size breaker should I use?
Well, a wall circuit is 110V at 15 amp, so a standard 20 amp breaker should be enough as long as the heater is the only thing on that circuit.
If you hardwire it, keep in mind that during the summer you should switch it off at the breaker to avoid phantom power draw.
You'd really be better off getting a screw-together Edison plug from Home Depot or Lowes or your local hardware store and just repairing the cable. However, copper 12/2 wire should be fine as long as it has a ground. Do not attempt to hardwire anything without a ground-- there's a reason the standards have changed.
Friendly reminder- turn off the breaker before you start messing around in the wall. 
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