I want to put three 220/20 outlets in my garage. I have the PVC and wire run over to the breaker panel. My question is this instead of putting in a sub panel for the outlets I have four 20 amp breakers for my electric heat all on single breakers. Can I replace a couple of the single heat breakers with double 20amp breakers.
The outlets will be used for a mill, drill press, table saw.
THANKS
ZIGGY

Ziggy, I apologize for not responding sooner.

As to your question, what type of heat do you have? Normally household heat is 240VAC, not 120 VAC.

Regardless, you need to know the amperage required by each machine. If any one of the three machines require more than 20 amps to safely operate you task becomes a little more difficult.

As to changing out the existing single pole breakers, the answer is yes they can be replaced by single throw tandem breakers (one on top of the other).

If any of the machines require a greater amperage than 20 amps, then the breaker and the correct wiring need to be installed. For example if the Saw needs 30 amps, then the breaker needs to be 30 amp and the wiring needs to be #10 AWG.

You need to make absolutely certain that the electrical installation instructions are followed for each machine.

If they are 240 volts, you may need a quad breaker, like this:
THOW - Circuit Breakers by Cutler Hammer (http://livewiresupply.com/circuit-breakers/cutler-hammer/THOW?gclid=EAIaIQobChMIyPD53a_F1QIVCzFpCh1VsAf0EAQ YAiABEgIb_fD_BwE)

If you are using a single Neutral for 2 circuits on opposite phases, you would use one like that as well(Breakers Tied)