While you are drawing no more than with a portable drill, you are drawing it continuously and thusly should be using an extension cord rated for 15 amps (the reason you only draw 12 amps is because electrical code only allows 80% of max draw to be taken continuously, and if having smaller wires inside the wall would be dangerous at 12 amps continuous, having smaller wire outside the wall surely would be, too). It is also important to note that the extension cord should NOT be coiled up, as this can generate heat potentially causing problems up to and including fires. This risk may be mitigated by having a cord rated for 15 amps, but I am no expert and the opinions I have come across vary.