Henry,
The reason for this is because the Amp Draw listed as 40 Amps is the "running" amps for this compressor. However, at start-up, the running amps increase significantly. This is called the "starting" amps. In this case, the starting amps increase to the point where the compressor requires an 80 AMP Breaker in order to handle the starting amps. We do not recommend using this compressor on a circuit with less than an 80 AMP Breaker.