What model do you have? looks like most of Sperry models only handle up to 10 Amps. Anyway in the case you want to test I would recommend you take the manufacturer's word for it. An inexperienced user could really mess up there motor trying to do so. In fact you need a similar test environment to perform manufacture type measurements accurately. The last thing you want to do is hold you motor at stall unnecessarily just to get that max current measurement because you can easily burn it up.