It will work. You won't get quite the same precision and (assuming that we are talking about Spyder CTs) the channel will read zero at and below a higher non-zero power draw than it would with a 20A CT. The optimal Spyder setup will use 20A CTs for all lines except those expected to see currents higher than 20A. Remember, too that a '40A' 230V breaker is looking at both 'sides' of the circuit, so each 'side' can be treated as if rated at 20A.
