when should a powermax pm4b drop into float mode

Ok, that explains a lot! The wire from the Trimetric shunt to the neg batt post has to carry all the amps from everything that is turned on at the time. It has to be the fattest shortest wire in the set-up. I use 12" of #1. This lets me run the microwave when the furnace is on or whatever.

You can also run the PM's wire's over to the inverter's terminals if they are close together, then they share the inverter's fat wires to the battery and the same fuse.