B)Modifying the instrument would be the cleanest and best way, but you would have to find and replace its internal resistor (since they are selected for 5mA CTs). After changing those resistors it may need recalibration.
Z?) without touching too much the pcb you could modify the internal 5A CT by winding a few tens of "thin" magnet wire thus reducing its primary current from 5A to a more manageable level, (this would also be less accurate).
I have meter ordered and waiting delivery. AFAIK the meter have option in menu to select CT ratio. From the photo I see internal CT is 5A/2.5mA so the ratio is 1/200. I have CT 50A/1V, but in datasheet I see that it's 1/1000 ratio, with internal burden, and I can remove that burden as it's easy reachable, and current adjustment can be done in meter.
Now about accuracy. Magnitude can be verified and adjusted. But I'm afraid that phase cannot be adjusted. So I think my options are:
1. Add external CT with 10 or 20 turns of thick wire and connect to unmodified meter. With 20 turns I got range from 0 to 25A with better magnitude precision, (25A fuse, and I don't care for short overcurrent precission). I assume that two CT's in series will produce more phase differences.
2. Keep my CT as is but remove burden, and in internal CT make 50 turns of thin wire. Should have same current range, but unknown phase diff.
3. Replace internal CT with my just removing burden from my CT. Original have 1/200 ratio and mine 1/1000 ratio. So without changing internal burden I got 25A range. And as there is only one CT I assume the phase will be better.
I don't know how much phase differ in two CT's in series, but I need as precise as can, as meter will operate near zerro power. (adjusting grid tie inverter to not supply power to grid.)