Minimum current to measure from a 30kW motor?
There is an old rule of thumb that no load current (NLA) is about one third of full load current (FLA).  So \$NLA = \frac{1}{3}*FLA\$.  As I said, this is just a rule of thumb.  As the number of poles goes up, the ratio of NLA to FLA will also go up.  The NLA/FLA ratio also varies inversely with motor power.  

Your NLA/FLA ratio is 39 A / 109 A = 0.35, which is pretty close to 1/3.  Assuming you have a 4 pole motor and given it's power of 30 kW, I'd say that is pretty typical.