Long Cable Runs and Voltage Drop

Your cable manufacturer should have published data which will help you solve the problem. If your manufacturer doesn't (this would be a worry) then try contacting Olex cables or Pirelli Ericson. The older versions of AS3000 were a brilliant source of this data and almost anything else to do with installation. I can't remember which year they gutted it, but now we use the above mentioned books. If you can't get anything to help yourself with sing out and I'll look it up when I can. The important other thing to remember you need to identify the type of cable accurately as some cables such as Siemens have better ratings, due to stranding etc.

I think you should do a re-calc, By canadian standard it should be 300MCM-350MCM I have a voltage drop calc excel sheet that could help you, you just punch in the data and it will give you the percent voltage drop. I would add it here if I knew how!!!

From your description, it sounds like this is a three phase system, and you can tolerate a 5% voltage drop in each leg. Given that, it comes down to Ohm's Law and wire tables. 5% of the 400V is 20V, and at 150A, this gives a per-leg resistance of 0.1333333 Ohms. For a 650 meter distance, this would require a wire with a resistance of 0.205 ohms per 1000 meters. AWG wire size 000 (3/0) with a diameter of 0.41 inches (10.4 mm) has a resistance of 0.203 Ohms / 1000 meters. If you need to keep the losses lower than 20V per leg, 4/0 wire (0.46 inches, 11.7 mm, 0.16 Ohms / km) would give you about 15.7V per leg drop at 150A.

From BS7671 your volt drop using 70mm cable is 58 volts. If you uprate to 300mm you will still have a 18 volt drop. Change to 2 off 120mm per phase will give you an 18 volt drop at a cheaper price. Is the cable in one piece, if not you will also have to make allowance for the voltage drop across the joints.