Selection of Wire Diameter

Cable manufacturer's selection chart will guide you the open air rating.then you may have to apply the derating factors depending on temperature,method of installation,etc.

Assuming a nominal temperature of 75 C, and 150% safety factor, per NEC Table 310-6, use a #3 AWG Cu or a #1 AWG Al conductor for each phase and the neutral.

http://www.electricaltraining.co.uk/Books.htm

Typically you would size the wire for a 3 phase motor at total amps plus 125%. In your case that would be 77.5 amps. The standard AWG size with 75 degree insulation would be #4AWG rated at 85 amps. Without knowing the distance to compensate for voltage drop this is correct for full load continuous running operation.

Guest,

In addition to the good and concise answer above from Who_Knows, the wire size is also chosen according to the temperature rating of the insulaion and the environment the wire is running through. The most common types of wire in the USA have a thermoplastic and nylon insulation, rated for 75 deg.C in a wet environment and 90 deg.C in a dry one. Because the electrical current creates heat (from the wire's resistance), the insulation gets hotter with more wires in the conduit (pipe), or the conduit is run through a fairly hot area. In the National Electrical Code (USA) section 310 has all these tables and correction factors. The 85 amp limit for the wire is from the 75-deg.C table. The highest temperature you could run your motor's wires through is 35-deg.C; but if the conduit is dry the maximum current is 95 amps and you can run it through a 50-deg.C environment. However, if you have 4-6 current-carrying conductors in that conduit instead of the three for the one motor, the wire can only carry 80% of its rating, and if you have 7-9, the limit is 70%. With more than 3 conductors, the wet environment needs a larger wire and the dry environment is limited to an unrealistic 25-deg.C if you have no more than 6 conductors. (With #3 wire, your wet temperature limits are 45-deg.C for 3 wires or 30-deg.C for 4-6 wires; while your dry temperature limits are 60-deg.C for 3 wires, 40-deg.C for 4-6 wires, and that unrealistic 25-deg.C for 7-9 wires.)

This is the type of stuff that electricians have to learn before they are considered to be safe in their trade, and why training (on the job and in classes) can take as long as 5 or more years before they are eligible to be tested for a Master level of skill. Apprentices must always have actively supervision and journeymen need checking before and after they do the work.

On the topic of voltage drop, mentioned by Who_Knows, there is no hard-and-fast answer on how far the wires can run. A common rule in practice is to allow 2% voltage drop in the wiring from the utility to your motor controller, and 3% more to the motor. If you have less at the first area, you can have more in the second. Using Ohm's law equation E=IR, with I = 62A and R = 0.31 ohms/1000 ft, you have E = 19.2 volts/1000 feet of wire. If you are allowed 5% drop on the 440 volts available, then you can drop 22V, so 22/19.2 is 1140 feet. This has to be the distance both ways, so half that distance is what you are allowed, which is about 570 feet (about 175 meters). Even at 3% voltage drop your maximum 1-way distance is about 340 feet (about 105 meters). That is very likely to be OK.

I have known of rare situations where wells with extremely deep submersible pumps have had the supply voltage boosted by hundreds of volts to compensate for this voltage drop so the motor can run at its nameplate rating.

Sorry, if the length of this post is boring. I asumed from the original post being by a guest and the wording of the question, that greater detail in the explanations and answer would be best.

--John M.