SYSTEM | POWER FACTOR | |||||||||
| 1.00 | .98 | .90 | .80 | .70 | ||||||
| Single phase | 1.00 | .98 | .90 | .80 | .70 | |||||
| Two phase (4 wire) | 2.00 | 1.96 | 1.80 | 1.60 | 1.40 | |||||
| Three phase (3 wire) | 1.73 | 1.70 | 1.55 | 1.38 | 1.21 | |||||
NOTE.—This table is for finding the value of the current in line, using the formula I = W ÷ (E × T), in which I = current in line; E = voltage between main conductors at receiving or consumers' end; W = watts. For instance, what is the current in a two phase line transmitting 1,000 watts at 550 volts, power factor .80? I = 1,000 ÷ (550 × 1.60) = 1.13.
Ques. Since there is no saving in copper in using two phases, what advantage has the two phase system over the one phase system?
Ans. It is more desirable on power circuits, because two phase motors are self-starting.
That is to say, the rotating magnetic field that can be produced by a two phase current, permits an induction motor to start without being equipped with any special phase splitting devices which are necessary on single phase motors, because the oscillating field produced by a single phase current does not produce any torque on a squirrel cage armature at rest.
Ques. For equal working conditions, what is the comparison between the single, two and three phase system as to size and weight of wires?
Ans. Each wire of the three phase system is half the size of one of the wires of the single phase system, hence the weight of copper required for the three phase system is 75% of that required for the single phase system. Since in the two phase system half of the load is carried by each phase, each wire of the three phase system is the same size as one of the wires of the two phase system, hence, the copper required by the three phase system is 75% of that required by the two phase system.
| Diameter squared | = circular mils | |
| Circular mils | × .7854 | = square mils |
| .000003027 | × circular mils | = pounds per foot |
| .003027 | × circular mils | = pounds per 1,000 feet |
| .0159847 | × circular mils | = pounds per mile |
| .003879 | × square mils | = pounds per 1,000 feet |
| .33033 | ÷ circular mils | = feet per pound |
| .0000002924 | × circular mils | = pounds per ohm |
| .342 | ÷ circular mils | = ohms per pound |
| .096585 | × circular mils | = feet per ohm |
| 10.353568 | ÷ circular mils | = ohms per foot |
Breaking weight of wire ÷ area = breaking weight per square inch.
Breaking weight per square inch × area = breaking weight of wire.