TWO-PHASE VS. SINGLE-PHASE
TWO-PHASE VS. SINGLE-PHASE
(OP)
WHAT IS THE CHARACTERISTIC DIFFERENCE BETWEEN TWO-PHASE & SINGLE-PHASE POWER? NOT ON MOTORS BUT IN THE CASE OF A HEATER. WHAT IS THE DIFFERENCE BETWEEN USING A 120V SINGLE PHASE HEATER AS OPPOSED TO PULLING FROM 2 LEGS OF A 240 THREE-PHASE LINE IN? WHAT IS THE RMS VALUE OF PULLING THESE 2 LEGS? IS THERE A PHASE INBALANCE OR DOES IT EVEN MATTER?






RE: TWO-PHASE VS. SINGLE-PHASE
The first is 208 volt, three-phase which is obtained by connecting to all three legs. This is the most efficient voltage, and maintains balance across the phases.
The second would be 208 volt, single phase which is obtained by connecting across two legs. This is the next most efficient because of a higher voltage (than 120 volt) and the sharing of the load across two legs.
The third configuration is 120 volt, single phase which is obtained by connection between one leg and the neutral.
As you probably know, the higher the voltage (all other variables equal), the lower the current. Thus, the 208 volt configuration requires less current than a 120 volt configuration. This allows you to use a smaller circuit to carry and equal load.
Also, when using single phase circuits, you should be careful to balance the loads across the panelboard/switchboard bussing as to keep the three phase load in balance. This allows you to achieve full capacity from the panel as well as minimize voltage variations between phases.
Hope this helps!
RE: TWO-PHASE VS. SINGLE-PHASE
120/240 volt, three-phase voltage would indicate 120 volts from leg to neutral and 240 volts from leg to leg.
Based on my above post, and assuming a 5kVA single phase heater (basically 5000 watts), the following should hold true:
Current = Power/Voltage
Current at 240 volts = 5000/240 = 20.83 amps
Current at 120 volts = 5000/120 = 41.67 amps
Therefore, much larger wire would be required for the 120 volt configuration.
Just for comparison, if the heater was 5kVA three phase:
Current = 5000/(240x1.732) = 12.03 amps
The three phase voltage calculation includes a square-root of 3 factor... adds a little complexity, but a good reference book could help you understand why.