TWO-PHASE VS. SINGLE-PHASE 1

[JOYLE]

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?

 

[SCELECTRICAL]

First of all, both voltages that you refer to are considered single-phase.&nbsp;&nbsp;Assuming a 120/208 volt, three-phase voltage -- three voltage configurations are possible.<br><br>The first is 208 volt, three-phase which is obtained by connecting to all three legs.&nbsp;&nbsp;This is the most efficient voltage, and maintains balance across the phases.&nbsp;&nbsp;<br><br>The second would be 208 volt, single phase which is obtained by connecting across two legs.&nbsp;&nbsp;This is the next most efficient because of a higher voltage (than 120 volt) and the sharing of the load across two legs.<br><br>The third configuration is 120 volt, single phase which is obtained by connection between one leg and the neutral.&nbsp;&nbsp;<br><br>As you probably know, the higher the voltage (all other variables equal), the lower the current.&nbsp;&nbsp;Thus, the 208 volt configuration requires less current than a 120 volt configuration.&nbsp;&nbsp;This allows you to use a smaller circuit to carry and equal load.<br><br>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.&nbsp;&nbsp;This allows you to achieve full capacity from the panel as well as minimize voltage variations between phases.<br><br>Hope this helps!

 

[SCELECTRICAL]

I just read the rest of your questions...<br><br>120/240 volt, three-phase voltage would indicate 120 volts from leg to neutral and 240 volts from leg to leg.<br><br>Based on my above post, and assuming a 5kVA single phase heater (basically 5000 watts), the following should hold true:<br><br>Current = Power/Voltage<br><br>Current at 240 volts = 5000/240 = 20.83 amps<br>Current at 120 volts = 5000/120 = 41.67 amps<br><br>Therefore, much larger wire would be required for the 120 volt configuration.<br><br>Just for comparison, if the heater was 5kVA three phase:<br><br>Current = 5000/(240x1.732) = 12.03 amps<br><br>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.