×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

TWO-PHASE VS. SINGLE-PHASE

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

First of all, both voltages that you refer to are considered single-phase.  Assuming a 120/208 volt, three-phase voltage -- three voltage configurations are possible.

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

I just read the rest of your questions...

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources