Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Sizing Circuit Breakers?

  1. Oct 19, 2006 #1
    Hi all,

    I am curious to know if anyone can provide me with a formula or a rule
    that sizes the circuit breaker so the circuit is protected during
    inrush currents. The supply is 415VAC, 50Hz. The current comming from
    the buildings main distribution panel is around 60 Amps. The
    parameters on the motor nameplate are as followed:
    FLA=37.5 amps

    The VFD is a 40 HP AC drive.

    Any help would be appreciated?

    Kind Regards,

  2. jcsd
  3. Oct 20, 2006 #2
    Since I am a girl and as you can see the most brave one to be the first one to tackle your question I would say the breaker should be rated 40Amps bcs you say that nameplate specs are 37.5 Amps. If there is a high starting momentum then max Amperage is increased to 45 A. Your wire gauge= wire size should be #8. Hope it helps, or if someone knows more please help us.

    *****One more thing; you say Distribution Panel and current comming is 60 Amps. What do you mean by that?
    Let me analyse it; 60 Amps on the panel is disconnect rating of 60 Amps, or did you measure with the Amprobe and the reading is 60 A?
  4. Oct 20, 2006 #3


    User Avatar
    Gold Member

    The cb protectes the conductors , i think the conductors should be sized at least 125% of motor flc.
  5. Oct 20, 2006 #4
    Yes CB protects conductors but the main protection is for the appliance and/or whatever is connected to those conductors and CBs.

    Choosing CB we have to read manufacturers specs and if specs on the namplate are 37.5 Amps that would be the main starting factor in deciding CB size, IMO.

    But, you right for 125%
  6. Oct 22, 2006 #5

    Thank you for your responses.
    The current CB was already sized to 40amps. I think the 125% of FLC would suit for any CB rating.

    ****The current comming in the building is 60 amps which means that that 60 amps is split into 40 amps for the three phase and the remaining is 20 amps for lights, aircon, heat, recepticles, alarm system).

    Sorry for not including details for the main supply.

  7. Oct 23, 2006 #6
    Hi again,

    Just wanted to say thank you again Nikola-Tesla for your help along with Wolram.

    Kind Regards,

  8. Oct 4, 2011 #7


    User Avatar
    Gold Member

    Hmmmm....I'm not sure I agree with above answers.

    Motors have a huge inrush of current essentially because the motor is just one long wire with very little resistance....until the magnetic field builds up.

    Generally speaking....you need to size your circuit breaker 2.5 times the full load current.

    That being said.....a 100 amp circuit breaker with #2 wire would be most appropriate.

    This is the general rule of thumb unless the nameplate or specs .....tell you the exact circuit breaker size.

    The 125% rule is pretty much doesn't apply in this case since the breaker I am talking about is 250% of full load current.

    More than likely the 40 amp circuit breaker that was mentioned above failed over and over on start up.
  9. Oct 4, 2011 #8


    User Avatar
    Gold Member

    Also, you say that the main distribution panel is 60 amps......not enough.

    You would need 100 amps just to start the motor alone.

    If you are saying that 20 amps is for resistive elements....thats ok....use your 125% rule there........125 amp main breaker should work for whole system.

    Ironically I just sized a 30 HP VFD with 38 full load amps for a AHU motor on a single line. 100 amp circuit breaker with #2 wire with a 80 amp fuse.

    Psparky, P.E.
    Last edited: Oct 4, 2011
  10. Oct 4, 2011 #9

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    and go to your breaker catalog for time-current curves. Motor will pull starting current until it accelerates the load to near synchronous speed.. you dont want nuisance trips.

    On motor nameplate is a code letter that tells starting KVA, which is several times running .
    That's why Sparky's number seems so high at first.

    Google "motor kva code"
    you can pay a little more and get motors that are easy to start.

    old jim
  11. Oct 4, 2011 #10


    User Avatar
    Gold Member

    Ahhh...nice to see some sense in this thread. Although the younger engineers above are clearly stating what they learned in school and in early years of engineering.

    Motors are a different flavor. The inductive nature of them makes them much trickier. The basic rules you know for lights and receptacles no longer apply.

    KVA, power factor, reactive power, inrush current.....and the use of starters .....are pieces to the puzzle.

    Here's a question for the younger engineers.

    Say a motor has a 10 KVA and full load current of 100 amps......with a power factor of .7. If you correct to power factor to .95 with a capacitor in parallel....

    What is the new current and KVA of the motor?
  12. Oct 5, 2011 #11


    User Avatar
    Gold Member

    The above question should actually read....

    Say you have a three phase 10 KVA motor with a full load current of 100 amps......with a power factor of .7. If you correct to power factor to .95 with a capacitor in parallel....

    What is the new current and KVA of the motor?
  13. Oct 10, 2011 #12


    User Avatar
    Gold Member

    Really, no one?

    Take a stab at it if you don't know it. You will learn something.
  14. Mar 27, 2012 #13
    FLA = kVA/[sqrt(3)*kV*pf]

    sub in FLA = 100, kVA = 10, pf = 0.7

    This yields your reference voltage, which is .0825kV, or 82.5V.

    Now, change pf to 0.95. Your kVA will remain the same.

    Solve for FLA. I got FLA = 74A
    Last edited: Mar 27, 2012
  15. Mar 27, 2012 #14


    User Avatar
    Gold Member

    You are right about the KVA staying the same....however you missed on the full load amps. Don't feel bad...most people miss this question.

    Nice try.........try again or ask questions.

    Draw a picture of a voltage source in parallel with a motor.
    Now draw another picture of a voltage source in parallel with a capacitor....and a motor.
    Does your amp calcluation still make sense?
    Last edited: Mar 27, 2012
  16. Jun 14, 2012 #15
    Dear VSDguy,

    the solution on your problem is this

    40 amps x 125% of the FLA since it is a motor so you get 40x1.25 = 50 amp.
    this is including the starting torque of the motor
  17. Jun 14, 2012 #16
    The kVA will stay the same, while the current will change by the ammount of amps required by the caps to provide the additional reactive power.

    The initial reactive power,

    [tex]Q_{i} = 10sin(arccos(0.7)) \approx 7kVAR[/tex]

    The final reactive power,

    [tex]Q_{f} = 10sin(arccos(0.95)) \approx 3.1kVAR[/tex]


    [tex]Q_{cap} = Q_{f} - Q_{i} = \frac{V_{c}^{2}}{X_{c}}[/tex]

    Vc should be known since you should be able to solve for the voltage from the initial case and the voltage will remain the same whether or not the capacitors are present. Once you know Xc you know Ic.

    The new PF will provide you with the new power (more than previous) given to the motor.

    With this one can find the current flowing into the motor. (The capacitors aren't receiving any of this power)

    The summation of these two currents will be the new full load current.

    I am hesitant to run the actual numbers for everything as I am worried about forgetting those troublesome √(3)'s everywhere.

    Hopefully my theory/understanding is in check! :uhh:
  18. Jun 14, 2012 #17


    User Avatar
    Gold Member

    Almost........you are 99% correct......

    This statement is not correct. The motor has no clue the capacitor is there and receives the same voltage, current and KVA as it did before the capacitor. But like you said...the power plant is now sending different current and less reactive power.
  19. Jun 14, 2012 #18
    The only reason I think the power has changed is because of the relationship shown within the power triangle.


    [tex]P_{i} = S cos(\theta_{i})[/tex]


    [tex]P_{f} = S cos(\theta_{f})[/tex]

    If S is to remain the same, it must be such that,

    [tex]P_{i} \neq P_{f}[/tex]

    How can this be true if you claim that the motor is receiving the same KVA, voltage and current?

    Am I missing something?
  20. Jun 14, 2012 #19


    User Avatar
    Gold Member

    In simplest terms....we basically have a voltage source in parallel with a cap and motor.

    What was the voltage across the motor before the cap?

    What is the voltage across the motor after the cap?

    The voltage across the motor before and after is obviously 480 volts. The load of the motor has not changed....

    How can the current and KVA change in regards to the motor?

    The current and reactive power coming from the source is now much different.....but...the motor doesn't have a clue this is happening....think about it.
  21. Jun 14, 2012 #20
    I agree with what your saying and I believe the intuition is correct, but my problem is that I should see such agreement within my calculations.

    Where is the discrepancy?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook