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

AC Motors

AC Motors

AC Motors

I'm trying to figure-out what would be the approximate max. or by calculation of phase currents I1,I2,I3 in a 3 phase-four wire system, with a given 1875KVA/380Vac, rated generator.(Balance load)
The question is for me to determined the type of CT's to be installed in 3 phases for add/shedding application. Pls. assist--Thanks


RE: AC Motors

just under 3000Amp

RE: AC Motors

That answer comes from I = S/(SQRT(3)*V)=1875000/(SQRT(3)*380)

RE: AC Motors

A quick rule of thumb for 380V 3ø circuits is 1.5 amps/kVA.

{220V … 2.6A/kVA}
{480V … 1.2A/kVA}
{208V … 2.8A/kVA}

RE: AC Motors

Thanks for your confirmation cbarn,electricpete and busbar.

I would rather choose to look for a whole package power monitoring device that will feed information to my program which monitors status of the loading condition of my genset.
I have variable 220Volts load with respect to ground.

I understand that total current from power formula is equal to all phase current say-- Ia,Ib,Ic at full load condition right?


RE: AC Motors

If everything is balanced (the normal assumption), then all three phase currents will be equal.  I'm not sure what you mean by total current. The current value that we use in the formula: P=sqrt(3)*I*VLL*powerfactor is the magnitude of any of the three phase currents (they're assumed equal).

RE: AC Motors

Hi electricpete--

I mean to say total current as per specification from 1875KVA. Would it be It=Ia=Ib=Ic?
Thanks for being prompt.


RE: AC Motors

Once again I'm not sure of any meaning for total current.
It is likely that the only current of interest are the phase currents which are assumed equal, and can be used in the formula Stotal=sqrt(3)*VLL*Iphase.  
(where S is va)

Note that if you represent the load as equivalent wye, the voltage across each phase is VLL/sqrt(3). We find the power across each of the single wye legs as:

S_singlephase=IV = Iphase*VLL/sqrt(3)

The total three-phase power is
S_three_phase = 3*S_singlephase
              = 3*Iphase*VLL/sqrt(3)
              = sqrt(3)*Iphase*VLL.
I'm not sure if that helps.

RE: AC Motors

In support of electricpete’s comments, I’ve not seen any reference in IEEE or ANSI stuff for the term “total current,”  but total apparent power {voltamperes} in a 3ø circuit is the sum of the apparent powers for all phases.  I.E.;

      S =  Van*Ia + Vbn*Ib + Vcn*Ic

or, the description of apparent power is the sum of the products of each phase-to-neutral voltage and its associated phase current.  Clear as mud, right?  

RE: AC Motors

Thanks for the remarks indeed i used a wrong identifier.
It helps me really.


RE: AC Motors

I believe CSR was asking if the VA rating of a three phase device is for one phase or for all three? I can't recall but seems to me if a generator or transformer is rated then this applies to the total power so for each leg you have to divide the rating by three to come up with the number for an individual leg. Can someone confirm this for me too? I agree that terminology for three phase hardly ever involves total current. Total current may be something the power company is interested in or some other parties for sizing the neutral conductor in a four wire system. Thanks Buzz

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! Already a Member? Login


White Paper - Design for Additive Manufacturing
With a growing number of parts manufactured directly by additive manufacturing techniques, it is important to lay down design principles suitable for such manufacturing processes and to ensure parts are designed for additive manufacturing. There are several factors that are to be considered at the design stage. Few such design issues in additive manufacturing are discussed in this paper. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close