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# AC Motors

## AC Motors

(OP)
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

C.A.A.

### 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

(OP)
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?

C.A.A.

### 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

(OP)
Hi electricpete--

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

C.A.A.

### 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

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

csr

### 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

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