Current calculation

[solomon]

which fomular is best and simple way to calculte a 3 phase 4 wire current.
I measured a 3 phase current as follows: Red 189A, yellow-210A and blue 196A.
In an unbalance circiut/load, how do i sum up the total current in the above?

 

[Smart $]

which fomular is best and simple way to calculte a 3 phase 4 wire current.
I measured a 3 phase current as follows: Red 189A, yellow-210A and blue 196A.
In an unbalance circiut/load, how do i sum up the total current in the above?

You don't.

You can calculate neutral current, but you'll have to make an assumption, which in the real world is seldom true.

You can also calculate total kVA... but you didn't provide the voltage.

 

[Julius Right]

As Smart $ already said, I don?t think a common total current has any importance. The total power will be sum of the product: ?[ Iph(i)*Vph(i)*cos(Iph(i),Vph(i))].
If you need to appreciate the current for calculating the allowed ampacity the maximum current has to be calculated-it is not the actual current [now] but maximum possible in this circuit. See, for instance, NEC Art.430 for motor overload or Art. 450 for transformer overcurrent protection.
:happyno:

 

[Tony S]

For neutral
In = ?((Ia?+Ib?+Ic?)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic)))
This doesn?t account for harmonics.

 

[Smart $]

For neutral
In = ?((Ia?+Ib?+Ic?)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic)))
This doesn?t account for harmonics.

Nor does it account for unequal power factors.

 

[Julius Right]

For neutral
In = ?((Ia?+Ib?+Ic?)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic)))
This doesn?t account for harmonics.

The neutral current-as Smart$ already said- Io=-(IA+IB+IC) in complex [but considering the same angle origin for all currents]. You have to draw a current phase diagram [considering the phase voltages as symmetrical and using a single phase wattmeter] and then to state the angle origin

 

[Thaddeus H. Black]

Phasor theory and nonlinear loads

Phasor theory and nonlinear loads

It is understandable that one would ask such a question, because three-phase calculations are not easy. However, the question as asked admits no straightforward answer. There is no "total current." There is a neutral current, but one doubts that the neutral current is the answer you seek.

The full answer to your question involves phasor theory, which is nontrivial to learn. See for example T. Wildi, Electrical Machines, Drives and Power Systems, 3rd ed., Ch. 8., 1997.

Of more practical interest may be what the NEC has to say on the question, in particular in article 310.15(B)(5), 2011. The NEC however will not help you to understand what is going on here, nor why your question regrettably does not quite make sense.

I wish that it were possible in a brief format like this to give you a more informative answer. Other than to caution you that you may need to take into account the matter of nonlinear loads to get an accurate answer to your question -- especially if a VFD is involved -- and to acknowledge that you may not know what a nonlinear load is, why, if you lack the time, preparation and/or aptitude to study the relevant electrical theory in considerable detail, then all I can do is to direct you to an engineer or, if this is not realistic, to wish you luck.

 

[iceworm]

Her are two posts from a similar question:

Given a 3-phase wye circuit (3 conductors and grounded neutral), with phase currents of L1=58A, L2=74A, L3=64A and N=12A, how does one compute the total current?

... Here is a suggestion on nomenclature:

Assuming 65kva, 208V Wye: Output current is 180A. Forget the "amps per phase". Forget the "total current". Those terms don't mean anything when discussing 3 phase.

For 3 phase balance load (fully loaded), put a clamp-on on each phase conductor, and each ammeter will read 180A. When you are asked what is the current? And you say 180A. You are saying the current is balanced and nominal 180A measured on each phase conductor.

For unbalance load, list the currents just the way you did, "phase currents of L1=58A, L2=74A, L3=64A and N=12A". That is perfect. There is no total current. They are vectors - if you add them, they better equal zero.

As others have said. The currents are vectors. And, yes, if the four currents are added together, the Sum(I) =0. Look at it this way:
There are four wires running from the panel to the equipment. Any current delivered on on phase comes back of one of the other phases ()or comes back on the neutral). So, if you put a clamp-on around all four conductors, it will read zero.

What you did was exactly right:

I measured a 3 phase current as follows: Red 189A, yellow-210A and blue 196A.

In an unbalance circiut/load, how do i sum up the total current in the above?

Exactly the way you did L1 = 189A, L2 = 210A, L3 = 196A, N = ???A.

That tells everything.

Well, except power factor - but that is the next step.

ice

 

[Smart $]

... Any current delivered on one phase comes back of one of the other phases ()or comes back on the neutral). ...

Because of the phase offset, the amount of current on each of the other wires during a single instance will vary, dependent on the configuration and draw of the load. It is easily possible during a single instance that current delivered through one phase conductor is returned on one, or in part through two or all three other conductors.