Silly question about neutral current In as sum of phase currents Ia, Ib, Ic

[akyip]

Hey guys,

I have what might possibly be a stupid or silly question regarding neutral current as the sum of phase currents.

Under exam standards, which is more correct for In as the sum of phase currents?

In = Ia + Ib +Ic

OR

In = -(Ia + Ib +Ic)

I was always used to the former because I always visualized the three phase currents as coming into the neutral point, and then the neutral current flowing out of the neutral point node. Hence, my KCL visualization was:

Ia + Ib + Ic - In = 0 --> In = Ia + Ib + Ic

But as I'm doing practice exam problems, I see some questions have the solution as:

In = -(Ia + Ib +Ic)

Is one preferred over the other? Or is there something I might be missing or incorrect about?

Thanks for any time answering this question!

 

[Zach Stone P.E.]

Hey guys,

I have what might possibly be a stupid or silly question regarding neutral current as the sum of phase currents.

Under exam standards, which is more correct for In as the sum of phase currents?

In = Ia + Ib +Ic

OR

In = -(Ia + Ib +Ic)

I was always used to the former because I always visualized the three phase currents as coming into the neutral point, and then the neutral current flowing out of the neutral point node. Hence, my KCL visualization was:

Ia + Ib + Ic - In = 0 --> In = Ia + Ib + Ic

But as I'm doing practice exam problems, I see some questions have the solution as:

In = -(Ia + Ib +Ic)

Is one preferred over the other? Or is there something I might be missing or incorrect about?

Thanks for any time answering this question!

Hi @akyip It's really a matter of preference. Most technical analysis books use the same direction of reference for the neutral as the three line currents which is from source to load. That's where the negative sign comes from. However, most engineers from a practical application perspective think of the reference direction of the neutral current as the return path from load to source.

When you use KCL to solve for the neutral current:

Current leaving = Current entering

                        In = Ia + Ib + Ic

You're solving for the neutral current flowing back to the source from the load as a return path.

To flip the reference direction to calculate the neutral current from the source to load, you multiply by negative 1 (which is the same as rotating the phase angle by plus or minus 180 degrees:

In = -(Ia + Ib + Ic)

The good news is that questions are most likely only going to ask you for the magnitude of the neutral current. You'll need to include the phase angle of each of the line currents in your KCL formula but both methods will produce the same neutral current magnitude.