3 Phase Transformer Neutral Load

[industrial1003]

Hello Everyone,

I have a quick question about the current that a neutral conductor will see if a 3 phase panelboard were to be fully loaded with single phase loads.

For example:

A 3 phase delta to wye transformer 480V HV, 208Y/120 LV, 30KVA rating

Assuming a balanced load of 10KVA per line, each line is capable of drawing 83A at 120V

If all loads are in simultainous operation, would the neutral conductor then see 83A + 83A + 83A = 249A? Or does the fact that the lines are 120deg offset because it is a 3 phase panel keep the neutral at 83A? Or the third option, would this require vector math to resolve (actual answer somewhere between 83 and 249)?

Supposing the worst case result (option 1 249A) would the transformer be capable of this type of load at its neutral connection?

Any insight would be helpful. Thanks.

 

[Ingenieur]

Hello Everyone,

I have a quick question about the current that a neutral conductor will see if a 3 phase panelboard were to be fully loaded with single phase loads.

For example:

A 3 phase delta to wye transformer 480V HV, 208Y/120 LV, 30KVA rating

Assuming a balanced load of 10KVA per line, each line is capable of drawing 83A at 120V

If all loads are in simultainous operation, would the neutral conductor then see 83A + 83A + 83A = 249A? Or does the fact that the lines are 120deg offset because it is a 3 phase panel keep the neutral at 83A? Or the third option, would this require vector math to resolve (actual answer somewhere between 83 and 249)?

Supposing the worst case result (option 1 249A) would the transformer be capable of this type of load at its neutral connection?

Any insight would be helpful. Thanks.

if balanced it would see 0 (or there abouts)
yes, requires phasor/vector math or a formula
worse case is 10kva on 1 phase 0 on the others, so 83 A on N

 

[kwired]

If the loads are linear loads the neutral will see zero/very near zero current.

If there is harmonic producing loads you may have some harmonic currents adding up in the neutral.

 

[Tony S]

This doesn’t account for harmonic currents in the neutral

I_n = ±√((I_a²+I_b²+I_c²)-((I_a*I_b)+(I_a*I_c)+(I_b*I_c)))

 

[roger]

Use this formula for a "close enough" total

SQRT I^2A + I^2B + I^2C - (IA x IB) - (IB x IC) - (IC x IA)

Roger

 

[industrial1003]

Everyone,

Thanks for the input, I was concerned that if a panel was loaded with all single phase circuits the neutral conductor would need to be much larger than the line conductors (Neutral It = I1+I2+I3) as it would be every loads path back to the transformer. But from the replies it looks like a 100% neutral would be OK.

 

[Carultch]

Use this formula for a "close enough" total

SQRT I^2A + I^2B + I^2C - (IA x IB) - (IB x IC) - (IC x IA)

Roger

Your formula appears to have all the same terms as KWwired's formula, yet you are missing the definition of what is part of the square root operation. Is your formula intentionally a different one? Also, it is not unit kosher the way it is written.

 

[roger]

Your formula appears to have all the same terms as KWwired's formula, yet you are missing the definition of what is part of the square root operation. Is your formula intentionally a different one? Also, it is not unit kosher the way it is written.

I think you meant Tony's formula and you are absolutely correct, the problem is, the way I originally entered it won't work because of forum problems, for example SQRT I�A + I�B + I�C - (IA x IB) - (IB x IC) - (IC x IA)


Roger