Neutral Loading

[jdd]

What current does the neutral take on a 3 phase system? For example if each line conductor pulls 100A, what does the neutral pull? Obviously not 300A, Bit confused, don't really work on 3 phase much.

 

[Guest123]

If each line conductor was drawing exactly 100A, there wouldn't be any current in the neutral conductor as all the phases are balanced.

Neutral current presents itself only when phases are unbalanced, the magnitude depends on how unbalanced they are.

 

[KFH]

If the load on the three phases is the same there should be no neutral current.

 

[jdd]

Thanks guys, often wondered, but never asked. Glad I did now. So if an installation was severely imbalanced I guess this could cause over heating in the neutral conductor?

 

[Farmelectrics]

I would say no as long as there was no loose connections

 

[Farmelectrics]

When an installation is designed the board is designed as balanced on all phases so sa the lads say no current in nuetral but in are works main kitchen area a re disgn of the kitchen which didn't include any design people cos of cost has left the board unbalanced on all phases but there is no over heating or damage to the nuetral

 

[Gordon]

Neutral current can never exceed the current of 1 phase which is why neutral conductor is usually same size as phase. Single phase
is as out of balance as you can get(100A Line = 100A Neutral). Currrent going into the neutral from 3 phases cancel each other out.
ie if all phases are balanced and put same current into neutral conductor they cancel each other out.

 

[Darkwood]

Thanks guys, often wondered, but never asked. Glad I did now. So if an installation was severely imbalanced I guess this could cause over heating in the neutral conductor?

Excluding all harmonics etc or loose connections, if you load up 1ph with 100amp and left ph2 and 3 empty your N current would be 100amps, adding load to ph2 or 3 will reduce the neutral current so as long as the neutral is the same csa as the phase conductor it will if the installation is designed correctly never overload with inbalance.
Many newby's get the impression that 100amps per phase may in extreme unbalanced situe put 300amp on the Neutral which isn't the case.

 

[Knobhead]

In=√(Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic))

So say:
Ia=65A, Ib=80A, Ic=40A In=35A

 

[SPARTYKUS]

In=√(Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic))

So say:
Ia=65A, Ib=80A, Ic=40A In=35A

Proper Bo.