return line from the transformer to substation

[fredspence1]

How is it that the grounded return line going back to the substation from the primary side of a transformer is frequently smaller than the ungrounded phase lines? I've had some tell me that it is because it's grounded. Others have said it is because the 3 phases are mostly cancelled out and the return just carries the unbalance, both of which I'm uncomfortable with. I don't have any utility line experience and it's bothered me for awhile.

 

[charlie b]

Re: return line from the transformer to substation

Others have said it is because the 3 phases are mostly cancelled out and the return just carries the unbalance . . . which I'm uncomfortable with.

You can take comfort in that explanation. It is valid. If the loads on the three ungrounded conductors are perfectly balanced, the grounded (i.e., "neutral") conductor will carry no current at all. In the case of an "extreme" unbalance, such as "Phase A = 0 current and Phases B and C carrying the same current," the neutral will carry the same current as B and C. So if the neutral is not the same size as the others, then it might be overloaded in this case of unbalance. That is a key reason that we are encouraged to balance the loads.

 

[LarryFine]

Fred, it's also why the neutral in your house is permitted to be smaller than the hots.

 

[Pierre C Belarge]

Charlie
Could we also add that with Utility type voltages, the ground conductor is also somewhat supplemented with the use of earth?

 

[charlie b]

Could we also add that with Utility type voltages, the ground conductor is also somewhat supplemented with the use of earth?

Yes. And we can say that, here too, the neutral current that flows through planet Earth is just the unbalanced current from the three phases. (BTW, I think you meant "grounded.")