Hot and neutral service entrance conductors

[williamsmithaud]

I understand that the neutral service conductor to a panel must be "equal to, or no less than two sizes smaller" than the ungrounded hot service entrance conductors.

What I would really like to understand is the technical reason why the neutral conductor can be as much as "two sizes smaller". I was asked this question recently by an associate, but was unable to provide him a technical or scientific explanation.

I hope somone can provide an easy to understand answer to this question.

 

[roger]

Re: Hot and neutral service entrance conductors

William, it can be more than two sizes smaller.

This is covered in 220.22, but for a short answer to why it can be smaller, loads such as water heaters, some pump motors, some A/C equipment etc... are "phase to phase" or "leg to leg" loads and do not utilize a neutral. These loads can be ignored when sizing the neutral.

Roger

 

[djtazjr]

Re: Hot and neutral service entrance conductors

Well, if you want to get scientific you will have to understand how electricity works. But to have a simple understanding, "neutral" current, or the unbalanced load or the unused current returns on the white or neutral wire. With linear loads the voltage dropped at each point is different and so the unused voltage that the and equipment doesn't use returns, 120 volts 20 amps out but the equipment but the equipment only requires 110 volt 12 amps. so the remaining amps return to the source.

Whatever the service ungrounded conductors are, be it 2/0 for 150 amps or so on, the neutral conductor can be "derated" or smaller then the ungrounded conductors because the return, or unbalanced load is smaller the "supply".

This is only true for 120/240 volt systems. 120/208 is another story. You can not derate the neutral on this system.

If this helps you any good, but I really can't give a real drawn out explanation. The others have more experience then I, and I too would like to see more post on this subject.

 

[Ed MacLaren]

Re: Hot and neutral service entrance conductors

William,
Here is a simplified sketch representing a typical 120/240 volt, 3-wire feeder.
The neutral carries only the unbalanced load (the difference in the "hot" conductor currents).

If the 120 volt loads were perfectly balanced, there would be 0 amps in the neutral conductor.

Ed

 

[iwire]

Re: Hot and neutral service entrance conductors

Originally posted by djtazjr:
This is only true for 120/240 volt systems. 120/208 is another story. You can not derate the neutral on this system.

I know of no code article that prevents downsizing the neutral on 120/208 systems.

Am I missing something?

[ November 23, 2003, 05:54 AM: Message edited by: iwire ]

 

[williamsmithaud]

Re: Hot and neutral service entrance conductors

Thanks to all of you for your replys. This makes complete sense and now I understand.

 

[djtazjr]

Re: Hot and neutral service entrance conductors

Iwire, I am wrong about lots of things so correct me if I am. I didn't want to get into the Non linear loads for the topic.

With that system 3 phase 4wire wye containing only nonlinear loads its not recommended to de-rated to neutral for harmonic currents could be introduced into the neutral causing it to overheat.

Once again, I always appreciate constructive criticism.

 

[iwire]

Re: Hot and neutral service entrance conductors

I agree if the load served has Non linear loads that would be something to design for.

Lets say you are providing a service for a pumping station with a 208/120 service.

95% of the loads are 3 phase pumps, the only line to neutral loads are the lighting and some outlets for service work.

In a case like this I think the only thing limiting how small a neutral conductor you could run for the service would be 250.24(B)

I have never done any service with reduced size neutrals I just believe code wise it can be done.

 

[roger]

Re: Hot and neutral service entrance conductors

DJ, for a quick example calculation to see why your statement

With linear loads the voltage dropped at each point is different

is wrong try it with (4) 240 ohm resistors at 60 watts each, what is the voltage dropped at each load.

You can put as many loads as you want in the question.

In a parallel circuit the sum of the current of the loads is the total current.

There is no "excess" current left over some where.

Check out Kirchoff's laws.

This is actually the same in single or three phase.

Figuring neutral current in a Wye configuration is a different method than what Ed shows in a single winding, but has nothing to do with whether it can be reduced, it can.

Roger

[ November 23, 2003, 10:38 AM: Message edited by: roger ]

 

[djtazjr]

Re: Hot and neutral service entrance conductors

Yes, Roger Kirchoff's Law. I ran into a similar situation applying Kirchoff's law with grounding.

Thanks, its good to have many minds to get the bigger picture.