Sizing grounded conductor

[Andrew A]

I am installing a 100 amp, 230 volt single phase feeder to a 20 circuit outdoor panel. The connected load will be 1-2 hp, 230 volt motor - 12 amps, 1-3/4 hp 230 volt motor - 6.9 amps, 1-led light - .5 amps and 2 convenience outlets. Currently there are no definite plans for additional electrical load(s). I plan on using # 3 copper for the ungrounded conductors.
Questiion - Does the grounded conductor have to be "full size", or can it be derated. What code sections govern this type of installation?

 

[kingpb]

Forget the code minimum safety requirements, it is not a design guide. Run a #3 and be done. You've probably though about it longer than it's worth.

But why are you running 100A cable when your load is no where near that and you have no intention of future, if you are trying to save cost, then run what you need and no more.

 

[texie]

I am installing a 100 amp, 230 volt single phase feeder to a 20 circuit outdoor panel. The connected load will be 1-2 hp, 230 volt motor - 12 amps, 1-3/4 hp 230 volt motor - 6.9 amps, 1-led light - .5 amps and 2 convenience outlets. Currently there are no definite plans for additional electrical load(s). I plan on using # 3 copper for the ungrounded conductors.
Questiion - Does the grounded conductor have to be "full size", or can it be derated. What code sections govern this type of installation?

220.61 covers this. Also, in no case the feeder neutral can't be smaller than that required by 250.122 per 215.2(A)(2) or if this were a service the neutral could not be smaller than required by 250.66.
Usually just using a neutral 2 sizes smaller than the ungrounded conductors is a good guess for applications like yours.

 

[Dennis Alwon]

You don't really need a neutral however if you run one it cannot be smaller than the equipment grounding conductor.

 

[texie]

You don't really need a neutral however if you run one it cannot be smaller than the equipment grounding conductor.

You missed the fact that the OP has some neutral load. You must be sleeping this morning.

 

[Dennis Alwon]

You missed the fact that the OP has some neutral load. You must be sleeping this morning.

Most days I waddle thru somehow. Getting lazy with the reading-

 

[qcroanoke]

Welcome to the forum!

 

[Ponchik]

From your description you don't have that much neutral load, but then again you might want to plan for future.

So, you can do a full #3 or you can do the minimum #8 and as stated earlier can not be smaller than the EGC #8.

 

[Andrew A]

grounded conductor sizing

grounded conductor sizing

Forget the code minimum safety requirements, it is not a design guide. Run a #3 and be done. You've probably though about it longer than it's worth.

But why are you running 100A cable when your load is no where near that and you have no intention of future, if you are trying to save cost, then run what you need and no more.

Although the homeowner is not sure what he would like to do down the road, he wants a 100 amp sub panel installed. The cost of the full size grounded conductor is not the issue. Minimum code compliance for this installation has become the topic of much discussion with inspectors lining up on both sides. Specific to the installation described, does the 2011 NEC permit the use of a grounded conductor that is less than full size? If your answer is yes, what section(s) of the code would support your position.

Thank you,

Andrew

 

[augie47]

220.61 is the applicable Code Article (along with 215.2).
To me the problem becomes the "calculated load". Did you perform the load calculations per Art 220 and submit them to the AHJ ?
Rightly or wrongly, many inspectors are reluctant to accept reduced neutrals on smaller residential feeders as the possibility of future loads being added by persons who will not attempt to perform any load calculation is so great.

 

[Ponchik]

Although the homeowner is not sure what he would like to do down the road, he wants a 100 amp sub panel installed. The cost of the full size grounded conductor is not the issue. Minimum code compliance for this installation has become the topic of much discussion with inspectors lining up on both sides. Specific to the installation described, does the 2011 NEC permit the use of a grounded conductor that is less than full size? If your answer is yes, what section(s) of the code would support your position.

Thank you,

Andrew

You can have smaller grounded conductor that is based on your calculated neutral load.
Take a look at 215.2(a)(2) and 220.61.

In fact you can use table 310.15(b)(7) assuming you meet the table requirements.

 

[Andrew A]

220.61 is the applicable Code Article (along with 215.2).
To me the problem becomes the "calculated load". Did you perform the load calculations per Art 220 and submit them to the AHJ ?
Rightly or wrongly, many inspectors are reluctant to accept reduced neutrals on smaller residential feeders as the possibility of future loads being added by persons who will not attempt to perform any load calculation is so great.

Hello Augie, I believe the load calculation complies with article 220. The total calculated load is 18.9 amps at 230 volts, (2 motor loads using table 430-248) and 3.77 amps at 110 volts (1-led pool light and 2 convenience outlets). Without getting into the minutia, the AHJ believes that 260.61 applies to services not feeders.
Assuming any additional wiring is code compliant, can you describe a scenario where an installation consisting of 100 amp rated ungrounded conductors (#3 copper) and a 70 amp rated grounded conductor (#6 copper) could lead to an overload of the grounded conductor.
Given the code requirement (manufacturers listing and labeling instructions) that limits circuit breaker use to 80% of its value (continuous load), I cannot.

Thanks for your input,

Andrew

 

[Ponchik]

Directly from NEC 2011

220.61 Feeder or Service Neutral Load
(A) Basic Calculation. The feeder or service neutral load shall be the maximum unbalance of the load determined by this article. The maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor.

 

[GoldDigger]

If the calculated load is 80A per phase and the three phase load is 10A or more, that would make the maximum imbalance only 70A or less. But does the wording of the section allow you to do that?

Tapatalk...

 

[augie47]

I can find no reason from the info given that your job would not be Code compliant with a #8 grounded conductor.
However, if the inspector does not believe 220.61 applies to Feeder Neutrals, since that is the Title, you obviously have a struggle on your hands.
Many inspectors would ask for a written load calculation to file with their paperwork just to cover themselves in the event of future changes.
Beyond that, I know nothing you can do other than appeal his decision.

 

[Andrew A]

Welcome to the forum!

Thank you

 

[Andrew A]

If the calculated load is 80A per phase and the three phase load is 10A or more, that would make the maximum imbalance only 70A or less. But does the wording of the section allow you to do that?

Tapatalk...

Hello, thanks for your response, sorry if I wasn't clear, this is a single phase installation.

 

[Andrew A]

I can find no reason from the info given that your job would not be Code compliant with a #8 grounded conductor.
However, if the inspector does not believe 220.61 applies to Feeder Neutrals, since that is the Title, you obviously have a struggle on your hands.
Many inspectors would ask for a written load calculation to file with their paperwork just to cover themselves in the event of future changes.
Beyond that, I know nothing you can do other than appeal his decision.

I am preparing to do so.