Neutral Conductor Sizing & 310.15(B)(7)

[Dennis Alwon]

I don't understand your question. I would think the neutral calculation already includes "load diversity."

The point is if I have a calculated load on a service of 100amps I could use all #4 service conductors however, if the calculated load for the service is larger than 100 amps but the calculated load for the neutral is 100 amps then I cannot use T. 310.15(B)(7) even though it is the same calculated load as the service above.

Here is why-- If the calculated load for a dwelling is 100 amps the chances are pretty good that the neutral conductor will not see 100 amps or anywhere close to it. If the neutral calculated load is 100 amps then that means there is a better chance that the neutral will see 100 amps although I doubt, in reality, it ever will be close.

 

[jumper]

I don't understand your question. I would think the neutral calculation already includes "load diversity."

My point is: that if this "load diversity" concept allows 175 amp conductors to carry a calculated resi load of 195/200 amps on the hot wires, why would the same not apply to the reduced neutral according to its calculated load of 100 amps?

If you use 310.16, load diversity is gone.

 

[david luchini]

My point is: that if this "load diversity" concept allows 175 amp conductors to carry a calculated resi load of 195/200 amps on the hot wires, why would the same not apply to the reduced neutral according to its calculated load of 100 amps? If you use 310.16, load diversity is gone.

I don't think "load diversity" is why the give the permitted reductions in T310.15(B)(7). The load diversity is already figured into the demand factors in the Feeder and Service Load calculations in 220.40. I'm under the impression that they give you these permitted reductions for 120/240V, 3 wire feeders because the feeder only has two current carrying conductors (the neutral is not counted as a CCC per 310.15(B)(4)(a)) and the allowable ampacities in T310.16 are based on 3 CCCs in a cable or raceway. Note that the permitted reduction does not apply to a 120/208V, 3 wire feeder, where there a 3 CCCs. But yes, the way the section is written, the permitted reduction would not apply to the neutral where the neutral is a reduced size. There is not a discernible logic to everything in the code.

 

[Dennis Alwon]

I don't think "load diversity" is why the give the permitted reductions in T310.15(B)(7). The load diversity is already figured into the demand factors in the Feeder and Service Load calculations in 220.40. I'm under the impression that they give you these permitted reductions for 120/240V, 3 wire feeders because the feeder only has two current carrying conductors (the neutral is not counted as a CCC per 310.15(B)(4)(a)) and the allowable ampacities in T310.16 are based on 3 CCCs in a cable or raceway. Note that the permitted reduction does not apply to a 120/208V, 3 wire feeder, where there a 3 CCCs. But yes, the way the section is written, the permitted reduction would not apply to the neutral where the neutral is a reduced size. There is not a discernible logic to everything in the code.

I disagree the load diversity is the main reason for 310.15(B)(7)

 

[jumper]

I disagree the load diversity is the main reason for 310.15(B)(7)

What Dennis said.:thumbsup:

 

[david luchini]

I disagree the load diversity is the main reason for 310.15(B)(7)

Are you saying load diversity is the main reason for 310.15(B)(7)?


Then why doesn't it apply to 120/208V, 3-wire single phase dwelling feeders?

 

[jumper]

David, he's referring to the inherent load diversity plugged into (B)(7), that we lose if we use (B)(16).

The calculated neutral load does not include diversity, although it had been demanded down.

Yep.

 

[Dennis Alwon]

Are you saying load diversity is the main reason for 310.15(B)(7)?


Then why doesn't it apply to 120/208V, 3-wire single phase dwelling feeders?

That I have no idea but the reason why this only applies to the main service and not sub panels is because of diversity. Once a sub panel is introduced the diversity for the house is changed since they have no control on how the circuits are run.

 

[david luchini]

David, he's referring to the inherent load diversity plugged into (B)(7), that we lose if we use (B)(16).

The calculated neutral load does not include diversity, although it had been demanded down.

I understand that, but where does 310.15(B)(7) say that the reduction is permitted because of "Load Diversity?" It doesn't. The dwelling unit feeder or service already includes "load diversity" when you apply the permitted demand factors in 220.40 when calculating the service/feeder load.

I believe the reductions in (B)(7) are not because of "load diversity" but because the feeder will have only two (2) current carrying conductors while the allowable ampacities in T310.15(B)(16) are based on three (3) current carrying conductors in a raceway or cable. Two ccc's would produce less heat giving a higher allowable ampacity. Thus the reductions (B)(7) for a 120/240V 3 wire feeder.

The reductions in (B)(7) do not apply to a 120/208V 3 wire feeder, because that feeder would have three (3) ccc's.

 

[david luchini]

That I have no idea but the reason why this only applies to the main service and not sub panels is because of diversity. Once a sub panel is introduced the diversity for the house is changed since they have no control on how the circuits are run.

Thats a good point, but if load diversity was the only reason, you would expect that it would also apply to 120/208V feeders. There may be some other reason it doesn't apply to sub feeders. Or maybe its a combination of the two concepts.

 

[Dennis Alwon]

It cannot apply to feeders because one could have all the a/c units and/or heating loads on that sub panel. That would defeat the purpose of the diversity.

 

[david luchini]

It cannot apply to feeders because one could have all the a/c units and/or heating loads on that sub panel. That would defeat the purpose of the diversity.

That doesn't sound right to me. If that was the case, then the reduction should also apply to 120/208V main feeders or services. The diversity on the main panel wouldn't be different if the service was 120/240 or if it was 120/208 single phase.

But as I noted, the load diversity for the service is applied in 220.40. A sub panel feeder will also be able to apply the demand factors in 220.40, but would likely result in a smaller % reduction if all the a/c units were on the sub panel. So the question of load diversity is addressed in the service and feeder calculation section.

The only difference between a 120/240V 3 wire feeder and a 120/208V 3 wire feeder is the number of current carrying conductors.

 

[dana1028]

The point is if I have a calculated load on a service of 100amps I could use all #4 service conductors however, if the calculated load for the service is larger than 100 amps but the calculated load for the neutral is 100 amps then I cannot use T. 310.15(B)(7) even though it is the same calculated load as the service above.

Here is why-- If the calculated load for a dwelling is 100 amps the chances are pretty good that the neutral conductor will not see 100 amps or anywhere close to it. If the neutral calculated load is 100 amps then that means there is a better chance that the neutral will see 100 amps although I doubt, in reality, it ever will be close.

I think it is finally starting to sink in.


The "calculated load"...

Dennis - correct me if I am wrong....previous posts have discussed this in detail.

If I remember correctly, we never could use T310.15(B)(7) when the calculated load exceeded the ampacity of the conductors [in T310.15(B)(7)].

Per the table we can use a #4 for 100A service conductors but only IF the calculated load did not exceed the ampacity value of the #4s. [I don't think I'm nuts about remembering this topic in other forum discussions].

So - per the original posting, the neutral was actually "calc'd" at 100A which does exceed the value of a #4.....thus, we have to use T310.16.

 

[jumper]

I think it is finally starting to sink in.


The "calculated load"...

Dennis - correct me if I am wrong....previous posts have discussed this in detail.

If I remember correctly, we never could use T310.15(B)(7) when the calculated load exceeded the ampacity of the conductors [in T310.15(B)(7)].

Per the table we can use a #4 for 100A service conductors but only IF the calculated load did not exceed the ampacity value of the #4s. [I don't think I'm nuts about remembering this topic in other forum discussions].

So - per the original posting, the neutral was actually "calc'd" at 100A which does exceed the value of a #4.....thus, we have to use T310.16.

But the calculated load for the hots exceed 310.16. In my mind it is use one table or the other according to the install parameters.

If you want a bigger neutral, I want a bigger set of hots/ungrounded conductors.

 

[dana1028]

Derek - you are correct - we never could use the T310.15(B)(7) values for our service conductors IF our calculated load exceeded those values...we were always required to go to T310.16 in that situation.

 

[jumper]

Derek - you are correct - we never could use the T310.15(B)(7) values for our service conductors IF our calculated load exceeded those values...we were always required to go to T310.16 in that situation.

Me correct? Are you nuts?

Where in the Bay area are you?

 

[Dennis Alwon]

Derek - you are correct - we never could use the T310.15(B)(7) values for our service conductors IF our calculated load exceeded those values...we were always required to go to T310.16 in that situation.

That is not an NEC requirement.

 

[George Stolz]

If I remember correctly, we never could use T310.15(B)(7) when the calculated load exceeded the ampacity of the conductors [in T310.15(B)(7)].

Per the table we can use a #4 for 100A service conductors but only IF the calculated load did not exceed the ampacity value of the #4s. [I don't think I'm nuts about remembering this topic in other forum discussions].

I don't recall seeing that discussion, but I disagree with that conclusion.

The illogical yet appropriate use of the table is straightforward; calculate the load, as you would anything else, and then use that number to select a service rating. Then, throw caution to the wind and connect a conductor to the service OCPD with an insufficient ampacity for the load in an approved fashion.

That doesn't sound right to me. If that was the case, then the reduction should also apply to 120/208V main feeders or services. The diversity on the main panel wouldn't be different if the service was 120/240 or if it was 120/208 single phase.

Here is what the panel has said about this section over the years.

Panel Statement: The original data that was used to establish 310.15(B)(6), formally Note 3, was actual utility company data for 120/240 volt 3-wire single phase systems. It established that the conductors specified in the table could be used on a calculated dwelling unit load as shown. The submitter has not provided any technical data to show that this is true for 120/208 4-wire three phase systems.

The submitter is incorrect in his assumption that the conductors in 310.15 (B)(6) are not permitted to be paralleled in accordance with 310.4. The conductor ampacities listed in 310.15(B)(6) are based on the diversity of the total load of an individual dwelling. This means that the conductors of a 120/240-volt, single-phase dwelling service or feeder with a calculated load of 200 amps will never carry 200 amps. Due to this fact, the language and table in 310.15(B)(6) will permit the use of a 2/0 conductor, which has an ampacity of 175 amps in the 75 degree C column...

The language in this section, along with the table, has been in the code for many years and, as the submitter states, has been modified to provide more clarity over the past several cycles. When appropriately applied, there has been no evidence that the sizing of the conductors shown in the table creates a problem. Table 310.15(B)(6) deals with service loads, not with ampacities.

Now, they are inconsistent - in one response they say it does not list ampacities, and in the next breath they say it does. But the Table is based upon load diversity. There was a proposal in the last cycle to delete 310.15(B)(6) and just give a 10% reduction on the load calc to achieve the bonus for diversity, but it was rejected for being arbitrary.

I'm going to try again this cycle.

 

[Little Bill]

But the calculated load for the hots exceed 310.16. In my mind it is use one table or the other according to the install parameters.

If you want a bigger neutral, I want a bigger set of hots/ungrounded conductors.

First, I agree with the table. But you can't always use a reduced neutral. Which is stated in (B)(7)..."provided the requirements of 215.2,220.61, and 230.42 are met. 220.61 is what is most clear to me. The neutral is sized on the maximum unbalanced load between the neutral and any one ungrounded conductor. For example, if there were no 240V loads and under severe unbalanced conditions, the neutral would carry the same current as an ungrounded conductors, and would need to be sized accordingly.

 

[nathanaelhn]

Nathanael N

Nathanael N

Im not sure what version of the NEC you are using. I have a 2008 NEC copy. In 2008 Article 310.15(B)6 it states at the end "The grounded conductor shall be permitted to be smaller than the ungrounded conductors..." THis is in reference to the Table right below it, so id say that #4 copper is fine using that Table. I understand some of your points in theory and this is undersizing the nuetral in a sense but the code seems to indicate that it is fine in residential situations, why... I dont know maybe they figure it will not likely be a problem in a home? Anyway my version of the code seems clear, from looking at yours posts it looks like you may have a different version than I do.