Dwelling Unit Optional Calculation

[jaggedben]

.... would this be correct, or wrong?

Correct. And I would even say you could use #4 according to 310.12.

 

[Grouch1980]

Correct. And I would even say you could use #4 according to 310.12.

ah. and what about the fusing? let's say 220.82 brings my connected load of 60 amps down to 41 amps. Can i use a 45 amp breaker / set of fuses? or does the 100 amp wire have to be protected at 100 amps (let's assume i'm using #3 wire)?

 

[jaggedben]

ah. and what about the fusing? let's say 220.82 brings my connected load of 60 amps down to 41 amps. Can i use a 45 amp breaker / set of fuses? or does the 100 amp wire have to be protected at 100 amps (let's assume i'm using #3 wire)?

I suppose there's no code rule against that, if the breaker is not a service disconnect or a detached building disconnect. (e.g. it's in a multi dwelling building). However you won't be able to land #3 on that 45A breaker that you special ordered inadvisadly. And, what would be the point?

 

[Grouch1980]

I suppose there's no code rule against that, if the breaker is not a service disconnect or a detached building disconnect. (e.g. it's in a multi dwelling building). However you won't be able to land #3 on that 45A breaker that you special ordered inadvisadly. And, what would be the point?

Yeah I got it now. I find this section a little 'off', but I got it. Understood about the #3's not being able to connect to the 45 amp breaker. Was just curious about the numbers aside from the physical limitations. Thanks for the help!

 

[david luchini]

Correct. And I would even say you could use #4 according to 310.12.

As Dennis pointed out in post #8, it appears that the language in 220.82 would preclude its use with #4 conductors.

 

[jaggedben]

As Dennis pointed out in post #8, it appears that the language in 220.82 would preclude its use with #4 conductors.

In my opinion the wording in 220.82 should be read as whatever 310 says you can use for 100A. Therefore, see 310.12.

 

[david luchini]

In my opinion the wording in 220.82 should be read as whatever 310 says you can use for 100A. Therefore, see 310.12.

The wording in 220.82 doesn't say anything about "whatever 310 says you can use for 100A."

It says "service or feeder conductors with an ampacity of 100 or greater."

According to Art 310, #4 conductors do not have an ampacity of 100 or greater.

 

[jaggedben]

The wording in 220.82 doesn't say anything about "whatever 310 says you can use for 100A."

It says "service or feeder conductors with an ampacity of 100 or greater."

According to Art 310, #4 conductors do not have an ampacity of 100 or greater.

Three points:

First, 310.15(A) begins: "Ampacities for conductors rated 0 volts to 2000 volts shall be as specified in the Ampacity Table 310.16 through Table 310.21, as modified by 310.15 (A) through (F) and 310.12." (Emphasis mine.) Pretty direct contradiction of your last sentence.

Second, "whatever 310 says you can use for xxA" is as good as any guidance you'll find in the NEC on how to determine what 'ampacity' is when other articles refer to it. The article 100 definition of ampacity really sheds no light on the contradiction between the way it is used in 310.12 vs everywhere else I've noticed.

Finally, when the language of 310.12 (formerly 310.15(A)(7)) was revised a few cycles ago, there clearly was no intent to change the application of 220.82.

 

[david luchini]

Three points:

First, 310.15(A) begins: "Ampacities for conductors rated 0 volts to 2000 volts shall be as specified in the Ampacity Table 310.16 through Table 310.21, as modified by 310.15 (A) through (F) and 310.12." (Emphasis mine.) Pretty direct contradiction of your last sentence.

No it doesn't.

Second, "whatever 310 says you can use for xxA" is as good as any guidance you'll find in the NEC on how to determine what 'ampacity' is when other articles refer to it. The article 100 definition of ampacity really sheds no light on the contradiction between the way it is used in 310.12 vs everywhere else I've noticed.

If 310.12 said that the ampacity of #4 was 100A, there would be no issue. But it doesn't say that. 310.12 says you can use a conductor with an ampacity of not less than 83% of the service rating. The ampacity of #4 is 85A, which is not less than 83% of 100A, so 310.12 permits #4 to be used for a 100A dwelling feeder or service. But the ampacity of the #4 is still 85A, which precludes the use of 220.82.

The only modification to the ampacity in 310.15(A) through (F) and 310.12 which gets the ampacity of #4 to 100A would be an ambient temperature correction factor for 50degF or less.

Finally, when the language of 310.12 (formerly 310.15(A)(7)) was revised a few cycles ago, there clearly was no intent to change the application of 220.82.

Which supports my position. The application of 220.82 has always been contingent on the service/feeder conductors having an ampacity of not less than 100A, and #4 conductors have never had an ampacity of 100A. 310.12 has always permitted a smaller conductor size for services.

220.82 could have said "set of 3-wire service or feeder conductors for a service rated 100A or greater" and there would be no issue. But that's not what the Code says. It specifically mentions the ampacity of the conductors.

310.12 could have said "shall be permitted to have their ampacity increased by 20%" or 310.12 could have said "for a 3-wire service or feeder the ampacity of the conductors shall be as follows: #4-100A, #3-110A, #2-125A...etc." then there would be no issue. But that's not what the Code says.
Instead it permits the use a conductor with a lower ampacity than the service rating. The ampacity of the #4 conductor per 310.12 is still 85A, but it is permitted to be used on a 100A rated service.

There is (and always has been) a disconnect between 220.82 which requires a service/feeder conductor ampacity of 100A or greater and 310.12 which permits a service/feeder ampacity which is lower than 100A.

As written, the Code would not permit use of the 220.82 optional method on a 100A service with #4 service conductors. As I noted above, it would require a simple change to the language to remedy the situation.

 

[jaggedben]

No it doesn't.

...

If you can't address that point more substantively then I really need not say much more. What I quoted says that 310.12 modifies the ampacity of conductors. That's what it plainly says. Which makes everything else you said fairly irrelevant.

If 310.12 said that the ampacity of #4 was 100A, there would be no issue.

...
There is (and always has been) a disconnect between 220.82 which requires a service/feeder conductor ampacity of 100A or greater and 310.12 which permits a service/feeder ampacity which is lower than 100A.

Before the old 310.15(B)(7) was modified with it's inconsistent use of 'ampacity' then there were no contradictions. Again, see 310.15(A) in the 2020 NEC.

 

[david luchini]

If you can't address that point more substantively then I really need not say much more. What I quoted says that 310.12 modifies the ampacity of conductors. That's what it plainly says. Which makes everything else you said fairly irrelevant.

I did address it more substantially in the following sections. If you chose to ignore that there's not much more I can say.

Before the old 310.15(B)(7) was modified with it's inconsistent use of 'ampacity' then there were no contradictions. Again, see 310.15(A) in the 2020 NEC.

Neither 310.12 in the 2020 nor the old 310.15(B)(7) tell you that the ampacity of #4 conductors is 100A. If you could point to the language in either of those sections that tells you the ampacity of the #4 conductors has increased to 100A, you would have a point. But no such language exists. You're reading what you want the sections to say rather the what they actually say.

Maybe you could provide an explanation as to why a conductor that has an ampacity of 85 for general purposes could suddenly have an ampacity of 100 when supplying a dwelling unit? Does 100A generate less heat on dwelling unit feeders than ot does on commercial feeders?

 

[Grouch1980]

I'm just glad I understood 220.82. that was the brain buster for me. and it all just comes down to the code's language and how it's worded.

 

[jaggedben]

... If you could point to the language in either of those sections that tells you the ampacity of the #4 conductors has increased to 100A, you would have a point. But no such language exists. You're reading what you want the sections to say rather the what they actually say.

310.15(A) (or whatever it was numbered previously) is the language I'm pointing to. It has always said that the "Ampacities of conductors ... shall be as specified in [various 310 tables] and as modified by [other parts of 310 including] 310.12" (or whatever it was previously numbered). I don't need to cite language in 310.12 because 310.15(A) explicitly encompasses 310.12. As far as I can see, it is the primary NEC guidance as to what the ampacity of a conductor 'is' when that is referred to in any other article. The only confusing thing about it is that they've been constantly changing the section numbering.

It's right there in plain English. You are simply choosing to ignore it.

Is it your view that I can ignore temperature derating and number of conductors in a raceway have no bearing when asking what 'ampacity' is referred to by (for example) 220.82? That if I had (hypothetically) 20 current carrying conductors in a raceway that were all for 3-wire feeders to dwellings that 220.82 would still apply to them if they were all #3? Because that is part of the same clause in the same sentence.

 

[david luchini]

I don't need to cite language in 310.12 because 310.15(A) explicitly encompasses 310.12.

You "don't need to cite it" because you can't. It doesn't exist. There is no language in 310.12 that says that the ampacity of #4 conductors is 100A. It's that simple. You have to pretend the section says something it doesn't to support your position.

 

[jaggedben]

Once again, 310.15(A) says that 310.12 modifies the ampacity of conductors. Stop ignoring that.

 

[david luchini]

Once again, 310.15(A) says that 310.12 modifies the ampacity of conductors. Stop ignoring that.

Once again, the text of 310.12 doesn't say anything about modifying the ampacity of conductors. Stop ignoring what 310.12 says.

Let's review:

310.12(A): For a service rated 100 amperes through 400 amperes, the service conductors supplying the entire load..shall be permitted to have an ampacity not less than 83 percent of the service rating.

For a 100A rated service, you are permitted to use a conductor with an ampacity of 83. A conductor with an ampacity of 83 is NOT a conductor with an ampacity of 100, as 83 does not equal 100. That's elementary arithmetic.

If you could explain what magic mechanism increases the ampacity of a #4 conductor by 18% when it is connected to a dwelling units vs. when it is connected to any other type of structure, I'd be happy to hear it.

Since you are so caught up in the language in 310.15(A) ("as modified by 310.15(A) through (F) and 310.12") that it causing you to ignore the plain language in 310.12, let's look at the other sections mentioned in 310.15(A).

310.15(A): There is nothing in the rest of 310.15(A) that modifies the ampacity of conductors. (0-1)
310.15(B): Ampacities...shall be corrected. This actually modifies conductor ampacity. (1-1) - batting .500
310.15(C): Ampacities...shall be be reduced. Two in a row modifying ampacity. (2-1)
310.15(D): Nothing that modifies the ampacity of conductors (2-2) - back to even
310.15(E): Nothing that modifies the ampacity of conductors (2-3) - sub .500
310.15(F): Nothing that modifies the ampacity of conductors (2-4)
310.12: Nothing that modifies the ampacity of conductors (2-5)

How is it possible that of all of the sections that 310.15(A) says will "modify the ampacity of conductor," only two actually provide direction to do so? Could it be that the language "as modified by 310.15(A) through (F) and 310.12" isn't an absolute direction that those sections will change the ampacity of a conductor, since most of the listed sections don't provide any direction on modifying the ampacity of a conductor?

Maybe the Code writers themselves could provide some insight, since they revised the language of 310.15(B)(7) in the 2017 Code to include 120/208V. The Committee Statement said "The load diversity in residential applications is similar whether the residence is fed with 120/240 or 120/208; therefore, 120/208 will be added to 310.15(B)(7)."

This statement doesn't suggest to me that the Code writers are suggesting that conductors have higher ampacities when they are connected to dwelling units vs other structures, but that they believe that actual loads on the dwelling conductors will be lower than what the Code's dwelling unit load calculations dictate so that a conductor with lower ampacity can be used without concern of overload.

 

[jaggedben]

...

310.15(A): There is nothing in the rest of 310.15(A) that modifies the ampacity of conductors. (0-1)
310.15(B): Ampacities...shall be corrected. This actually modifies conductor ampacity. (1-1) - batting .500
310.15(C): Ampacities...shall be be reduced. Two in a row modifying ampacity. (2-1)
310.15(D): Nothing that modifies the ampacity of conductors (2-2) - back to even
310.15(E): Nothing that modifies the ampacity of conductors (2-3) - sub .500
310.15(F): Nothing that modifies the ampacity of conductors (2-4)
310.12: Nothing that modifies the ampacity of conductors (2-5)

I would say in passing that some of this I do not agree with. For example, the 'rest of 310.15(A)' clarifies that 110.14(C) may modify ampacity. 310.15(E)(F) may modify ampacity in as much as they affect the calculations required by (C). But, to be clear, this is moot with respect to any of the other points I will make.

How is it possible that of all of the sections that 310.15(A) says will "modify the ampacity of conductor," only two actually provide direction to do so?

Indeed, why mention all of those sections if they do not determine ampacity? In fact, doesn't the language pretty much say, in slightly different words, that the end result of following all those sections determines ampacity? That is how I read it. I realize you don't, but try to see my point of view. You can read it as pretty much just saying that.

Could it be that the language "as modified by 310.15(A) through (F) and 310.12" isn't an absolute direction that those sections will change the ampacity of a conductor, since most of the listed sections don't provide any direction on modifying the ampacity of a conductor?

Could it be that, regardless of how all those sections are internally worded, 310.15(A) is telling you that they all specify and modify ampacity? That's more or less what it literally says.

Since you are so caught up in the language in 310.15(A) ("as modified by 310.15(A) through (F) and 310.12") that it causing you to ignore the plain language in 310.12

Could it be that you are so caught up on the internal wording of those various sections that it is causing you to ignore the plain language of 310.15(A)? Could it be you're missing the forest for the trees?

Maybe the Code writers themselves could provide some insight, since they revised the language of 310.15(B)(7) in the 2017 Code ...

Yes let's give that a try again. But the important change with respect to 310.12 was made between the 2011 and 2014 codes. In the 2011 code, that section (then 310.15(B)(7)) stated more or less what you paraphrased in post #29, that " conductors shall be as follows: #4-100A, #3-110A, #2-125A ... ". It made no use of the word ampacity and yet it rather clearly determined the ampacity in the covered application. So let's look at what the documentation says about how that got changed in the 2014 ...

Interestingly for this discussion, the original proposed language that led to the change (proposal 6-49a) was as follows:

(7) 120/240 Volt, Single-Phase Dwelling Services and Feeders. For service
and feeder conductors of 120/240-volt, single-phase, individual dwelling unit
one-family, two-family, and multifamily service ratings from 100 through 400
amperes, an adjustment factor of 0.83 of the service ampere rating shall be
permitted to be used to determine the size of the ungrounded conductors.

This proposal was accepted in the report on proposals. Would you not agree that the language changes the ampacity of the conductors?

Somehow the above wording, which I think is clear, got changed during the comments process to what we have now with "permitted to have an ampacity not less than 83%" as you quoted above. Unfortunately the report on comments does not give us much indication as to why, other than saying things like "[this] language is clearer and easier to understand" or " language is submitted to further clarify the requirements and to address the inadvertent exclusion of feeders to individual units in multifamily buildings." Certainly at no point in the comments does anyone express a concern along the lines of 'the proposed language would make installation under 220.82 less safe.' In fact, a simple revision to the above proposal changing 0.83 to 83 percent was Accepted in Principle. And a lot of proposals asking to delete 310.15(B)(7) entirely were rejected.

In other words, there was no intent that the new language not modify the ampacity of conductors for purposes such as applying 220.82. Or, if there was, there is no evidence for it in the revision documentation for the 2014 NEC. It seems like an accident that no one thought about, and the intent of CMP was otherwise.