310.15(B)(7) contradiction?

[mbrooke]

This keeps coming up and Im not sure how to answer it. 310.15 B 7 lets one use smaller cables for a 120/240 service to a dwelling. Clearly this table seems to be based around the 90*C table of 310.15 (B) (16)since most of the gauges are equal to or within 10amps of the 90 degree column.



My question is, if this table allows for wire above 75*C, then isn't it a violation to terminate theses cables to a 75*C rated main breaker? Most residential load center mains appear to have a 75*C rating, yet all use this sizing table regardless?



How does this work? :dunce:

 

[jumper]

This keeps coming up and Im not sure how to answer it. 310.15 B 7 lets one use smaller cables for a 120/240 service to a dwelling. Clearly this table seems to be based around the 90*C table of 310.15 (B) (16)since most of the gauges are equal to or within 10amps of the 90 degree column.


My question is, if this table allows for wire above 75*C, then isn't it a violation to terminate theses cables to a 75*C rated main breaker? Most residential load center mains appear to have a 75*C rating, yet all use this sizing table regardless?



How does this work? :dunce:

Actually, IIRC the tables are based on historical usage data provided by POCOs for residential services.

 

[david luchini]

This keeps coming up and Im not sure how to answer it. 310.15 B 7 lets one use smaller cables for a 120/240 service to a dwelling. Clearly this table seems to be based around the 90*C table of 310.15 (B) (16)since most of the gauges are equal to or within 10amps of the 90 degree column.



My question is, if this table allows for wire above 75*C, then isn't it a violation to terminate theses cables to a 75*C rated main breaker? Most residential load center mains appear to have a 75*C rating, yet all use this sizing table regardless?



How does this work? :dunce:

I don't believe that the table is based on the 90*C rating.

To me it seems that the allowance is based on the fact that there are only two current carrying conductors, and table 310.15(B)(16) is based on three current carrying conductors. Fewer ccc's equals less heat equals higher allowable ampacity. You'll notice, 310.15(B)(7) only applies for 120/240V, not for a single phase feeder of service that is 120/208V which would have three current carrying conductors.

 

[mbrooke]

I don't believe that the table is based on the 90*C rating.

To me it seems that the allowance is based on the fact that there are only two current carrying conductors, and table 310.15(B)(16) is based on three current carrying conductors. Fewer ccc's equals less heat equals higher allowable ampacity. You'll notice, 310.15(B)(7) only applies for 120/240V, not for a single phase feeder of service that is 120/208V which would have three current carrying conductors.

True, but even where MWBC are involved, no conductor is allowed to go over 75*C, in fact I cant think of any application other than this. Not saying your wrong just the way I see it.


As for the rating while not explicitly stated, it is near identical to the 90 degree column.

 

[augie47]

I agree with posts above. The allowances are based on load diversities and studies of actual loads on whole house services and feeders along with the reduced neutral load on a 240/120 system (as compared to two phases and a neutral of a 208/120 system). Termination temperature, IMO, did not play a part.

 

[Dennis Alwon]

The fact that the table seems to be using the table at 90C is coincidental. I stated in another forum that the reason is diversity in residential loads. I know this is hard to get used to but since very few loads in a residence are considered continuous the cmp members felt like they could give the residential service conductors a different rating. Seems odd but we know the conductor can handle the loads thru experience and the conductor is not really the issue the insulation is. Apparently it never gets hot enough for long periods of time to cause issues.

 

[mbrooke]

I agree with posts above. The allowances are based on load diversities and studies of actual loads on whole house services and feeders along with the reduced neutral load on a 240/120 system (as compared to two phases and a neutral of a 208/120 system). Termination temperature, IMO, did not play a part.

But why then go to the trouble of calling it a 200amp service? I know your right, its been this way for decades but its always something Ive found strange.

The fact that the table seems to be using the table at 90C is coincidental. I stated in another forum that the reason is diversity in residential loads. I know this is hard to get used to but since very few loads in a residence are considered continuous the cmp members felt like they could give the residential service conductors a different rating. Seems odd but we know the conductor can handle the loads thru experience and the conductor is not really the issue the insulation is. Apparently it never gets hot enough for long periods of time to cause issues.

I do agree though, if load calcs are done correctly it will never go over 80%. Just a really odd rule IMO.

Out of curiosity, I saw the post on ET, what issues did the old table not take into account?

 

[winnie]

I like to think of 310.15(B)(7) as indirectly saying that _for the specific residential loads described_ a conductor with ampacity X may be used with a breaker of ampacity Y > X . 310.15(B)(7) does not change the allowed temperature ratings or the ampacity of the conductors. So a 175A conductor may be used for a 200A service protected by a 200A breaker. This is just like the use of larger breakers with smaller conductors for devices such as welders...simply annoying because it is indirect.

-Jon

 

[mbrooke]

I like to think of 310.15(B)(7) as indirectly saying that _for the specific residential loads described_ a conductor with ampacity X may be used with a breaker of ampacity Y > X . 310.15(B)(7) does not change the allowed temperature ratings or the ampacity of the conductors. So a 175A conductor may be used for a 200A service protected by a 200A breaker. This is just like the use of larger breakers with smaller conductors for devices such as welders...simply annoying because it is indirect.

-Jon

Indeed annoying:lol: The thing is welders are on a duty cycle, guess that applies here since a dwelling never does pull 180 amps for hours. But why not just make the minimum service requirement 90amps and call 200 amp services 180amps? (Think aloud) :angel:

 

[iwire]

I agree with winnie. Basically they know that the load calculations where very conservative so they made up for that with this odd ball table.


That said check out the changes in 2014.....

 

[augie47]

I agree with winnie. Basically they know that the load calculations where very conservative so they made up for that with this odd ball table.


That said check out the changes in 2014.....

Yep! If you found the Table "strange" look at 2014

 

[Dennis Alwon]

Out of curiosity, I saw the post on ET, what issues did the old table not take into account?

The old tables allowed a certain size conductor in se cable but didn't take into account the de-rating that may come into play. So I could have 4/0 al seu or ser that may need to be rated at 60C and still use it for 200 amps. At least that is the way many of us saw it since it doesn't say anything about de-rating. Now the 2014, as I mentioned at the other forum, works on an 83% of the load needed but the de-rating will come into play.

I believe Gus and his Tennessee buddies have been enforcing the de rating before the 2014 came into play

 

[ActionDave]

Yep! If you found the Table "strange" look at 2014

More proof that they are running out of things to do and are searching for ways to justify selling code books every three years IMO.

 

[George Stolz]

I believe Gus and his Tennessee buddies have been enforcing the de rating before the 2014 came into play

How? It wasn't an ampacity table capable of applying the derating factors - as you noted, hence the change.

If I wasn't pretty disgusted of the process I would consider trying yet again to get (B)(7) deleted and a simpler version put into Article 220 where it belongs.

As for the "two current carrying conductor" theory mentioned above, unless someone produces historical evidence to the contrary I will continue to believe that it is a smokescreen that the CMP has put forward to try to stem the proposals that always come along begging for 208 to be included in. Considering the blank check residential feeders had prior to 2014, and the fact that nowhere else in the codebook does derating begin until 4 conductors, I'm pretty sure that the reason (B)(7) is reserved for 120/240 is because it always has been. I think that's about it.

 

[Little Bill]

The old tables allowed a certain size conductor in se cable but didn't take into account the de-rating that may come into play. So I could have 4/0 al seu or ser that may need to be rated at 60C and still use it for 200 amps. At least that is the way many of us saw it since it doesn't say anything about de-rating. Now the 2014, as I mentioned at the other forum, works on an 83% of the load needed but the de-rating will come into play.

I believe Gus and his Tennessee buddies have been enforcing the de rating before the 2014 came into play

I've never had any inspector say anything about the table unless someone tried to use it to size conductors for a subpanel. Never heard of any derating being done or required.