service conductor sizing for 310.15(B)(7)

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electricguy61

electricguy61

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Remotely running business in Bay Area, CA from Med
As a preface, I'm in CA, so we are just now adopting the 2014 code.

I'm getting ready to replace a 100 amp residential service (usually would replace with 200amp, but PG&E has some strange requirements for upgrades adjacent to gas meters. They'll let you install the same size, but not upgrade).

Just for hoots, I looked up the minimum wire size in my new 2014 NEC Handbook. Whereas in the 2011 handbook, Table 310.15(B)(7) clearly gave you wire sizes for services, in the 2014 there are only formulas (basically you can use wire ampacity 83% of the service rating).

Which at 75 degrees C would be 83 amps for a 100 amp service (#4 THHN), 104 amps for a 125 amp service (#2 THHN), 125 amps for a 150 amp service (#1 THHN), and 166 amps for a 200amp service (2/0THHN).

Which is the same as the previous table, only harder to figure out.

Anyone know why the powers that be removed the simpler to read table? Or if my calcs above are incorrect, please inform me.

Thanks
 
I think that one reason is that the old table didn't account from variables like parallel conductors. Your calcs look good and should be no different than the table in the 2011 and earlier versions of the NEC.
 
The other motivation is that the table caused some to assume that correction and adjustments did not need to be applied to the table results, while the 83% formulation makes it clearer that temp corrections and fill adjustments must be taken into account.

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jumper said:
It is George Stoltz's fault. Rumor has it that he drank 2 pots of coffee, was bored, and decided that the nice simple chart should be replaced by a formula that is irritating to remember.:)
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It didn't really go down like that. George did the greatest rewrite of 310 ever imagined. It didn't get accepted because nobody wanted to admit how bad they screwed the article with all their needless changes since 2005.
 
jumper said:
It is George Stoltz's fault. Rumor has it that he drank 2 pots of coffee, was bored, went outside and kicked a kitten, told several mothers that their babies were ugly, stole some candy from some kids (and littered the wrappers), told a homeless guy to get a job, dumped several gallons of used motor oil down the storm drain, and decided that the nice simple chart should be replaced by a formula that is irritating to remember.:)
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FIFY ;)
 
GoldDigger said:
The other motivation is that the table caused some to assume that correction and adjustments did not need to be applied to the table results, while the 83% formulation makes it clearer that temp corrections and fill adjustments must be taken into account.

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I don't understand why we have a different ampacity rule for service conductors in the first place. It makes as much sense as raising the speed limit by 20%, while also calibrating the speedometers to falsely display a speed that is 20% higher.

If the reason why service conductors can be smaller is due to taking credit for load diversity in a service, then the service calculation should account for that.
 
Carultch said:
I don't understand why we have a different ampacity rule for service conductors in the first place. It makes as much sense as raising the speed limit by 20%, while also calibrating the speedometers to falsely display a speed that is 20% higher.
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I agree wholeheartedly, all it is doing is making things more complex.
 
Carultch said:
I don't understand why we have a different ampacity rule for service conductors in the first place.

If the reason why service conductors can be smaller is due to taking credit for load diversity in a service, then the service calculation should account for that.
Click to expand...

I agree, welcome to the NEC. :slaphead:
 
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