120/240 residential sub panel 100a wire size

[rustynailhead]

Im kinda confused....I'm installing a 100a sub panel for a friend in his residence. He has a roll of #4 THHN Cu 90degree and i'm gonna pull it in pipe....i said that would work for a 100a. When i double checked 310.16 states that #4 cu is good for 95 amps.....but 310.15 b7 says 100a for dwelling services and feeders (2011 code cycle) can anyone clear it up.

 

[infinity]

Since this a sub-panel you would use 310.16 {2008} or 310.15(B)(16) {2011} to figure out the ampacity of a #4 conductor.

Welcome to the Forum.

 

[DARUSA]

Im kinda confused....I'm installing a 100a sub panel for a friend in his residence. He has a roll of #4 THHN Cu 90degree and i'm gonna pull it in pipe....i said that would work for a 100a. When i double checked 310.16 states that #4 cu is good for 95 amps.....but 310.15 b7 says 100a for dwelling services and feeders (2011 code cycle) can anyone clear it up.

#4 is rated only for 85 amp based on table 310.16 0n the 75 degrees colum the 90degrees table is only used for derating purpose. this is assuming that you run a single phase feeder .

 

[Dennis Alwon]

Im kinda confused....I'm installing a 100a sub panel for a friend in his residence. He has a roll of #4 THHN Cu 90degree and i'm gonna pull it in pipe....i said that would work for a 100a. When i double checked 310.16 states that #4 cu is good for 95 amps.....but 310.15 b7 says 100a for dwelling services and feeders (2011 code cycle) can anyone clear it up.

First off you cannot use the 90? C rating as your final ampacity rating since the lugs on almost everything is 75?C or less. Since there is no derating then the conductor is only good for 85 amps. If the calculated load is 85 amps or less then you can use a 90 amp breaker. 240.4(B).

Now the table for residential will allow #4 to a sub panel if the sub panel carries the entire load of the service. So if you had a 100 amp service that consisted of a meter and a 100 amp disconnect- no branch circuits, then you could run #4 as a 100 amps to the second (sub) panel.

 

[rustynailhead]

thanks, for some clarity....no matter how much time we spent on code in the appreticeship there always seems to be questions!!! Knowing the calculated load is less than 85 amps....I will go with a 90a breaker....the important thing is to do it correctly, quality work always