Service Wire and Feeder ampacity

[sparkync]

For years I have used the table that says for residential services and feeders, you can use #2 for 100 amps, and 4/0 for 200 amps. I got turned down on a feeder that I had a 100 amp breaker on. I had to put in a 90 amp breaker instead. Obviously that rule no longer exist. I guess that means that 4/0 is no longer acceptable for 200 amp, and I'll have to go to 3/0 copper or 250 aluminum. Is this correct?
Thanks

 

[don_resqcapt19]

It still exists. The table was not there for a cycle or two, but there was a rule that permitted the conductor to have an ampacity of 83% of the service rating.
Starting with the 2020 code the table is back but in a new location. 310.12.
In the 2017 the rule, without a table, was in 310.15(B)(7). The table was in Example D7 in the 2017 code.

 

[texie]

For years I have used the table that says for residential services and feeders, you can use #2 for 100 amps, and 4/0 for 200 amps. I got turned down on a feeder that I had a 100 amp breaker on. I had to put in a 90 amp breaker instead. Obviously that rule no longer exist. I guess that means that 4/0 is no longer acceptable for 200 amp, and I'll have to go to 3/0 copper or 250 aluminum. Is this correct?
Thanks

Is this "feeder" carrying the entire load of the dwelling?

 

[wwhitney]

For years I have used the table that says for residential services and feeders, you can use #2 for 100 amps, and 4/0 for 200 amps. I got turned down on a feeder that I had a 100 amp breaker on.

Was this either (a) a feeder on a 100A residential service or (b) a 100A feeder that carries the whole load of the residence? If so, #2 Cu is fine for 100A, absent any ampacity correction or adjustment.

But if it was, say, a 200A service for one residence, you've never been allowed to use #2 Cu for a 100A feeder on that service. Only the 200A service and any 200A feeders.

Cheers, Wayne

 

[sparkync]

Here in NC, we are still going by the 2017 code in residential. In 310.15 (B) 7 (1) through(4), it says that we can take 83% of the service and feeder rating. 83% of 100 amps would be 83 amps, thus allowing #2 aluminum which is good for 90 amps, to be used as a service wire or feeder wire in residential, and 83% of 200 amps would be 166 amps, which is less than the 180 amps that 4/0 aluminum is good for in Table 310.15 (B)(16), thus allowing 4/0 to be good for 200 amps for service ore feeders wires. What am I missing? Thanks again.

 

[sparkync]

texie, sorry, I missed your post. No, the #2 Aluminum feeder was just going to a subpanel, not the entire load of the dwelling, so I can see where I was wrong there. If it were going to the entire load of the house, according to the 83% rule, would it not be alright then?, thanks

 

[texie]

Yes, you got it now. I will say though that I have seen many AHJs accept the 83% rule for sub panels such as the typical basement panel. I might add though that some allow it out of ignorance.

texie, sorry, I missed your post. No, the #2 Aluminum feeder was just going to a subpanel, not the entire load of the dwelling, so I can see where I was wrong there. If it were going to the entire load of the house, according to the 83% rule, would it not be alright then?, thank