If 2/0 Copper MTW/THHN/THWN is rated at 195A max - why is it allowed to be on a 200A breaker?

Which table of NEC 310 are you using?

There is no 195 amp standard breaker. For those sizes you are allowed to round up under certain ampacities.

#2/0 has a *usable* ampacity of 175 amps at 75° C. It’s rated for 195 at 90° C but that ampacity is typically only allowed to be used for derating purposes because no equipment is rated for conductor use at its 90° C.

175 amps is a standard size from table 240.6 so rounding up to 200 amps is not permitted, however for dwelling service entrance conductors and feeders carrying the entire load you can use conductors with only 83% of the OCPD rating. {Code change 2014 NEC, 310.15(B)(7)}

For 2011 {Table 310.15(B)(7)} and earlier versions of the NEC you could use a separate table for dwelling conductors which allowed:

#4=100 amps

#3=110 amps

#2=125 amps

#1=150 amps

#1/0=175 amps

#2/0=200 amps

Thanks for the information

You’re welcome. Some have grumbled that by removing the simple table and replacing it with a calculation the NEC has become less user friendly but the old table didn’t take into account other potential installations like those using parallel conductors and derating. Now with the 83% rule those are covered.

Also the sizes in the old table are still accurate so for example in the OP you could just look at the 2011 NEC and find that #2/0 Cu conductors can be used at 200 amps.

Great Points Robert.

I might add that I (Opinion Alert) and many others on the various CMP’s believe that Table 310.15(B)(7) did and always intended to require the adjustments and corrections of 310.15(B)(2) and (B)(3) where applicable.

The “grumbling” will more than likely, at the least, get the table back into the Informative Annex D7. However, we will have to see how it shakes out in San Diego in November. Personally, I like the 83% math…makes it easier when dealing with feeders in my opinion, the real confusion people had was knowing what the heck a Main Power Feeder was…lol