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I have a Cutler Hammer CH30E 150 Amp panel. It is fed from a 150 main disconnect mounted on the wall next to my meter can. The wire sizes are 2#2/0 AL THW & #1/0 AL THW NEUT & #6 CU GND buried in a 2" PVC pipe. My house was built in 1981.

Checking my wire ampacity chart, I'm seeing that 2/0 Al is good for 135 Amps. Should my feed have been 3/0 AL?

PS. I would have never checked this before joining this site.

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    I'll leave it to the experts to formally answer, but basically you get an 83% derate. See diy.stackexchange.com/questions/157043/… for a not-quite-duplicate. Dec 5, 2019 at 17:30
  • Unlike regular circuits in the house, a professional is almost always involved in the installation of the main cable because the electric company inspects that before attaching the meter. There's no cut off between the meter and the main breaker and the meter is attached with a tamper-proof seal. All that is to say that an experienced electrician picked/inspected that wire size.
    – JPhi1618
    Dec 5, 2019 at 18:39

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Yes, you should immediately downbreaker to a 135A breaker. Hold on. Those aren't made.

Oh dear. Well, per NEC 240.4(B)(2), you should round up to the next available breaker size.

You'll need to fit a 150A breaker ASAP. Sorry to put you to the expense! :)

Seriously, there's one other thing that may apply here. Since all the power for the entire domicile comes through this feeder cable, it actually gets to be derated like a service. Per 310.15(B)(7), you apply a (favorable) 83% derate, meaning the 150A breaker only needs wire rated for 124.5 amps.

Alternately, take your 135A wire ampacity and divide by 0.83 to give 162.7A if it's handling all the power for a domicile. That again rounds up to 165, 170 or 175A depending on which breaker sizes are actually made.

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