The disconnect switch under the electrical meter has a 125 amp breaker, the panel in the home has a 100 amp main breaker. It is an underground service at approximately 50'. Do I size the wire for the 125 amp rating or 100 amp rating?

Thank you

125 Amp

The breaker cannot be reated higher than the wire, breaker is to be the weak link, would size for 125 amps.

Thanks for the quick response. I know that there are tap rules, but didn't know if this applied.

Thanks again.

Tap rules?

Tap rules are only 10 and 25 foot with very, very, did I say very, specific purposes, the longest is 35 feet for an industrial plant. Taps are allowances for using smaller wire fed by a much larger breaker to a smaller breaker very close downstream in the circuit.

There are other very somewhat rare, specific reasons a smaller wire can be protected by a larger breaker for large motor or refrigeration loads.

Yours is not any of these.

You have a feeder. The wire needs to be rated to the overcurrent protection device upstream.

Ok, Thanks again.

If ever in doubt, go the larger size wire to reduce voltage drop, opposed to getting away with the smallest you can get by with. Code was written for safety, not necessarily what is best or most efficient. There are some things code says I can do, but choose other methods that still comply with code.