What size disconnect for a 3.5 ton AC unit?
Tonnage does not size the disconnect; the nameplate does. A typical 3.5 ton residential condenser has a minimum circuit ampacity around 20 to 26 amps and a max overcurrent protection of 35 to 45 amps, and the standard answer in the field is a 60 amp non-fused pullout disconnect. Here is how to justify it.
Read the nameplate, not the tonnage
Two numbers matter. MCA, minimum circuit ampacity, sizes the conductors. MOCP, maximum overcurrent protection, caps the breaker or fuse. The disconnect itself must be rated at least 115 percent of the rated-load current of the equipment per NEC 440.12(A)(1), and it has to be capable of interrupting the load. Since the smallest common air conditioning pullout is 30 amps and the next step is 60, and a 60 amp non-fused disconnect costs nearly the same, the 60 amp pullout covers practically every residential condenser through 5 tons.
Fused or non-fused
If the condenser nameplate says maximum fuse rather than maximum breaker or HACR breaker, the overcurrent device must be fuses, which means a fused disconnect at the unit or fuses upstream. If it lists a max circuit breaker and your panel breaker matches, a non-fused pullout at the unit is fine. Match the nameplate language exactly; inspectors read it.
Location rules under 440.14
The disconnect must be within sight of the equipment and readily accessible. Within sight means visible and not more than 50 feet away per Article 100. It cannot be mounted on panels designed to allow access to the equipment, so do not screw it over the service panel of the condenser. Mounting it on the house wall adjacent to the unit, at working height, keeps everyone happy. Maintain the 110.26 working space in front of it.
Field notes
Wire size follows MCA using 75 degree copper, so a 26 amp MCA runs fine on 10 AWG. The breaker can be larger than the conductor ampacity here, up to the MOCP, because 440.22 allows short-circuit and ground-fault protection above conductor ampacity for hermetic compressor loads; the compressor overloads protect against overcurrent. A 35 amp max breaker on 10 AWG copper to a condenser is compliant and common, and it surprises people who have never read Article 440.