I removed an electric slide-in range. The box was wired with 8-3 stranded copper on a 60a circuit breaker. I am installing a cooktop with a built-in oven. All I have is the one 8-3 wire in the wall. What are my chances of using that wire for both the cooktop and oven? Cooktop is a 40 amp unit. Not sure of the oven. Edit.. oven is a 30amp appliance. Neither will be used even close to their full potential.
That 8/3's overbreakered no matter what
The 8AWG copper you have in your walls is rated for 40A at 60°C or 50A at 75°C as per Table 310.16, so you'll have to swap that 60A breaker out for a 40A (NM cable) or 50A (armored cable, SE cable, wires in conduit) no matter what else you do.
40A is adequate for most ranges though
However, due to the Table 220.55 range load demand factors, which also apply to a separate cooktop and oven on a single circuit, 40A is adequate for the vast majority of range setups, and even those that don't fit on 40A have no trouble with a 50A circuit, unless you have a very large cooktop. In particular, a 12kW nameplate range load only is treated as 8kW of actual load for circuit sizing purposes, and every additional kW or major fraction thereof adds 5% to that 8kW. So, knowing that a 40A circuit gives us 9.6kW and that ranges are not treated as continuous loads by the NEC, we get 10% over 12kW, or a 13.2kW maximum range load on a 40A circuit. Going up to 50A gives us 12kW, which translates into a 14.4kW maximum range load.
#8 wire on the 75 degree table Is good for 50 amps but of the range is rated as a 50 amp load you need to go up to #6, a 40 amp load rated range should be put on a 50 amp breaker as that is 125% of the load. If your circuit rating is 40 amps the receptacle can be 40 or 50 amp per table 210.21.b.3. If the range requires 4 wire you may need to pull new wires if you have black, red, white a new separate ground can be pulled as long as it taps a circuit from or goes back to the same panel. If you have 3 & a bare copper and your new range requires 4 wire most locations require a new cable to be pulled.