Replacing Electric Cooktop 30A #10 Wire (manual says needs #8 wire)
Cooking appliances are a bit weird

The NEC treats household electric cooking appliances (ranges, cooktops, ovens) somewhat differently due to their rather uneven loads.  This starts in section and table 220.55 of the NEC, which applies an 80% demand factor to the load of a single household cooking appliance rated not more than 8.75kW.  As a result, your JEC9530's nameplate rating gets this factor applied to it, causing it to be considered a 6kW cooktop for the purposes of load calculations, as 220.14(B) invokes 220.55, and 220.10 invokes 220.14 in turn.

From here, we divide by 240V to get 25A for the maximum demand load of the cooktop, then apply 210.19(A)(3) to allow us to use 10AWG wire and a 30A breaker, as the conductors only need to be large enough to serve the load, with no 125% continuous load adjustment in sight.