I want to upgrade my kitchen range. I currently have an old 1987 kenmore drop-in. The manual lists it as a "30 inch drop-in", but when I lift up the cooking surface, the distance from counter top to counter top is about 29.5 inches (give or take a few cm). I spoke with several appliance retailers and I get varying answers. Some say that the cut-out should be slightly less than 30, others say they won't install unless it is 30 or more. Which is it? Also, I want to upgrade to an induction cooktop. The circuit breaker for the stove is rated for 50 amps, but my old stove is only 20 amps so I don't know if they installed the proper gauge of wiring for 50 amps or cut corners and used wire that was rated for 20 amps.
Typically a "drop-in" range will rest on the adjoining counter top surfaces. That the actual cabinet is narrower, gives you the wiggle room you need to drop in the unit to install it.
If you want to change to a cooktop installation, that is something that is only supported by the counter top, then you'll need to build a cabinet, and provide a counter top with cutout to fill that space.
In any case, as others have suggested, check the exact measurements of any product that you select. In this age of the internet, most manufacturers have their Installation Guides online, you can pull a copy in spite of the retail staff.
There should be fairly standard dimensions for this type of thing but the best advice is to carefully measure the existing cutout and then take that information with you when you shop for a new unit. There may very well be unit to unit differences that will need to be taken into account. One factor to consider is how much of a hanging flange the new unit has. Often the width of the flange can accommodate a range of cutout width.
Also you should evaluate how the existing cutout is situated in the cabinet and what type of counter top there is. Sometimes it is totally possible to widen a cutout gap of 29.5 inches out to 30 inches if the cabinet sides below are wider apart than 29.5 inches. Of course you would only do that if needed but do know that widening may be all from one side or could be some from each side.
As far as the electrical wiring goes the cable jacket should have marking along it that shows the gauge of the wire. You can then determine the ampacity from the size of the wire. If you are at all uncomfortable with working with the electrical wiring the best advice is to hire a professional electrician.
The cutout in our soapstone counter is 30 3/8" wide. We have a GE slide-in electric range with single oven that is exactly 30" wide with an overhang on top of 3/8" on each side.
The range circuit is protected by the original GE 50-A 240-V double-pole breaker, now 47 years old. The range is connected with a cord and plugs into a 4-wire receptacle. The wire is some very thick stranded aluminum, not sure about the exact gauge.
The 30" specified in the description of appliances is due to standard cabinet sizes 12" 18" 24" 30" 36" 48". It does not mean the unit is that size exactly, in all cases the nominal and maximum size of hole in countertop is specified in the installation instructions. Be warned these sizes can range wildly between brands. Always double check install sizes required for any appliances in the manual before purchase. It will save you headaches later☺