You'll need a dedicated 240 volt run but the amperage depends on the cook top. Rule of thumb seems to be 40 amps for most and fifty for the bigger ones.

It's not the voltage that matters in this situation - since you have an electric cooktop already, you should have a 240 volt circuit. Just to confirm, this is a cooktop only with the oven separate, not a range, correct? If so there is a good chance you only have a 30 amp circuit which will limit your options for induction. Head down to the breaker panel to check out your situation and post back what you find!

"Ruining" is a tad strong. There is still debate here about whether induction or gas is the best. However, if the alternative is electric coil, whether exposed coil or buried under-glass coil, then induction is certainly superior in just about every way but cost, and even that might be partially refunded by much higher efficiency than coil, and significantly easier clean up. The cooking dynamics are much like gas, so one gains the control capabilities of gas with improved safety.

kas

My kitchen was 20 years old and had a glasstop electric cooktop, but it was on a 40-amp circuit. We replaced it with induction and are thrilled with it. Prices are coming down. It will change your life!

I just checked specs at a major online retailer for a 36" (i.e., biggest models) of popular brands. Then I found the specs for 30", same brands. Look at the specs and installation instructions for more info at the makers' websites.

36" Induction Cooktops
Thermador:
240/208V AC, 60 Hz
Required Circuit Breaker: 40 Amp
Conduit: 39" Flexible 3-Wire
Shipping Weight: 62 Lbs.

GE:
240/208V AC, 60 Hz
Required Circuit Breaker: 40 Amp
Conduit: 39" Flexible 3-Wire
Shipping Weight: 62 Lbs.

Electrolux:
Amps@240/208 Volts: 46.0/46.0 A
Connected Load@240/208 Volts: 11.1/9.6 kW
Electrical - Single phase 3 or 4-wire cable, 120/240V or 120/208V, 60Hz, fused on both sides of line with ground: Required
Armored Electrical Cable: Included

Kitchenaid:
Electrical Requirements:
A 4-wire or 3-wire, single phase,
240 volt, 60 Hz., AC only electrical
supply is required on a separate,
50-amp circuit, fused on both
sides of the line.

Miele:
208-240 Volt (Specify on order), 50 Amp 3-wire power supply connects rear left bottom

30" Induction Cooktops
KitchenAid:
Electrical Requirements:
A 4-wire or 3-wire, single phase,
240 volt, 60 Hz., AC only electrical
supply is required on a separate,
40-amp circuit, fused on both
sides of the line.

Electrolux:
Power Supply Connection Location: Right Center Rear
Connected Load @ 240/208 Volts: 8.4/ 7.3 KW
Amps@240/208 Volts: 35/35.1
Minimum Circuit Required: 40 A
Required Electrical Components: Single phase 4-wire cable, 120/240 Volts or 120/208 Volts, 60 Hz, fused on both sides of line with ground.
Armored Electrical Cable: Included

Miele:
208-240 Volt (specify on order), 40 Amp 3-wire power supply connects rear right bottom

Thermador:
Electrical Supply: 240/208V AC, 60 Hz
Required Circuit Breaker: 40 Amp
Conduit: 39" Flexible 3-Wire

GE:
KW Rating @ 240V: 7.4KW
KW Rating @ 208V: 5.6KW
Amps: 40Amps