amp rating a circuit breaker
Circuit breakers such as the Minn Kota MKR-19 you point to are rated to disconnect at a particular current …...not at a particular load power (V*A), so the voltage is not used as any part of the specified trip current.    

The Voltage IS important however, since this tells you what voltage the breaker is rated to 'break' without sustaining damage at the rated current. This is normally to do with the ability of the breaker to extinguish any arc that might form. In high voltage breakers the design includes special non-conductive plates that increase the arc length which results in the arc dying out quickly. In low voltage breaker they may simply have a fast acting separation of the contact points over a distance incompatible with sustaining the arc. 

There will always be an arc, and so breakers that have tripped many times do eventually erode the contacts which results in a higher voltage drop across them, and ultimately failure of the breaker.