Re: 856 key switch max amperage?

A better solution is to use a relay. Power the relay coil from the IGN or ACC key switch terminal. Run a #12 wire from the battery (or from the alternator side of the ammeter if you tractor has an ammeter) to one relay contact terminal. (For safety to provide a fuse in the line near the battery or ammeter connection.) Then run another #12 wire from the other relay contact terminal to the controller box.

Now the key switch will see only a fraction of 1 amp from the relay coil – the relay will switch the control box power on and off with the key switch. (Added bonus: This setup will also provide slightly higher voltage to the controller box than the key switch can).

For the relay suggest using a fog light relay or similar. These relays are readily available at any parts store, they will easily handle 20 amps and they are cheap!