I agree with Dennis that the old war horse ZWs are great, even for modern layouts.
Their internal thermal breakers are unreliable, and if they reset automatically, the current will keep coming on and overloading it, as well has hazarding the trains' wiring and parts.
Roger, you question does not have an easy answer. The internal breaker in a ZW is on the wire to the U post. If you were to set the left handle at 6 volts and the right at 20, and connect the A&D posts, you'd have a dead short fed by 14 volts. It wouldn't be too long before your transformer, and possibly your house, went in a puff of smoke. This is the situation that can exist in a loco passes from a section controlled by one lever to a section controlled by another, while its rollers span the fiber pin.
So most users add a 10-amp or less breaker or fuse in each feed coming from A,B,C, & D. However, this can permit up to 40 amps to be drawn from the ZW, which yields the result noted above.
For maximum and proper protection, I'd put a 10-amp breaker in the line from U, and a 5, 7, or 10 in each of A,B,C, or D--the minimum that will run the trains. Do be aware that fuses and breakers do not open immediately upon the current exceeding their rating: time delay depends on the percentage and duration of overload.
On my layout, where I never turn smoke on, I use 7 and 5 amp breakers. But most all my lighted acrs are now LEDs, and I want the breakers to open ASAP on a short. The 7 amp runs 3 trains at a time; the 5 easily runs 2.