Pool, your question suggests you need to learn the trade.
We're not plumbers. We don't downsize our wire after each load because the rest of the circuit won't "need" all that electricity.
Instead, we -usually- size the wire according to the size of the breaker (or fuse) we have protecting the circuit. A 20-amp breaker needs a circuit with '20 amp' wire for the ENTIRE circuit.
This is one of the basic principles of electric work. The code won't directly spell it out - but the code will tell you how to size your wire.
