Ok, so here is what I put together for a simple, 240V only, 1 element, 1 PID controller.
View attachment 330741
If your spa panel has larger than a 30A breaker feeding it from the main panel, then you need to wire from your main panel to the spa panel, and the spa panel to your control panel with wire large enough for the main panel breaker rating, and have the 30A breaker (or 30A fuses) in the control panel. For a 50A main breaker you need 6AWG wire, and for a 40A breaker you need 8AWG. If the breaker in the main service panel is 30A, then you don't need the breaker in the control panel, and you can wire from main panel to spa panel, and spa panel to control panel with 10AWG wire.
The "fat" wires in the control panel schematic after the breaker are all 10AWG. The thin wires can be down as low as 20AWG.
Component PN's are examples only. Equivalent parts can be substituted.
Let me know if you have questions.
Brew on