S trap and piping sizes

I'm just cleaning out where the HWS used to be - that got moved - to create a workspace under the house. Because I can (sewer and water access dead easy) I was thinking of putting a small sink in the workbench I'm about to build. To wash brushes, hands, not get into trouble trashing the laundry tub, etc.

My question is what determines the size of the PVC to drain (i.e. pipe and S trap diameter)? It is going into terracotta, so any size up to 90mm is possible (but real silly), and my house has every diameter from 30mm up to 60mm on various sink/tub outlets, but the logic escapes me.

I'd have thought 40mm would be absolutely fine, but there must be more to it than that!

Thanks

Compleat


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