You're talking about switching the low voltage side?

Yes; I’m talking about putting a switch of some kind in the run of low voltage wire that will be in conduit from the fixtures to the transformer. The transformer is in a weatherproof housing and plugs into a GFCI-protected receptacle on the side of the house. That receptacle is contained in a weatherproof box that complies with current code. I’m not talking about doing anything that switches the house voltage.

The sequence would be: fixtures; low voltage wire in 10’ of conduit; switch in a conduit body or receptacle box; low voltage wire continuing in 30’ of conduit; low voltage transformer.

In general, you have more flexibility with voltage under 50 volts. In your case its probably 12 volts (common for landscape lighting) or 5 volts (becoming more common because its also used for many LED lights).

There are many types of weatherproof switches used to switch 12 volts in trucks, cars and boats, and you might want to go online to see what is available. Another good place to check is boating supply stores. Most of these switches use a simple round hole for mounting, and you could put it on whatever side of the conduit body is most convenient.

Bruce

Yep, google weatherproof switch. You have to watch the amperage, but most landscape lights are LED these days so it usually isn't a problem.

Marine shops are a great source: https://www.westmarine.com/buy/bep-marine--water-resistant-toggle-switch-off-on-spst--17951914?recordNum=1

Thanks for everyone’s input. Even though it’s just 40 watts total on a 12 volt line, I wanted to make sure it was feasible.