It's usually a Black Landscaping Lighting Wire it should have an outdoor rating.

Also if this is a composite deck, check with your composite deck manufacturer what stair stringer spacing should be usually its 12" on center, so I would check the stringer spacing (it looks like it's more than 12") to make sure that's what you have there, if not additional blocking will be needed.

What kind of "normal" wiring - UF, NM - it should say on the cable jacket.

Landscape lighting wire should be fine - and for LEDs I would think 18AWG would be heavy enough and a lot easier to work with.

If it's "interior" wiring then it probably isn't rated for use in wet conditions. I'd have him replace it with something more appropriate or just let whoever you use to install the lights do it..

Hello,

I would recommend first checking that the cable which your contractor originally installed is rated for outdoor uses; if it is not, it will more than likely degrade overtime and need to be replaced. The cable typically used for low-voltage landscape lighting applications is insulated, non-polarized, and is rated for direct-burial applications outdoors.

For your project, the gauge of cable won't necessarily be of concern; as long as it is able to be safely ran through openings while distributing the proper voltage range to the fixtures.

-Evan K.

Industry Liaison - VOLT® Lighting