Can anyone provide guidance on how large a timer I need for a 400W
metal halide grow lamp? The vendor suggested a 15 amp grounded
outlet, the stated he likes 20 amps ones. However, I recently ran
across some nice, grounded 10 amp, outdoor timers ( Intermatic . On
the side they list capcaity as 1200 amps R Resistive or 1200 amps
Tungsten. I know that Metal halides are different, but do I really
need a 15 amp timer or will these 10 amp ones work?

Years ago, when I had the money to spend on things like this, I had two
400-watt halides over a large sal****er aquarium (the corals LOVED it). I
don't recall the brand, but I bought a timer that was designed for
high-draw inductive loads. A couple of years later, I sold off the
aquarium, and the timer was still working just fine.

steve