Is my light timer powerful enough???

Theres a thread started by an electrician in indoor growing, he would be best to answer such a question. I was going to link it, but I can't find it now.

Its more than fine.
If you run on 120V you need a 4A timer (W\V=I | (400+30)\120 = ~3.6A. You'll need it a bit higher when you light your light up)
Your timer is 10A, you can run 2x400W light on it safely as well, taking 20% more than you need for safety reasons.

Have fun

The only thing that would concern me is that there seems to be no ground plug.

Unless your balast is drawing some stupid amount of current you should be fine, 10A is pretty high(for a balast).

your ballast should be a three prong plug and the timers you are showing are two prong.

It's ok to hook your 400w light to your timer. If you plan to run more amperage for lights in the future you should install a relay. My rule of thumb is never draw more than half of what the timer or relay is rated for. Make sure you pay attention to polarity, the wide side of the 120vac plug in is the common and it is bonded to the ground in the main breaker box and in your light transformer box. So if you're cutting off the ground lead be sure the common side of your plug goes into the common side of the pluggin or your ballast box will be hot.

Most 400w ballest will only pull about 3amps, also most have a 3 prong (ones the ground).

Oh sorry the link was for the exact timer I have except mine is 3 prong, theyre on amazon somewhere. Thanks every1 great help looks like i'm safe until I upgrade to 1000w, +reps. Bird I can't understand what your sayin, my ballast is sometimes pretty warm but never hot, and I keep it up as high as possible as recommended by a few people.

In Canada in a 120vac circuit there are three wires. The ground(which is the wire without any plastic coating) attaches to the green lug. In appliances the ground wire is usually green. The common which is the white wire is attached to the silver lug. The common side of the 120vac outlet is the wider side. The common and the ground wire are attached together(bonded) in the main breaker box. You cannot get a shock from the cold side(ground, common) of the circuit because they are grounded. It is refered to as cold because it's grounded and fairly safe. The hot side of the circuit is the black wire and in 240vac the red wire as well. These two wires are not grounded and the current being carried by these wires can cause death. Because they are not grounded they are considered to be the hot side of the plane. If you plug in an appliance backwards that has a grounded case, the grounded case of the appliance will be on the hot plane instead of the cold plane. I hope this helps everyone understand electrical polarities and why they are so important to be observed. I was lucky enough to score some cheap timers with ground plug also.( "and I keep it up as high as possible as recommended by a few people." ), this is good advice for ballasts etc.