You are just fine with that timer a 400 watt lite will pull 3.3 amps well under the rated amp draw of the timer. As long as you do not exceed the 15 amp rating of the timer you will not have a problem. All a timer is, is a switch on or off that all it does except that it is time controled. It will not change the voltage to the light it just turns it on and off. If you every wonder about what amp draw a light will draw just take the watts and divide by the voltage that is being supplied witch in most cases 99.9% of the time to be 120 volts you get the amp draw for example 400 watts divided by 120 is about 3.3 amps. hope this help Bizzy.