Use your hand to test the heat of the lamp. Put the back of your hand under the lamp and slowly move it away until you get to a comfortable distance that isn't too hot.
If it's too hot for your hand it's too hot for plants.
I would say keep it at about 18 inches to 2 feet away, but some people have a 400w as close as 1 foot away from the plants. This is OK if you have good cooling system.
GTC21 Reviewed by GTC21 on . tell me what u think all advice is appreciated :) i got 1x 400w philips 1 timer set on 18/6 the 3 healthy one's r Powerplant and the small not healthy looking 1 is K2 plz give me some help 2cnd time growing 1st plants where confiscated by police :( when they where 3weeks into grow thx for anny respond :o Rating: 5