400w HPS Proximity To Foliage

What is a safe distance to place plants from a 400W HPS with cool tube and two inline fans attatched to duct on it (one for intake, one for exhaust) in a very small grow box.

The reason I ask is because I only have about 3 feet height to work with, so I don't want these to get too big, so how close should I put them to make them smaller?

Hold your hand under the light. If it's too hot keep moving your hand down till it's warm not hot and that the distance you want.

I am using a 400w in a cool tube and am keeping it just about 4 inches away with no heat issues or problems but hold the back of your hand at the tops of the plants and feel the heat like blzbob said.

Thanks, but I didn't make it clear what I ment. The plants that I flowering under the HPS were only vegged for 16 days, because I am growing sativas, with only 3 feet of vertical space to work with, so I want to keep them short. I vegged them with CFL's, and I know the closer they are to the CFL's, the more compact and less space between the nodes, thus smaller in size. I want to keep them as short as possible (keep in mind that they are sativas), so would the same be true with HPS? I know that you get more vertical growth with HPS lights, so if I put them closer to the light, would that still keep them bushy & small, or would vertical growth still be a problem, no matter how far you put it from the light?

3 ft of space isn't enough room for sativa unless you tie the tips down. all my sativas got at least 4-5 ft tall thru flowering and they were only 6" when I started budding

Good info. How do I tie the tips down, or even crop them?

I just started flowering on my sativa grow....and I hope like hell I dont run out of room.

when your plants are getting to tall tie the tip down as far as your stem will allow without breaking, this will make the side shoots grow toward the light.

Zombie...That is a cool ass avatar...I likes... I am using a 400HPS with a can fan (some cfl's also)and I keep it about 4 - 6 inches away. If you keep it that close make sure you check everyday so the plants wont grow up into the light.... I keep it 2 -4 inches away during veg to prevent stretching Happy New Year