Light Distance From Plant

What distance should a 400 watt hps be from seedlings, clones and older plants? I've read on the roll it up faq 4 feet but that just seems too far. On a youtube video this grower said 30 inches for smaller plants and 20 inches for bigger plants. I just want to clear things up before I burn or stretch my plants.

Thanks!

what they said sounds alright, i'd do 2 1\2 to 4 feet. remember you are using a very strong light. Like takin part of the sun and putin it in a room.

Distance is only 1 part of the equation really. Cooling plays a big factor in it. Basically you want your light as close as you can get it to the tops of the plants without making them too hot & also without getting so low that your light/reflector doesn't get light to the plants on the outside of edge of the room. But generally speaking heat is the limiting factor. For plants vegging & beyond you want them to be around 70-75 degrees F ideally. Above 90 degrees can be lethal. So a 400 watt in 1 room w/ good cooling should go alot closer than a 400 watt in another room w/o adequate cooling. So put a thermometer w/ your girls & keep an eye on temp @ plant level & adjust your light according to that.

As far as clones go, maybe someone else can give you better info on this... But what i do know is that to develop roots clones like to have the soil temp slightly warmer than the air temp. That's why they make special little heating pads for them. & they should not need a ton of light as they aren't vegging yet so I would guess w/ clones you could back the light off alot???

Clone under fluros, about 18"-2' above.
I've got my 600 at 27" above the canopy w/just a fan blowing across, between the light and plants. I think you can get to at least 2' above.
What you wanna watch for is bad tip burning on the new upper growth.

Here's a little data I'm still collecting. https://www.rollitup.org/indoor-growing/109801-gathering-hps-data-calling-all.html