That would work,3 rows of four lights.

So running 10 would be better then 12 in a room 20' long 12' wide and 12' hight

Pufn247 said:

So running 10 would be better then 12 in a room 20' long 12' wide and 12' hight

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DE's at 1000 watts have a 5 x 5 footprint. My new setup I am running 2 rows of 5 lights, that gives me a 10 x 25 foot print in a 15 x 28 room. This gives me very little space to move around in. Something that you should consider when setting up.

In my humble opinion that is a very small space for an 8 light. I'd run 2 rows of 4 with a middle walkway spanning the entire room. I have a couple layouts just like that and it can't justify more light.
What I thought would be cool if unlike me you didn't have ducting running the middle of your room I'd add a 315 lec or two on a light mover just for spectrum but the room should be packed 8 lights like no room to do anything other than slide to the other side.

Will a 5 ton be enough for a room that big ?

So im gonna widen it up but thats the basic rundown of what in doing

i use a 4 ton for 12.5 kw in michigan and i think i could run 14kw off this ac.im in a 12 x 20 x 9.5h building.no ac needed from about nov 15th to march 1st unless we get real warm days like above 50f

Like I said, a row of 5 DE's is going to give you a 5' x 25' foot print. So for 10 lights I'd go with at least 13 x 28. I went with 15 x 28 because it worked well in my shop. I have two identical rooms right next to each other, both at 420 sq ft. :D

This is a row of 6 nanos on 5x5 centers with a 2' walkway on one side inside a shipping container fully framed, insulated, drywalled. They throw light so good at 1200w that my canopy almost completely covers the walkway. I'm vegging with 1 nano @ 825 on a light mover and have great results lighting a 7'x11' canopy.
We just built a room for a buddy that was 17x12 and we just did two rows of 3 lights with a 2' walkway down the center. We just split the 17' into three even spots so his lights are on a 5x5.6 or something like that and his canopy filled in amazing. He also had the canopy over take his walkway down the center.
I think the 5x5 is a conservative number. Gavita says 4x6 or something like that. I would not pack them in any tighter it is just a waste of power. Part of the reason you get bigger yeilds with d/e's is because they light a bigger area, not because they grow bigger buds. If you over light it you will not see any positive difference, probably just problems like Wentworth says.
Just my opinion Wich usually doesn't matter, gavitas are WAY overrated. Plastic sockets, super shitty lighting controller, and almost 100$ more vs a nano. I cant say but I have heard gavitas come with better bulbs, although I haven't hit less then 2#/ with my lowest yeilder and well over 3 with my best yeilder with nanos.
If you live in a warm area run 1/2 ton per light at least. I would put a 5 ton on 8 lights and never look back. I'm running 2-2 ton fujitsu's on 6 and I can make that place any temp I want any time no matter how hot it is outside. BTU rating of the lights doesn't account for all the other heat generated from everything else you have plugged in.
Good luck, go grow some weed