It seems that the 4 ft. x 4 ft. mark is the grey area between a 400w and a 600w system. All thing being equal, I would prefer the 600w; however, I do have two concerns:

1. Heat. It seems that many of you with a 400w can get away with using a standard fan to cool the bulb. Will a 600w require glass and cooling ducts... just glass with a standard fan... just a fan? If it matters, there is plenty of room above.

2. Power spikes. I appears that the difference in overall power consumption is negligible between the two. But, what about when the bulb ignites? Some of the recent threads say that under a 1000w is nothing to worry about, but is there a noticeable difference between a 400w and a 600w.

I'd appreciate any thoughts.

The 400 watt will give you around 42000 lumens, the 600 wat will give you 90,000 lumens. 50% more power consumption, 100% more light. The difference in heat output will reqiure that you put it about 2" higher than a 400 watt.

So, the 600w will not require glass and air cooling ducts?

go with a 600 digital ballast..they produce withing 5% usable light as a 1000w magnetic core ballast. a 400w is to small for a 4X4 area in my opinion. im in a 4X4 area and just switched to a 600w from a 1000w. and this is coming from someone who owns a 400w, 600w, and 1000w.

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Drunk Drivers Kill People; High People Just Miss Their Exits

Personaly for 4x4 area i would use 1000watt AIRCOOLED digital ballast MH/HPS conversion. My grow room is a 5f x 6.4f x 6.8f i have a grow tent in there a big one. and i use the grow tent for veg-flower, and outside the tent is where the mothers chill with some 4 foot T-5's and teh clones.

in the veg-flower area i use dual 600watt's mh/hps digital, everything is aircooled and ventilated, with a coco carbon filter. and the mother and clones done give off enough smell to put another cococarbon. but with one t-5 4f 8 bulb HO FLuro, and two 600watts im lucky my rents dont catch on. but i do a good job in keepin the rest of the houses lights off. and we use CFL's for everything pretty much. but my setup works fucking wicked, but is currently not in use untill the next couple days

just somthing you can build up too, in total i've spent $2,000 canadian over time.

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Ok, the consensus is 600w. Can I get away with using a standard fan to cool or will this require duct-cooling?

for optimum setep you are gonna want duct cooling but its not a must. I would highly suggest it.

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Drunk Drivers Kill People; High People Just Miss Their Exits

600 watt is what i would go with if i had it to do over. Yes you will need to vent. It is SO important. Proper venting and air flow could mean the difference between a great crop and a diaster.

If my memory serves me correctly, the 400 watt system is only good for a 3.2x3.2' area. Your area is slightly bigger. So, yes the 600 watt system. You can get one at a good price. Get the glass too.

videoman is correct get one with the glass that will help you vent the extra heat