Running a 600w hps at 400w

Some say that running a 600w bulb with a 600w ballast dimmed to 400w is harmful to the bulb. I'm wondering if anyone has an explanation of why. It seems to me that if the ballast was operating as a 400w ballast (which is exactly what they advertise) the lower wattage wouldn't be harmful. I don't run mine at 400 often, just an a warm day because of heat. If I had to turn the light off and let it cool, then put a 400w bulb in, it would hardly be worth the trouble.

When you start it up it will hit it at 600w, but just for start up!So yes its harmful to a 400w bulb.But it will work.

But is not recommended for long term usage, if you don't care for the life of the bulb than okay fine use it.

209 Cali closet grower said:

When you start it up it will hit it at 600w, but just for start up!So yes its harmful to a 400w bulb.But it will work.

But is not recommended for long term usage, if you don't care for the life of the bulb than okay fine use it.

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How do you know it will hit it at 600w when you start it up?

that's what I have read.
thought that's what jd said, but I guess I must of read that.

But when you turn it on it's going to strike at a higher volt.

I'm running a 400 mh on my 600 ballast no problem