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I have been running a 400w MH bulb against my 1000w ballast for a lot of years -- 16 years on this one. Probably 30 years altogether. It's a noisy proposition -- sounds like a herd of africanized killer turtles stampeding thru the laundry room -- but neither the bulbs nor the plants seem to mind either the setup or the noise. I replace bulbs about every year on general priciples, but never because I blew out or fogged a bulb. Now I'm thinking this isn't the most efficient way to use a 1kva power supply, but it's what I've got, and I can add a 2nd bulb to the mix if I care to without adding a ballast. And I can tell small children there's a herd of africanized killer turtles stampeding in the lanudry room, and they stay out of it.
You can't just wire in a second bulb with out wiring in another set of caps. MH bulbs still need a cap to boost the first few seconds of power to light the gas inside the bulb. Even if you have a 1000w transformer with a 400w bulb. Sure the bulb will work I have done it my self for many years but trying to run 2 bulbs from the same 1000w transformer same cap will not work. You need to wire in a second set of caps to run parallel