# Question about overdriving T5HO tubes



## Church (Sep 14, 2004)

I've never done this before, so I'm slightly unsure what to expect... basically my issue is this:

I am about to modify my coralife T5NO fixture to turn it into a single-tube T5HO fixture. I've already decided that finding a ballast to run a 31w T5HO tube is pretty difficult, so I know I have to overdrive. The closest thing to 31w is the very common 39w, so that seems like the obvious solution to me here... just use a 39w ballast and overdrive the 31w bulb to 39w. Right?

But then I found out that I can get a ballast for a 54w tube at a much cheaper cost then the 39w ones, through my work. The cost savings are enticing me to try overdriving the 31w tubes to 54w instead of 39w. Of course, I know that means the bulbs will burn out extremely fast. But my real issue is whether or not a 31w bulb being overdriven to 54w will actually be to much light over a 20 long?

So my questions are:

Am I correct in understanding that connecting my 30 inch, 31 watt T5 tube to a ballast designed to run a 36 inch, 39 watt tube will actually cause it to run at 39w? Or does the ballast know through voltage resistance that there is a lesser wattage tube at work? :icon_redf

If so, then would overdriving it to 54w just seem like the better idea, since I'm already overdriving at all? I should mention here that the aquarium has CO2 already.

Would running a single bulb at 54w over a 12" tall aquarium, and not even raised up off the top, be a bad idea? That just seems like a lot of light, which makes 39w seem like the proper way to go.

Any thoughts?


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## Hoppy (Dec 24, 2005)

A 12 inch high tank should not have a T5HO light over it, unless it is a reef tank. Even one bulb would give you much too much light. The best lighting for a 12 inch high tank is T8, one bulb for a 12 inch front to back depth, or two widely separated bulbs for a 24 inch front to back depth.


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## Church (Sep 14, 2004)

Seriously? I just had a 2 x 24w T5HO over this tank for the past month or so, and had nothing but fantastic results. I just don't want a dual bulb fixture anymore. I want a one-bulb, strip light style fixture, so I can open and close the glass top easily.

So yeah, I'm curious, why do you say that?


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## HolyAngel (Oct 18, 2010)

Well it won't actually be driven at 54watts but a little more then the 39w it normally draws. At most it should only increase par/light by 25%. definitely not more then that. You should be fine but may have to raise a bit if algea shows up. If you're still gonna use the coralife fixture/reflector you may be fine where it is.


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## Church (Sep 14, 2004)

^ Yeah that's another thing I forgot to mention... since I'll be using the crappy coralife t5no fixture, I figure the light won't be as well-reflected as it could otherwise be, hence less light.

Also, like I said, I was just using a 2x24w fixture from the hydro store, and it has really good reflectors, over each individual bulb, and my tank loved it. The glosso carpet was just thick, lush, green, and FULL of oxygen bubbles. And it was sitting directly on the glass top... no legs. So I can't imagine a single 31w (or 39w) bulb would be too much?

And just to reiterate, for the record... I'll be using a 31w tube, the kind that only Coralife sells, for their 30 inch T5HO fixtures, but since I cant find a 31w ballast I will have to either overdrive it to 39w or 54w. I will not be using 39w bulbs, because they won't fit in the 30" fixture.


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## HolyAngel (Oct 18, 2010)

Ah yeah I meant 31w, it still won't draw the full 54watts though due to the bulb itself(the resistance and such) but yeah more then the 31w it normally draws.

I Agree with hoppy though if the fixture had nice individual reflectors per bulb, should be way too much light with that.


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## Wasserpest (Jun 12, 2003)

It depends on the ballast... there is a wide variety of them out there and all of them behave differently.

For example, a Fulham WH7 ballast can light up 4' 54W bulbs. But it can just as well drive 2' 24W bulbs at their correct wattage. Now if you connect a 4' 28W bulb it will be overdriven to ~54W.

Generally I would not expect a 39 or 54W ballast to overdrive a (shorter) 31W bulb. But as I said... depends on the ballast.


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## nameless? (May 28, 2011)

For what its worth ice cap ballasts overdrive 39w bulbs to 60w and 54w to 80w, so I wouldn't worry to much.


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## HolyAngel (Oct 18, 2010)

I'm pretty positive my workhorse 7 is overdriving my 2x24w bulbs no matter what I do. They are *much* brighter then they were with the stock ballast..sadly I can't truly verify since I don't have a par meter, but just looking, it's noticeable. But agreed Wasserpest ^^


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## Wasserpest (Jun 12, 2003)

If you want to know, I suggest to get one of those Kill-A-Watt meters, often for sale for less than $20 shipped at meritline or dealxtreme etc. You can just plug in the fixture in and it tells you the wattage being drawn. Of course you have to take a few watts for the ballast itself into account, but that way you can get a fairly good idea.

It's possible that the WH7 overdrives smaller bulbs a bit...

Sorry OP this is all a bit off topic. Perhaps you can get a ballast in a B&M store with the opportunity to test it and return if it turns out too bright or doesn't work for some reason.


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## Church (Sep 14, 2004)

Thanks everyone, this is very helpful information!

Wasserpest, thanks for the advice. And I actually sell those killawatt things at my work, so I will definitely do that. Here's the deal, since it might help explain things better:

I get good prices on T5HO fixtures, but they only come in the 2', 3', and 4' lengths. But my 20 long is currently in a spot, under a shelf, where I don't even have a full 3' to work with. I pretty much only have the length of the tank (30"). So this means I have to go with either 24w tubes or those 31w tubes that Coralife makes. Since I would prefer not to have any shadows on the ends of the tank, I figure I should try for the 31 watters.

So I was looking around Fulham's website at the workhorse ballasts, and noticed that all the wiring diagrams just talk about 24w, 31w, and 54w configurations. I then did some googling and found out people overdrive their tubes all the time. And that was when I discovered that I would more than likely be able to wire up a 31w tube to a 39w ballast.

THEN it dawned on me that I wouldn't have to mail order a ballast after all... I could simply purchase a 39w strip light and take it apart! They come with electronic ballasts, and as a bonus, it is made to fit inside a narrow tube, just like the one I'm replacing out of the coralife strip light. This means I can still close up the coralife fixture and keep it internally-ballasted!!

Here is the fixture I will be taking the ballast from:











I figure, it's only gonna cost me about $21 or something, so I'll chance it. If it doesn't work, big deal. But hopefully it _will_ work, and I'm an optimist. :biggrin:


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## Church (Sep 14, 2004)

I just finished the conversion! :biggrin:

I gutted the 39w strip light and stole its ballast, then retrofitting it to the coralife one. I tested it after wiring it up and it worked fine, so I know the wiring is cool. But the problem is I was only able to test it with 39w and 24w tubes, not the 31w that I plan on using. Here are the results I had from trying out the various T5 tubes I had on hand:

First, after wiring everything up but before installing it inside the fixture, I tested it by holding the end caps tightly on the ends of a 39w HO tube. It fired right up! 

Then, I held the end caps tightly on the ends of an 18w NO tube. Nothing happened. 

The last type of T5 I had lying around was a 24w HO tube. Connected the end caps, it fired right up!  I couldn't tell that it was necessarily being over driven though. I don't know if the ballast detected that it was a 24w tube and only fired it at 24 watts, or if it was driving the tube at 39w. I would need a kill-a-watt to find that out.

So what I learned is, it _will _fire 24w and a 39w tubes, which won't fit in my fixture, but it _won't_ fire a 18w normal output tube which DOES fit this fixture. I guess the ballast doesn't work with NO tubes?

I sure hope it works with the 31w tube I designed this thing for. I have to go buy one now so I can find out..


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## Church (Sep 14, 2004)

IT WORKS! :biggrin:

I just got back from the LFS with a 31w tube, and I just plugged it in. Success! I now have a single-tube T5HO strip light that actually fits over the top of a 20 long. I took a few pictures along the way, mostly so that I could put things back together again if I got lost, so I will be posting a how-to thread about this in the diy forum.


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## Wasserpest (Jun 12, 2003)

Awesome. If you measure the wattage, let us know...


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