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I gather this is generally possible, as long as bulbs draw less power than the ballast can provide, but are there limits on how low you can go?

I currently have an electronic ballast for 1 or 2 54W T5 F54T5HO bulbs and two F13T5 bulbs. The specifications on the ballast don't list anything close to that wattage.

What are the consequences of running 1 13W bulb on a ballast that can support 2 54W (besides wasting the hardware)?

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That's not how it works.

Unlike incandescent bulbs, florescent bulbs do not draw a particular amount of power - they are rated to handle a particular amount of power. They are actually "negative resistance" devices - which basically means they will draw an infinite amount of power if you let them. (Specifically: The more current that goes into them the lower their resistance, which then allows even more current to flow into them, reducing the resistance even more.)

The job of the ballast is to prevent them from doing so and refuse to feed them more power than they are rated for.

So it's the ballast the controls the power NOT the bulb, and if you tried what you are suggesting you will put 54W of power in a bulb rated to handle 13.

  • I see. Answer to this question: diy.stackexchange.com/questions/27738/… effectively says otherwise ("You could put a regular bulb in an HO fixture, but not vice versa.", unless it just needs to be qualified) and I've seen other similar notions on the internets, but I can't find anything official. – Eugene Jan 20 '15 at 17:42
  • You answer (negative resistance) makes sense though. – Eugene Jan 20 '15 at 17:44
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    @Eugene That person is wrong. I left a comment telling them so. – Ariel Jan 20 '15 at 20:32
  • This answer is correct all HID lighting works on this principal where the ballast controlls the power to the lamp if the lamp value is not listed on the ballast it may work for a short time but may damage the ballast and or have a shorter bulb life.+ – Ed Beal Nov 2 '17 at 15:48
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I gather this is generally possible, as long as bulbs draw less power than the ballast can provide

Nope, you're thinking in voltage. With fluorescent tubes, you must think in current.

Anything you plug into mains power must be a constant-voltage device. Heaters are, and incandescent bulbs are close enough. That's where you get the misconception.

Fluorescent tubes are a member of the "arc discharge light" family. Once the arc strikes, they are nearly a dead short. The word for that is "non-linear". They need an additional part, a ballast, to limit current through them to avert their destruction. The ballast outputs the correct amount of current for the tube it is designed for... At whichever voltage that turns out to be.

However the ballast has practical limits to its voltage range. That limits which tubes it can work with. It may also have dual circuits to drive two tubes independently (and can work with one), or not. The upshot is the ballast has a data sheet (that link wasn't the data sheet) listing every combination of tube sizes it can drive. You need to go find that data sheet and look.

Now that we're thinking in current, the obvious answer is "probably not", the HO ballast would push more current than the NO tube is designed for.

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