How to mount a UVB fixture

Three positions, and what each one costs you

Every care sheet gives you a distance. Almost none of them tell you that the distance is only valid for one mounting position, which is why two keepers can follow the same figure and end up with UV index readings that differ by a factor of two.

The mesh column is the whole reason this page exists. Published Ferguson zone guidance and the lamp manufacturers both state distances with a mesh assumption baked in - the figures we carry on this site assume somewhere between 30% and 45% of the output is blocked, depending on the species page they came from. Take the mesh away and you have not improved the setup. You have moved the lamp effectively closer, and the published distance is now wrong.

Mounting on the mesh: the default, and the arithmetic it implies

Resting the fixture on a mesh lid is what most keepers do and it is a perfectly good answer. The published through-mesh distances assume exactly this, so you can use the figure as written: 11 in for an Arcadia T5 HO Desert 12% over a bearded dragon, 10-14 in for a ReptiSun T5 HO 5.0 over a ball python, and the lower ShadeDweller figures for geckos. The full table lives on UVB distance and Ferguson zones and is not repeated here.

Two things to get right in this position. First, the fixture needs to sit flat and stay put - a tube that slides two inches off the basking zone over a week has quietly halved the UV index the animal receives, and nothing about the enclosure will look different. Second, leave the fixture room to shed heat. A T5 ballast is warm in normal use, and a fixture pressed under a closed hood or crammed against a heat lamp dome runs hotter than it was designed to.

Mounting inside the enclosure: the correction nobody applies

Fixing the fixture inside the enclosure is legitimate and sometimes necessary - a solid ceiling leaves you little choice, and arboreal setups often want the tube lower than the lid. Arcadia's ProT5 kit is sold as fixable inside a vivarium, rested on a mesh-topped terrarium, hung above an enclosure or hung within it where safe.

The correction is mandatory. With no mesh between tube and animal you need the no-mesh figure: 16-17 in rather than 11 in for a Desert 12%, 17-18 in rather than 11-12 in for a Dragon 14%. Keep the 11 in figure and you have put a dragon well above its UVI 4.0-4.5 at the basking surface target rather than on it.

There is a second constraint that only applies inside the enclosure, and it is a physical one. Retailer documentation for the reflector-equipped Arcadia kits warns that the fixture should not be within 12 inches of an animal when a reflector is fitted, with the caveat that an arboreal setup where the animal can move away is less of a concern. So an internal mount has to satisfy both numbers: far enough for the UV index target, and far enough that an animal that climbs cannot reach the tube. In a short enclosure those two requirements can conflict, and when they do the honest answer is that the enclosure is too shallow for an internal mount.

Suspending it: the most adjustable mount, and the easiest to get wrong

Hanging the fixture from a shelf, a hook or a frame above the enclosure is the most flexible option, because the distance becomes a dial rather than a fixed property of your lid. It is how we would set up a tall enclosure where the basking branch is well below the ceiling.

The failure mode is the opposite of the internal mount: people overshoot. A fixture hung six inches above a mesh lid has added six inches to a figure that already accounted for the mesh, and UV output falls off fast with distance. If you are suspending the fixture, the published distance is the distance from the tube to the point the animal's body occupies when basking - the top of the basking platform plus the animal's own height, or the branch a crested gecko rests on. Measuring to the substrate overstates your distance by several inches, which is enough to matter.

Mechanically, suspend from something that cannot shift. Chain and S-hooks through a shelf, or a bracket screwed into a stud. Not tape, not zip ties around a cable, and never hanging by the power cord itself - the cord is not a structural member and the strain relief at the fixture is not rated for the weight.

Solid ceilings: the case that forces a decision

PVC and wooden enclosures do not have a mesh lid to rest a fixture on, which is why the material you chose ends up deciding the lighting mount. You have two options and a non-option.

You can surface-fix the fixture inside the ceiling with its clips or brackets, applying the no-mesh distance. Or you can mount it outside, shining through a vent or a cut aperture - which keeps the fitting out of the humidity but costs you output through whatever covers the opening, an unmeasurable amount unless you own a meter. The non-option is leaving the fixture resting loose on top of a solid roof, where the light goes nowhere.

If you are building the enclosure yourself, decide the lighting mount before you cut the roof. How to build a reptile enclosure covers the panel work, and reptile enclosure ventilation covers the apertures - the two decisions interact, because the vent you cut for airflow is often the one the lamp ends up shining through.

Orientation: along the length, over the warm end

A linear tube is the whole point of a T5 fixture - it lights a zone rather than a spot - so run it along the long axis of the enclosure, positioned over the warm end. Mounted across the width, a 22 in tube in a 48 in enclosure produces a narrow band of UV with nothing either side of it, and the animal has to choose between the heat gradient and the UV gradient instead of finding them in the same place.

How much of the length the tube should cover is a sizing question rather than a mounting one, and it has its own page: what size UVB bulb for your tank. The short version is that the tube should span roughly half to two-thirds of the enclosure length, leaving a genuinely shaded end, because the gradient is the point.

Reflectors change the number

A reflector is not a cosmetic add-on. It redirects output that would otherwise be lost upward back down into the enclosure, which raises the UV index at the basking site substantially - and the published distances for reflector-equipped kits already assume one is fitted. The practical consequences are simple and frequently missed.

If a published figure assumes a reflector and you run the tube bare, you are below target. If a figure assumes no reflector and you add one, you are above it. And if you retrofit a reflector to an existing setup that was measuring correctly, you have just changed the setup and need to re-measure. That is not an argument against reflectors - we recommend them - it is an argument for knowing which case your figure came from.

Fire, heat and the electrical side

A UVB fixture is a low-wattage device and it is not the fire risk a basking lamp is, but two rules carry across from heat lamp fire safety and are worth stating. Leave clearance around the ballast so it can shed heat, in line with the NFPA's general guidance on heating equipment, and never run the fixture resting on dry substrate or against a wooden surface inside the enclosure.

Keep the UVB fixture and the heat lamp on separate circuits of your timer and controller setup, too. The UVB tube runs on a plain plug timer for the species photoperiod - 12-14 hours of light per day, seasonally adjusted for a bearded dragon - and must never be wired through a dimming thermostat, which is for the heat source only. UVB hours per day covers the timer side.

Verify the mount, do not trust it

Every number on this page is a published estimate for a mounting position. The only way to know what your animal actually receives is to read the UV index at the basking site with a meter, and the reading is what tells you whether your mesh, your mount and your reflector combination landed on the species target.

Without a meter, the published advice for a bearded dragon is to add 1 to 3 inches to the figure to allow for the animal's own height, and to err on the far side rather than the near side. Then re-check the geometry every few months, because fixtures sag, branches get moved and tubes decline before they fail.

The mount, in order

Decide the position first, because it selects the distance figure. Read the figure for that position from the distance table, not from the box. Mount along the length, over the warm end, with the fixture fixed rather than balanced. Measure to the animal's back. Then verify with a meter, or add the margin and accept that you are estimating.

For the fixture itself rather than its mount, best T5 fixtures is the scored roundup, and the bearded dragon UVB and heat setup shows the whole lighting assembly in one place for the species that is least forgiving about getting it wrong.