Short answer: match the bulb to how your species basks, then mount it at the correct distance. Shade dwellers use the 70 watt. Partial baskers use 70–100 watt. Open and mid-day baskers use 100–160 watt. The 275 watt is a professional zoo bulb and is not intended for home enclosures.
Getting this wrong matters in both directions. Too little UVB and your animal cannot synthesise D3, which leads to metabolic bone disease. Too much — or a bulb mounted too close — can burn its eyes. The guidance below comes directly from Mega-Ray.
Step 1 — Find your reptile
| How it basks | Species | Bulb |
|---|---|---|
| Shade dwellers | Corn snakes, leopard geckos, crested geckos, Burmese pythons, anoles | 70 watt |
| Partial sun / occasional baskers | Red-footed tortoises, skinks, Chinese water dragons | 70–100 watt |
| Open / partial sun baskers | Bearded dragons (full setup guide), red-eared sliders and other basking turtles, monitors, some caimans | 100–160 watt |
| ‘Mid-day’ sun baskers | Uromastyx, iguanas, chuckwallas, tegus | 100–160 watt |
Within a range, choose based on enclosure size and how far the bulb will sit from the basking spot.
Step 2 — Set your mounting distance
Distance is measured from the face of the lamp to the basking surface or the animal.
| Bulb | Minimum | Optimum | Maximum |
|---|---|---|---|
| 70 watt | 6″ | 8–10″ | 12″ |
| 100 watt | 12″ | 14–16″ | 20″ |
| 160 watt | 18″ | 20–22″ | 24″ |
| 275 watt | 30″ | 36–42″ | 48″ |
At minimum distance a bulb delivers roughly 150–200 µW/cm² of UVB and 900–1200 µW/cm² of UVA. At maximum distance that falls to 50–70 µW/cm² of UVB.
Three things that change the answer
- Screen tops deflect 20–40% of UVB, heat and light. If your bulb sits above a screen, your animal receives noticeably less than the figures above.
- Minimum is not optimum. In Mega-Ray’s words: “All too often our clients mistake ‘minimum distance’ with ‘optimum distance’.” Mount at the optimum and move closer only with good reason.
- Your fixture must be ceramic AND rated above the bulb’s wattage. These bulbs run hot enough to melt a plastic lamp socket. Critically, most reptile clamp lamps are rated to only 150 watts, which is not enough for the 160 or 275 watt bulbs. See the fixture table below.
Check your fixture rating before you order
Mega-Ray is specific about this: “All of our bulbs require a ceramic base and require a higher amount of wattage and voltage than the bulb you need.” The fixture must be rated above the bulb it holds — not merely equal to it.
This catches people out, because the popular reptile clamp lamps sold by Fluker’s, Zoo Med and similar brands are typically rated to a 150 watt maximum.
| Bulb | Minimum fixture rating | Typical 150 watt reptile clamp lamp |
|---|---|---|
| 70 watt | Above 70 W | ✓ Suitable |
| 100 watt | Above 100 W | ✓ Suitable |
| 160 watt | Above 160 W — 250 W+ recommended | ✗ Not suitable |
| 275 watt | Above 275 W, ceramic/porcelain | ✗ Not suitable |
If you are running a 160 or 275 watt bulb, check the rating stamped on your fixture. A 150 watt fixture holding a 160 watt bulb is the most common cause of melted sockets and premature bulb failure — and it is a genuine fire risk. If you are not sure what yours is rated for, ask us before you install it.
Why the 275 watt is not on this list
Mega-Ray is explicit about it: “This bulb is a zoologist bulb. Not intended for personal use unless you need UVB within the optimum distance of the bulb for your habitat.” Its optimum distance is 36–42″, so it only suits very tall enclosures and institutional installations. In a typical home enclosure it would sit far too close and deliver a dangerous overdose. See the 275 watt if you are outfitting a tall or professional habitat.
Still not sure?
Tell us your species, your enclosure height and whether there is a screen between the bulb and the animal, and we will tell you exactly which bulb and what mounting distance. We would rather send you the correct bulb than the expensive one. Get in touch.
Common questions
How often should I replace the bulb?
Every 6–12 months. UVB output falls steadily and invisibly long before the bulb stops producing light — a bulb that still looks bright can be delivering far less UVB than it did new. In UV Guide UK’s independent testing, a self-ballasted Mega-Ray lost only 15% of its UVB output in its first year and 17% after 18 months (F. Baines, 2006) — we still recommend 6–12 months, or a Solarmeter reading. The full test data, methods and limits are on our Mega-Ray UVB test results page.
Can I use one bulb for two enclosures?
No. UVB drops off sharply with distance and angle. Each basking area needs its own correctly positioned bulb.
My reptile is a shade dweller — does it even need UVB?
Shade dwellers need far less, not none. The 70 watt at its optimum 8–10″ provides an appropriate low-level gradient. As Mega-Ray puts it: “If the husbandry is set up properly, all animals can use our bulbs.”
What if I mount it further away than the maximum?
Output drops below the useful threshold and your animal may not synthesise enough D3. Stay within the range, and remember to account for a screen if you have one.
