T5 vs T8 tubes: diameter, length and base differences

How T5 and T8 fluorescent and LED tubes differ in size, base and circuitry, why they are not interchangeable, and what to check before buying.

From a distance the two tubes look the same: white, long, with metal pins at each end. Put them side by side, though, and the difference is clear: one is noticeably slimmer. Ask for a "tube light" in a shop and the first question will be exactly this: T5 or T8? The two standards are close relatives, but they do not swap in the same fitting.

Diameter: what the name means

The number in a tube's name gives its diameter in eighths of an inch.

• T8: eight eighths of an inch, about 26 mm. A little thicker than a finger.
• T5: five eighths of an inch, about 16 mm. Slightly thicker than a pencil.

You feel the difference as soon as you pick them up. Older fittings may also have thicker T12 tubes, but these are now rarely used.

Base: G13 and G5

The tubes differ not only in diameter but also in the pins at their ends.

• T8 tubes use the G13 base, with about 13 mm between the two pins.
• T5 tubes use the G5 base, with about 5 mm between thinner pins.

So a T5 tube physically cannot go into a T8 lampholder, nor a T8 into a T5 holder. That is a useful difference that prevents accidental mistakes.

Length: close, but not the same

Lengths differ too, which becomes decisive when adapters come up. T8 tubes are close to round figures such as about 600, 1200 and 1500 mm. T5 tubes are about 50 mm shorter than their T8 counterparts; for example, the T5 equivalent of a 1200 mm T8 is roughly 1150 mm.

This is because T5 fittings were designed from the start around a different sizing system. As a result, a T5 tube does not match the distance between the lampholders in a T8 fitting.

Circuitry: choke or electronic ballast

Fluorescent T8 tubes can run on either an old electromagnetic choke with a starter or an electronic ballast. Fluorescent T5 tubes, however, are designed to run on electronic ballasts only. They have no starter.

The difference comes up again when you move to LED:

• LED T8 tubes come in the widest range: models for choke circuits, for electronic ballasts and for direct mains connection.
• LED T5 tubes are less common. Most are either compatible with specific electronic ballasts or require direct wiring.

In both cases, buying a tube without knowing what is inside the fitting is a gamble.

You do not always need to dismantle the fitting to find out what is inside. There is often a label with a diagram and technical data on the cover or inside the housing, showing the tube type, wattage and ballast type. If you can see a small cylindrical starter on the fitting, it has a choke circuit and takes T8 tubes. If there is no starter, the circuit uses an electronic ballast and the tube may be either T8 or T5. If the label has worn off, open the cover with the power off and read the marking on the ballast.

Can you switch from T5 to T8 or vice versa?

There are adapter kits that let you fit a T5 tube in a T8 fitting. They clip into the T8 lampholders and contain a small electronic ballast. These kits were once popular as an energy-saving measure. Today, converting a T8 fitting directly to an LED T8 tube is usually simpler and more reliable: no extra device, no extra contacts and no extra point of failure.

In the other direction, fitting a T8 tube into a T5 fitting, there is no sensible solution. If you want LED in a T5 fitting, look for an LED T5 compatible with its ballast or replace the fitting.

Quick comparison and choice

• Diameter: T8 about 26 mm, T5 about 16 mm.
• Base: T8 uses G13, T5 uses G5.
• Length: T5 is slightly shorter than the matching T8.
• Circuitry: fluorescent T5 on electronic ballasts only, T8 on either.
• LED options: wider for T8.

Before buying, remove the old tube, measure its diameter and pin-to-pin length, and look at the wiring diagram on the fitting's cover.

The Zəfər İşığı catalogue lists LB-18 series fluorescent tubes in two lengths and an S2 starter. Knowing which standard your existing fittings use makes it possible to choose correctly between fluorescent and LED tubes.