Fluorescent Tube Types: T5 vs T8 and Fluorescent to LED Tube Retrofit Wiring
T5, T8 and T12 fluorescent tube sizes, 840–865 colour codes, magnetic vs electronic ballasts and the wiring steps to convert a fluorescent fitting to LED tubes.
Fluorescent tubes are named by diameter: the number after `T` is the diameter in eighths of an inch (T8 = 26 mm, T5 = 16 mm). Most existing fluorescent fittings are now renewed with LED tubes that fit the same lampholders; the conversion depends on the ballast in the fitting and must be wired accordingly.
> Related Category: Browse lengths and colour temperatures in LED Tube Lamps, Fluorescent Tube Lamps and Bulbs.
1. Fluorescent tube types and sizes
| Type | Tube diameter | Cap | Common power and lengths | Note |
|---|---|---|---|---|
| T12 | 38 mm | G13 | 20 W / 60 cm, 40 W / 120 cm | Obsolete; no longer made |
| T8 | 26 mm | G13 | 18 W / 60 cm, 36 W / 120 cm, 58 W / 150 cm | Most common; offices, workshops, weatherproof fittings |
| T5 | 16 mm | G5 | 14 W / 55 cm, 28 W / 115 cm, 35 W / 145 cm | Electronic ballast only; about 5 cm shorter than T8 |
| Compact fluorescent (PL) | — | G23, G24, 2G11 | 9–55 W | Downlights and wall lights |
T5 and T8 tubes are not interchangeable: lengths and pin spacing (G5: 5 mm, G13: 13 mm) differ.
2. Reading the colour code
In the three-digit code the first digit is the colour rendering class and the last two the colour temperature.
| Code | Colour rendering (CRI) | Colour temperature | Appearance |
|---|---|---|---|
| 827 | 80–89 | 2,700 K | Warm white |
| 830 | 80–89 | 3,000 K | Warm white |
| 840 | 80–89 | 4,000 K | Neutral white; office and workshop standard |
| 865 | 80–89 | 6,500 K | Daylight (cool white) |
| 765 | 70–79 | 6,500 K | Daylight, lower colour quality |
| 940 / 965 | 90 and above | 4,000 / 6,500 K | Colour matching, printing, textiles |
For colour temperature choice see the Bulb Base Types and Kelvin Guide.
3. Ballast types
| Ballast | How to recognize | Property |
|---|---|---|
| Magnetic (choke) | Heavy iron core; the fitting also has a starter | Flickers at start, 50 Hz flicker, own loss 6–10 W |
| Electronic | Light box; no starter | Instant start, flicker-free, low loss |
A round removable starter in the fitting means a magnetic ballast; the LED conversion method depends on it.
4. Fluorescent vs LED tube
| Property | T8 fluorescent 36 W (120 cm) | LED tube 18 W (120 cm) |
|---|---|---|
| System power (with ballast) | 42–46 W | 18 W |
| Luminous flux | ≈ 3,350 lm (all round) | 1,800–2,700 lm (downward) |
| Life | 10,000–20,000 h | 25,000–50,000 h |
| Start | Delayed; weak when cold | Instant; full light when cold |
| Mercury | Yes | None |
| Frequent switching | Shortens life | No effect |
An LED tube emits downward over about 120–160°, so in fittings without reflectors it gives similar light on the work surface despite fewer lumens. Rough equivalents: 18 W fluorescent → 9 W LED tube (60 cm), 36 W → 18 W (120 cm), 58 W → 22–24 W (150 cm).
5. Converting a fluorescent fitting to LED tubes
Before starting, switch off the circuit breaker and read the wiring diagram on the tube label; the feed (single-ended or double-ended) depends on the maker.
A. Fitting with magnetic ballast (with starter):
- Remove the fluorescent tube and the starter.
- Fit the LED starter (fuse link) supplied with the tube in the starter holder.
- Insert the LED tube. The ballast stays in circuit and still loses a few watts.
- To remove that loss, bypass ballast and starter and wire mains directly to the lampholders (method B).
B. Fitting with electronic ballast (direct mains wiring):
- Remove the electronic ballast. Mains-voltage LED tubes do not work with it, and its high-frequency output damages the driver.
- Single-ended tube: line and neutral go to the two contacts of the same lampholder; the far holder only supports the tube.
- Double-ended tube: line to the holder at one end, neutral to the holder at the other.
- Label the fitting "converted for LED tubes, do not fit fluorescent tubes".
- Only special LED tubes marked as electronic-ballast compatible work without removing the ballast.
- T5 fittings are converted with T5-length G5 LED tubes, usually with the ballast removed.
- In damp and dusty areas the fitting itself matters too; see the IP Rating Chart for Luminaires.
6. Status of fluorescent lamps and waste
Fluorescent tubes contain mercury. The European Union ended the placing on the market of T5 and T8 fluorescent tubes for general lighting in 2023; spares are still needed for existing fittings, but new installations use LED. Do not vacuum a broken tube, ventilate the room; spent tubes go to a waste collection point, not household waste.
Frequently asked questions
What is the difference between T5 and T8 fluorescent tubes?
T8 is 26 mm in diameter with a G13 cap; T5 is 16 mm with a G5 cap and about 5 cm shorter. T5 runs only on electronic ballasts and is more efficient; they are not interchangeable.
What wattage LED tube replaces a 36 W fluorescent?
An 18 W LED tube of 120 cm. A 9 W tube replaces the 60 cm 18 W fluorescent and a 22–24 W tube the 150 cm 58 W fluorescent.
Must the ballast be removed for an LED tube?
With a magnetic ballast, swapping the starter for an LED starter lets the ballast stay. An electronic ballast must be removed and mains wired directly to the holder, or the LED tube will fail.
What do 840 and 865 on a tube mean?
The first digit is colour rendering (8 = CRI 80–89), the last two the colour temperature: 840 is neutral white 4,000 K, 865 is daylight 6,500 K.
Why does an LED tube flicker or glow faintly?
Common causes are an old starter left in place, an electronic ballast not removed, and an illuminated switch passing a small current when off.