**Choose a ballast by its approved lamp combinations, supply voltage, and wiring requirements.** Matching physical size or total wattage alone is insufficient.

The easiest replacement is the same model as the original, or a manufacturer-approved equivalent—provided the fixture still uses the intended lamps.

1. **Read the tube markings.** Record the complete lamp designation, such as `F32T8`, `F40T12`, or `F54T5/HO`. Length and diameter alone do not establish compatibility; standard-output and high-output tubes can require different ballasts. Lamp color codes such as “830” or “841” generally do not affect ballast selection.

2. **Count the lamps operated by each ballast.** A four-tube fixture might contain two two-lamp ballasts. The replacement must explicitly support your lamp type **and the number of lamps you will connect**. A ballast advertised for “one or two lamps” can support either configuration, using its specified wiring diagram. Manufacturer selection tools, such as [Fulham’s lamp and wiring-diagram finder](https://www.fulham.com/contact-us/wiring-diagrams/), identify the approved combinations.

3. **Match the electrical supply.** Check input voltage and frequency against the fixture’s supply. “Universal voltage” means the stated range, such as 120–277 V; it does not mean every voltage worldwide. Do not confuse input voltage with the ballast’s lamp-output voltage.

4. **Check wiring and lamp holders.** The new ballast may have a different wiring diagram or different numbers of wires. Follow **its** diagram rather than reconnecting by wire color alone. Lamp holders—often called *tombstones*—may be shunted, meaning their two contacts are electrically joined, or non-shunted. Rapid-start and programmed-start systems generally require separate connections to both lamp pins; a change in starting method may therefore require new holders or rewiring.

5. **Check fit and operating conditions.** Confirm case dimensions, mounting holes, permitted mounting arrangement, temperature ratings, and any special requirements for dimming or emergency lighting.

For an upgrade while retaining fluorescent tubes, compare these features:

| Feature | How to choose |
|---|---|
| **Electronic ballast** | A compatible electronic model can replace an older magnetic ballast, usually reducing hum and visible flicker. Conversion may require rewiring. |
| **Starting method** | Instant start suits lamps left on for long periods. Programmed start preheats the electrodes and is usually preferable for frequent switching or occupancy sensors. |
| **Ballast factor** | Indicates relative lamp light output. Match the old value for similar brightness; a higher value generally increases brightness and power consumption. It is different from *power factor*. |
| **Cold-start rating** | Must suit the lowest expected temperature, especially in garages or outdoor fixtures. |
| **Dimming** | Requires a ballast compatible with the particular control system. Ordinary ballasts are not automatically dimmable. |

Manufacturers list lamp combinations, ballast factor, and starting-temperature limits separately; compare those specifications rather than relying on “latest” or “high efficiency” in a product description. [Example manufacturer specification sheet](https://www.assets.signify.com/is/content/Signify/BALLAST).

If you want **LED tubes**, choose the conversion approach before buying a ballast:

- **Type A:** Runs from a ballast; the exact ballast must appear on the LED tube manufacturer’s compatibility list.
- **Type B:** Bypasses the ballast and requires rewiring according to the LED tube’s instructions.
- **Type C:** Uses a separate LED driver.

These distinctions are explained in [LEDVANCE’s LED tube guide](https://www.ledvance.com/en-us/professional-lighting/insights/blog/product-guide/led-tube-types).

Before opening or rewiring the fixture, isolate power at the breaker and verify it is de-energized. Have a qualified electrician handle the replacement if you cannot safely verify the supply or interpret the wiring diagram.

To identify a specific replacement, provide the old ballast’s model number, the tube markings, the lamp count per ballast, and the supply voltage.