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As you know when replacing a bulb you normally check the bulb, the vehicle handbook or online to find out which bulb is required. In this question I will be using the T10 501 W5W Bulb as an example and hopefully, by answering this, it'll help me understand the letters and numbers across a range of different bulbs.

Currently, I assume the following:

  • T10 is the size of the bulb? However, you can also have T10 bulbs in the home or office which are completely different.
  • 501 is the type bulb?
  • W5W is Wedge 5 Watts?
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W5W Bulb

T10

Refers to tubular glass, 10 mm diameter, according to this forum post.

W5W

The W5W defines the bulb "category", which is the standardized filament lamp design, as explained in ECE Regulation 37.

There is no explicit mention of the first "W" referring to "Wedge", but all bulb categories that start with a "W" share the same wedge-style base in the linked document.

Incidentally, W3W and W5W have the same dimensions, the former rated for 3 W output, the former rated for 5 W.

W5W Table

501

I couldn't find any reference to 501, but the bulb "type" is plausible.

The document distinguishes between "category" and "type":

2.1.1. Definition of "category"
The term "category" is used in this Regulation to describe different basic design of standardised filament lamps. Each category has a specific designation, as for example: "H4", "P21W", "T4W", "PY21W"or "RR10W".

2.1.2. Definition of "type"
Filament lamps of different "types" are filament lamps within the same category which differ in such essential respects as:

2.1.2.1. Trade name or mark (Filament lamps bearing the same trade name or mark but produced by different manufacturers are considered as being of different types. Filament lamps produced by the same manufacturer differing only by the trade name or mark may be considered to be of the same type);

2.1.2.2. Bulb design and/or cap design, in so far as these differences affect the optical results;

2.1.2.3. Rated voltage;

2.1.2.4. Halogen.

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  • I think 501 could be a UK or EU thing only because looking at eurocarparts.com/interior-bulbs they have many different codes that I can not find on Wikipedia or US based websites.
    – Simon Hayter
    Mar 13, 2018 at 17:07

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