At a glance
| Cap | What the code means | Often seen in | Check with the maker |
|---|---|---|---|
| E14 | Edison screw, 14 mm thread | Candle holders and small pendants (common) | Rating per holder, lamp length, closed glass, dimming |
| E27 | Edison screw, 27 mm thread | Larger globes and pendants | As E14; lamp diameter in the glass |
| B22, B15 | Bayonet, about 22 mm or 15 mm cap | Older UK fittings | Push and twist: support a glass arm |
| G9 | Two pins, 9 mm apart | Small cups and wall lights | Rating, size, closed glass |
| G4 | Two pins, 4 mm apart | Small cups and wall lights | Supply: often low voltage; driver |
| GU10 | Two pins, 10 mm apart; push and twist | Spots; rare in glass fittings | Rating, heat, beam |
The letter is the cap type and the number a size in millimetres. The cap does not fix voltage, rating or heat. ‘Often seen in’ is general practice, not sourced: the maker states the caps each fitting uses.
Read the cap, then read the fitting
A lamp cap code has a letter and a number. The letter is the cap type: E for Edison screw, B for bayonet, G for pins. The number is a size in millimetres: the thread diameter of a screw cap, the pin spacing of a pin cap. The Lighting Products Regulations define G4, GY6.35 and G9 as two small pins 4, 6.35 and 9 mm apart (Schedule 1). The standard that fixes every cap, IEC 60061-1, has a sheet for each, including E14, E27, B22, B15, G4, G9 and GU10; this sheet does not reproduce its dimensions.
The cap tells you only that a lamp will physically go in the holder. It says nothing about the supply voltage, the power the holder can take, whether the lamp fits inside the glass, or whether a closed shade lets it run safely. Those are separate questions, and the table below is a checklist of the questions to ask the maker.
The rating on the fitting is the limit
The rating on the fitting is the limit. The Electrical Equipment (Safety) Regulations 2016 say the essential characteristics that let equipment be used safely must be marked on it or on an accompanying document, that temperatures which would cause a danger must not be produced, and that in foreseeable overload it must not endanger persons or property (Schedule 1, paragraphs 1(a), 2(b) and 3(c)). The Electricity at Work Regulations 1989 say no electrical equipment shall be put into use where its strength and capability may be exceeded in a way that gives rise to danger (regulation 5). The luminaire standard is where the marking rules sit: BSI lists BS EN IEC 60598-1:2024+A11:2024 as current and says the 2024 edition changed the marking requirements. We did not read its clauses, so we give no marking wording from it: confirm against the current edition which one your supplier tested to.
The rating is normally given for each lamp holder, so a fitting with several holders has a limit for each and a total. Write the rating on the shop drawing, and put the lamp type in the specification with the fitting rather than leaving it to the operator. Eight checks before any lamp is chosen or changed:
| Check | Why | Who confirms |
|---|---|---|
| Cap, and how many holders | The cap must be the one the holder takes. A fitting can mix holders. | Shop drawing |
| Maximum rating per holder | Marked on the luminaire or given in its papers. A limit, not a guide. | Maker |
| Length and diameter of the lamp | A lamp of the wrong size can be damaged, or the lamp, fitting or control gear can overheat (HSE HSG38, para 78). | Maker; check against the glass |
| Weight | The lamp standard asks for a note where a lamp is much heavier than the one it replaces, because it can strain holders and lamp retention (Electrical Safety First, quoting BS EN 62560). | Lamp supplier |
| Supply: mains or low voltage, and driver | The cap does not fix the voltage. A lamp pack must show the cap and the supply separately (SI 2021/1095, Sch 3, para 6(2)(d)). | Lamp supplier; electrician |
| Closed or enclosed glass | A lamp made for conditions other than 25 degrees Celsius ambient, or needing thermal management, must say so on its pack (Sch 3, para 6(2)(k)). Closed glass is such a condition to ask about. | Lamp supplier, in writing |
| Dimming | A pack must warn if the lamp cannot be dimmed, or only with listed dimmers (Sch 3, para 6(2)(l)). | Lamp supplier; controls designer |
| Light output, colour temperature, colour rendering | Luminous flux, colour temperature and colour rendering index are shown on the pack (Sch 3, para 6(2)). | Lighting designer |
Replacing filament or halogen lamps with LED
Match light output, not watts. An LED lamp uses far fewer watts than the filament lamp it replaces, so the rating on the fitting is not the guide to what to fit. The Regulations let a lamp claim an old filament equivalent only if it reaches the light output in this table (Schedule 8, paragraph 2, Table 10; intermediate values are interpolated):
| Old filament lamp it is compared with | Light a lamp must give to claim the comparison |
|---|---|
| 15 W | 136 lm |
| 25 W | 249 lm |
| 40 W | 470 lm |
| 60 W | 806 lm |
| 75 W | 1,055 lm |
| 100 W | 1,521 lm |
Halogen capsule lamps are a special case. Schedule 3, Table 2 gives G9, G4 and GY6.35 halogen lamps a row of their own only for the period before 1 September 2023. After that date such a lamp falls into the “other light sources” row, with a threshold of 120 lumens per watt, which on our reading a halogen capsule cannot meet; there are exemptions for specific uses (Schedule 4, paragraph 3(2)). So do not specify halogen capsules for new work, and ask the supplier for the status of any halogen lamp in an existing fitting.
A retrofit LED lamp in an older fitting changes what the fitting is tested for. Electrical Safety First’s guide on retrofit LED lamps (2014) records a fatal fire in 2013 that investigators put down to a faulty self-ballasted LED lamp in a luminaire over a bed, and asks for lamps made to BS EN 62560 (BSI lists BS EN 62560:2012+A11:2019 as current), with the supply switched off or the fitting unplugged before any lamp is changed. Ask the maker to name, in writing, the lamps each fitting was assessed with, and to supply the specified lamp with the fitting. For drivers and dimming see LED drivers for decorative luminaires and dimming protocols.
Heat on glass and in closed shades
Where the heat goes changes when the lamp does. A US Department of Energy fact sheet (2007) gives, for a 60 W incandescent lamp, 8 per cent of the input as visible light, 73 per cent as infrared and 19 per cent as heat by conduction and convection; for the white LEDs of that year, 15 to 25 per cent as light, almost no infrared and 75 to 85 per cent as heat that has to be conducted away through the lamp’s base. Those are 2007 figures and today’s LEDs turn more input into light, but the principle holds: a filament lamp warms the glass it faces, and an LED lamp warms its own base and the air around it. Excess heat shortens LED life and shifts its colour and output, which is why the lamp supplier must say if a lamp is not for closed fittings.
Hand-blown glass is annealed, not toughened, and the Glass Museum Murano says glass is a poor conductor of heat, so uneven cooling leaves tensions in it. A float-glass maker’s bulletin explains the same thing from the other side: a hot centre against a cooler edge puts the edge in tension, and heat traps make it worse. A lamp that is within its rating in open air may not be within it in a closed cup: ask the maker for the maximum lamp rating for the glass, not only for the holder, and see glass types for lit joinery and luminaires. HSG38 says information on high-wattage halogen lamps can be sought from the luminaire manufacturer (paragraph 77). The conservation literature on glass chandeliers records that 18th-century candles cracked glass candle tubes and suggests low-voltage lamps, which give off less heat, as one way to lessen the load on old glass.
Colour through coloured glass
Coloured glass is a filter. It lets some wavelengths through and holds back others, so the light that comes out is a different colour and a fraction of what the lamp gave. The drawing models that for seven glasses and two white sources. The pattern is clear even though the numbers are not measured: cobalt blue and ruby red let through a small share of a white lamp, amber and green more, opal spreads and holds back light, and the warmer or cooler lamp shifts the share a little. Glass colour comes from added metal compounds; the Murano museum lists cobalt or copper for blues, gold, selenium or copper for reds, and chromium or copper for greens.
- Choose the lamp for the room, then approve the glass lit. A colour rendering index describes how a lamp shows surfaces, not how it looks through red glass.
- Plan for less light. Where the glass is strongly coloured, count on the light coming from other fittings, and ask the maker for a measured transmittance curve for the approved sample.
- Dimming moves the colour. A lamp that warms as it dims changes the light reaching the glass, so approve the glass at the levels it will be used at.
For how colour varies between batches see Murano glass colour and acceptance criteria for handmade glass; for colour temperature and colour rendering see colour temperature by space, CRI, R9 and TM-30 and dim-to-warm, tunable white and RGBW.
Specification wording
Paste into your schedule or specification and replace anything in [square brackets] with your own figures. Wording is a starting point: your specification governs.
Light source for each fitting tag: [LED lamp], cap [E14], [5] per fitting, rated [230 V], [2,700] K, colour rendering index [90] minimum, [470] lm each, dimming [trailing-edge] compatible with [control and protocol]. Lamp length not more than [ ] mm and diameter not more than [ ] mm for the glass supplied. The lamp is to be listed on the shop drawing and on the care sheet.
The maker is to state on the shop drawing the maximum lamp rating per holder [ W], the lamp types and caps the fitting was assessed with, and whether a closed or enclosed lamp is permitted in each glass element. No lamp above the marked rating, and no change of lamp type, without the written approval of the maker and [designer]. The lamp supplier is to confirm in writing that the lamp suits operation in the closed glass [element], and the maker is to state the temperature at the lamp holder [ ] with that lamp. No halogen capsule lamps.
Glass colour [ref] is to be approved lit, with the specified lamp [type, K, CRI], at the dimming levels [ ], on a sample [ref] before production. The maker is to supply a measured transmittance curve for the approved sample on request. The lamp colour is not to be changed after approval without a new approval.
Terms used on this page
- Lamp base (cap)
- The standard fitting at the end of a lamp. E14 (small Edison screw) is common in chandeliers, E27 is the larger screw, G9 is a small pin type, and GU10 is a spotlight twist-lock.
- Maximum lamp rating
- The most power a lamp may draw in a holder, marked on the luminaire or given in its papers. It is a limit set by the maker for heat and safety.
- Enclosed use
- A lamp running inside closed glass or a sealed housing, where heat cannot escape freely. The lamp must be made for it.
- Luminous flux
- The amount of visible light a lamp gives out, in lumens. Compare lamps by lumens, not watts.
- Transmittance
- The share of light, at each wavelength, that passes through a glass. A measured curve shows what a coloured glass does to a lamp.
Standards and sources
- BS EN IEC 60598-1:2024+A11:2024: BSI Knowledge: Current, published 31 Mar 2025; clauses not read; confirm the edition your supplier tested to.
- IEC 60061-1 (lamp caps); BS EN 62560:2012+A11:2019: sheets listed in amendment 63 (2022); BS EN 62560 Current per BSI (31 Mar 2019).
- SI 2021/1095 Schedules 1, 3, 4, 8; SI 2016/1101 Sch 1; SI 1989/635 reg 5: read on legislation.gov.uk, 6 Nov 2026.
- HSE HSG38 Lighting at work (1997), paras 76 to 80: read 6 Nov 2026.
- Electrical Safety First, Best Practice Guide 9 (2014); US DOE PNNL-SA-51901 (2007): retrofit LED lamps; LED heat. Dated: see unverified.
Questions we are asked
Can I put any E14 LED lamp in a chandelier?
No. The cap only means it will go in the holder. Check the rating per holder, the lamp’s length and diameter against the glass, its weight, whether it is made for closed glass, whether it dims on your control, and its light output and colour. Ask the maker which lamps the fitting was assessed with.
What is the maximum wattage for a chandelier lamp?
It is the figure marked on the luminaire or given in its papers, normally per holder, and it is the limit. It differs between fittings, so there is no general figure. The Electricity at Work Regulations 1989 say equipment must not be used where its strength and capability may be exceeded in a way that gives rise to danger.
Can I replace a halogen lamp with LED in a glass fitting?
Only if the maker confirms the lamp and the fitting. Match light output, not watts (a lamp must give about 470 lumens to claim comparison with a 40 W filament lamp). Halogen capsule lamps with G9, G4 or GY6.35 caps lost their own efficacy row after 1 September 2023, so ask the supplier about any in stock.
Is an LED lamp cooler than a halogen one in closed glass?
It is a different heat problem. A filament or halogen lamp gives out mostly infrared, which warms the glass it faces. An LED turns most of its input into heat that is conducted away through its base, and a closed shade can trap it. Ask the lamp supplier whether the lamp suits enclosed use and the maker for the limit for the glass.
Why does a blue or red glass fitting look so dim?
Coloured glass is a filter that holds back the other colours. In our model a strongly coloured glass lets through a small share of a white lamp’s light. Approve the glass lit, with the specified lamp, and plan the room light from other fittings.
Does colour rendering apply through coloured glass?
A colour rendering index describes how a lamp shows eight standard pastel colours on surfaces. It does not describe the colour of light that has passed through a coloured glass. Specify it for the lamp, and approve the lit result for the glass.
Download the factsheet
A two-page A4 PDF with the lamp cap table, the eight lamp checks, the heat and colour points and wording to paste into your specification.