LED bulb shapes and base types

Bulb shape codes

A bulb code is a letter for the shape followed by the diameter at its widest point, in eighths of an inch in North America: an A19 is the classic bulb shape, 19 eighths or about 2⅜ inches across. Europe uses millimeters instead, so the same shape is called A60.

The letters have specific meanings: A once stood for "arbitrary", G is globe, B or C is candle, ST is "straight tapered", BR is bulged reflector, PAR is parabolic aluminized reflector, MR is multifaceted reflector and T is tubular. ANSI C79.1 defines the North American shape names.

Lamp base codes

The base code gives the type and size: E means Edison screw and the number is the diameter in millimeters, so E26 is the standard North American base and E27 the European one. B is bayonet, G and GU are two-pin bases, and the number after them is the pin spacing.

Suffix letters add detail: d means two contacts and s a single contact, and a U in GU or a Y or X in GY and GX marks a variant of the pin design. Bases are standardized internationally in IEC 60061 and in North America in ANSI C81.61. Never force a lamp into a different socket: E26 and E27 bases interchange mechanically, but a lamp must also suit the local mains voltage.

Choosing by lumens, not watts

Choose by lumens, not watts. A 40 W incandescent equals about 450 lumens, 60 W about 800, 75 W about 1,100 and 100 W about 1,600. LED versions use roughly 5, 8–9, 11 and 14–16 watts. For reflector lamps, compare center-beam candlepower and beam angle as well.

These equivalents come from the U.S. Federal Trade Commission's lighting label rules, and the lumen equivalents table covers halogen and CFL too. Reflector lamps are harder to compare by lumens alone, because a narrow beam concentrates the same light into a brighter spot; the center-beam candlepower (CBCP), in candelas, describes that intensity.

Reflector lamps and beam angles

BR lamps have a soft-edged wide beam for general downlighting, PAR lamps a tighter, more controlled beam for accent and outdoor flood lighting, and MR16 and GU10 lamps a compact spot for display. Packaging codes such as SP, NFL, FL and WFL indicate spot, narrow flood, flood and wide flood.

Names vary between manufacturers, but spots are roughly 10–25°, floods roughly 36–60° and wide floods wider still; the beam angles table shows typical uses and spacing. An MR16 runs on 12 V from a transformer, which must be LED-compatible; many flicker problems with MR16 retrofits come from old magnetic or electronic transformers with too high a minimum load. GU10 lamps run directly on mains voltage, which makes them simpler to retrofit.

LED tubes: Type A, B, C and A/B

LED tubes replace fluorescent T8 and T5 lamps in four ways. Type A plugs into the existing ballast, Type B bypasses the ballast and runs on mains wired to the sockets, Type C uses a separate external driver, and hybrid Type A/B works either way. Type B avoids ballast losses but requires rewiring.

• Type A (plug and play): works with a compatible electronic ballast. It is the easiest swap, but the ballast still uses power and can fail.
• Type B (ballast bypass): mains power is wired straight to the sockets. Single-ended and double-ended versions need different wiring, and the fixture should be relabeled; a mismatch can be a shock hazard.
• Type C (remote driver): a separate LED driver powers the tubes at low voltage. It is the most efficient and dimmable option, at the highest cost.
• Type A/B (hybrid): runs on a ballast now and can be rewired later when the ballast fails.

In North America, Type A tubes are certified under UL 1993 and Type B and C retrofits under UL 1598C.

What to check before you buy

Before buying, check four things: the base, whether the lamp physically fits the fixture, enclosed-fixture and damp or wet ratings, and dimmer compatibility. Enclosed glass shades trap heat that shortens LED life, so use lamps rated for enclosed fixtures, and on dimmers choose lamps marked dimmable from the maker's compatibility list.

• Fit: LED lamps can be longer or heavier than the bulbs they replace because of the driver and heat sink; measure shades, harps and recessed cans.
• Location rating: use damp-rated lamps in bathrooms and covered porches, and wet-rated or IP65 products where water can reach them (see IP ratings).
• Color: pick one color temperature per room and CRI 90 or more for living spaces (see the CRI and CCT guide).
• Dimming: non-dimmable lamps can flicker, buzz or fail on a dimmer (see LED drivers and flicker).

Sources and further reading

1. IEC 60061-1, Lamp caps and holders together with gauges for the control of interchangeability and safety: Lamp caps, International Electrotechnical Commission (IEC).
2. ANSI C79.1, Nomenclature for Glass Bulbs Intended for Use with Electric Lamps; ANSI C81.61, Electrical Lamp Bases: Specifications for Bases (Caps) for Electric Lamps.
3. ANSI C78.20 (A, G, PS and similar shapes) and ANSI C78.21 (PAR and R shapes), American National Standards for electric lamps.
4. U.S. Federal Trade Commission, Energy Labeling Rule, 16 CFR Part 305 (Lighting Facts label and lumen equivalents).
5. UL 1993, Self-Ballasted Lamps and Lamp Adapters; UL 1598C, Light-Emitting Diode (LED) Retrofit Luminaire Conversion Kits.
6. U.S. DOE, "Lighting Choices to Save You Money," Energy Saver.
7. NEMA SSL 7A-2015, Phase-Cut Dimming for Solid State Lighting: Basic Compatibility.

Image credits

• Assorted LED lamps: NIST, public domain. Edison screw bases: Medvedev, CC0. Bayonet cap and holder: Jnestorius, CC BY-SA 3.0.
• GU10 spotlight: Matt Kieffer, CC BY-SA 2.0. MR16 lamp and LED tube: Dmitry G, CC BY-SA 3.0. All via Wikimedia Commons.

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