IEC 60061-1: Lamp Caps and Holders — Gauges for Interchangeability (Part 1: Caps)

E27: How a 27 mm Thread Diameter Ensures Global Lamp Interchangeability

IEC 60061-1 specifies lamp cap dimensions, threads, and gauges. Every E27 screw-base bulb worldwide fits into every E27 socket because of this standard. It may seem “low-tech,” but it is the lighting industry only mandatory mechanical interchange standard.

Core concept: GO/NO-GO gauges. Every lamp cap thread must simultaneously pass two checks: a GO gauge (maximum material limit — verifying the cap is not too large) and a NO-GO gauge (minimum material limit — verifying it is not too small). This ensures that the “tightest socket” still accepts the “loosest bulb” without jamming or falling out.

The LED challenge: An incandescent lamp cap was purely a mechanical interface. An LED lamp cap is also a thermal conduction path — higher wattage means more heat must be dissipated through the cap. But IEC 60061-1 dimensions are fixed (for interchangeability). This forces LED manufacturers to innovate in heatsink design — the lamp cap dimensions are a hard constraint.

TN Lab — One thread standard lets every bulb screw into every socket, worldwide.