What are standard Lamp Thread Sizes

Understanding Standard Lamp Thread Sizes

The American standardization of lamp thread sizes in the early 1900’s enabled the mass production of lighting. Recommended by the American Society of Mechanical Engineers these standardization's brought unity to the lighting world. Systematic dimensions were used. Most table lamps and bridge lamps were put together with 1/8 IP fittings while large reflector floor lamps and chandeliers often used ¼ IP fittings.

Standardized thread sizes provide a consistent connection between various lamp parts adding strength and stability to lighting. It is imperative to know the thread size when replacing any antique lamp parts. List of common lamp threads and there uses provides a quick reference.

Lamp Thread Sizes

• 8-32 - 5/32" outside diameter, used as holder screws with some glass shades.
• 1/4-20 - 9/16" outside diameter
• 1/4-27 - 7/32" outside diameter, used on tops of lamp harps & most lamp finials.

• 1/8 IP-27 - 3/8" outside diameter. This is a very common lamp thread size. Used with most lamp-sockets, table lamps, and lamp-finials with a large opening. Interior allows electrical wire to pass through pipe.

• 1/4 IP-18 - 1/2" outside diameter, used on floor lamps, clusters, ceiling fixtures, and floor lamp bases.

Lamp Terminology

• F: designates a female thread - part is tapped having interior threads.
• M: designates a male thread - threaded with exterior threads.
• Watts: Volts multiplied by Amps
• Amps: Watts divided by Volts
• Volts: Watts divided by Amps