Shunted vs Non-Shunted Lampholders

Updated 10/10/23 by 1000Bulbs Staff

The difference between shunted and non-shunted sockets is how the electrical current flows through the fixture. Power flows through a shunted socket through a single path. For non-shunted sockets, the electricity flows through multiple paths.

These sockets are sometimes called lamp holders or shunted tombstones and non-shunted tombstones.

Using the wrong type of socket is a fire hazard and increases the possibility of an electrical short. Not understanding the important difference between shunted vs. non-shunted sockets can also result in bulbs burning out quickly and void the UL rating of the lamp.

A diagram of a shunted socket vs a non-shunted socket

In fluorescent fixtures, shunted sockets are only used with an instant start ballast. Non-shunted sockets are used with dimming, pre-heat, program start, trigger ballasts, and rapid start ballasts.

Row of overhead light fixtures with visible tombstone sockets.

The ballast provides a sufficient voltage amount to start the lamps and regulates the electrical current to keep them operating effectively.

Essentially, the ballast is a bouncer for the club (fixture) which allows the right number of people (volts) to get through the lines (sockets) to get the party started (turn on the lights).

Without a ballast, the fluorescent tubes would draw an unregulated electrical current, which would over heat the lamp and burn it out.

How to Identify a Shunted or Non-Shunted Socket with a Voltage Meter

You can identify whether you have a shunted or non-shunted socket by testing the electrical contacts for continuity (complete path of current flow) with a voltage meter or a voltage meter set.

A shunted socket has internally connected electrical contacts, which means continuity is present for both of the separate contacts.

With a non-shunted socket, there is no continuity present between the electrical contacts since the two contacts are not connected internally. These sockets require separate power leads.

Depending on the voltage meter used, an identifier or beeping noise will ring if continuity is present. Typically, shunted sockets have two holes and non-shunted sockets have four holes, but since there are exceptions it's safer to use a voltage meter to figure out which socket style is installed in the existing fixture.

Electrician bypasses ballast to make fixture compatible with LED tubes, since the ballast manages the voltage

LED retrofit lamps don't usually require a ballast to produce a high start-up voltage like fluorescent tubes. LED tubes are offered in two different versions, either a direct wire or plug-and-play option.

Most direct-wire LED lamps require non-shunted sockets while plug-and-play LED lamps typically require shunted sockets. For the direct wire function, the ballast must be bypassed (removed) so the LED bulb can operate off the line voltage.

The plug-and-play option is a game changer since many of these LED tubes are capable of operating with specific ballasts, including instant start ballasts or programmed start ballasts.

Fluorescent bulbs with visible lamp pins.

How to Tell if You Need Shunted Sockets or Non-Shunted Sockets

The ever changing world of technology has allowed manufacturers to tweak and create different generations of each lamp type.

In order to get the best functionality from your overhead lighting, whether fluorescent or LED, it is best practice to reference the specification sheet or manufacturer brochure the so that you aren't left in the dark by an electrical short or damaged light bulb caused by using the wrong socket.

While purchasing and installing light bulbs in your existing fixtures, be sure your sockets translate the proper voltage to avoid any possible mishaps or headaches.

Contact Us at 1000bulbs.com For Your LED Lighting Needs

If you still have questions, or would like help figuring out the socket needs for your existing light fixture, don't panic. Contact our customer service team at 1-800-624-4488 Monday through Friday (7 am to 7 pm CST) with your questions.

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