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When to Use Shunted vs. Non-Shunted Lampholders with Your T8s

Pairing the incorrect lampholder with an incompatible ballast or bulb in a fixture can result in premature lamp failure, damage to the ballast, or void any warranty on the components. The best way to avoid compatibility issues and blunders is by knowing what lampholder to use with which ballasts and bulbs.

Fluorescent

There are two types of lampholders, shunted and non-shunted. In non-shunted lampholders, or tombstones, current flows through multiple paths and in shunted tombstones current flows through a single path. Rapid start, programmed start, and dimming ballasts use non-shunted lampholders to provide power through the lamp’s cathodes before the lamp is turned on. These ballast types require each end of the lamp to be separately connected to the ballast. Instant start ballasts do not preheat the lamp before starting, but instead provide a high initial voltage to start that lamp because of this the bulbs cannot be dimmed and require shunted lampholders to direct flow of power. This type of ballasts needs only one wire between it and both lampholders.

Recent developments in ballast technology have produced an increase in performance and efficiency. Modern electronic ballasts operate above a 20,000 Hz frequency which causes an increase in light output and eliminates the hum and flicker often associated with fluorescent lighting.

Shunted
Non-Shunted
Rapid Start, Programmed Start
X
Dimming Ballast
X
Instant Start
X
LED Plug-and-Play*
X
LED Direct Wire*
X
Double Ended LED Direct Wire*
X
X

*In cases where the ballast seems to require a different lampholder than the lamp, please defer to the needs of the lamp rather than the ballast.

LED

Shunted lampholders are required for fixtures using instant start ballasts. Many fluorescent fixtures use shunted lampholders that may need to be replaced when switching to LED depending on the type of tube you want to use. Plug-and-play LED tube lights are compatible with some fluorescent ballasts and allow you to use the existing sockets. Bypassing the ballast and direct wiring the tube requires non-shunted lampholders are required for LED tubes that are powered on one end. When retrofitting a fluorescent fixture with shunted sockets, you either need to rewire the fixture for non-shunted sockets or use a plug-and-play LED tube. Double ended LED direct wire bulbs are an exception. Wired at both ends, they can be used with either shunted or non-shunted sockets.

Upgrading to LED Tubes

If you’ve decided to upgrade your fluorescent fixtures to LED, the type of ballast and sockets your fixture has may impact which LED tubes you pick. If you have an instant start ballast, plug-and-play T8 tubes will be the easiest solution to start. This type of LED tube simply installs straight into the fixture without needing to rewire or change your fixture. Keep in mind, because of the average lifespans of fluorescent ballasts and LEDs, the ballast will fail long before your bulbs do.

For installing direct wire LED bulbs, the ballast will be removed entirely and line voltage will be wired directly to the non-shunted sockets on one side of the fixture with the sockets on the other side just helping to hold the lamp in place. If bypassing an instant start ballast, the shunted sockets will need to be replaced with non-shunted ones. Recently, there have been direct wire LED tubes manufactured that are compatible with both shunted and non-shunted tombstones. These T8 bulbs are primarily designed for applications with hard to reach sockets.