When fluorescent lighting first came out, a four-foot long lamp and fixture actually measured 48” and an eight-foot lamp 96”. Since 48” equals 4 feet and 96 inches equals 8 feet, planning for installation of a four-foot or an eight-foot fixture was fairly straight-forwards. However, by the late 1990s advancements in fluorescent technologies led to the development of the T5 lamp, which could provide the same amount of light as much larger fixtures, allowing for lower installation costs, and by the end of the century T5 sized fixtures had become extremely popular.
However, the technological changes impacted the overall size of the fixtures, and where a four-foot length was standard for the older fixtures, T5 fixtures that used to be 48” became 46” and eight-foot measured 92”. To accommodate the change the industry adopted “nominal" sizing.
Today, with LED fixtures becoming more the norm, linear boards measure between 22” and 23” long, meaning the lighting fixtures are correspondingly shorter to prevent dark spots and allow for even illumination. Since most linear LED fixtures are closer to T5 lengths, they have become the new de facto “four-foot” fixture, even though they may be a few inches shy of that.
Why is this important? Well if you are planning to install a pendant-mount 8’ fixture you might think that your junction boxes and cables should be set up for a 96” fixture. This is going to cause you troubles when your 92” fixture arrives.
The moral of this story is that you need to pay close attention to your lighting fixture catalog sheets, as our industry has not yet come up with a way to properly define today’s ‘nominal’ fixture sizes. To be completely sure, only trust your fixtures’ actual specifications.