Help Understanding IRC Code Lighting Requirements
The IRC code per 303.1 mentions that there should be, for habitable rooms, an average minimum of 6 candlepower (65 lux) at 30″ above the floor.
I understand that total lumens is calculated by taking area of room and multiplying it by the desired candlepower. BUT, what’s the 30″ for? Is that to ensure that the light output is full across that plane, there’s no dark spots (30″ above the floor)?
If I have a bedroom that’s 12′ x 14′ I’d need a minimum of 1,008 total lumens spread across the room such that it’s fully lit at 30″ above the floor?
NOTE: I’m not looking to minimize lighting, just wanting to understand what the code is really asking for. Geeking on code?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
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Mark, I don't know for sure but it seems logical that 30" corresponds to the height of a desk or dining table. If you're sitting in a chair reading, your hands are probably a bit less than 30" off the floor. If you're cooking, the kitchen countertop is a little more than 30" off the floor. I have never had an inspector worry about this one bit; they just want to see a code-required light or switched outlet for every room. I'd call the 6 footcandles a good guideline. Like most things in the code, it's probably a bare minimum.
Lights are rated in lumens. A lux is one lumen per square meter. The further you get from a light the lower the lux output, as the radius of the sphere increases the area increases as well. The 30" tells you how far from the light the measurement should be taken.
I don't expect anyone is out there with a lux meter, you just take the lumens and the ceiling height and calculate the lux at that distance.
Yeah, that was my understanding. I tracked down some equations* and when I crunched numbers I came up with 160 candlepower needed at the source (ceiling less one foot down to account for candlepower being rated at 1' [distance for light to travel was computed to be 66" -> 9' - 30" - 1']). Trying to apply this number seems to produce unreasonable values: 160 candlepower x room area generates numbers that are clearly too high.
* I = C/(D^2), where I = intensity, C = constant, and D = distance
I doubt that any inspector would ding me on anything I'd end up going with (but perhaps the permitting folks would scrutinizing and make sure I'll meet code mins). Might be that the architect that I use can reliably spec out for meeting code: I'll for sure ask how it's done!
Mark,
It's an interesting topic to think about just for fun, but it's way into the weeds as something to worry about. I'll bet your architect gives you a completely blank look when you mention it, as it's something that simply isn't an issue and never comes up. The only way it's g0ing to become a problem with your building inspector is if you pursue it with them.
Mark,
Typically your architect will provide a lighting plan with the location, size and sometimes the manufacturer the light. I find that to be the easy part. The tougher part is spec'ing the actual fixture if you want to customize. There a lot more options available today whereas 10 years ago it was typically a Halo, Cooper or Lightolier canned high hat, bulb optional. Nowadays there are a lot more options. Personally for general lighting I like 3" and 4" regressed LED's from Halo and Lotus lighting. The Halo's allow for various color temps via a dip switch on the juction box (no can). Lotus have regressed gimbals which are great for areas where you want to light up an object on a wall without having a portion of the gimbal exposed below the ceiling. For non-can fixtures, I like WAC Lighting. I find them to be of high quality, well-designed, stylish and easy to install. Light fixtures are the new furniture to me, as they can be a focal point of the room. Then there are switches and outlets which are no longer limited to just the decora rocker and plain old outlet. I am currently using a combination of Legrand Adorne and Lutron Caseta switches and outlets. They both can be controlled via a WiFi connected hub, Alexa, Google, etc. Most of my lights are single-pole and I am using remotes to create 3-way switches. The Lutron remotes are battery powered and can be wall mounted to look like a standard switch or placed on a table mount. Lutron remotes can also be used to control Sonos sound systems. The Legrand remotes need line voltage, but they don't need to be on the same circuit as the lights they are controlling. Adorne has WiFi outlets which I am using to turn certain things off at night and on the morning via a programmed scene. I am also using their Pop Out outlets in my kitchen and my first floor where outlets are required. I placed them in baseboard and with the flat cover, the are much less noticeable than standard outlets.