Lighting Calculations

[timm333]

I have a question about the lighting calculations. We have to light an area for which we have some regular lights (7000 lumen each light) and some battery back lights (6000 lumen each light). As the lumen are not uniform, so how much lumen/lamp we should put in the software for the lighting calculations, can we put 6500 lumen/lamp?

 

[d0nut]

If your software uses the zonal cavity method to estimate the average illuminance, what you are proposing would work. You would also get the same result if you used one luminaire with the total lumens for all of the luminaires planned for the space.

If your software is more sophisticated or provides point-by-point calculations, you will need to input each luminaire type separately with the appropriate lumens and light loss factors applied.

 

[timm333]

Thanks. A few more questions: In addition to the lights mentioned above, there will also be one spotlight in the room. Can the spotlight be put on the same switch/circuit as the other regular LED lights?

As LED lights don’t have ballast, so can we calculate the current of the LED lights directly from the watts? (for example, 50 W at 120V = 0.14 A).

Also, is the lighting contactor only used on the outdoor lighting circuits?

 

[d0nut]

1. Without knowing the exact loads or how the space is to be used, I would assume there is capacity on the circuit for the room lights and the spotlight. You could theoretically control the lights together, but it would probably make more sense to control them separately. I can easily envision scenarios where the people using the space would want the room lights on with the spotlight off or vice versa.

2. Most LED manufacturers have nominal lumen output and nominal wattage in their catalog numbers. The exact power use and light output can typically be found in the luminaire cutsheets. Sometimes the power used varies slightly by the input voltage.

3. I don't know about your particular project, but I personally haven't designed a contactor based control system for interior lighting for at lease a decade. I haven't even used a lighting control panel in the past 7 years or so. Most everything is distributed, addressable lighting controls. Newer energy codes would be difficult to meet in most applications without distributed, addressable controls.

 

[timm333]

Thanks. The lighting is for an industrial building whose ceiling height is 10 feet. The calculation shows that the maximum spacing between adjacent fixtures is 6.5 feet, and the maximum spacing of the last fixture from the wall is 3.25 feet. The problem is that there is 4 feet wide HVAC duct next to the wall. So if we keep 2 feet spacing between the duct and the fixture, the total spacing of the fixture from the wall would be 6.5 feet, which is too much. Also as the mounting height of the fixture is above the bottom of the duct, so the duct will obstruct the light and the area close to wall would be darker. Is it normal to place the fixtures far away from the wall in this kind of situation?

 

[d0nut]

If you are worried about the wall under the duct being dark, either build some supports to mount the lights under the duct at the spacing you want or supplement with some wall mounted lights.

How bright are you trying to make the area? 7000 lumen lights on 6.5' centers sounds like a ton of light. I would also have some concern with the potential for glare with 7000 lumen lights in a 10' ceiling.

 

[timm333]

Trying to make it 35 foot-candles. Each fixture is fluorescent, 2 feet long. The output of each fixture is only 3600 lumen, as I am using smaller fixtures for getting uniform light distribution.

How much should be the minimum spacing between the fixtures and duct? Also can the batteries of the "battery pack lights" and "exit lights" be on the same circuit (circuit coming directly from breaker in lighting panel, with no wall switch in the room) ?

 

[d0nut]

If you are using fluorescent lights, you are probably doing the owner a disservice. I would design the lighting using LED lighting, which will give better control over the light distribution and may allow you to increase the spacing and eliminate some lights while still meeting your target illuminance.

If the ductwork is blocking the light from your ceiling mounted luminaires, that wall will look dark no matter how close you put the lights to the ducts. High angle light distribution to make the tops of a wall lit makes the space feel brighter and larger. It may be acceptable to have that wall dark, but the only way to get light on the wall (especially high on the wall) would be to mount some lights under the duct or on the wall to light that area.

The battery packs and exit signs should be on the same circuit as the general lighting, but connected ahead of any local switching. If you are using the circuit breaker as the switch, that approach would not work. Every time you turned off the breaker, the batteries would discharge.

Depending on the size of the space, I would recommend using LED lights with an inverter for the emergency egress lighting. I would use a UL 924 listed relay like a Wattstopper ELCU-200 or something equivalent from a different manufacturer to have the lights from the inverter follow the same switching as the normal power room lights.

 

[timm333]

I have 5 lanes of lights in the process area. If I use larger LED lights (4 feet long) in the center lane, and smaller LED lights (2 feet long) on the other 4 lanes, will it be normal, or will it look odd?