# NCEES Power Handbook Lighting Error



## RedRaider2020 (Jan 25, 2021)

Page 25 in the NCEES Handbook
It seems like the Flux equation flux = II*W*L without being divided by the Coefficient of utilization would be correct.
Is this correct and the handbook is wrong? The three equations just don't seem to agree with each other if you divide the flux equation by CU since the initial illumination equation is already divided by CU.
The last picture is from the IES Fundamentals of lighting manual and restates the MMI formula from the handbook.


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## akyip (Jan 27, 2021)

Myself speaking, I have been a bit confused by some of the lighting formulas in the reference handbook. So I just usually stick with:

Luminous flux (lm) = (E * A) / (CU * MF) = (E * A) / (CU * BF * LLF)

E: illuminance (lumens over unit area)
A: area
CU: coefficient of utilization
MF = BF * LLF: maintenance factor
BF: ballast factor
LLF: light loss factor

This is the main luminous flux equation that I've seen throughout the lighting practice problems that I work with...


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## Dothracki PE (Jan 27, 2021)

I remember this was brought up in a similar thread. I don't know if it was ever submitted as a question to NCEES

Thread 'NCEES Problem 123: Illumination' NCEES Problem 123: Illumination


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## RedRaider2020 (Jan 27, 2021)

Dothracki PE said:


> I remember this was brought up in a similar thread. I don't know if it was ever submitted as a question to NCEES
> 
> Thread 'NCEES Problem 123: Illumination' NCEES Problem 123: Illumination


Thank you for the help. 


akyip said:


> Myself speaking, I have been a bit confused by some of the lighting formulas in the reference handbook. So I just usually stick with:
> 
> Luminous flux (lm) = (E * A) / (CU * MF) = (E * A) / (CU * BF * LLF)
> 
> ...


Thanks, that's a good point and I agree. I'll email NCEES with this and see what happens. The formulas they have provided certainly don't agree with each other. I noticed that they have already changed the equation once in the handbook revisions from Flux=I*W*L/CU to Flux=II*W*L/CU. The IES book that I have is good at explaining most of the subject but it doesn't have this equation.


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## RedRaider2020 (Jan 27, 2021)

Dothracki PE said:


> I remember this was brought up in a similar thread. I don't know if it was ever submitted as a question to NCEES
> 
> Thread 'NCEES Problem 123: Illumination' NCEES Problem 123: Illumination


Thank you, that is very helpful. I hate to spend too much time on this but I'm also curious. That curiosity is what gets me in trouble sometimes and causes hours and hours to waste away.


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## akyip (Jan 27, 2021)

RedRaider2020 said:


> Thank you, that is very helpful. I hate to spend too much time on this but I'm also curious. That curiosity is what gets me in trouble sometimes and causes hours and hours to waste away.


I understand what you are saying about wanting to better understand the illumination equations and concepts. Keep in mind though that lighting is a relatively small portion of the exam specifications


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