# Exterior Lighting Power Calculation for Building Facades

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Can anyone with experience showing compliance with ASHRAE 90.1-2004
section 9.4.5 enlighten me as how the baseline calculation should be done?

The confusion I?m having is with the definition of an illuminated wall or
surface length when applying the 5.0 W/linear foot allowance for building
fa?ades. How is the lineal footage of an illuminated wall determined?

For example, if a 100? wide wall has one exterior lighting fixture on it
at the center, is the entire wall considered to be illuminated and thus
the baseline lighting power allowance is 500W? Or is there some method of
determining the actual portion of the wall that is illuminated?

This came up on a LEED project where I had applied table 9.4.5 as
conservatively as possible and only considered a portion of the wall to be
illuminated at each fixture.

Any help would be greatly appreciated.

Robby Oylear, LEED? AP

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Robby,

My understanding is: since this is a non-tradable surface, base line and proposed design should be the same even though it is well below allowances.

Thanks,

Weixiu

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Weixiu,

The problem I am having is that I do not understand how to correctly
calculate the baseline power allowance. Part of the mandatory provisions of
section 9.4.5 states that you cannot exceed the baseline exterior lighting
power allowance with your proposed building. By incorrectly calculating the
baseline exterior lighting and showing this in my model the LEED reviewer
has flagged my EAp2 credit for denial due to not complying with the
mandatory provisions.

In short, I need to reply back with a corrected baseline lighting power
allowance, which I am having trouble with due to the unclear definition of
?illuminated wall?.

Robby Oylear, LEED? AP

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I would divide the 500 Watts by the entire 100 lineal feet of the wall the
fixture is on. I don't see what you would gain by just considering the lit
area of the wall but I can see where you might be penalized. Since it's just
an exterior load there aren't any interactive effects to worry about either.

Carol

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Joined: 2010-10-05
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Yes, due to the fact that I didn?t want to claim unfair credit over the
baseline I only considered a very small portion of the wall to be ?lit?.
This caused my baseline allowance to be lower than my proposed which set
off a red flag for the LEED reviewer due to Exterior Lighting Power
density being a mandatory provision of ASHRAE 90.1-2004.

Robby Oylear, LEED? AP

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Weixiu
Are you sure about your interpretation of the non-tradable surfaces. If I understand correctly what you are saying is that even though my proposed design uses less lighting power compared to the baseline for the non-tradable surfaces I cannot get an benefit from those savings.

My understanding was that total of tradable and nontradable should be less than the overall allowance +5% additional unrestricted allowance.
Each category of tradable does not have to comply prescriptively and can be higher than base allowance in some cases and have increased savings in another. For example my overall lighting energy use can be higher for say parking area compared to the base allowance as long as I compensate that additional power by increased savings in another category say plaza area.
On the contrary one has to meet the prescriptive requirements for each category of non tradable surfaces and cannot exceed the allowance. Increased building fa?ade lighting cannot be compensated by reduced gatehouse inspection station lighting.

In my model I have significant savings in non-tradable lighting and I have been modeling Ashrae allowance for non tradable area based on area being lit in base case and actually proposed design wattage and lighting power in the proposed case which is much lower.

Thanks

ARVINDER DANG, LEED? AP

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I agree with Carol. If your exterior lights are evenly placed along the wall, you may use the entire length to calculate allowances. This is to show compliance. In your energy model, your base line should be the same as the proposed design, because it is a non-tradable surface and you can't claim savings. Also you can't claim benefit for other categories that don't comply with the code.

Thanks,

Weixiu

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Arvinder,

I got the interpretation from one of the LEED reviewers. And I think it
agrees with how you show compliance for non-tradable surfaces.

Weixiu

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I am not a LEED reviewer but I agree with Arvinder's approach below. Model the actual exterior lighting design for the proposed model and use the maximum allowable exterior lighting for the baseline for each surface that is modeled for the proposed design. From the ASHRAE Standard 90.1-2004 and 2007 User's Manuals, p G-18:

"Other than cases where the lighting is exempted, credit may be taken for improvements in exterior lighting efficacy or wattage. If the proposed building does not have an exterior lighting application (for instance a parking lot that is not lighted), the baseline building shall not have the exterior lighting application either. If the lighting application exists, however, then the proposed building can take credit for a more efficient system."

To answer Robby's question, I agree with Carol Gardner that for an "illuminated wall" you should use the entire length of the wall to calculate the lighting power allowance for compliance with 9.4.5, even if you only have a single fixture in the center.

Regards,

William Bishop, EIT, LEED? AP

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Joined: 2011-09-30
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The intent of the standard seems to not allow lower non-tradable lighting
power in the proposed model compared to the baseline, because this would in
effect make it subject to trade off. Below is an abstract from 90.1 user
manual with the relevant text highlighted in red.

Exterior Building Lighting

Power (? 9.4.5)

The Standard specifies power limits for

many exterior lighting applications

including parking lots, walkways and

plazas, building entrances and exits,

canopies and overhangs and outdoor sales

areas. For these applications, an exterior

lighting power allowance is calculated for

the entire project and a lighting budget is

established.

provided for other lighting applications

such as building facades, automatic teller

machines, guard stations, drive through

windows and parking near retail

establishments, but these are use-it-or lose-

it allowances and no tradeoffs are

permitted. There is a 5% adder that may

be applied to the exterior lighting power

allowance.

Maria

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Joined: 2011-09-30
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If you go by ASHRAE?s definitions, a non-tradable surface

(LPD calculations for the following applications can be used only for the specific application and cannot be traded between surfaces or with other exterior lighting. The following allowances are in addition to any allowance otherwise permitted in the ?Tradable Surfaces? section of this table.)

If I am interpreting this correctly, this means that for a building to comply, you have to comply for each tradable surface considered. So for example if you have two illuminated facades 100 feet long, and you had 2W/lft on one of them and 5.5w/lft on the other, you would be in violation of this because they are not tradable (even though the overall average is below 5w/lft).