To determine a buildings power allowance for exterior lighting, one needs to determine within which ‘lighting zone’ the project is located. The lighting zone is a description of the area surrounding the project and is meant to indicate the expected level of ambient lighting that will be experienced. The lighting power limits are then set based on these zones under the belief that less light is needed in areas that have less ambient light with which to compete.
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Most codes and standards use either a 5 or a 4 zone system; the IECC uses a 4 zone system as shown here.
Let’s make a couple of notations here:
1. Lighting zone 3 is the catch‐all, used for projects that are not adequately described by either lighting zone 2 or 4 but it is expected that the ambient light will fall reasonably between a typical residential area and the downtown of a major metropolitan area.
2. Light zone 4 may be used only when the area has been officially designated as such by the local zoning authority; otherwise lighting zone 3 is the highest level allowed.
Now let’s see where we use these lighting zones.
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The IECC places a limit on how many watts per square foot one may use to illuminate exterior areas.
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The total exterior lighting power allowance for all exterior building applications is the sum of the base site allowance plus the individual allowances for areas that are to be illuminated and are permitted in Table 505.6.2(2) for the applicable lighting zone.
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Tradeoffs are allowed only among exterior lighting applications listed in the Tradable Surfaces section of Table 505.6.2(2). The lighting zone for the building exterior is determined as shown above unless otherwise specified by the local jurisdiction. =====================================================================
The areas that were exempted on the efficacy requirement are also exempted here.
Let’s now look at the Base Allowance
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The base allowance is for the entire project and may be used in or split up among any of the areas listed in either the Tradable or the Non‐tradable sections of the table.
As shown here, this base allowance varies as a function of the lighting zone.
This base allowance is intended to be of the most use to smaller projects where irregular shaped lots and easements may lead to needing these additional watts.
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Now let’s look at the areas where the wattage allowances may be traded. This means that watts allowed but not used in one area, such as a parking lot, may be used in some other tradable area such as another parking lot or in an outdoor sales area.
One can see here that as more ambient light is expected (i.e. a higher lighting zone has been specified) more light is allowed. A parking lot in a national park gets 0.04 watts per sq.ft while that same parking lot in Manhattan get more that three times that amount at 0.13 watts per sq.ft.
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The allowances for these surfaces (non‐tradable that is) are referred to as “Use‐It‐Or‐Lose‐
It”. These allowances are calculated based on the actual illuminated surface area and may only be used to illuminate that area.
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Let us assume we have a project with the following:
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• we are in lighting zone 3 with
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•7,200 sq.ft of parking lots,
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•A single point of entrance that is 6’ wide and covered with a canopy that 10’ x 12’,
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•450’ of walkway that is 8’ wide AND
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We plan to light the front façade of our 50’ wide building that is 22’ high.
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What is our maximum power allowance for the exterior lighting of this project.
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(1)Let’s now look at the various components of our answer.
The base site allowance is 750 watts since our project was stated to be in a lighting zone 3.
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(2)We then see the various calculations that go into our tradable and non‐tradable wattage allowances; let’s look at some particulars:
You’ll notice that at the one door we get both 30 watts per linear foot of doorway and 0.40 watts per square foot of canopy at the doorway.
The sum of 1,308 tradable watts may be used anywhere among the tradable surfaces.
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(3) Next we show here two calculations for the one façade we are lighting – by area and by length. We are free to use the maximum of these two.
For facades under 25’ in height, the linear calculation (by length) will produce the most allowance.
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We get a total power allowance of 2,245.5 watts where the 187.5 from the façade calculation may only be used on that façade. But what if we want to use 220 watts on the façade; may we? Yes, while we can’t use any of our ‘tradable’ watts there, we can use the 750 base site allowance in either tradable or non‐tradable surfaces.
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