What is EPA ?
While a little technical, this concept is very relevant when it comes to solar outdoor lighting, solar lighting, or solar security camera mounting, especially regarding structural design. The concept in question is called the Effective Projected Area (EPA) and relates to the wind force acting on objects, such as solar panels and battery boxes, which create force on the pole.
Understanding Effective Projected Area (EPA)
The Effective Projected Area (EPA) is a basic principle in structural engineering. It measures a three-dimensional object on a two-dimensional area. This principle applies to solar lighting systems, solar security camera racks, and solar racking design, construction, and installation.
EPA helps determine the pole strength needed to support a solar lighting system during wind events. The calculation considers the total area that the solar power system and light fixture occupy at the top of a pole. It also helps engineers and manufacturers choose the right pole size, anchors, foundation, and brackets needed to keep the system secure during strong winds.
Role of EPA and AASHTO Standards
EPA and AASHTO Standards are key when calculating pole requirements for any solar lighting or power system. These standards determine the pole’s size and thickness to ensure it can withstand specific wind speeds and conditions.
The EPA varies with the angle, shape, and size of system components. Even the pole shape affects EPA — square poles have higher EPA values than round poles. When designing a solar power and lighting system, the system’s angle influences both EPA and solar power production. All these elements must be considered during project design to ensure the installation lasts for years.
Why EPA Matters
When designing a solar power system, the total EPA must be calculated to ensure the assembly remains stable during a storm. Without proper calculation, the pole could bend or fail due to excessive surface area exposure.
To find local AASHTO wind load ratings, check online or consult your pole manufacturer, solar lighting specialist, or local engineering firm. Since weather varies by region, a local engineer will understand soil conditions and microclimates that affect calculations.
Regional Wind Considerations
Different locations, such as mountainous, coastal, or Great Lakes regions, have unique wind speed requirements. Consulting a local authority ensures you select equipment that can handle the area’s wind conditions.
Steps to Determine Project Requirements
- Determine the site location.
- Calculate the total weight and EPA of the equipment.
- Identify the wind load requirement.
- Consult your manufacturer to confirm the pole’s capacity.
Understanding the Drag Coefficient
The EPA is the projected area combined with the appropriate drag coefficient. Depending on the object’s size and shape, the drag coefficient will vary. It represents the resistance an object creates when moving through air. The lower the drag coefficient, the less resistance the object faces.
For example, a round surface object has a drag coefficient of 0.5 (AASHTO LTS-5 Table 3-6), while a rectangular object has 1.2. The rectangular object creates about 60% more resistance for the same area than the round one. Knowing these values helps calculate wind force on each object and design poles accordingly.
Calculating Wind Force
Wind force on an object is calculated by multiplying the EPA by the wind’s velocity pressure, as defined in AASHTO Standard Equation 3-1 for structural supports. These wind pressures are based on decades of studies.
It’s important to use the correct wind pressure because wind force can far exceed an object’s weight. For instance, a banner weighing only 25–30 lbs can experience over 400 lbs of force in high winds. Sun-In-One™ strongly recommends consulting a qualified local engineer for proper pole and foundation design.
Final Recommendation
If you are unsure about the calculations, hire a local engineering firm. Engineering reports usually cost between $500–$1000 and include signed and sealed calculations.
In the end, consulting your manufacturer and local engineer ensures you select the correct pole type and size to withstand local wind conditions. Understanding these details will help you make better decisions about your solar system design.