What Is EPA and Why Do You Need To Know?

The Effective Projected Area (EPA) wind rating of a specific light fixture refers to the strength of wind velocity an arm-mounted light fixture can withstand. The fixture must be durable enough to endure windy conditions, ensuring safety and dependability. But how can you determine their capability to withstand such conditions? This is where EPA ratings become essential.

EPA ratings are employed to assess whether lighting fixtures can adhere to the standards required by specific geographical regions. The Effective Projected Area is a measure used to determine the wind forces a lighting fixture might face.

Projected Surface Area

The projected surface area of an object is like the shadow it makes on the ground when light shines on it. For instance, imagine a cone. When you look at its shadow from the side, it looks like a triangle. But from above, the shadow looks like a circle. By looking at shadows from different angles and putting them together, we can figure out the shape of the object's shadow. This is what we call the projected surface area.

The drag coefficient is a value that comes from analyzing fluid dynamics. Standard variables, including the object's shape and density, indicate the extent to which an object is affected by drag forces. An object with a higher drag coefficient typically has higher wind resistance.

How Is Wind Pressure Measured?

A fixture's wind rating and drag profile are established by the highest wind pressure values it can withstand. These values take many years to collect and are crucial in calculating the force exerted on the fixture and its mounting equipment at any given moment. 

The American Society of Civil Engineers (ASCE)  has developed a wind speed map for the United States to simplify identifying the wind speed in a particular area. According to the ASCE, wind ratings can usually range from 70-110, with a few “special” regions exceeding that range.

Overhead Lights and EPA Ratings

With thousands of people in a stadium, in a parking lot, or on a street,  ensuring that the EPA wind rating meets the requirements for safety is important. Since stadium lights are typically larger and heavier, those poles will often move slightly. But they could snap and break if they are too tight and thick.

It's crucial to understand the wind forces you might face when installing an outdoor overhead light such as a stadium light, street light, or parking lot light. 

This ensures the setup is secure, with poles of the right size and bolts that fit correctly. A proper installation can also lessen the wind's impact on the light. Several things influence the force the wind has on a light. The wind's speed and the air's weight depend on where the light is installed. The size of the light and its shape, which affects how it interacts with the wind, are also important, although we won't go into detail about these here.

To calculate the wind's force on something, we use a formula that considers the object's front size, its shape's influence on airflow (known as the drag coefficient), the weight of the air, and the wind's speed. This force is measured in Newtons. For these calculations, we typically use an air density that matches the sea level conditions with a temperature of 15°C. 

The formula squares the wind speed, showing that it significantly affects the force. We look at the object's biggest side facing the wind to consider the most challenging wind conditions. The drag coefficient shows how the object's shape affects its wind resistance; it changes with the object's shape and the wind's direction and is accurately determined through tests. A lower drag coefficient means the object resists the wind less.