One of the little things you might not be thinking about (but your certified lighting professional should be) is the concept of effective projected area (EPA). EPA is an important consideration when you retrofit your outdoor lighting fixtures but it doesn’t represent what you might think when you first hear it. EPA sounds like the amount of light projected onto the ground (or target area) but this is actually NOT the case. Nor is it the Environmental Protection Agency (perhaps more obvious). The EPA is a two-dimensional projection of a three-dimensional object. Think of it like the object’s shadow. Since it is a two-dimensional area, the effective projected area is measured in square feet (ft2).
When it comes to outdoor lighting retrofits, the EPA is important because it helps to indicate the wind resistance of the particular luminaire (and thus the forces it will impose on the pole). It is essentially a factor important in calculating loads/strain on the light. The EPA, when used in combination with the luminaires coefficient of drag, relates to the force a particular fixture will impose on the pole it is mounted on when it comes into contact with the wind. It helps determine the structural capacity of outdoor light poles to withstand wind forces due to the torque created by the lighting fixture sitting on top of them. It is important to understand in order to prevent the pole from failing when the environment is excessively windy. Generally speaking, the larger the fixture the higher the EPA rating. Similarly, the higher the pole and/or the faster the wind speed, the more torque is being placed on the pole. Note that objects of different shapes (such as a circle and a square) might have the same EPA value but a significantly different drag profile (generally the circle will have a much lower coefficient of drag). In other words, EPA is important to understand but it’s not the entire picture.
Every luminaire has an EPA rating. Similarly, light poles are rated to withstand luminaires up to a particular EPA rating. When you’re buying poles and luminaires together it makes sense to check the compatibility of the luminaire and the pole. You want to ensure that the luminaire has an EPA less than or equal to the rating of the pole it will be fixed to. What you might not think of, however, is what to do when the pole’s rating is unknown. This is often the case when retrofitting outdoor light fixtures that require new lights but whose poles are fine. What do you do when buying a new luminaire for an existing pole without a known EPA rating? Will the poles withstand the loads consequent from the new fixtures? In this case the best thing you can do is to ensure that the luminaires have an EPA not to exceed the EPA rating of the luminaire being replaced (the fixture/light currently being used). This assures with a reasonable certainty that the wind load rating of the new luminaire won’t potentially compromise the pole. And it’s a lot easier to look up the fixture than the pole if you’re not the original owner/builder. Typically luminaires with an EPA at or below .9 ft2 will comply with this general guidance.