The Effective Projected Area, or EPA, wind rating of a fixture is one of those complicated, technical-heavy concepts of outdoor lighting that are nonetheless important. It’s so technically heavy that to some degree, the value must be estimated for worse case scenarios. Why should property owners care about it then? The truth is, it’s not something that property owners will need to study in any detail, but it is something that they should expect their lighting providers to know. And it’s something that clients should expect their lighting providers to observe when selecting and installing fixtures.
But what is an EPA wind rating?
There’s a lot of math involved, but here’s the condensed version. Every fixture has a unique drag profile when it is exposed to fast moving air. This drag profile is determined by the exact properties of the air at the time of measuring – factors like the air’s density and its velocity – and by the shape and size of the fixture. All of these factors come together to determine how the fixture will behave in inclement conditions, and a fixture whose EPA wind rating isn’t well-suited for areas where inclement conditions are common will likely fail before long. And in this case, failure means the fixture snaps off a pole or suffers a major technical failure.
No surprise, it gets a little more complicated than that. Factors like air density and velocity are easy to measure and are well known. Their values are usually set at a worse-case number, depending on where the fixture is installed. Air densities and velocities tend to be greater near oceans, for example.
But the fixture’s unique drag profile is harder to determine. The Effective Projected Area of the fixture is one factor in its drag profile, and this is calculated by mapping the fixture to a two-dimensional space. Its cross-sectional area is computed, and this is multiplied by the fixture’s drag coefficient.
The drag coefficient is where lighting manufactures have to also substitute in worse case values for certain fixture shapes, like flat plates or spherical shapes. The reason for this is that drag coefficients are determined by a complex array of minute factors that can only be measured experimentally, such as bringing the fixture into a wind tunnel. This is, of course, prohibitive for a fixture manufacturer, hence the subbed in values.
Okay, so what does all of that mean? It all adds up to an EPA wind rating, which is the wind velocity at which a fixture may suddenly fail when installed at a given height (usually 30 feet for standard lighting poles). It may go without saying, but a property owner does not want fixtures that are likely to fail, so it’s a critical element of the fixture selection process. This is a particular concern in areas along the coast and areas close to mountains, as air velocities tend to be higher and spike more frequently.
In all, while an EPA wind rating isn’t something that property owners should research exhaustively, it should be a point of discussion with lighting providers.