What is the Electrical Datum Plane?
The Electrical Datum Plane (EDP) is a theoretical horizontal plane from which heights and depths can be measured. We’re concerned with heights. In relation to waterfront structures like docks, the National Electric Code (NEC) defines the Electrical Datum Plane (EDP) as a specified distance above the highest water level that will occur under normal circumstances. Above EDP, electrical equipment can be installed and electrical connections can be made.
Although the NEC states that section 555 does not apply to private boat docks, the purpose of the EDP is to keep electrical equipment well above the water... always a good idea! The EDP is covered in detail in in NEC 555.3 Electrical Plane Datum Distances
Radically different highest water levels can exist, sometimes within fairly short distances or within a single body of water. For this reason, it’s important that they put forth their best effort in determining the most accurate EDP for the location of the properties they inspect.
Storm surge influenced water levels can vary up to 15 feet (5 meters) from one end of Lake Erie to the other.
Factors Affecting Highest Water Levels
In addition to coastal shorelines, shorelines that experience tidal fluctuations can include homes and residential developments on the shores of harbors, canals, and the near-coast portions of rivers. Shorelines closest to the coast will be most affected by tides and so will have shorter distances between changing EDP.
The effect of tides on rivers can vary according to a number of factors. Maximum water heights may not be limited to tides or seasonal flow volume (or the combination of the two) but may be affected by tidal bores. Those inspecting homes along such rivers should be careful in identifying local conditions that may affect EDP.
EDP by Dock Type
The EDP will vary depending on whether the dock being inspected is:
1. Floating Docks
Floating docks rest on floats and are anchored in place, meaning that they’re designed to have little or no lateral movement. As long as dock floats don’t develop leaks and begin to sink, the decks of floating docks will maintain a constant distance above the water. For their inspection, the EDP is always a horizontal plane 30 inches (762 mm) above the water surface beside the dock.
2. Fixed Docks (non-floating) in tidal or non-tidal locations
At fixed docks in locations subject to tidal fluctuations, the EDP is 2 feet (606 mm) above the local highest tide level (high water mark) that would occur under normal circumstances.
In addition to tides, the forces that affect water levels can vary depending on the nature of the body of water on which the dock is located.
Factors Affecting High Water Levels
Along freshwater shorelines, local highest water levels can vary significantly according to a variety of factors, both environmental or man-made. These factors will be more influential on smaller bodies of water like lakes.
Environmental factors may include:
- Amount of precipitation;
- Natural inflow volume from upstream lakes, rivers and streams;
- Natural outflow volume;
- Surface run-off from the drainage basin;
- Evaporation from the lake surface;
- Inshore bathymetry (near-shore bottom shape);
- Directional orientation of the shoreline; and
- *Seasonal surge from storms/wind.
*This factor requires some explanation. “Normal circumstances” as used by the NEC means that storm surge from hurricanes, or natural or man-made disasters is not taken into account in determining EDP. In areas like some of the Great Lakes, some degree of seasonal storm/wind surge is normal, and authorities having jurisdiction (AHJs) in various places do their best to make educated and informed decisions as to what the “normal” high-water level could be.
Man-made factors may include:
- Regulation of lake inflow and outflow volumes;
- Regulation of the flow volume of contributing lakes, rivers, and streams;
- Consumption of water; or
- Diversions into or out of the basin.
The Great Lakes
The Electrical Datum Plane can vary considerably in a single body of water. In very large lakes, like the Great Lakes that lie between the US and Canada, local variations can be considerable. In addition to seasonal fluctuations
due to surge., so it’s important that inspectors find accurate EDP information for inspection locations.
the EDP is usually considered to be determined by water levels under normal circumstances, For example, in relatively shallow Lake Erie, water levels from one end to the other can vary up to 15 feet (5 meters).
How do I identify the local Electrical Datum Plane?
Accurate identification of EDP is difficult or impossible unless a pin or marker listing an EDP elevation has been installed on the property during a survey. Inspectors should include a disclaimer to that effect in the report.
Those inspecting docks typically look for high water staining on the bulkhead to identify the high water mark.