Seven years ago, in my home cruising grounds of Chesapeake Bay, high voltage electricity arced from a powerline to the mast of a 35-foot sailboat while the mast was still 12 feet from the wire. The people jumped into the brackish water and there were no serious injuries, but the boat burned to the waterline.
Per the National Electrical Safety Code (NESC), if the surface area of a body of water is between 200 and 2,000 acres, the minimum powerline clearance is 34 feet, on water bodies over 2,000 acres, the minimum clearance is 40 feet. On reservoirs over 2,000 acres and in coastal water, the minimum is 52 feet for powerboats only, and 57 feet if sailboats are present. However, “sailboat waters” can exclude coves, shallows, and channels in marinas registered as power boat facilities.
NESC clearances for electrical lines in areas posted for rigging or launching sailboats also vary depending on surface area of the body of water and must be 5.5 feet greater than the minimums, but this is not always the case.
Lines can also sag in the summer under high air conditioning loads (the wires get warm and stretch). The rating is supposed to allow for this, but exceptions have been known. Lines also sag after installation. More reasons you need a safety factor.
The NESC Code for minimum clearances based on waterbody size are physical distances from the high water level to the wire. But remember, electricity can arc between conductors. The higher the voltage, the farther electricity can arc. Sailboat masts make great conductors, so just because you won’t physically touch the wire does not make it safe to pass under it. Here are minimums per the Occupational Safety and Hazard Administration (OSHA).
OSHA MINIMUM CLEARANCE DISTANCES
Up to 50 kV 10 ft.
> 50 to 200 kV 15 ft.
> 200 to 350 kV 20 ft.
> 350 to 500 kV 25 ft.
> 500 to 750 kV 35 ft.
> 750 to 1,000 kV 45 ft.
*Clearances for voltages greater than 1,000 kV are established by
engineers on site.
The clearances assumes that the line is in compliance with regulations, and that the appropriate notifications were made. Around marinas, ramps, reservoirs, and smaller rivers, don’t bet on this.
Note the authorized clearance on your charts. If not stated, look at a smaller scale chart, call the power company, or just don’t pass under it. Even more than bridge clearance, wire height is notoriously difficult to gauge from deck level, and just because another boat passed under it does not mean it won’t arc to your mast on a different day under different conditions.
I sail on Lake Conroe, Texas. There is a power line crossing the lake. I researched the NESC vertical clearance guidelines for powerlines. However, the guidelines are not that clear as to what water level (pool elevation, spillway elevation, top of spillway elevation, crest of dam elevation, or the maximum design water surface area elevation) they use to determine the air gap (vertical clearance). I contacted the powerline company and they didn’t know either. They would have to measure it. I have been trying to get them to do it but no luck so far. My question is do you have an answer on what elevation they use. I understand the air gap measurement and water depths when talking about water bodies where there is a tidal change but no clear on what is used in a lake. As a registered Professional Engineer, I know what elevation I would use, but that might not be what they used. Thanks. Bob lenhart, P.E.
Do you think it might not be a good idea to be sailing (or even in your boat at all such as at the dock) under conditions where the water level was at the crest of the dam or maybe even the top of the spillway? Electricity and powerline clearance may not be the biggest problem.
Excellent article to put the word out. High-voltage powerlines are serious hazards! Chart No. 1 has symbols to cover high-voltage powerline, but as Darrell describes, not all charts will show the symbol or be updated. Suprising the marina folks did not know about something this dangerous nearby. Solution would be to call the USCG so they can put this into the weekly local notice to mariners (LNM). Next, post a sign on the docks and inside the marina office, along with a written warning in the mooring / berthing contract.
Wow, thanks for reminding us of arcing!
A professional specialty of mine has been arcing at small distances, of say a mm or um. Now I have a sore spot on my forehead where I slap myself while saying “dummy”. Duh, that’s what lightning does. I’ll pay more attention to my mast and the voltages involved.
I have to suspect salt vs. fresh water is also a factor. Overall conductivity to ground is the issue, and is lower through fresh water than salt water, suggesting greater clearances over salt water. Then there’s the “common sense” factor–missing aboard a 26′ cruising sloop some years ago when it left part of its forestay coiled around the power line across the river near my dock (25′ clearance on the chart). The mast probably reached over 35′. Part of the neighboring town went dark for a while after the explosion–amazingly nobody was hurt that I know of. The coil remains.