Crane Power Line Clearance (OSHA 1926.1408)
The 20 ft everyone carries is a DEFAULT you take when you have not determined the voltage -- one of three options, and usually the most expensive. Determine the line voltage and Table A applies instead, starting at 10 FT for lines up to 50 kV, which is most distribution: on a 12 kV service that is 10 ft of working radius bought back for the price of a phone call to the utility. The trap runs the other way too. The 20 ft default is only for lines UP TO 350 kV; above that it is 50, and Table A climbs to 25, 35, and 45 -- so on transmission the remembered number is not conservative, it is wrong in the dangerous direction, and the tile flags a clearance that would satisfy a default which does not apply. Over 1,000 kV there is no number at all: the utility or a qualified engineer sets it.
Formula and source
required clearance = 0 where the line is deenergized and visibly grounded; the Table A distance where the voltage has been determined (up to 50 kV 10 ft, over 50 to 200 kV 15 ft, over 200 to 350 kV 20 ft, over 350 to 500 kV 25 ft, over 500 to 750 kV 35 ft, over 750 to 1,000 kV 45 ft, over 1,000 kV set by the utility or a qualified engineer); otherwise the default, which is 20 ft for lines up to 350 kV and 50 ft above that.
OSHA 29 CFR 1926.1408, Subpart CC - Cranes and Derricks in Construction. A US federal regulation in the public domain, so Table A is reproduced rather than paraphrased.
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