In my company the Electrical Engineers handle the interpretation of the NESC which gives the required clearance. I'm not sure what code governs the distance in your country. You might ask one of the EE's in your company. I would guess that the value depends on elevation and many other factors like humidity and pollution. The value given by equations is probably very conservative because Engineers tend to be safe when designing systems.

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I have been called "A storehouse of worthless information" many times.

Thanks Transmissiontowers

We follow Indian code IS-5613 for lines in our country. We are doing lot of works for other countries also where we follow EN , NESC etc. Most of the time client specify required distance. As per my observation it is same as live metal clearance required under no swing position. What distance you provide as per NESC ? As per NESC 1% extra clearance is required above 1000m for every 100m. So for line at 1500m altitude , extra clearance is (1500-1000)/100 * 1 = 5% extra clearance require. Correct me if I am wrong.

I really only look at the structural parts of NESC and don't get into the EE realm. I'm in Houston, Texas and our maximum elevation is about 50 feet so we don't have to worry about much of an elevation change (or earthquakes). We just have hurricanes every once in a while.

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I have been called "A storehouse of worthless information" many times.