By Ken Jorgustin – Modern Survival Blog
Any nuclear detonation above 25 miles will cause a high altitude EMP (electromagnetic pulse).
A nuclear weapon detonated at an altitude of 200 miles could affect all unprotected electrical equipment within the continental United States.
Nuclear weapons at that altitude could be detonated from a satellite, an intercontinental ballistic missile, or from a ‘Scud’ type missile launched from a freighter. Depending on the location and size of the blast, the effect would be to knock out the power grid across most, if not all, of the continental United States.
Read more at Modern Survival Blog: High Altitude EMP