The effects of an EMP occur within line of site of the burst. The higher in the atmosphere the atomic bomb explodes the larger the area covered by the EMP is. To calculate the extent of the effect of the EMP the following formula can be used.
R(tangent)=R(earth) cos-1 ( R(earth) / (R(earth) + HOB))
where R(earth) equals approximately 6371 km
Here you can see the approximate effects radius according to the height of the burst:
40 km (height of burst) => 712 km
50 km (height of burst) => 796 km
100 km (height of burst) =>1,121 km
200 km (height of burst) =>1,576 km
300 km (height of burst) => 1,918 km
400 km (height of burst) => 2,201 km*
If an EMP hit a large part of the United States it would not cause just a temporary power outage. It could take months or even years to get power back to most of the United States. Electronic equipment and power generating equipment in power plants themselves would be damaged and there will be no way of easily replacing this equipment. Even if the blackout were only for weeks, the disruption to the United States would be immense.
A major EMP over a large part of the United States would cause all major transportation systems to break down. Most means of communication would not be possible.
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