CONUS
Found this.
https://doh.wa.gov/sites/default/files/legacy/Documents/Pubs/320-090_elecpuls_fs.pdf
High-altitude nuclear detonations and electromagnetic bombs can generate EMP that
has the potential to damage or destroy electronic devices over widespread areas.
Electric power systems would also be at risk from surges produced by such weapons.
However, the EMP from a kiloton-range surface nuclear explosion would not be
expected to produce serious damage outside the radius of severe destruction from
blast.
A 1.4 Megaton bomb launched about 250 miles above Kansas would destroy most of
the electronics that were not protected in the entire Continental United States. During
the brief return to atmospheric testing in 1962, a 1.4 megaton nuclear weapon was
detonated over Johnston Island at an altitude of about 250 miles. The effects of EMP
were observed in Hawaii, 800 miles east of the detonation. Streetlights and fuses failed
on Oahu and telephone service was disrupted on the Island of Kauai.