[pierrem15]

a) The closest we have to a 'real world' EMP test was the Starfish Prime test in 1962, which used a thermonuclear warhead with a yield of 1.4 megatons.

b) The largest fission-only bomb known to have been constructed yielded 500 kilotons: this was probably close to the maximum deliverable size for such a weapon;

It's not clear to me then whether a fission weapon, even with greater efficiency at creating EMP effects, can generate an EMP effect as large as a fusion weapon; or one sufficiently large to cause nationwide EMP.

[Zathras]

Yep.
What isn’t commonly known is the 500k atomic device was so dangerous that no one would ever use it in a real weapon.
It contained (I think) 4-10x critical mass of U235.

Just dropping it could have triggered a nuclear detonation.

[Ancesthntr]

The EMP from Starfish Prime was not expected. So we have a 1.4 megaton nuke, the percentage of which yield was from fission I don’t know, and which WASN’T “tuned” to put more of its energy into gamma rays than explosive force and heat.

I wouldn’t be lulled into a false sense of security by those facts.