We actually have little real data on EMP's over land. How much protection is actually required is debatable, whether or not a Faraday cage or an EMP "bag" will actually do anything is really unknown. A Faraday cage has a limit to how much EM radiation it will attenuate and how much will get through. The energy has to be dissipated by resistive heating, reflection and reradiation. In order for an EM shield to perform at its most effective ability requires the shield to be grounded with the Earth. Military grade shielding has multiple large gauge grounds wires to copper coated steel rods pounded deep into the earth. This allows the shield to absorb the energy from an EM wave and convert it into current and voltage and drain the energy into the earth. Even then, a high enough magnitude field will literally melt the Faraday cage and turn it into molten metal. An EMP that is large enough to affect a wide area requires a massive release in energy, think nuclear bomb size. And EMP will reach a much wider area than the direct effects of a nuclear bomb. The best information that we have is the atomic bombs tests from the 50's in the atolls in the Pacific Ocean. Hawaii was hundreds of miles away and the EMP still blew street lamps bulbs. This was travelling over seawater that is highly conductive and draining energy from the wave over the entire distance. These were filament lamps, not LED's. LED's are solid state devices and are far more susceptible to electrical discharge. IC chips are even more delicate. What this all means is that we have no idea how a large nuclear detonation over land will affect the entire US, but it doesn't look good.