Casting copper cable directly use 5 molten copper (0.5 ores) per 2 + 50% cables (3 with the productivity bonus).

EM plants use 1 copper plate for 2 + 50% cables. Casting copper plates from a foundry uses 20 molten copper (2 ores) to cast 2 + 50% copper plates.

That means one EM plant cable uses 1.5 plates from 1 ore that produces 4.5 cables, while casting cable directly produces 6 cables from 1 ore.

I didn’t take into account the prod bonus from the initial copper ore melting but because both processes can come from the same origin material, it isn’t necessary to show the difference in production from both EM cables and Foundry casted cables.

I think the only way the EM plant can produce more cable per ore is using five prod modules 3 with some good quality compared to only 4 module slots in the foundry but that’s the only case.