Belt can carry 30 items/sec.

Green circuit uses 3 cables in 0.5 sec, so 6 cables/sec.

Therefore max green factory per belt is 30/6 = 5.

Cable factory makes 2 cables in 0.5 sec, so 4 cables/sec.

Therefore need 30/4 = 7.5 cable factories to keep 5 circuit factories busy.

Green circuits also uses 1 iron in 0.5 sec, or 2 iron/sec, or 10 iron/sec for 5 factories.

30 iron/sec can feed 15 circuit factories.

Cable needs 2 copper/sec for 4 cables/sec, so 30 copper/sec can also feed 15 circuit factories.

Conclusion: Two red belts, one each of copper and iron can make 30 cables/sec from 15 circuit factories and 22.5 cable factories.