Oh, I also just noticed you are putting your CopperCable into the crafting recipe as an ItemStack. It should be an Item. Just do CopperCable.getItem()
I am using the experimental API, and it appears you have the CopperCable Item name incorrect. For my version (2.0.157 ex) it should be: insulatedCopperCableItem
Here is the full code, tested this time:
Code
ItemStack circuit = new ItemStack(Items.getItem("electronicCircuit").getItem(), 2);
ItemStack CopperCable = Items.getItem("insulatedCopperCableItem");
// Creates a Electronic Circuit
GameRegistry.addRecipe(circuit, new Object[]{
"XXX",
"RCR",
"XXX",
'C', Item.diamond, 'R', Item.redstone, 'X', CopperCable.getItem()
});