Observer said:You're sure I can get 2 x 4mm conductors in the terminals of an FCU?
Yup, they'd be unsuitable for use on 4mm² radial circuits if you couldn't 3x2.5mm² or 2x4² conductors is what they can take (technically thats for BS1362 sockets but there is no reason for an FCU to be different, as they are installed on the same types of circuits as sockets)