Passing another circuit through back box

[SparkyChick]

Whats not required?

A warning notice about voltages...

514.10.1 "Every item of equipment or enclosure within which a nominal voltage exceeding 230 volts to earth exists and where the presence of such voltage would not normally be expected, shall be so arranged that before access is gained to a live part, a warning of the maximum voltage to earth present is clearly visible".

Unless I'm much mistaken, that's the only regulation about voltage warnings. So for a standard low voltage three phase supply you don't need one. Yes there is 400v phase to phase but to earth it's only 230v so a notice isn't required. But as has been pointed out already in this thread, the regs are a starting point. There's nothing stopping you putting a 400v warning sticker on things, it's just not required.