After a preliminary analysis of the power requirements of the installation, a study of the piping of each of the circuits and of its electrical protection must be undertaken, starting at the origin of installation to lead to terminal circuits.
Each pipe-protection combination must simultaneously meet several conditions that ensure the safety of the installation:
to carry the permanent operating current and its normal transient peaks;
do not generate voltage drops liable to adversely affect the operation of certain loads, such as motors during the start-up period, for example. In addition, the protection, circuit breaker or fuse, must:
protect the pipe or the busbar for all overcurrents up to the short-circuit current; ensure the protection of people against indirect contact, in particular in TN and IT systems, where the length of the circuits can limit the level of short-circuit currents, thus making possible the risk of a non-instantaneous automatic shutdown.
Note: in the TT scheme, this condition is naturally satisfied by the compulsory installation of non-timed RCDs on the terminal circuits.
The section of the conductors is defined by the general method described in the Practical determination of the minimum section of a pipe section. Specific receivers may require oversizing the power cable and its protection.