This website provides convenient tables for fused amperage given conductor cross-section area (mm^2), number of conductors, and temperature.
The cable in question is a 5 conductor (L1,L2,L3,N,PE) 10 mm^2. 20 meters long, connecting the main panel to a secondary panel. It is currently using 16 A fuses (Hager FS MBN116A). It is installed "in the air", i.e. in the walls but not buried in concrete or plaster.
I want to upgrade it's fuses with an appropriate amperage rating. Considering this is a domestic residence, and the subpanel will feed yet another subpanel in the garage, which is expected to run large amperage single-phase power tools occasionally.
From the first table, first column, I get 10 mm^2 -> 73 A.
From the second table, given normal ambient temperature of 30 C -> 1.0 factor -> 73 A.
From the third table, 5 conductors -> 0.75 factor -> 54.75 A.
The remaining tables seem not applicable.
I have very little background knowledge on this topic. Is 54.5 A per phase a reasonable value? I plan on fusing each phase 32 A (Hager MBN332A).
Thank you.
Best Answer
I suggest you not to use fuses on 3-phase, just go for a magnato-termic breaker (L1,L2,L3 + N). Fuses have to be in fixture that if a single phase breaks, all 4 wires have to go out of power (if you have PE separated from neutral = both green-yellow and blue), or onlt 2 wires if you have a single PEN conductor.
Note that in tables usually they consider only 'active' conductors -> 5 wire cable is '4 poles' wire as PE isn't an 'active' conductor
PE = green yellow
N = blue
PEN = usually blue marked with green-yellow tape at ends and in accessible boxes