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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.

A similar, simpler table here

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  • There may be some confusion in or caused by the use of the terms "fuse" (a thin piece of wire in a non-conducting glass or ceramic capsule, the wire melts under overcurrent conditions) and "breaker" (a device in an electrical panel which usually includes both thermal and magnetic sensors to break overcurrents on one or multiple line conductors)
    – RedGrittyBrick
    Commented Apr 12, 2018 at 14:16
  • @RedGrittyBrick. Right, my bad on the poor terminology. Should have said breaker. They are "FS" in German, whatever that means.
    – Tyson Hilmer
    Commented Apr 12, 2018 at 14:19
  • Honestly, the links you are providing aren't a lot of help to English speakers. Take America - I can show you tables that'll authorize anything from 25A to 40A on a 12AWG wire (3.31mm2), but for building wiring there's a statutory rule of 20A, end of subject. So it's just a matter of finding the right table - or rather, the wrong one.
    – Harper - Reinstate Monica
    Commented Apr 12, 2018 at 16:06
  • @Harper. I'm an American living in Germany. I recognize those VDE references as "building code". I've yet to find a German-language equivalent to stackexchange; just crummy forums with pages of no-reputation advice.
    – Tyson Hilmer
    Commented Apr 12, 2018 at 16:10

1 Answer 1

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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

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  • does it help that the Hager MBN332A does indeed bind all three phases to a single phase tripped?
    – Tyson Hilmer
    Commented Apr 12, 2018 at 9:10
  • @TysonHilmer -- yeah, common trip is the norm for such a breaker.
    – ThreePhaseEel
    Commented Apr 12, 2018 at 11:46
  • It's 32A per phase (for a perfectly evened circuit it's 96A in total). Also amperages are given for single wire, not for the 'composed' cord
    – DDS
    Commented Apr 12, 2018 at 11:54
  • *Thermal derates also disregard neutrals in multi-phase arrangements. That is because if balanced, neutral is 0. If imbalanced, it's cooler still. Heat is proportional to amperage squared, so two wires handling 2 and 13 amps will always be cooler than the same wires with 15 and 0 amps.
    – Harper - Reinstate Monica
    Commented Apr 12, 2018 at 16:02

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