Location: Central Europe, voltage between phases: 400V. Voltage between phases and ground / neutral: 230V. Frequency = 50 Hz.

When looking at an old 3 phase 1 ground socket with wiring diameter of 3mm2 I discovered that the safety fuse (old, separate fuse for each phase) used to secure this socket is rated at 40A. The cable is installed in a pipe going through insulated walls. Assuming the 3 phases are used to power a machine the maximum capacity per phase according to Wiring capacity is 20A.

Is this the wrong type of fuse?

Or is the capacity of the wires higher under symmetric load where only minimal current is dissipated through the neutral?


Fuse used: Fuse 40A

  • 1
    Is there only one fuse? There should be 3 for a 3-phase circuit.
    – jwh20
    Mar 19 at 11:40
  • No there is one 40A fuse for each phase..Sorry for the confusion
    – Rubus
    Mar 19 at 17:28
  • 1
    Was that socket dedicated to one particular device? If so, and that device had a large inrush current, this might be okay. But you're right, for general purpose use, those fuses should be smaller.
    – Nate S.
    Mar 19 at 17:42
  • I guess so but I'm not sure (can't read the description beneath the fuses). So the capacity rating of the wires also holds for symmetric loads?
    – Rubus
    Mar 19 at 18:03
  • I agree with nate , with the NEC code motor driven devices with fuses may be up to 175-300% if needed for starting inrush. Motors are the exceptions. NEC table 430.52.
    – Ed Beal
    Mar 19 at 18:12

3mm2 is a very unusual wire size for that location.

Since many years the standard sizes are e.g. 1.5, 2.5, 4.0, 6.0, 10.0 mm2. Maybe the size is 4mm2?

But even for 4mm2, the maximal current would be 40A, but only if the cable is running in free air and only for 2 loaded wires, certainly not within a pipe and/or insulation and certainly not for a 3 phase cable (= maximal 3 wires under full load).

A pipe and insulation will reduce the maximal allowed current to 20 to 30 ampere, dependent on neighbouring cables/wires and the dimensions of the pipe and distance to the insulation.

--> Yes, this fuse/cable installation combination is not suited to protect against fire hazards or cable damages, it is not in line with the EU standards.

The Neutral wire is never loaded with more current than a single Phase. Exactly that is one of the important advantages of the 3 Phase power system. If only 1 Phase and Neutral is used, i.e. 230V, obviously both are loaded with 40A in this example of maximal unsymmetry. If all 3 Phases are used in a symmetric load of 3 x 40A, f.e. for a motor in a star configuration, the Neutral current is zero, since all 3 phase currents add to zero current. The Neutral can not be loaded with >40A current.

ABB has a good overview of wire installations and the maximal load.

  • Thx for the answer. The wire is indeed 3mm2 I checked about 5 times from different angles using a caliper gauge..I will leave the question open to get some additional feedback when I have a picture of the fuses used..
    – Rubus
    Mar 20 at 13:51
  • @Rubus Maybe it helps to check the same caliper gauge with known cables of size 2.5 and 4mm2. Is it normal copper cable of type NYM xxx or an old flat cable type? It could be also a completely unsuited cable type for car or low voltage applications, if the size is 3mm2.
    – xeeka
    Mar 20 at 14:14
  • 1
    I cross checked with 1.5mm2 NYM cable and multiple 3mm screws..The caliper gauge seems to be accurate..Its a NYM type cable with rigid conductors
    – Rubus
    Mar 20 at 14:30
  • One nit: it is possible to have more load on the neutral than on the phases in three phase wye installations in this day and age, due to something called triplen harmonics (odd multiples of 3). (In short, the 3rd, 9th, 15th, and so on harmonics don't cancel out across phases, so it's very possible in an environment with nonlinear, non-PFC loads everywhere to have a need for a fatter neutral than the phase conductors) Mar 20 at 15:50
  • 1
    @ThreePhaseEel That is why in the EU non-linear loads of a certain size need 1. an approvement of the net or power supply company and/or 2. a power factor compensation/filtering in order to avoid those non-linear effects. Otherwise, import and use is not allowed. With the normal electronic household devices (monitor, router, PC, TV) the minimal breaker size of 10A (nearly always 16A) will be large enough to avoid an overload of the Neutral line, which has no breaker. But even those low power devices tend to have electronics to reduce the non-linearity, f.e. active power factor compensation.
    – xeeka
    Mar 20 at 20:23

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