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Is 4x2.5 mm Cable Sufficient for an 11 kW Motor?

kiecio 59838 35
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Is 4x2.5 mm² cable enough for an 11 kW motor, or what cable size should be used?

4x2.5 mm² is usually too small for an 11 kW motor; the thread’s main recommendation is 4x4 mm², with 6 mm² suggested for extra margin. For a motor in a simple three-phase + PE setup, one reply says 4x2.5 mm² may only be acceptable for short runs laid on a wall or floor, up to about 50 m, while in conduit 4x4 mm² was suggested and up to about 90 m [#5380129] Another reply notes that an 11 kW motor has a rated current of about 23 A, so 4 mm² may warm up under full load, and recommends 6 mm² because starting current is much higher [#5387486] A later reply also says that in most cases 4x4 is enough for an 11 kW motor [#5388241] If the motor uses star-delta starting, more conductors are needed than a 4-core cable provides [#5380129]
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  • #31 5402367
    JK60
    Level 14  
    Posts: 65
    Help: 5
    Rate: 27
    Daj wrote:

    And I don`t agree with the suggestion JK60 replacing the fuse behind the meter with a slow-blowing C25 fuse because there is such a thing as gradation of protections and why do you need two C25s one after the other and you never know which one will work first.

    Redundant with B25 characteristics for an 11kW motor?
    And what is the selectivity now - before the C25 counter, after B25?
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  • #32 5407966
    kiecio
    Level 11  
    Posts: 25
    Rate: 13
    Today I finally started this engine under full load (two circular saws in wood) and it turned out that it has absolutely no power, it stops on thin rolls of material (wood) and, what is important, it does not eject the fuses, it only stops when under load :/ will it be Is it the fault of the cable which, as I wrote earlier, is 20m 4x1.5mm and 30m 4x2.5mm?
    And one more question for my friend @Daj, are you sure a 25A switch is enough because today in the store a guy argued with me that 40A is the minimum? ?
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  • #33 5408046
    marian_em
    Level 25  
    Posts: 843
    Help: 60
    Rate: 182
    Start by measuring the voltage at the terminals while the engine is under load. Then measure the voltage at the beginning of the first cable (or preferably at the first protection in the switchgear) and you will know how much is lost along the way.
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  • #34 5417606
    00marian00
    Level 11  
    Posts: 9
    Rate: 2
    Hi,

    as for the low power, it may be caused by the fact that when you connect this motor in a star, you have 1/3 of the power, the next reason is the low supply voltage. Torque on the motor: M = flux x current, flux depends on voltage, so with star connection and reduced voltage, the motor power will be approximately 1/4 of the power. Also, only a triangle connection gives a reasonable solution, the power will not be maximum anyway. And I would also choose 4x4 wires.
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  • #35 5417638
    Daj
    Level 14  
    Posts: 85
    Help: 3
    Rate: 34
    As I wrote earlier, I was wrong when I said that you can use the "2 times" boot, I corrected my previous post.

    This motor is not suitable for Y/Δ starting because it must operate in Y with a phase-to-phase voltage of 400 V, so switching it to Δ will cause it to burn out because the motors were designed for a voltage of 290 V.

    And its lack of power is caused by voltage drop on too thin wires
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Topic summary

✨ The discussion centers around the suitability of a 4x2.5 mm cable for powering an 11 kW motor. Responses indicate that while a 4x2.5 mm cable may suffice for short distances (up to 50 meters), a larger cross-section, such as 4x4 mm or 5x6 mm, is recommended to accommodate starting currents and ensure safe operation. The necessity of a fifth wire for grounding (PE) is debated, with some asserting that only three phase wires and a ground are needed. Concerns about voltage drop and the motor's operational characteristics when connected in star or delta configurations are also highlighted. The importance of proper cable sizing based on distance, load, and installation conditions is emphasized, with suggestions for using protective devices and ensuring adequate current ratings.
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FAQ

TL;DR: For an 11 kW three-phase motor (~23 A rated) [Elektroda, SZYMKA10, post #5387486], “4 × 2.5 mm² works only up to 50 m” [Elektroda, Luklukowaty, post #5380129]; most installers choose 4 × 4 mm² copper in star with PE. Use correct protection and match nameplate voltage.

Why it matters: Correct sizing prevents stalled starts, nuisance trips, and burnt windings for workshop or farm machinery.

Quick Facts

• Rated current: 17–23 A depending on 500 V or 400 V supply [Elektroda, HeSz, #5388688; SZYMKA10, #5387486] • Long-term ampacity: 2.5 mm² Cu, 25 A in free air at 30 °C [Elektroda, HeSz, post #5388688] • Typical cable pick: 4 × 4 mm² Cu ≤ 60 m run [Elektroda, Marian_em, post #5390567] • Starting current: up to 10 × In (≈230 A) [Elektroda, Madrik, post #5380323] • Rewind or derate 290/500 V motors on 400 V grids [Elektroda, Daj, post #5401880]

What cable cross-section is best for an 11 kW motor under 60 m?

Use 4 × 4 mm² copper. It carries 23 A with margin, keeps voltage drop reasonable, and meets installers’ experience [Elektroda, JK60, #5388241; Marian_em, #5390567].

Do I need a neutral wire for three-phase motors?

No. The motor needs three phases plus PE. Neutral (N) is unused unless ancillary 230 V loads share the cable [Elektroda, marian_em, post #5380268]

Why did my motor stall on light timber even though fuses stayed intact?

Thin mixed wiring (20 m of 4 × 1.5 Al + 30 m of 4 × 2.5 Cu) caused voltage drop, cutting torque. Flux drops with voltage, so torque falls (M = Ψ·I) [Elektroda, kiecio, #5407966; 00marian00, #5417606].

Should I use a star-delta (Y/Δ) starter on a 290/500 V nameplate?

No. The motor is designed for 500 V in star; switching to delta on 400 V would over-voltage the windings (400 V per phase) and burn them [Elektroda, Daj, post #5401880]

What happens if I undersize the cable further, say 1.5 mm²?

Insulation may overheat in minutes; one test boiled 1.5 mm² insulation after two minutes on 11 kW load [Elektroda, Madrik, post #5380323]

Three-step checklist to size cable and protection quickly?

  1. Read nameplate voltage and full-load current.
  2. Select cable with ampacity ≥ 1.25 × In and ≤ 3 % voltage drop for run length.
  3. Choose breaker: C-curve at 1.1–1.25 × In and ensure upstream selectivity. Tools like Moeller Suwak PKZ automate this [Elektroda, JK60, post #5387686]

What standard tables back these numbers?

PN-EN 60364-5-523 gives ampacity and short-circuit density; example: 2.5 mm² Cu, 25 A, 143 A·s/mm² [Elektroda, HeSz, post #5388688]

Edge case: private transformer vs public grid—does it matter?

A private substation tolerates direct starts better because voltage sag stays local. Public networks may flicker lights or trip upstream fuses [Elektroda, Akrzy74, post #5389002]
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