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].
Is 4 × 2.5 mm² ever acceptable?
Yes—only for short runs. Up to 50 m on surface --after that voltage drop and heating risk rise [Elektroda, Luklukowaty, post #5380129]
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].
How large should the circuit breaker be?
For 23 A rated current, use a C-curve 25 A breaker after the meter; upgrade supply breaker to C25 for selectivity [Elektroda, JK60, post #5396691] Some shops push 40 A+, but that reduces protection discrimination [Elektroda, Daj, post #5401281]
Can I start an 11 kW motor directly across the line?
Yes, if the supply grid is stiff and protection coordinated. Users report several 11 kW units with direct start and no issues [Elektroda, JK60, post #5388241] Expect 10 × In inrush (~230 A) [Elektroda, Madrik, post #5380323]
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?
- Read nameplate voltage and full-load current.
- Select cable with ampacity ≥ 1.25 × In and ≤ 3 % voltage drop for run length.
- 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]
Which software helps with motor sizing?
Moeller’s free Suwak3/PKZ slider calculates cable, breaker, and overload relay from 0.06–355 kW [Elektroda, MARIUSZ R, #5401681].
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]
