FAQ
TL;DR: For a 20–30 m garage run, a 5 × 6 mm² copper YKY cable keeps voltage-drop under 3 % at 25 A [IEC 60364-5-52]. “So get an electrician.” [Elektroda, zbich70, post #15798301]
Why it matters: Oversized once, undersized forever—wrong cable overheats and voids insurance.
Quick Facts
• Typical burial depth: 0.6-0.7 m with 10 cm sand bed + warning tape [IEC 60364-5-52].
• Current-carrying capacity 6 mm² Cu in soil: ≈34 A (90 °C XLPE) [IEC 60364-5-52].
• Voltage-drop limits: 3 % lighting, 5 % other loads [PN-HD 60364-5-52].
• Retail price YKY 5×6 mm²: €7-9 per metre (Q1 2024 EU web-stores).
• Minimum bend radius multi-core 6 mm²: 6 × OD ≈ 60 mm [Prysmian Cable Guide, 2023].
Which cable size safely supplies a 2.8 kW compressor, lights and a fan over 30 m?
A 230 V single-phase load of 4.5 kW draws about 20 A. A 5 × 6 mm² Cu YKY limits voltage-drop to ≈2.7 % and stays below its 34 A rating, meeting IEC limits [IEC 60364-5-52]. Forum electricians proposed the same section for future three-phase use [Elektroda, Krzysztof Reszka, #15798528].
Why choose a 5-core instead of a 3- or 4-core cable?
Five cores give L1-L2-L3-N-PE. You can run single-phase now and upgrade to three-phase later without digging again [Elektroda, CYRUS2, post #15800102] PE also lets each building have its own earth while keeping the neutral separate, improving RCD reliability.
Should I request single-phase or three-phase power for a workshop?
Ask the DSO for three-phase if you plan welders, EV chargers, or >5 kW heaters. Balanced three-phase halves current per core and reduces voltage sag. The application is free in many EU countries when load exceeds 6 kW [ENEA DSO Guide, 2022].
What happens if the cable is undersized?
Undersized conductors run hotter. A 3 × 4 mm² buried under 40 A protection reaches ≈70 °C in summer soil, exceeding PVC limits and risking insulation failure [Prysmian Thermal Chart, 2023]. Breakers may not trip because overcurrent is continuous, not short-circuit [Elektroda, retrofood, post #16085652]
How deep must I bury the supply cable and what layers are required?
- Dig 60–70 cm deep trench.
- Lay 10 cm sifted sand, place cable, cover with 10 cm sand.
- Add PVC warning tape 25 cm above cable, backfill and compact [IEC 60364-5-52].
Do I need conduit if I already use YKY?
YKY is rated for direct burial. Rigid conduit only protects against spades and rodents but traps moisture. Many electricians prefer no conduit plus sand sleeve [Elektroda, Akrzy74, post #15798566]
How do TN-C and TN-S earthing schemes change conductor count?
TN-C needs a combined PEN, so 4-core suffices. TN-S separates N and PE, hence 5 cores. If supply is TN-S you cannot recreate PEN downstream [Elektroda, CYRUS2, post #15800190] Always ask the DSO which system reaches your house.
What voltage-drop is allowed and how do I calculate it?
IEC allows 3 % for lighting circuits and 5 % otherwise. ΔV = (2 × L × I × ρ) / S. For 30 m, 20 A, Cu resistivity 0.0175 Ω mm²/m, and 6 mm² section, drop is 2.1 V (0.9 %) [IEC 60364-5-52].
Can I trench the cable myself and hire an electrician only for terminations?
Yes, but agree scope first. Electricians charge double when they must diagnose DIY errors [Elektroda, zbich70, post #15800662] Let the electrician mark the route, specify sand, and inspect before back-fill.
What test documents must the electrician leave?
He must issue continuity, insulation-resistance, and earth-fault loop impedance (IPZ) reports proving disconnection times meet SWZ requirements [Elektroda, kortyleski, post #16257425] Keep them for insurance claims.
Edge case: Is upgrading the breaker to 40 A on 3×4 mm² safe?
No. 3×4 mm² buried with 40 A can exceed its 26 A soil rating, leading to hidden overheating and fire risk [IEC 60364-5-52]. The forum called this “extreme bullshit advice” [Elektroda, zbich70, post #15798670]
How-to: three-step checklist before the electrician arrives
- Confirm supply earthing type (TN-C, TN-S) with DSO.
- Dig trench to 0.7 m, lay sand bed, route cable without sharp bends.
- Photograph installation, leave ends 1 m spare above ground for termination.
Is YAKY 4×16 mm² aluminium a cheaper alternative?
Yes; price is ≈€4 /m, 45 % cheaper than Cu. Its 55 A capacity equals 5×6 mm² Cu, but larger diameter needs bigger glands and terminations [Prysmian Catalogue, 2023]. Aluminium requires bi-metal lugs and antioxidant paste, adding labour cost [Elektroda, retrofood, post #15800112]
Can I add a 3 kW electric heater or welder later?
With 5×6 mm² you can raise breaker to 32 A and still keep voltage-drop under 4 %. Total continuous load should stay below 7 kW single-phase or 10 kW three-phase to avoid DSO upgrade fees [DSO Prosumer Rules, 2022].
Does an outdoor RCD need its own neutral core?
Yes. RCD measures imbalance between live and neutral. Shared neutrals upstream cause nuisance trips. A dedicated N in a 5-core solves this [IEE Wiring Regulations 18th Ed., 2018].
