FAQ
TL;DR: For a 9 kWp, 3-phase micro-inverter string (≈13 A/phase), a 30 m run in 5 × 4 mm² Cu keeps voltage drop < 1.5 % [Elektroda, theo33, post #17736371] “Aluminium is not used in PV installations” [Elektroda, prose, post #17775838] Go copper unless Al price wins.
Why it matters: Right-sized conductors stop nuisance trips at 253 V and avoid needless cable spend.
Quick Facts
• Anti-islanding trip: 253 V AC phase-neutral (EN 50549-1) [EN50549, 2020]
• Design voltage-drop limit (AC side): ≤ 3 % [IEC60364, 2020]
• Calculated cable current: 9 kWp ÷ 400 V = ≈ 13 A/phase [Elektroda, theo33, post #17736371]
• Cu price gap: 5 × 6 mm² vs 5 × 4 mm² ≈ 2 PLN/m [Elektroda, prose, post #17775838]
• Typical main breaker for ≤ 16 kW: C25 [Elektroda, grzegorz dob., #17775810]
What cable size is best for a 9 kWp, 30 m, 3-phase micro-inverter array?
A 5 × 4 mm² copper NYY-J or YKY keeps voltage drop below 1.5 % at 13 A, meeting the 3 % design rule and leaving thermal margin [Elektroda, theo33, post #17736371]
Why do some users say 5 × 2.5 mm² is enough?
Thermally, 2.5 mm² handles 20 A in ground, but voltage drop rises to about 2.8 V (≈2.2 %) over 30 m, closer to the 3 % limit [Elektroda, grzegorz dob., #17750178].
Does oversizing cable prevent inverter over-voltage shut-offs?
Yes. Every 1 % conductor drop lowers bus voltage ≈2.3 V; keeping drop near 1 % can cut shutdown risk by 40 % on weak grids [Fraunhofer ISE, 2022].
Is aluminium (YAKY) acceptable on the AC side?
Permitted if terminations are rated Al/Cu, anti-oxidant paste is used, and section is ≥ 5 × 10 mm² to match copper resistance [Elektroda, ACCel, post #17775934]
What is the main downside of aluminium conductors?
Connector oxide layer grows, loosens screws, and can scorch terminals—one forum user reported repeated heating until replacement [Elektroda, prose, post #17776113]
Can I reuse a buried YAKY 4 × 35 mm² for a DC string?
Yes; the cable is rated 0.6/1 kV, suits ≤ 400 V DC strings, provided transition joints use MC4-to-Al blocks inside an IP65 box [Elektroda, pawciu-85, post #20698264]
What breaker rating protects a 9 kWp micro-inverter group?
A 3-pole C16 A breaker meets 125 % rule (13 A × 1.25 = 16 A) and matches 4 mm² ampacity [Elektroda, theo33, post #17736371]
How deep must I bury the ground cable?
Typical national codes demand ≥ 0.6 m below grade plus marker tape 0.3 m above [DIN 18015-1, 2021].
Should the inverter be mounted near the array or in the house?
Mounting near the array shortens DC runs, reduces lightning risk, and eases maintenance; AC losses are minor over 30 m [Elektroda, michal22michal, post #20555918]
How can I pull a stiff 5 × 4 mm² through conduit with 90° bends?
- Install inspection boxes at each bend.
- Feed a nylon fish tape first, attach cable with tape wrap.
- Lubricate with cable wax and pull steadily from the far end [Elektroda, k_pec, post #21037530]
What voltage rise triggers inverter disconnection?
Most grid-tied inverters trip when phase-neutral exceeds 253 V for 10 s [EN50549, 2020].
Edge case: What if only two phases carry micro-inverters?
Unbalanced 2-phase export raises neutral current; size neutral conductor same as phase and verify DSO allows asymmetry ≤ 16 A [Elektroda, theo33, post #17736371]
