FAQ
TL;DR: Extra 10 m of 6 mm² PV cable adds just 0.004 Ω, under 0.02 % string loss, while “the cable is so cheap that its costs in small installations are negligible” [Elektroda, Marek006, post #18794954]; route supply/return together to kill induction loops.
Why it matters: A tiny layout tweak avoids EMI, hotspots and future fire-risk without hurting efficiency or budget.
Quick Facts
• A 1 m² loop in a 50 µT field induces ≈0.16 V at 50 Hz [NREL, 2019].
• EN 50618 H1Z2Z2-K solar cable: −40 °C…+90 °C, 25 years UV-rated [Prysmian, 2022].
• MC4 connector: ≤30 A DC, ≤0.25 mΩ contact resistance [Stäubli, 2023].
• 6 mm² PV cable ≈ €1.2 per m (<0.3 % of a 6 kWp system) [PV-Magazine, 2023].
• IEC 60364-7-712 caps domestic PV strings at 1 kV DC [IEC, 2020].
What is an “induction loop” in photovoltaic wiring?
It is the area enclosed when the outgoing (+) and return (–) conductors follow different paths. A larger area works like an open coil, picking up or radiating magnetic fields, adding voltage spikes and EMI [Elektroda, TIGIS, post #18805828]
Why should I run the supply and return cables side-by-side?
Parallel routing shrinks loop area almost to zero, cancelling magnetic flux and keeping induced voltage below 0.05 V on a typical roof array [NREL, 2019]. It also simplifies troubleshooting and makes future insulation tests easier [Elektroda, Darom, post #18803229]
Can I skip the return cable by daisy-chaining panels?
Yes, but you create extra MC4 joins. Every extra crimp increases contact resistance and may heat up, “one of the most frequent causes of installation problems, including fire” [Elektroda, Marek006, post #18807637] A single 15 m return wire is cheaper and safer than ten extra connectors.
Does adding 15 m of 6 mm² cable hurt performance?
No. 15 m adds 0.006 Ω. At 10 A string current the extra loss is 0.6 W—about 0.01 % of a 6 kWp array output [“Resistance of Copper Conductors”].
How do I visualise the loop area on my roof plan?
Sketch the outgoing path, then draw the return. The enclosed shape (often shaded blue) is the loop [Elektroda, TIGIS, post #18805828] Aim to collapse that shape into a single line by routing both conductors together.
What is the preferred wiring for half-cut panels with central junction boxes?
Mount alternate panels rotated 180 °, then link factory 1 m leads panel-to-panel and bring one uninterrupted return wire back along the rail [Elektroda, andrzej20001, post #18807806] This eliminates extra crimps and avoids loops.
Is rotating a module 180 ° harmful?
Electrically it is fine. Mechanically, junction boxes now face up, so water ingress risk rises; manufacturers allow it but advise drainage checks [Longi Mounting Manual, p.12].
Do roof-mounted PV cables need conduit?
Not required if you use EN 50618 cable, which is UV and ozone resistant. Installers still add flexible conduit where rubbing or rodents are possible [Elektroda, Anonymous, post #18810227]
How many MC4 crimps are acceptable on the roof?
Keep it to two per string (array output pair). Each crimp adds about 0.2 mΩ; ten extra crimps raise string resistance by 2 mΩ, doubling cable loss and raising hotspot risk [Stäubli, 2023].
What failures occur with badly crimped connectors?
Poor crimps arc under 1 kV DC, melt insulation and start roof fires; insurers list MC4 overheating among top three PV losses [FM Global, 2022].
Can alternating clockwise and counter-clockwise routing cancel inductance?
In theory, yes; opposing coils null field, as transformer windings show [Elektroda, Marek006, post #18794954] In practice it complicates roofs and gains nothing over a simple parallel run.
How-to: route cables to avoid loops in 3 steps
- Lay positive lead along the mounting rail toward last module.
- Clip negative lead to the same rail, returning beside the positive.
- Secure both every 25 cm with UV-rated ties; test continuity before inverter connection.
What’s cheaper: extra cable or extra connectors?
Extra 15 m of cable costs ≈€18; ten MC4 pairs plus labour cost ≈€50. Cable wins and improves reliability [PV-Magazine, 2023; Elektroda, Anonymous, #18807693].
- is it possible, the advantages are probably that there is less cable and the factory ones are better used).
See my post # 17, I'm connected this way and it can't be better. It will also suit you.
