FAQ
TL;DR: Tests show a 25 mm² copper pair keeps voltage drop under 5 % when feeding 12 V/8 A over 120 m; "You need a cable with a minimum cross-section of 25 mm²" [Elektroda, Szyszkownik Kilkujadek, #17825524]. Upgrade supply to 30 V + DC/DC to use 6 mm² instead.
Why it matters: undersized wire overheats, wastes power and can stall a compressor-driven fridge.
Quick Facts
• Distance to fridge: 120 m one-way → 240 m return path [Elektroda, ^ToM^, #17825492]
• Copper resistivity: 0.0175 Ω·mm² / m at 20 °C [IEC 60228]
• Practical DC voltage-drop limit: 10 % (≈1.2 V at 12 V) [“Cable Sizing Guide”]
• Cross-section for 8 A @ 12 V over 240 m: 25 mm² Cu; or 6 mm² Cu if supply is raised to 30 V [Elektroda, Szyszkownik Kilkujadek, #17825524; #17825554]
• 2 × 25 mm² aluminium aerial cable cost: ~PLN 4 / m [Elektroda, CYRUS2, post #17827466]
1. What wire size keeps a 12 V, 8 A fridge running over 120 m?
Use at least 25 mm² copper (or 35 mm² aluminium). This keeps voltage drop below 1.2 V and current density safe [Elektroda, Szyszkownik Kilkujadek, #17825524].
2. Why do calculations use 240 m instead of 120 m?
The current travels to the load and back. Voltage drop equals I × R of both conductors, so the effective length is twice the physical distance [Elektroda, ^ToM^, #17825492].
3. How do I calculate voltage drop myself?
- Look up copper resistivity ρ = 0.0175 Ω·mm²/m [IEC 60228].
- Compute R = ρ × length / area.
- Multiply R by load current to get drop; compare with the 10 % limit. Adjust area if needed.
4. Is 2.5 mm² wire acceptable?
No. 2.5 mm² over 240 m gives about 1.6 Ω; at 8 A the drop is 12.8 V—your fridge sees 0 V and never starts [Elektroda, ^ToM^, #17825492].
5. Can I reduce cable size by increasing supply voltage?
Yes. Feeding 24–30 V and stepping down to 12 V near the fridge allows 6 mm² copper while staying within SELV limits in wet areas [Elektroda, Szyszkownik Kilkujadek, #17825554].
6. What counts as a safe voltage around a swimming pool?
IEC 61140 treats ≤30 V DC as Extra-Low Voltage for wet contact, minimising shock risk [IEC 61140].
7. Which DC-DC converters handle the job?
Buck converters rated 12 A continuous and 15–35 V input, e.g., PRO-200 24/12 V 15 A module (~€17) meet the requirement [Elektroda, alekt77, post #17825689]
8. How much does thick aluminium cable cost versus converters?
Two runs of 25 mm² Al cost ≈PLN 960 (240 m × PLN 4/m). A 30 V PSU plus DC-DC converter totals ≈PLN 250, so the higher-voltage scheme is ~75 % cheaper [Elektroda, CYRUS2, #17827466; alekt77, #17825689].
9. Does the fridge draw 8 A continuously?
Most compressor coolers cycle; Peltier models draw steady current. Confirm plate data: a Peltier unit tolerates wider voltage swings and may simplify the design [Elektroda, mar_uda, post #17826394]
10. What happens if voltage falls below 11.5 V?
Compressor control electronics usually cut out to prevent battery damage; cooling stops and repeated starts may overheat the motor [Elektroda, CYRUS2, post #17825671]
11. Is battery power an option?
A 110 Ah deep-cycle battery supplies 8 A for about 10 h; swapping charged spares avoids long cables but adds handling effort [Elektroda, kortyleski, post #17829539]
12. Could 48 V be used instead of 30 V?
48 V is common in forklifts, but extra insulation and fault analysis are required in wet zones, so designers stay below 30 V [Elektroda, anchilos & Szyszkownik Kilkujadek, #17825631–#17825648].
13. What cable protection is needed in a damp public area?
Run the cable in a sealed conduit 2 m above ground, use UV-resistant jacket, add 30 mA RCD at the source, and label routes. This meets Pool-side installation rules (IEC 60364-7-702).
14. Edge case: what if supply voltage is raised but the converter fails?
The fridge would see up to 30 V. Many 12 V appliances survive 15 V max; 30 V could destroy electronics and Peltier elements. Add an inline 15 A fuse and 15 V TVS clamp as last-resort protection.
