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[Solved] Choosing Cable Cross-Section for 12V DC, 8A Car Fridge: Installation Distance of 120m

mlewan 26766 44
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How do I choose the cable cross-section for powering an 8A 12V car fridge over 120 m?

You should not try to run the fridge directly at 12 V over 120 m; for a 240 m round trip the voltage drop becomes so large that one calculation in the thread put the required wire at about 25 mm² if you insist on 12 V [#17825524] A better solution is to send a higher voltage down the long cable and convert it back to 12 V right next to the fridge [#17825554] One suggested practical setup was a 230 V / 24 V DC supply at the start, then a DC-DC converter near the fridge, with cable sizes around 2×6 mm² or 2×4 mm² depending on the chosen voltage and acceptable drop [#17827508] Another reply calculated that if you only allow the fridge to see about 11.5 V minimum from a 15 V source, the cable size comes out around 16 mm² [#17825671] The topic author eventually confirmed that raising the transport voltage to 30 V solved the problem and allowed an acceptable cable size [#17836293]
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  • #31 17827466
    CYRUS2
    Level 43  
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    Szyszkownik Kilkujadek wrote:
    distance of 120 meters, now be careful: You need a cable with a minimum cross-section of 25mm2.
    You had a good feel.
    But .... use 2 x 25mm2 aluminum conductor (or cable)
    Price approx. PLN 4 / m gross.

    You also have to pay for the 2 x 4mm2 Cu conductor - about PLN 4 / m gross.
    For this additional converters - you have to pay.
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    #32 17827508
    ptero
    Level 24  
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    Hello all.
    First, give a transformer of 230/24, power 160-200VA. 2x6mm2 or 2x4mm2 cable. At the end, next to the refrigerator, a bridge with a capacitor and a step up / step down converter - 2.5-30V / 12V 10-15A, I once bought one on Ali for a solar panel. Even with a voltage drop of a few volts, it can handle it. There is a Peltier module in the refrigerator, which has a fairly wide operating range.
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    #33 17827807
    CYRUS2
    Level 43  
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    Can a colleague calculate what will be the cost?
    ptero wrote:
    At the end, a bridge with a condenser next to the refrigerator.
    Such "bungling" as a colleague suggests in a public facility is not acceptable.
  • #34 17827831
    Szyszkownik Kilkujadek
    Level 37  
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    Maybe let's wait for the author with further discussion? :-D
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    #35 17828268
    ptero
    Level 24  
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    CYRUS2 wrote:
    Such "bungling" as a colleague suggests in a public facility is not acceptable.


    Yeh, in fact, bungling. But it can be done "professionally" :)
    After all, it's just a loose idea, not a ready-made solution ...
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    #36 17828581
    William Bonawentura
    Level 34  
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    It may also turn out that, for example, the most sensible solution will be to freeze ice at home and bring it to the refrigerator without any power supply. Until the author writes more precisely what he wants to achieve, further prompting is probably pointless.
  • #37 17828587
    alekt77
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    Szyszkownik Kilkujadek wrote:
    Maybe let's wait for the author with further discussion?

    The author of the topic seems to have been effectively discouraged by such a wealth of good advice ;-)
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    #38 17828913
    CYRUS2
    Level 43  
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    ptero wrote:
    Yeh, in fact, bungling. But it can be done "professionally"
    You cannot, there are no such ready-made devices.
    The author got realizable solutions in posts # 15 and # 31.
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    #39 17829456
    Strumien swiadomosci swia
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    The main question is how much it gets at start-up.
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    #40 17829539
    kortyleski
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    The fact that there are no ready solutions does not mean that you cannot implement something new. The condition is proper execution and as-built documentation that will enable subsequent service.
    But the simplest solution will be two large batteries, a charger and a cart. 8A for 10h is 80Ah. For the comfort of the 110Ah battery. No cables, safe.
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    #41 17829739
    endi1940
    Level 11  
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    I suspect that the refrigerator has something like a thermostat and does not work all the time. One large (about 110ah) gel battery is enough, more resistant to cyclic operation. For this low battery alarm, just in case - there are ready for 12 volt batteries. It works as a source during the day and recharges at night. Possibly 2 Batteries to work in shifts. It is not the ultimate in comfort, but an alternative to cables and converters. It can be more expensive in terms of price.
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    #42 17829816
    Strumien swiadomosci swia
    Level 43  
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    mlewan wrote:
    I take the power supply from 230 V AC, the problem is that it is a wet environment where a lot of people hang around (swimming pool). There is a place to lay the cable, but generally wet and a lot of people. If no construction works are to be carried out (and they are not), only safe voltage is involved.

    And I wonder why you need a fridge at the pool, install it in the changing room or.
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    #43 17830035
    kortyleski
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    Strumien swiadomosci swia wrote:
    I wonder why you need a fridge at the pool,

    So that the customers of the pool have cold ones at hand, not in the changing room. I was making a whole booth in the pools. It was just a ride.
  • #44 17833942
    Szyszkownik Kilkujadek
    Level 37  
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    CYRUS2 wrote:
    The author got realizable solutions in posts # 15 and # 31.

    What a modesty. :-D
  • #45 17836293
    mlewan
    Level 20  
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    Dear colleagues :)

    Thank you for the hints and solutions, increasing the voltage to 30 V solves the problem. Let me close the topic.

    greetings :)

    Added after 1 [minutes]:

    The problem was solved by increasing the voltage to 30 V, which allowed the use of a cable cross-section of an acceptable size.
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Topic summary

✨ The discussion revolves around selecting the appropriate cable cross-section for a 12V DC car fridge drawing 8A over a distance of 120m. Key considerations include voltage drop calculations, with recommendations ranging from 6mm² to 25mm² depending on the voltage at the source and the acceptable voltage drop at the fridge. The use of a higher voltage (up to 30V) at the beginning of the cable is suggested to minimize losses, allowing for a smaller cable size. Various solutions are proposed, including using a step-down converter at the fridge and considering alternative power sources like batteries or gas-powered refrigerators. The importance of safety in a wet environment is emphasized, along with the need for proper installation practices.
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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, post #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^, post #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]

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^, post #17825492]

3. How do I calculate voltage drop myself?

  1. Look up copper resistivity ρ = 0.0175 Ω·mm²/m [IEC 60228].
  2. Compute R = ρ × length / area.
  3. 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^, post #17825492]

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.
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