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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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  • #1 17825441
    mlewan
    Level 20  
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    I have a 12 V DC car fridge; 8 A. I would like to carry a large piece of the installation to it, as much as 120 m. What cable cross-section should be chosen? I searched a lot on the Internet, I found various calculators, but the results range from 1 to 6 mm sq. Could someone explain it to me clearly? Thanks in advance :)
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    #2 17825489
    alekt77
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    What is the minimum voltage at which the refrigerator will work? 11V, 10V? Take from the tables for cables with different cross-sections the resistance of 1mb of the cable, count the resistance of the cable for 240mb of length (2x120m), multiply by 8A, you will have a voltage drop on the cable, subtract from 12V and check if this value is above the minimum operating voltage of the refrigerator, take a wire with a cross-section a little larger than in the calculations (just in case and for plug losses).
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    #3 17825492
    ^ToM^
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    mlewan wrote:
    If I don't get out in the sandbox, it'll be a miracle :)

    I have a 12 V DC car fridge; 8 A. I would like to carry a large piece of the installation to it, as much as 120 m. What cable cross-section should be chosen? I searched a lot on the Internet, I found various calculators, but the results range from 1 to 6 mm sq. Could someone explain it to me clearly? Thanks in advance :)


    So on quickly:
    120 m distance, therefore 240 m of cable where some voltage drop will occur at 8 A.
    How will you use a wire with a cross section 2.5 mm2 then the resistance of a 240 m cable will be 1.6 ?

    R = ??L / S
    R = 0.0168?240 / 2.5
    R = 1.6 ?

    Therefore, by giving such a wire and passing 8 A through it, there will be a voltage drop U = I * R, i.e. 8 A x 1.6 ? = 12.8 V

    Will you be able to learn from this?
    :)
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    #5 17825524
    Szyszkownik Kilkujadek
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    mlewan wrote:
    I have a 12 V DC car fridge; 8 A. I would like to carry a large piece of the installation to it, as much as 120 m. What cable cross-section should be chosen? I searched a lot on the web, I found various calculators, but the results range from 1 to 6 mm sq.

    I don't know where these results came from? They are wrong.
    If you want 12V, send 8A DC over 120 meters, now be careful: You need a cable with a minimum cross-section of 25mm2.

    If you want to know how to solve this problem, please ask. Write where you get the power from and why such a long cable. Basically what are you up to? :-)
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  • #6 17825534
    mlewan
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    Szyszkownik Kilkujadek wrote:
    Write where you get the power from and why such a long cable. Basically what are you up to? :-)


    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.
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    #7 17825543
    alekt77
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    mlewan wrote:
    And is this table authoritative?

    Here: Link you have a calculator, you can convert for different assumptions
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    #8 17825554
    Szyszkownik Kilkujadek
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    If we apply the most stringent safe voltage criterion, we can give a maximum of 30V. Then you can go down to 6mm2 cross-section.
    So you install a 230V / 24V DC power supply at the beginning of the cable, and a converter lowering the voltage to 12V next to the refrigerator.

    https://pl.wikipedia.org/wiki/Napięć_bezpieczne#Wartości_napięć_dotyczki_dopuszczalnych
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    #9 17825578
    zworys
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    24V is also a safe voltage. Only half of that voltage would be lost to heat in the cable. Next - the 8A refrigerator is charging continuously or periodically? I think that periodically such a simple method is omitted because with lower power consumption, for example, 19V will appear on the refrigerator and this can be fatal for the refrigerator. A converter should be used at the end of the cable, which would supply some 12V to the refrigerator when the load changes. I don't know if it would be cheaper to use a set of solar panel + controller + 12V battery.
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    #10 17825584
    alekt77
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    Due to the fact that the refrigerator probably consumes 8A continuously, the matter becomes more complicated because the voltage at the end of the cable from the refrigerator side will change in a fairly wide range.
  • #11 17825629
    Szyszkownik Kilkujadek
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    Mates @zworys and @ alekt77 please note that at the end of the cable I proposed a converter that reduces the voltage to 12V. Read the whole thing with understanding.
    zworys wrote:
    I don't know if it would be cheaper to use a set of solar panel + controller + 12V battery.

    Doubtful.
  • #12 17825631
    anchilos
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    Szyszkownik Kilkujadek wrote:
    If we apply the most stringent safe voltage criterion, we can give a maximum of 30V.
    48VDC is widely used as safe voltage even with direct contact (see industrial trucks). Solution . 230VAC / 48VDC converter, on the other side a step down converter to 12VDC with a continuous current of at least 12A (lower heating)
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    #13 17825648
    Szyszkownik Kilkujadek
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    anchilos wrote:
    48VDC is widely used as a safe voltage even with direct contact (see industrial trucks)

    Do lorry trucks run in the pool?
    Did you read what the author wrote? Here:
    mlewan wrote:
    the problem is that it's 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.


    48V DC can be used, but it requires deep analysis. I would not suggest it in the dark.
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    #14 17825663
    anchilos
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    Szyszkownik Kilkujadek wrote:

    Do lorry trucks run in the pool?
    Is the author going to let bare wires through the pool? Will he use the correct wire - a cable buried in the ground? More like a second version. I greet everyone. By the way
    Szyszkownik Kilkujadek wrote:

    Do lorry trucks run in the pool?
    They traveled in Cyprus. Directed by our so-called "elite". greetings again.
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    #15 17825671
    CYRUS2
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    mlewan wrote:
    If I don't fly out to the sandbox, it'll be a miracle
    Why ? Everyone has the right to ask on Elektroda. In the electrics section for everyone.
    Refrigerator - max voltage 15V because this voltage can be on the car battery.
    The minimum voltage on the refrigerator should be 11.5V.
    This is the output for the calculations.
    But what do you want to power it from?
    You need to purchase a DC 15V 10A-15A stabilized power supply.
    15V- 11.5V = 3.5V
    3.5V / 8A = 0.437?
    0.437? / 240 = 0.0018 ? / m
    The conductor cross-section is 12 mm?
    You will need a cable with a conductor cross-section of 16mm2.
    This is enough for a current at engine start of approx. 11.6 A.
  • #16 17825675
    mlewan
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    anchilos wrote:
    Is the author going to let bare wires through the pool?


    Rather not, the cable is going to be placed on the cornice, 2 m above the ground, in a protective tube. It will definitely be wet.
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  • #17 17825689
    alekt77
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    Szyszkownik Kilkujadek wrote:
    Read the whole thing with understanding.

    I am reading, which does not change the fact that it is troublesome to find a converter with a wide range of voltage changes at the input. For example, I found something like this on the web: Link
    It works from 15V input voltage, so with a power supply of not less than 24V DC on the primary side of the cable, you could be tempted to use a 4mm? cable, which is a reasonable size.
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    #18 17825742
    zworys
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    Szyszkownik Kilkujadek wrote:
    please note that at the end of the cable I proposed a converter that reduces the voltage to 12V. Read the whole thing with understanding.

    When I wrote my post, yours was not visible yet. I also mentioned the converter.
  • #19 17825774
    Szyszkownik Kilkujadek
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    alekt77 wrote:
    I am reading, which does not change the fact that it is troublesome to find a converter with a wide range of voltage changes at the input.

    Difficult? :-) https://www.elektroda.pl/rtvforum/topic3434421.html

    Added after 3 [minutes]:

    Or: https://sklep.avt.pl/zasilacz-regulowany-winners-xl4016-max-10a-35v.html

    Added after 5 [minutes]:

    Which does not change the fact that perhaps the best and most economical solution may be a gas-powered camping refrigerator:
    https://pl.wikipedia.org/wiki/Chłodziarka#Chłodziarka_absorpcyjna
  • #20 17825808
    alekt77
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    Szyszkownik Kilkujadek wrote:
    Difficult? :-)

    Maybe not so much :D
    If you read my post # 17 carefully about an hour ago, you would see that I found something suitable on the network and in a ready housing for about PLN 73 (gross)
  • #22 17825872
    Szyszkownik Kilkujadek
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    anchilos wrote:
    However, difficult. "maximum load 10A (continuous load up to 8A 70W)"

    mlewan wrote:
    I have a 12 V DC car fridge; 8 A.

    :-)
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    #23 17825890
    anchilos
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    anchilos wrote:
    8A 70W) "
    Szyszkownik Kilkujadek wrote:

    mlewan wrote:
    I have a 12 V DC car fridge; 8 A.
    12x8 =? W :D
  • #24 17825909
    alekt77
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    Szyszkownik Kilkujadek wrote:
    anchilos wrote:
    However, difficult. "maximum load 10A (continuous load up to 8A 70W)"

    mlewan wrote:
    I have a 12 V DC car fridge; 8 A.

    :-)


    8A x 12V = 96W> 70W :cry:
  • #25 17825963
    Szyszkownik Kilkujadek
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    Right. Anyway. The inverter is no problem.
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    #26 17826394
    mar_uda
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    Maybe it is worth for the author to write what kind of refrigerator it is about.
    There are models in which cooling takes place using a Peltier cell. Thus, there is no increased inrush current and the sensitivity to voltage changes is lower. Then you can choose a power supply with a correspondingly higher voltage and the converter will not be necessary, and the cable cross-section will be smaller.
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    #27 17826486
    acso
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    Or maybe two 230 / 24V transformers?
    One at the beginning of the cable and one at the end.
    A current of about 4A will give a voltage drop of about 6.5V with a conductor cross-section of 2.5mm2.
    The second transformer is connected to the wire by a secondary winding
    it should give you about 165V on the primary one.
    Most of the 12V switch mode power supplies can handle this voltage without any problems.
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    #28 17826550
    freebsd
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    Can it do something like this: a 1: 1 transformer and a 230 V earth cable (although located on the cornice)?
    In this thread, electricians familiar with the standards should have the final say :-)
  • #29 17827408
    Szyszkownik Kilkujadek
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    Generally, we have a consensus: you need to increase the voltage at the beginning of the cable and lower it at the end (at the refrigerator).
    It is also possible not to lay the cable and use a gas-fired refrigerator instead.
    The choice is up to the author of the topic.
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    #30 17827445
    ^ToM^
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    Szyszkownik Kilkujadek wrote:
    Generally, we have a consensus: you need to increase the voltage at the beginning of the cable and lower it at the end (at the refrigerator).
    It is also possible not to lay the cable and use a gas-fired refrigerator instead.
    The choice is up to the author of the topic.


    But there is no choice here because the author already has a refrigerator! Read in the first post. :)
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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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