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Calculation of the conductor cross-section - direct current DC, 12V

burczynski82 8160 15
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  • #1 20558185
    burczynski82
    Level 9  
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    Hi
    Please advise what cable to use for the following, thanks

    From a 12V 95Ah battery to a 1000W inverter at a distance of 2.8 - 3m
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  • #2 20558242
    teskot
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    Apart from the fact that this battery is too weak for this inverter:

    1000W/12V=~84A
    The efficiency of the inverter is about 90%, it gives a current of nearly 100A.
    3m inverter-battery distance equals 6m of cable. At 25mm² the voltage drop will be: U=100x0.018*6/25=0.432V. Power P=100x0.432=43.2W will be dissipated on such a wire.
    I would use at least 25mm² or even better 35mm².
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  • #4 20558279
    teskot
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    The manufacturer of the inverter presumably recommends a battery "not less than 100Ah". Such a good quality battery will have a total capacity of no more than 50Ah at maximum load, ie at maximum load it will last less than half an hour, and its lifetime will be severely affected, especially if it is not recharged immediately after a deep discharge. A reasonable battery capacity with such a load is 500Ah.

    Oh, I assumed that we were talking about a lead battery, and this is a lithium one. Yes, lithium can be charged more in the long term.
  • #5 20558298
    burczynski82
    Level 9  
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    The battery in the picture
    Calculation of the conductor cross-section - direct current DC, 12V
  • #6 20558311
    teskot
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    We return to the starting point :) Li-Ion is not Lion :P Your battery is a regular "lead" only with the electrolyte trapped in a glass mat.
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  • #7 20558321
    burczynski82
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    I understand that if I add another one with the same or similar parameters, it will be a bit better
  • #8 20558329
    teskot
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    Sure, but it's good to combine batteries with the same parameters and condition to avoid problems related to the different internal resistance of the cells. As for the discharge current and the capacity dependent on it, I recommend that you read the first catalog note of the "deep discharge" battery. Example:
    https://www.bto.pl/produkt/81850/zelowy-accumulator-12v110ah-dyno
    In short, it can be said that for starter and buffer batteries, the capacity is usually given for a twenty-hour discharge current, for traction and other "deep cycle" batteries for a five-hour current.
  • #9 20558335
    burczynski82
    Level 9  
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    I will buy exactly the same, the current one is almost new.

    Now I'm confused - should I still leave the 25mm2 wires?
  • #10 20558337
    teskot
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    Yes, cables connecting the batteries as short as possible or individual cables from each battery to the inverter.
  • #11 20558425
    modziul
    Level 33  
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    new_player wrote:
    daro31ie wrote:
    The 2.5 cable is the most suitable for your application.


    For both devices?
    Cable length in device 1: 0.8 m.
    Cable length in device 2: 0.5 m.

    Regards

    A more precise calculator with the ability to calculate short sections. http://ep.az.pl/KIS/pl/przedy.htm
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  • #12 20559527
    burczynski82
    Level 9  
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    Thank you for your help
    I just got an email from the manufacturer and he advises to use exactly 25mm²
  • #13 20559539
    Anonymous
    Level 1  
  • #14 20559626
    teskot
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    I will not agree. While 8A for 1mm² may be acceptable in some circumstances for 230VAC installations, it means nothing. Following this lead, 10mm² would allow for a continuous load of 80A, and this has nothing to do with the permissible values. What counts above all is the allowable voltage drop on the cable, regardless of the power dissipated on it. 0.5V at 230V is nothing but at 12V it becomes significant.

    Let's assume that we drop 0.5V on a given section of the cable, a 12V installation powered by batteries. The battery has 11.5V, 11V on the inverter and there is already a cut-off, so we do not have the full capacity of the battery available.
  • #15 20559716
    Anonymous
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  • #16 20659763
    Graceee
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    >>20558185 Sure, but it is best to combine the battery pack with the same parameters and conditions to avoid problems due to the internal resistance inside the battery. As for the discharge current and depending on its capacity, I suggest you read the first directory description for "deep discharge" batteries.
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Topic summary

✨ The discussion revolves around selecting the appropriate cable size for connecting a 12V 95Ah battery to a 1000W inverter over a distance of 2.8 to 3 meters. Initial calculations suggest that a minimum of 25mm² cable is required to handle the current, which can reach nearly 100A due to inverter efficiency. Concerns are raised about the battery's capacity, with recommendations indicating that a 100Ah battery is preferable for optimal performance. The importance of using batteries with similar parameters to avoid issues related to internal resistance is emphasized. The conversation also touches on the significance of voltage drop in low-voltage systems, with suggestions to keep cable lengths short and to consider the manufacturer's advice on cable specifications.
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FAQ

TL;DR: In 12V inverter runs, a 0.5V drop can trigger shut‑down; “What counts above all is the allowable voltage drop on the cable…”. Size cable by drop, keep leads short, and avoid one‑size amp/mm² rules. [Elektroda, teskot, post #20559626]

Why it matters: This helps 12V DIYers pick the right cable and battery so 1000W inverters don’t trip on low voltage or waste power.

Quick Facts

What cable cross‑section do I need for a 12V 1000W inverter over ~3 m?

Plan for ~100A input. For a 3 m run (6 m round‑trip), 25 mm² copper drops ~0.432V and burns ~43W. That is acceptable for many builds, but 35 mm² reduces loss and heat. Use fine‑strand copper, tight lugs, and short runs. [Elektroda, teskot, post #20558242]

How do I calculate DC voltage drop for my inverter cables?

Use copper’s resistivity in a simple formula. 1) Double the one‑way length for round‑trip. 2) Compute drop: U = I × 0.018 × L / S (Ω·mm²/m). 3) Check U against your inverter’s low‑voltage cutoff margin. Example: 100A, 6 m, 25 mm² → 0.432V drop; heat ≈ 43W (P = I × U). [Elektroda, teskot, post #20558242]

Does the “1 mm² per 8 A” rule work for 12V systems?

No. That ignores voltage drop, which dominates at 12V. “What counts above all is the allowable voltage drop on the cable, regardless of the power dissipated on it.” Even a 0.5V drop can force premature inverter cutoff. Size by drop, not a current‑only rule. [Elektroda, teskot, post #20559626]

Will a single 95–100Ah AGM battery run a 1000W inverter well?

Not for long. At heavy load the usable capacity can be ≤50Ah, giving under 30 minutes runtime at ~1kW. Deep discharges also shorten life if not recharged promptly. For reasonable runtime at this power, plan around 500Ah of lead capacity. [Elektroda, teskot, post #20558279]

My battery says AGM/VRLA—Is that lithium?

No. AGM/VRLA is a sealed lead‑acid battery with electrolyte absorbed in a glass mat. It is not Li‑ion. Expect good cranking/buffer roles, but capacity at high discharge is limited versus lithium packs of similar Ah. [Elektroda, teskot, post #20558311]

Can I add another battery in parallel to improve performance? Any matching rules?

Yes. Parallel batteries should match type, capacity, and condition to avoid unequal sharing from different internal resistances. Check datasheets: many starter/buffer batteries quote capacity at 20‑hour rate (C20), while deep‑cycle types often specify 5‑hour rate (C5). [Elektroda, teskot, post #20558329]

How should I wire two batteries to the inverter to share current well?

Keep the interconnect between batteries very short. Better yet, run individual equal‑length positive and negative cables from each battery to the inverter’s terminals to balance voltage drop. Use identical lugs and crimp quality. [Elektroda, teskot, post #20558337]

What wire size did Renogy recommend for its 12V 1000W inverter?

The manufacturer advised using 25 mm² battery cables for this specific model. Follow device guidance first, then verify voltage‑drop is within your target at peak load and surge. [Elektroda, burczynski82, post #20559527]

What happens if my cables are too thin or too long?

You get excessive drop and heat, and the inverter may shut down early. Example edge case: with a 0.5V drop, a battery at 11.5V leaves only 11.0V at the inverter, triggering low‑voltage cut‑off and stranding usable capacity. [Elektroda, teskot, post #20559626]

Do I count the return path in cable length (is 3 m actually 6 m)?

Yes. Current leaves and returns, so resistance and drop are based on round‑trip length. A 3 m one‑way run is 6 m of conductor for voltage‑drop calculations and power loss. [Elektroda, teskot, post #20558242]

Is there a calculator I can use for short cable runs?

Yes. Try the KIS voltage‑drop calculator referenced in the discussion. It lets you enter short lengths, currents, and conductor data to estimate drop and heat, then adjust cable size. [Elektroda, modziul, post #20558425]

What input current should I design for at 1000W?

Compute steady draw as P/V. 1000W/12V ≈ 84A. Include inverter efficiency: at ~90% efficiency, input rises toward 93–100A. Size cables and fusing for this current and any surge requirements. [Elektroda, teskot, post #20558242]
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