12/24/48 V DC Wire Size & Fuse Calculator

Current and voltage drop

For inverter or audio output, $P_{dc}=P_{out}/\eta$ and $I_0=P_{dc}/V_{min}$. Reserve $I_d=I_0/(1-d)$ for permitted fractional drop $d$. A current-limited solar controller uses its rated battery current directly. The resistance budget is $R_b=V_{min}d/I_d$; subtract the entered connection resistance before sizing copper.

Cable resistance follows $R=r_{20}(L_++L_-)[1+\alpha(T_d-20)]$, with $\alpha=0.00393\ \mathrm{K^{-1}}$. Auto mode uses $r_{20}=\rho_{eff}/A$. The input shows 20 mΩ·mm²/m, explicitly converted to $\rho_{eff}=0.020\ \Omega\,\mathrm{mm^2/m}$ is a screening estimate informed by OMERIN's bare class 6 product data; it is not a reproduced IEC resistance table or a guarantee for every cable. Custom mode uses the maker's maximum $r_{20}$.

The operating inverter current is the stable root $I=2P_{dc}/[V_{min}+\sqrt{V_{min}^2-4P_{dc}R}]$. Negative discriminants have no steady constant-power solution. Reference cards separately show Victron's first-pass, 20 °C arithmetic; the operating result retains feedback.

Thermal calibration and fuse selection

The maker's continuous rating $I_m$ at ambient $T_b$ and conductor $T_r$ calibrates a linear heat-loss model. Let $\Delta T_d$ be the smallest permitted rise from the entered rise, terminal limit and insulation limit. Then

No increase above the published current rating is credited. Routing multiplies allowable current; it is not a thermal simulation of a bundle. The trial rise evaluates resistance at the design temperature. A numeric upper bound is shown only when the trial rise does not exceed the permitted rise; otherwise the result reads “Outside thermal model validity”.

Select the smallest JLLN product rating at least $1.25I_d$, then the smallest listed cable meeting both $R\le R_b$ and $I_f\le I_z$. No eligible cable means failure, not permission to use the largest row. The 35–1200 A JLLN example has 20 kA DC interrupt capacity and 125/160 V DC rating depending on current range (2025-08-06 datasheet). The fuse and holder must also satisfy surge duration, overload clearing, available fault current and equipment instructions.

Alternator balance

At charging voltage, solve amplifier current with the same feed resistance. Subtract existing vehicle demand and reserve from hot alternator output. Charging voltage must lie within the declared battery-voltage limits. Usable continuous feed current is limited by derated cable ampacity, fuse rating divided by 1.25, allowed voltage drop at the minimum battery voltage, and the stable constant-power branch. Sustainable output is the smaller of the alternator-supported output at charging voltage and the feed-supported output at minimum battery voltage, both including I²R loss. Increasing alternator output does not resize the fuse or cable. No music duty-cycle discount is assumed.

• Copper only; DC steady operation, one positive and one return conductor of the same area. Lengths may differ. CCA (copper-clad aluminium), aluminium, parallel cable current sharing and skin-effect calculations are excluded.
• Auto ampacities are TEBAOFLEX 600 V yellow/black construction, 2 AWG through 4/0, at 30 °C ambient and 90 °C conductor. They are product data, not NEC/ABYC ampacity tables. Check suitability, terminations, listing, environment and continuous-duty interpretation with the maker before purchase.
• The 20 mΩ·mm²/m (0.020 Ω·mm²/m) resistance–area estimate includes an allowance above ideal copper. Confirm the purchased cable's resistance; use Custom mode if its data differ. OMERIN's 70 mm² resistance is supplied as a custom input example only: the 200 A ampacity is an explicitly unverified placeholder.
• The 0.8 routing factor and 0.6 mΩ extra resistance are design allowances. Neither can establish the temperature of a particular enclosed route or a bad lug.
• Lowest loaded and maximum charging voltages are independent design limits. Audio assumes 12.0–14.8 V unless overridden. Solar is the battery connection, not PV array/string protection.
• The 10 kA fault input must be replaced with a justified upper bound. Do not use BMS continuous current as short-circuit current or assume a BMS replaces fault protection.
• The first diagram omits isolation switches and grounding details for clarity. Positive-source fusing is shown; grounded/ungrounded system requirements and any protection at both source ends remain installation-specific.

Designed for nominal 12/24/48 V copper battery circuits (custom minimum 1–100 V; maximum 125 V). A result passes only the stated steady-current, resistance, simplified thermal, DC voltage and interrupt checks. The estimated conductor rise is not a field measurement. It is suppressed when the thermal-envelope check fails. Thermal calibration assumes the maker rating is applicable to continuous service and cooling can be approximated by a linear thermal conductance.

IEC 60228:2023, edition 4, defines conductor area, construction and resistance requirements; it does not supply a universal ampacity for an installed battery cable. No standard table or figure is reproduced. AWG areas use the geometric diameter law associated with ASTM B258; these are nominal copper areas, not a certification of strand construction. Check against the standard in force.

The 125% continuous margin is the stated design rule requested for this tool, not a claim of NEC or ABYC compliance. Always check fuse holder ambient derating, fuse minimum/maximum clearing curves, inverter capacitive inrush and surge duration. Cable short-circuit withstand, fuse let-through I²t, low-current fault clearing, selective coordination, DC time constant and equipment maximum fuse limits need a separate check. A battery-side fuse must be close enough to protect the unfused positive lead under applicable installation rules.

Big 3 is an explanatory charging/ground circuit: retain factory wiring and follow vehicle-maker battery-current-sensor and grounding instructions. Chassis path resistance must be measured and included. A larger alternator, upgraded wiring and a larger battery solve different constraints.

Data review: 2026-10-26. Installation work belongs to a qualified person who must verify cable-maker ratings and local installation requirements before energising the circuit.