DC wire size calculator

With a 3 % drop limit, copper and a conductor temperature of 30 °C, the formula A = (2 · ρ(T) · L · I) / ΔU_allowed gives 14.93 mm²; the next standard size is 16 mm², which lands at an actual drop of 2.80 %. At 24 V the requirement halves to 7.47 mm², so 10 mm² off the shelf. Enter the one-way run length — the factor 2 for the return conductor is already built into the formula.

Resistivity rises with temperature: ρ(T) = ρ_20 · (1 + α · (T − 20)). In the reference case, moving from 30 °C to 70 °C conductor temperature raises the required cross-section from 14.93 to 17.19 mm² — about 15 % more. Sizing with the cold resistance leaves the cable undersized.

No — this calculator always evaluates both governing criteria, voltage drop and ampacity, and the larger cross-section wins. Most free cable calculators only compute the drop and can return sizes that are thermally impermissible. Short-circuit withstand and the drop across connectors and terminations are outside the scope.

Because they answer different questions: voltage drop is an efficiency and function issue (too thin = losses, flickering lights, devices shutting down), ampacity a safety issue (too thin = overheating, fire risk). The recommended size is the next standard size above BOTH requirements — and the calculator says which criterion governed. For short, high-current runs ampacity often wins; for long runs, voltage drop.

Twice over: the power loss P = I² × R is burnt off in the cable — at 100 A and a tenth of an ohm that is 1,000 W, in the middle of the run. Over the year this adds up to lost yield, quantified by the “annual loss” tile. A cross-section step costs a few euros per metre once; the loss runs forever.

Because current flows through out AND return conductors: over a 5 m run it passes 10 m of copper. Forgotten return conductors are the most common error in forum rules of thumb — results are then optimistic by a factor of 2. The calculator takes the one-way distance as input and doubles internally.

Yes — if you enter your conductor's base ampacity from the code table (switch in the advanced assumptions). The page then checks both criteria in one step and states which one governs the cross-section. Without the value it keeps the honest statement that ampacity must be checked separately — an invented table value would be the worst thing in a safety-critical calculator.

For PV source circuits it computes both prescribed paths and takes the stricter: 125 % of the short-circuit current rated at 156 % (continuous path) against the value divided by the correction factors (conditions path). From 11 A Isc this yields 18.9 A of required ampacity. The switch only acts under NEC standards — IEC/VDE have no such construct.

Because it warns precisely that 30 °C is only a default. If you enter the conductor temperature deliberately — say 70 °C in a filled cable duct — you have replaced the assumption with a decision; in the example the required cross-section grows from 16 to 25 mm². Until 5 August 2026 the warning appeared always, even after a deliberate entry.

They translate loss power into energy: 10.1 W of continuous loss is about 10 kWh per year at 1,000 operating hours — energy the system must first generate. For a continuous runner (8,760 h) the same cable becomes an 88 kWh item, and a larger cross-section pays for itself via module cost. The field was previously unreachable.

Yes, with the charge current as input — the charge controller calculator hands it over directly via the follow-on link. For the PV side the string calculator hands over the best configuration's short-circuit current, because NEC 690 computes from Isc, not from operating current. Both handovers work since 5 August 2026.