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Why voltage drop matters in DC systems
Voltage drop is the voltage lost along a cable because the cable itself has resistance. In a 12-volt or 24-volt DC system that loss matters far more than most people expect, because the same absolute loss is a much bigger percentage of a small supply voltage. Losing 2 volts on a 230-volt circuit is under 1 percent and nobody notices. Losing 2 volts on a 12-volt circuit is nearly 17 percent, and a motor runs slowly, a light dims, a battery charger misreads the battery, and a solar charge controller can behave strangely.
The usual planning target for DC runs is 3 percent, with 5 percent as an outer limit for less sensitive loads. This calculator uses the standard DC formula: drop equals twice the current times the one-way length times the resistivity, divided by the cross-section. The factor of two is there because current travels out along one conductor and back along the other, so both lengths of cable add resistance. Copper resistivity is taken as 0.0172 and aluminium as 0.0282 ohm square millimetres per metre, which are the standard room-temperature values.
How length, current and wire size interact
Three things drive the drop, and they multiply. Double the current and the drop doubles. Double the length and the drop doubles. Halve the cross-section and the drop doubles. That simple relationship is why long DC runs are so unforgiving: a garden light 30 metres from a 12-volt supply, drawing 10 amps through thin cable, can lose a quarter of its voltage before the light even sees it. The fixes are equally simple in principle: shorten the run, move the supply closer, reduce the current, or use thicker cable. In practice, thicker cable is usually the cheapest fix.
A worked example
Take a 12-volt LED lighting run drawing 10 amps, with the lights 20 metres from the supply, wired in 12 AWG copper. The calculator gives a drop of about 2.08 volts, which is 17.3 percent of 12 volts. That is far beyond any sensible target, and the lights would be visibly dim. Switching to 4 AWG copper, with a section of 21.15 mm², drops the loss to about 0.33 volts, or 2.7 percent, which is inside the 3 percent target. The example is deliberately extreme, but it mirrors real off-grid and campervan jobs where thin cable and long runs quietly ruin performance.
A gentler example shows the same logic at 24 volts. A 5-amp load, 15 metres away, on 14 AWG copper, loses about 1.24 volts, or 5.2 percent of 24 volts. That is borderline: fine for a rugged load, too much for sensitive electronics. Moving to 12 AWG brings it to about 0.78 volts, or 3.2 percent, and 10 AWG brings it under 2 percent. The pattern to remember is that doubling the system voltage halves the percentage drop for the same power, which is one reason 24-volt and 48-volt systems are popular for longer runs.
Common mistakes in DC voltage drop
The classic mistake is entering the round-trip length instead of the one-way length, or the other way round. This calculator asks for the one-way length and doubles it internally, so entering 40 metres for a 20-metre run doubles the answer and sends you shopping for cable you do not need. The second mistake is using the AC mains mindset on a low-voltage system. A drop that would be trivial at 230 volts is a failure at 12 volts, so always read the percentage, not just the volts.
Frequently asked questions
Is 3 percent a law? No, it is a widely used planning target, and some codes and manufacturers allow 5 percent for certain loads. Sensitive electronics, motors and chargers deserve the tighter target. Does aluminium change the answer a lot? Yes, aluminium has about 64 percent higher resistivity than copper, so the same run drops about 64 percent more voltage. That usually means one or two gauge steps larger for the same result. Can I just raise the supply voltage? Raising the source voltage to compensate for drop is a hack that overvolts the load when the current falls, so fix the cable instead.
Safety first: an estimate, not a code approval
Every electrical tool on this page is an educational estimator only. It is never professional advice and never a substitute for a licensed electrician. Electrical codes differ by country, region and building type, and they change over time. A result that looks fine here can still fail a local inspection, and a result that looks tight here can still be acceptable under a rule this simple calculator does not model, such as temperature correction, grouping, or a special location rule. Before you buy cable, cut a wall, or touch a panel, check your local electrical code and talk to a licensed electrician. Breaker, conduit and box-fill results are estimates, not code-compliance guarantees. If you are unsure about any part of a job, stop and ask a professional. Electricity can injure or kill, and no free online calculator changes that fact.
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