Voltage Drop Calculator

Voltage Drop Calculator

How the Voltage Drop Formula Works

The calculator above runs one equation: Vdrop = 2 × I × R × L ÷ 1,000, where I is load current in amps, R is conductor resistance in ohms per 1,000 feet, and L is the one-way run length in feet. The 2 is the part people forget: current travels out to the load on one conductor and back on the other, so a 20-foot run means 40 feet of wire resisting the flow. R comes from NEC Chapter 9, Table 8, the code’s official conductor-properties table — 12 AWG uncoated copper is 1.93 ohms per 1,000 feet, 10 AWG is 1.21, 6 AWG is 0.491. Plug in a realistic 12-volt example: 20 feet of 12 AWG feeding a 15-amp load gives 2 × 15 × 1.93 × 20 ÷ 1,000 = 1.16 volts of drop. On a 120-volt circuit that’s under 1 percent — invisible. On a 12-volt battery bank it’s 9.6 percent, and the load sees only 10.8 volts, low enough to trigger the 10.5-volt alarm on many inverters. Same wire, same current, radically different outcome.

The 3% Rule Is a Recommendation, Not a Mandate

The pass/fail verdict in the calculator uses the National Electrical Code’s best-known guideline: keep branch-circuit voltage drop under 3 percent, and the combined feeder-plus-branch drop under 5 percent. Read the fine print, though — that figure lives in Informational Note No. 4 under NEC 210.19(A), and Section 90.5 says informational notes are explanatory only, not enforceable requirements. An inspector working from the base NEC cannot fail your wiring for a 4 percent drop. Two caveats matter. First, some jurisdictions and energy codes do adopt voltage-drop limits as binding rules, so local amendments can turn the suggestion into law. Second, the NEC itself makes drop mandatory in a few special cases — Article 647 holds circuits for sensitive electronic equipment to 1.5 percent on the branch and 2.5 percent combined, with the word ‘shall.’ For hobby and off-grid DC work, treat 3 percent as an engineering target rather than a legal line: motors run cooler, LED strips stay uniform, and chargers see the voltage they expect.

Why 12-Volt Systems Punish Thin Wire

Voltage drop punishes low-voltage systems twice. First, moving the same power takes more current: a 100-watt load draws 8.3 amps at 12 volts but only 0.83 amps at 120 volts, and drop scales directly with amps. Second, the allowance is smaller in absolute terms — 3 percent of 12 volts is just 0.36 volts of headroom, versus 3.6 volts on a 120-volt circuit. Stack the two effects and the numbers turn dramatic. Take 12 AWG copper carrying 15 amps: it stays inside 3 percent for about 62 feet one-way at 120 volts, but only about 6 feet at 12 volts. That’s why wiring in RVs, boats, camper vans, and solar sheds looks absurdly thick to anyone used to house circuits, and why a 25-foot LED strip fed with skinny wire dims and color-shifts at the far end. It’s also the argument for 48-volt battery banks: for the same load power, 48 volts tolerates sixteen times the run length of 12 volts on identical wire — four times from carrying a quarter of the current, four times from having a 1.44-volt drop budget instead of 0.36. When you can’t raise the voltage, raise the copper.

Copper vs Aluminum: The 1.65× Penalty

Flip the calculator’s material toggle and the drop jumps by roughly 65 percent. NEC Chapter 9, Table 8 puts aluminum at about 1.65 times the resistance of copper in every gauge: 12 AWG aluminum is 3.18 ohms per 1,000 feet versus 1.93 for copper, 10 AWG is 2.00 versus 1.21, 6 AWG is 0.808 versus 0.491, and 4/0 is 0.100 versus 0.0608. The working conversion is two gauge steps — aluminum two sizes larger matches copper almost exactly. Check it against the table: 4 AWG aluminum at 0.508 ohms sits a hair above 6 AWG copper at 0.491, and 2 AWG aluminum at 0.319 tracks 4 AWG copper at 0.308. Aluminum still earns its place on big feeders, where it costs far less per foot and weighs half as much, which is why 200-amp service-entrance cable is almost always aluminum. For small-gauge, low-voltage DC work the choice usually makes itself: aluminum below 8 AWG is hard to buy, terminations are fussier, and marine standards call for stranded copper throughout. When in doubt, run the number both ways above and let the percent column decide.

Ampacity and Voltage Drop Are Different Questions

A wire can be perfectly safe and completely inadequate at the same time. Ampacity — the current a conductor can carry without overheating — is what NEC Table 310.16 governs, and it doesn’t change with length: 12 AWG copper NM cable is protected at 20 amps whether the run is 5 feet or 500. Voltage drop grows with every foot. The 20-foot, 15-amp example above proves the gap: 12 AWG is thermally fine at 15 amps, yet on a 12-volt system it delivers only 10.8 volts to the load. To fix drop you upsize, and the math is friendly: resistance halves roughly every three gauge numbers, so each 3-AWG jump cuts the drop in half. Walking that same 20-foot, 15-amp, 12-volt run up the table: 10 AWG still drops 6 percent, 8 AWG drops 3.8 percent, and 6 AWG finally passes at 2.5 percent. A workable rule of thumb for 12-volt systems: any run past about 15 feet should be sized by voltage drop, not ampacity, because drop will demand more copper than heat does. Always satisfy both constraints — ampacity sets the floor, drop usually sets the answer.

When to Call an Electrician

This calculator is an estimator for hobby and low-voltage DIY work: RV and camper builds, boat wiring, off-grid solar, LED strips, trolling motors, garden lighting, ham-radio power runs. For those jobs it will keep you out of the classic traps — browned-out fridges, flickering strips, chargers that never finish. It is not a substitute for NEC compliance on mains wiring. Real branch-circuit design also involves ampacity with temperature and bundling corrections, terminal temperature ratings, conduit fill, overcurrent protection, and AC effects the DC formula ignores — Table 9 impedance values differ from Table 8 resistance once conductors get large or power factor drops. The math here uses DC resistance, which is accurate for DC systems and conservative-to-close for 120/240-volt AC at these gauges. If the project involves 120 or 240 volts in fixed wiring — a subpanel, a hot-tub feeder, a detached-garage run — it likely needs a permit, and a licensed electrician will size conductors against the full code, not one informational note. Use this page to sanity-check a quote or plan wire purchases, not to skip the pro.