DC Wire Size Calculator (12V, 24V and 48V)

Background.

Low-voltage DC wiring is not AC wiring with a smaller number in it. On a 240 V circuit a 3% voltage-drop budget is 7.2 V, which is a comfortable amount of room, and the conductor is usually decided by heat — by the ampacity tables an inspector enforces. On a 12 V circuit the same 3% is 0.36 V. Not seven volts: a third of one volt, for the whole out-and-back trip. That single difference is why voltage drop governs essentially every low-voltage DC run of any length, why the ampacity table barely participates, and why a 12 V circuit needs conductors that look absurd next to an AC circuit carrying the identical current.

The comparison is worth seeing concretely. Twenty amperes over fifteen feet on a 120 V AC branch circuit needs 12 AWG — an ordinary piece of house wire. The same twenty amperes over the same fifteen feet on a 12 V DC system needs 6 AWG, three trade sizes up, at roughly four times the copper. Nothing about the current changed. The only thing that changed is how many volts you can afford to lose.

That is the whole reason this page exists separately from the AC wire size calculator. Everything the NEC builds around ampacity — the 110.14(C) termination columns, the 240.4(D) small-conductor limits, the 125% continuous-load adjustment, the bundling derates — is most of that page and is close to irrelevant to a battery-to-inverter run. Here the arithmetic is one equation, and the interesting decisions are about system voltage, run length and how much drop the load will actually tolerate.

The system voltage field is the biggest lever on the page and is worth playing with before you buy cable. Because current falls as voltage rises and the allowable drop in volts rises at the same time, the copper needed for a given power over a given distance falls with the square of the voltage. The same 240 W load needs four times the circular mils at 12 V that it needs at 24 V, and sixteen times what it needs at 48 V. Anyone laying out an off-grid system with runs longer than a few metres is usually better off solving the problem with system voltage than with copper, and this calculator will show you the trade in one comparison.

Three scope limits, stated here rather than tucked into an FAQ. First, length is measured one-way and the calculator doubles it. ABYC's published tables are printed the other way round — their length column is source-to-device-and-back — so a number carried across from one to the other without halving or doubling is wrong by a factor of two, and that is the single most common mistake in DC wire sizing. Second, boats are not covered here at all. Marine DC wiring is governed by ABYC E-11, which has its own ampacity table, its own voltage-drop tables IX and X, its own resistance constant, and a 3%/10% split between critical and non-critical circuits. Its published tables are in places one trade size more conservative than any formula reproduces, so they must be read rather than calculated. Third, photovoltaic source and output circuits carry the extra 125% stacking of NEC 690.8 and energy-storage circuits fall under Article 706; this page sizes a conductor for a current you give it, and it does not compute that current for you.

And the limit that matters most: a battery bank is a fault-current source with no upstream breaker to save you. A shorted 12 V lithium pack will happily deliver thousands of amperes into a wrench. Conductor sizing is only one part of a safe DC installation, alongside correctly rated fusing at the source end, proper lugs, crimps and strain relief, and terminations that are actually listed for the conductor. Have a licensed electrician or a qualified installer sign off before anything is connected.

What is dc wire size calculator?

A DC wire size calculator answers the question of which conductor to run between a source and a load on a direct-current system, given the current, the distance, and how much voltage you are prepared to lose along the way.

The underlying physics is Ohm's law and nothing more. A conductor has resistance proportional to its length and inversely proportional to its area; current through that resistance produces a voltage drop; the drop is subtracted from what arrives at the load. On DC there is no reactance and no power factor, so unlike the AC case the calculation is exact rather than an approximation — the only modelling assumption is the temperature the resistance is quoted at, here 75 °C.

The trade form of the equation is VD = 2·K·I·L/CM. I is the current in amperes, L is the one-way length in feet, CM is the conductor's area in circular mils, and K is a resistance constant — 12.9 for copper and 21.2 for aluminium, both at 75 °C, both derived from NEC Chapter 9 Table 8. The 2 is there because the current has to come back: on DC there is no neutral doing anything clever, and the return conductor drops exactly as much as the supply conductor.

Ampacity still exists and still has to be checked. It is simply rarely the binding constraint down here: 14 AWG copper is good for 25 A on heat alone, and a 12 V circuit carrying 25 A over any useful distance will have been pushed to 6 AWG or larger by voltage drop long before heat becomes interesting. The calculator reports both so you can see which one decided the answer, because that tells you what to change — if drop governs, only more copper or a higher system voltage helps; if ampacity governs, the run is short and the current is large, and the real problem is probably at the terminations.

How to use this calculator.

1. 01Pick the system voltage. If you are still choosing one, run the same load at 12 V and again at 24 V before you buy anything — the copper needed falls with the square of the voltage, and on long runs that decision is worth more than any other.
2. 02Say whether you know the load in amperes or in watts, and fill in that field. The other one is ignored.
3. 03Measure the run one-way, source to load, along the route the cable actually takes rather than straight-line. The calculator doubles it for the return path.
4. 04Set the drop budget. 3% is the normal design target. 10% is defensible for cabin lighting and other loads that genuinely do not care, and is what ABYC E-11 permits for non-critical circuits — but check that your load really is one of those before you spend the allowance.
5. 05Read which constraint governed. If it says voltage drop, the only fixes are more copper, a shorter run or a higher system voltage. If it says ampacity, the run is short and the current is high, and your attention belongs on fusing, lugs and crimps rather than on gauge.
6. 06Check the ampacity floor's assumptions against your install. It is taken at 30 °C ambient with no more than three conductors together — an engine bay, a roof space or a bundle through a conduit is none of those, and needs the derating factors the AC wire size calculator applies.
7. 07Size the fuse or breaker separately, at the source end of the run, and have the whole installation signed off by a licensed electrician or qualified installer before it is connected to a battery.

The formula.

Start from resistance. A conductor's resistance is its resistivity times its length divided by its area. Expressing area in circular mils folds the awkward constants into a single number K, defined as the DC resistance of a 1,000-circular-mil conductor 1,000 feet long: 12.9 ohms for copper and 21.2 for aluminium, both quoted at 75 °C in NEC Chapter 9 Table 8. That gives resistance per foot as K/CM, and Ohm's law gives the drop.

The factor of 2 is the round trip. On a DC circuit the current leaves through one conductor and returns through the other, both of them the same size, so the drop is twice what a single length would give. Getting this wrong is the classic DC wire-sizing error, and it is made easier by the fact that ABYC prints its own tables against the round-trip length rather than the one-way length. This page asks for one-way and doubles it, and says so beside the answer.

Rearranged to size a conductor, CM = 2·K·I·L / VD_allowed, where VD_allowed is the system voltage times the drop budget. The calculator computes that area, then takes the smallest standard conductor whose circular-mil area reaches it — compared unrounded, so a requirement a hair above a size steps up rather than rounding onto it. The worked example is 2 × 12.9 × 20 × 15 / 0.36 = 21,500 circular mils, which 6 AWG's 26,240 covers and 8 AWG's 16,510 does not.

The ampacity floor is a separate lookup rather than a calculation: the smallest conductor whose NEC Table 310.16 figure, in the column matching your insulation rating, reaches the circuit current. It is taken at the table's own basis of 30 °C ambient and no more than three current-carrying conductors, with no correction or adjustment applied — a deliberate simplification, stated beside the result, because on a low-voltage DC system the drop constraint is normally several sizes above it anyway. Where it is not — a very high current over a very short run, which is exactly what a battery-to-inverter cable is — the floor takes over and the page says so.

The answer is the larger of the two, because a conductor has to satisfy both at once. Every comparison is made on the unrounded value and rounding to ten decimal places happens once, at the return boundary.

One relationship is worth having in your head, because it is the most useful thing on the page. For a fixed power and a fixed distance, the copper required falls with the square of the system voltage. Doubling the voltage halves the current and doubles the volts you can afford to lose, and those two effects multiply. The worked example's 240 W load needs 21,500 circular mils at 12 V and 5,375 at 24 V — a quarter as much, and the difference between 6 AWG and 12 AWG.

A worked example.

Frequently asked questions.

References& sources.

1. [1]National Fire Protection Association, NFPA 70 — National Electrical Code, 2023 edition. Chapter 9 Table 8 (Conductor Properties — circular-mil areas and DC resistance at 75 °C), Table 310.16 (allowable ampacities, used here as a floor), and 210.19(A) Informational Note No. 4 (the 3% branch-circuit recommendation, which is guidance and not enforceable code). The code text is gated behind NFPA's free-access registration and was not opened directly; the values used here were transcribed from reproductions and cross-checked. Retrieved 2026-07-29.
2. [2]American Boat and Yacht Council, Standard E-11, "AC and DC Electrical Systems on Boats", excerpts published by Paneltronics. Read directly from the PDF on 2026-07-29. E-11.14.2.6 requires a maximum 3 percent drop on panelboard and switchboard main feeders, bilge blowers, electronic equipment and navigation lights, and 10 percent on lighting and other circuits where drop is not critical; E-11.14.2.7 sizes conductors from Table VI and checks them against Tables IX and X, requiring the larger conductor where the two conflict. Cited as the scope boundary for this page: marine wiring must be sized from ABYC's tables, not from the NEC-based method used here. The tables also print their length column as source-to-device-and-back, which is the origin of the one-way/round-trip confusion this page warns about.
3. [3]Mike Holt Enterprises, "Voltage Drop Calculations". Retrieved 2026-07-29. Source of the K constants used here — 12.9 ohm-circular-mils per foot for copper and 21.2 for aluminium, defined as the DC resistance of a 1,000-circular-mil conductor 1,000 ft long at 75 °C and derived from NEC Chapter 9 Table 8 — and of the VD = 2·K·I·L/CM form. The constants were checked arithmetically against Chapter 9 Table 8: 12.9 ÷ 105,600 circular mils gives 0.1222 Ω per 1,000 ft against the table's 0.122 for 1/0 copper, and 21.2 ÷ 211,600 gives 0.1002 against 0.100 for 4/0 aluminium.
4. [4]NECA/IBEW Electricians, reproduction of NEC Table 310.15(B)(16) (renumbered Table 310.16 from the 2020 edition onward), "Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts, 60 °C Through 90 °C". Read directly from the PDF on 2026-07-29. Source of the ampacity floor and of the circular-mil areas, shared with the AC wire size calculator so the two pages can never disagree about a table value.