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

Background.

On a 12-volt system, the usual 3% voltage-drop target leaves you 0.36 V to lose, for the whole trip from the battery to the load and back. On a 240 V circuit the same 3% is 7.2 V. That twentyfold difference is why low-voltage DC cable looks oversized to anyone used to house wiring: a 20 A load fifteen feet from a 12 V battery needs 6 AWG copper to stay inside the budget, while the same current over the same distance from a 120 V DC bank gets by on 14 AWG.

Choose the system voltage, give the load in amps or watts, measure the run one-way and set the drop you will accept. The calculator works out the conductor area the drop budget demands, finds the smallest standard size that meets it, checks the current against the NEC Table 310.16 ampacity for your insulation rating, and recommends whichever size is larger. It also shows the drop you will actually get on that size and the voltage left at the load. If the cable comes out uncomfortably heavy, try a higher system voltage: for the same power, doubling it cuts the copper needed to a quarter.

The method is NEC-based, which sets its limits. Boats follow ABYC E-11 and its own tables, solar array circuits carry extra multipliers under NEC 690.8, and wiring inside vehicles follows SAE practice. A battery bank can also push thousands of amperes into a short with nothing upstream to trip, so the fuse at the battery end and the terminations matter as much as the gauge; have a licensed electrician or qualified installer check the design before anything is connected.

What is dc wire size calculator?

DC wire sizing means choosing the conductor between a direct-current source and its load so that, at the expected current, the voltage lost in the wire stays within an agreed share of the supply and the wire does not overheat. On DC the loss is plain resistance, with no reactance or power factor, so Ohm's law describes it exactly once the conductor temperature is fixed. Resistance rises with length and falls with cross-sectional area, which North American practice measures in circular mils, the area of a circle one thousandth of an inch across. AWG sizes are steps on that scale: 8 AWG is 16,510 circular mils, 6 AWG is 26,240 and 4 AWG is 41,740. Both conductors of a DC circuit carry the full current, so the length that counts is out and back.

How to use this calculator.

1. 01Choose the DC system voltage. If you have not settled on one yet, run the same load at 12 V and again at 24 V before buying cable.
2. 02Say whether you know the load in amps or watts and fill in that field; watts are divided by the system voltage. Use the continuous rating, not surge. A 2,000 W inverter on 12 V draws 166.67 A before its own losses are counted, and more once they are.
3. 03Enter the one-way length from source to load, measured along the route the cable will actually follow; the calculator doubles it for the return conductor.
4. 04Pick copper unless you have a reason not to. Aluminium needs about 64% more area for the same drop and terminations listed for it.
5. 05Set the drop budget: 3% is the normal design target; ABYC allows 10% for lighting and other circuits where drop is not critical.
6. 06Choose the insulation rating, which only moves the ampacity check. Battery and welding cable is often rated 105 °C but is not NEC building wire, so the 90 °C column stands in for it here.

The formula.

The area needed is CM = 2 × K × I × L ÷ (V × d). I is the current in amperes, L the one-way length in feet, V the nominal system voltage, d the drop budget as a fraction, and K the resistance of a conductor one circular mil in area and one foot long: 12.9 ohms for copper and 21.2 for aluminium, both at 75 °C and derived from NEC Chapter 9 Table 8. The 2 accounts for the return conductor, which loses as much voltage as the outgoing one. As a cross-check, 12.9 ÷ 105,600 circular mils gives 0.1222 Ω per 1,000 ft, against the table's 0.122 for 1/0 copper.

The smallest standard size with at least that area is the voltage-drop answer. The ampacity check is a separate lookup: the smallest size whose Table 310.16 rating, in the column for your insulation, reaches the circuit current. It uses the table's own conditions of 30 °C ambient and no more than three current-carrying conductors together, with no correction for heat or bundling. The recommendation is the larger of the two sizes, and the drop on it is VD = 2 × K × I × L ÷ CM.

For a fixed power P, the current is P ÷ V and the allowed drop is V × d, so the required area varies as 1 ÷ V²: 24 V needs a quarter of the copper of 12 V, and 48 V a sixteenth. Aluminium's larger K is where its 64% area penalty comes from, since 21.2 ÷ 12.9 ≈ 1.64.

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.