24V Wire Size Calculator
Estimate the conductor cross-sectional area and equivalent round-wire diameter needed to stay within a selected voltage-drop limit.
System and cable properties
Recommended wire size per cable
Required area 22.26 square millimeters; equivalent round-wire diameter 5.32 millimeters.
How to use this 24V wire size calculator
What this calculator does
This calculator estimates the minimum conductor cross-sectional area needed to keep steady-state voltage drop within a limit you choose, then converts that area to the diameter of an equivalent solid round conductor. It supports DC or AC single-phase circuits and AC three-phase circuits, copper or aluminum conductors, percent or volt drop limits, feet or meters, and Celsius or Fahrenheit. The result is a voltage-drop sizing estimate, not a complete code-compliance or ampacity determination. Real installations can also be governed by insulation temperature rating, bundling, ambient conditions, terminations, fault protection, local electrical rules, and manufacturer requirements.
Use it when planning a battery-fed 24 V load, checking a long low-voltage cable run, comparing copper with aluminum, or estimating the conductor area needed before selecting a standard commercial cable. Voltage drop matters because excessive conductor resistance reduces the voltage available at the load and wastes energy as heat; the U.S. Department of Energy discusses the practical effects of excessive distribution-system voltage drop.
How to calculate
- The calculator opens with a complete demonstration: DC / AC single-phase, 24 V source, 3% allowable drop, copper, 56 A, 25 ft one-way distance, and 50 °C maximum wire temperature. The results and a validated example XLSX are available immediately.
- Replace the demonstration values with your circuit values. If you change a unit selector, the current value is converted rather than merely relabeled. Results update live; no Calculate button is needed.
- Read Wire cross-sectional area (A) first. Then use Wire diameter (d) as the equivalent diameter implied by that area. Select an actual cable whose conductor area is at least as large, subject to applicable ampacity and installation rules.
- Choose Download Excel to export the current validated inputs and results to a real XLSX workbook. Reset deliberately clears the demonstration data. After Reset, export is disabled until every required field contains a complete valid state again.
Input guide
Electrical system is required and accepts either “DC / AC single-phase” or “AC three-phase.” The single-phase/DC model uses a factor of 2 because current travels to the load and back; the three-phase model uses √3 for the per-conductor area. A common mistake is selecting three-phase for an ordinary two-wire 24 V DC system.
Source voltage is required in volts and must be positive; 24 is the startup example. A higher source voltage generally allows a smaller area for the same percentage drop because the permitted drop in volts increases. When source voltage is above 50 V and the one-way distance is below 16 m, this calculator applies a 16 m minimum design distance to preserve the captured sizing behavior.
Allowable voltage drop is required. Enter a positive percentage below 100% or a positive voltage less than the source voltage; 3% is the example. Smaller allowable drop means a larger conductor. The unit selector converts between percent and volts using the current source voltage. Do not confuse source voltage with voltage drop: at 24 V and 3%, the permitted drop is 0.72 V.
Conductor material is required. Choose Copper or Aluminum. Copper starts from 1.68 × 10⁻⁸ Ω·m at 20 °C; aluminum uses 2.82 × 10⁻⁸ Ω·m at 20 °C. Higher resistivity increases required area. The temperature adjustment is linear around the ordinary wiring range; NIST provides deeper reference information on temperature-dependent copper resistivity.
Current (I) is required in amperes and must be greater than zero; 56 A is the startup example. Required area changes directly with current, so doubling current doubles the calculated area when all other values are unchanged. Use the circuit current relevant to the design case rather than an unrelated breaker or battery label.
One-way distance (D) is required and accepts feet or meters; 25 ft is the example. Enter the source-to-load distance once. The single-phase/DC formula already accounts for the return path, so entering round-trip distance would double-count cable length. Longer runs require more area in direct proportion to the effective distance.
Maximum wire temperature is required and accepts °C or °F; 50 °C is the example. The calculator converts to Celsius and adjusts material resistivity from the 20 °C reference. Higher temperature raises resistivity for copper and aluminum, which increases the calculated conductor area. Avoid substituting ambient air temperature when the expected conductor temperature is materially different.
Output guide
The summary pills labeled System, Voltage drop, and Material mirror the validated current state so you can confirm the active system, normalized percentage drop, and conductor material at a glance. They do not add separate calculations.
Wire cross-sectional area (A) is the primary estimate in mm². It is driven by system type, current, temperature-adjusted resistivity, one-way distance, and allowable voltage drop. A higher value means more conductor metal is needed to meet the same drop criterion. The result is a calculated minimum, so practical selection normally moves up to an available standard cable size rather than down.
Wire diameter (d) is shown in millimeters and is the diameter of a solid circular conductor having the calculated area, using d = √(4A/π). It is an exact geometric conversion of the area result, not a promise that a stranded, insulated, or commercially labeled cable has the same outside diameter. The nearby voltage-drop value is the actual ΔV used in the equation and is shown only to make the percentage conversion transparent.
Worked example
With the startup values, 3% of 24 V is 0.72 V. Copper at 50 °C is modeled at about 1.8781 × 10⁻⁸ Ω·m, and 25 ft converts to 7.62 m. For DC / single-phase, A = 2 × 56 × 1.8781 × 10⁻⁸ × 7.62 ÷ 0.72 = 2.2261 × 10⁻⁵ m², or 22.26 mm². The equivalent solid-round diameter is √(4A/π) = 5.32 mm. Those are the exact first-open displayed results and the same values stored in the startup workbook.
Learn more
For the broader engineering relationship among current, conductor resistance, length, cross-section, and voltage drop, see the Electrical Installation Guide's steady-load voltage-drop equations. Use the calculator as an analytical sizing aid and verify the chosen commercial conductor against the rules and ratings that govern the actual installation.
How the model works
Electrical resistance grows with conductor resistivity and length and falls as cross-sectional area increases. Rearranging the resistance and voltage-drop relationship gives the area directly. For DC and single-phase AC, the modeled circuit length is twice the one-way run, so A = 2IρL/ΔV. For three-phase AC, the per-conductor relationship is A = √3IρL/ΔV. These equations are useful for voltage-drop planning because they make sensitivity intuitive: more current, greater distance, or higher resistivity pushes the required area upward, while allowing a larger voltage drop pushes it downward.
Temperature matters because conductor resistivity changes with temperature. The calculator uses a linear temperature coefficient around a 20 °C reference: ρ(T) = ρ20[1 + α(T – 20)]. That is a practical engineering approximation for ordinary conductor temperatures, not a universal material law across all temperatures. For unusually hot, cryogenic, specialized-alloy, or precision applications, use manufacturer or laboratory resistivity data instead.
Common mistakes and interpretation
- Entering round-trip distance in a DC or single-phase calculation when the model already doubles one-way distance.
- Typing “3” volts when the intended limit is 3%; at 24 V those are very different constraints.
- Choosing a cable solely from the calculated geometric area without checking ampacity, insulation class, local rules, or equipment instructions.
- Ignoring temperature: resistance rises with temperature for copper and aluminum, so a hot conductor can require more area for the same drop target.
- Rounding downward to the nearest marketed cable size. Treat the calculated area as a minimum for this specific voltage-drop model.
Educational engineering estimate only. Electrical installations can present shock, fire, and equipment-damage hazards. Have a qualified professional verify conductor selection and applicable code requirements before installation.