Voltage Drop & Wire Size Calculator

Use this free voltage drop calculator to find the right wire size for solar panels, battery banks, inverters, and AC circuits. Choose a mode: find the smallest wire that works, check the voltage drop of a wire you already have, or find the longest run a wire can handle. It covers 12V, 24V, and 48V DC systems, plus 120V to 480V AC, in copper or aluminum from 14 AWG to 500 kcmil, and it checks ampacity at the same time so you never size a wire for voltage drop alone.

Last updated October 28, 2026. General information for planning. Not a code compliance check, electrical design, or inspection.

How to use this voltage drop and wire size calculator

1. Pick what you want to find. Use “Wire size” to get a recommendation, “Voltage drop” to test a wire you already own, or “Maximum distance” to see how far a wire can run.
2. Choose your system voltage. 12V, 24V, and 48V DC cover most off-grid and RV battery systems. Pick “Other DC voltage” for a solar string, or an AC option for house circuits.
3. Enter the load and the one-way distance. For a solar array, choose “PV array” and enter the module short-circuit current (Isc) times the number of parallel strings.
4. Set the voltage drop you can accept. 3% is a common planning figure. Solar and battery runs usually aim lower.
5. Read both checks. The result must pass voltage drop and ampacity. The table lists every wire size so you can see the trade-offs.

What makes this wire size calculator different

• It checks ampacity and voltage drop together and tells you which one decides the size.
• It has a solar mode that applies the 156% factor used for PV source circuits (125% x 125%).
• It works for DC and AC, copper and aluminum, with an option to change the resistance temperature.
• It shows every wire size side by side, including power lost as heat, so you can decide whether a bigger wire is worth it.
• It states its formulas, tables, and limits below instead of hiding them.

Voltage drop formula and worked examples

Voltage drop is the voltage lost in the wire itself. Current flows out to the load and back, so a two-wire circuit counts the distance twice. The calculator uses the DC resistance values in NEC Chapter 9, Table 8, corrected for temperature.

DC and single-phase AC: VD = 2 x I x R x L / 1000
Three-phase AC: VD = 1.732 x I x R x L / 1000
Percent drop = VD / system voltage x 100

Here I is the current in amps, R is the wire resistance in ohms per 1000 feet, and L is the one-way length in feet.

Example 1: 12V battery cable, 30 A, 15 feet

A continuous 30 A load needs ampacity for 37.5 A (30 A x 125%). Copper 8 AWG has enough ampacity at the 75 C column (50 A), but over 15 feet it drops about 5.8% of 12 volts, far above a 3% limit. Copper 4 AWG brings it to 0.277 V, or 2.31%. Tightening the limit to 2% moves the answer to 3 AWG (1.84%). Low-voltage circuits need thick wire because a small voltage loss is a large share of 12 volts.

Example 2: 240V single-phase circuit, 40 A, 100 feet

For a 40 A continuous load, copper 8 AWG gives 6.22 V of drop, or 2.59% of 240 volts, and has just enough ampacity for the 125% requirement (50 A).

Example 3: solar PV string, Isc 11.5 A, 200 V, 100 feet

PV source circuits are sized at 156% of Isc, which is about 18 A here. Ampacity alone would allow a very small wire, but the voltage drop target decides. At a 2% limit, copper 14 AWG drops 3.6% and 12 AWG drops 2.28%, so the answer is copper 10 AWG, with 2.85 V of drop (1.43%).

Ampacity can also decide. At 48V, a 100 A load over 10 feet would pass a 3% drop limit with 6 AWG, but the 125% continuous-load rule requires 1 AWG for ampacity. The calculator reports which factor governs.

Recommended voltage drop limits

Wire gauge amp chart: AWG ampacity and resistance

This chart shows the values the calculator uses: DC resistance in ohms per 1000 feet (NEC Chapter 9, Table 8) and ampacity for copper and aluminum at 60, 75, and 90 C (NEC Table 310.16). Ampacity values apply to not more than three current-carrying conductors in a raceway or cable at 30 C ambient. Hot locations and bundling reduce them.

Small conductors also have overcurrent protection limits (NEC 240.4(D)) that can be lower than their table ampacity. Choose fuses and breakers separately.

Solar and battery wiring: what to check beyond voltage drop

• Use the right current. For PV circuits, use Isc (not the operating current) times parallel strings, then apply the 156% factor. The calculator does this when you choose the PV circuit type.
• Keep battery cables short and thick. At 12V or 24V, a few extra feet can double the percentage lost. Place the battery, inverter, and charge controller close together where you can.
• Allow for heat and bundling. Rooftop conduit and tightly bundled cables carry less current safely. Use the ampacity factor in the advanced options to reduce the allowed current.
• Match terminals and connectors. Lugs, bus bars, and connectors (including MC4) have their own current and temperature ratings. The wire is only as good as the weakest part of the circuit.
• Protect every circuit. This tool does not choose fuses or breakers. Overcurrent protection, disconnects, grounding, and cable type must follow your code and the equipment instructions.

Common mistakes when sizing wire

• Entering the round-trip length. Enter the one-way distance.
• Sizing from an amp chart alone. A wire can be safe for the current and still lose too much voltage over distance.
• Mixing up DC and AC voltage, or using the wrong voltage for a solar string.
• Ignoring continuous load rules. A load that runs 3 hours or more needs 125% ampacity.
• Assuming aluminum matches copper. Aluminum has higher resistance and needs a larger size and terminals rated for it.

Voltage drop and wire size questions answered

What size wire do I need for a 12V solar or battery circuit?

It depends on the current and the one-way distance. For 30 A over 15 feet with a 3% limit, copper 4 AWG works (2.31% drop). With a 2% limit, it becomes 3 AWG. Enter your own numbers in the calculator above for your circuit.

How do you calculate voltage drop?

Multiply 2 x current x wire resistance (ohms per 1000 feet) x one-way length (feet), then divide by 1000. For three-phase circuits, replace the 2 with 1.732. Divide the result by the system voltage to get the percentage.

What is an acceptable voltage drop for solar?

There is no single legal number. NEC informational notes suggest about 3% on a branch circuit and 5% overall for feeder plus branch. Many solar designers aim for 1% to 2% on DC circuits because the lost energy is lost production every day. Treat these as design targets and check your local code.

Is the distance one way or round trip?

Enter the one-way distance. The formula already doubles it to account for the return conductor in a two-wire circuit.

Should I use copper or aluminum wire?

Aluminum has higher resistance than copper. For example, 4/0 AWG is 0.0608 ohms per 1000 feet in copper and 0.100 in aluminum, so aluminum usually needs a larger size. Terminals and lugs must also be rated for aluminum.

Why do different voltage drop calculators give different answers?

Differences usually come from the resistance values. This calculator uses NEC Table 8 values at 75 C for stranded wire. A calculator based on bare copper at 20 C will show a lower drop. For 8 AWG copper carrying 1.2 A over 300 feet, the drop is 0.56 V at 75 C and 0.46 V at 20 C. Choose 20 C in the advanced options to compare.

What is the 156% rule for solar wire?

NEC 690.8 sets maximum PV circuit current at 125% of the module short-circuit current, and conductors must then be sized at 125% of that value. Together that is about 156% of Isc. The calculator applies it when you choose the PV circuit type. Confirm with the NEC edition your area uses.

Can I use this for 120V and 240V house wiring?

Yes, for a quick resistive estimate. For AC it ignores reactance and power factor, which matter mostly on large conductors over long runs. For feeders and critical work, use NEC Chapter 9 Table 9 impedance values or ask an electrician.

Does this calculator replace an electrician or a permit?

No. It helps you plan and compare options. Code compliance, overcurrent protection, grounding, and inspection are your responsibility and the local authority’s. When in doubt, hire a licensed electrician.

Sources and methodology

The calculator runs entirely in your browser using fixed tables and the formulas shown above. NEC tables are published by NFPA and not freely available, so the values here were cross-checked against educational references that reproduce them. Verify against your edition of the NEC before relying on any value. Excerpts of the sources below were reviewed on October 28, 2026.

• Conductor resistance (NEC Chapter 9, Table 8) and temperature correction: Michigan State University Biosystems and Agricultural Engineering conductor table (educational reference); Mike Holt Enterprises voltage-drop sample chapter (code training publisher).
• PV circuit current and the 156% factor (NEC 690.8): EC&M magazine (trade publication); Penn State e-Education course notes (educational reference).
• Ampacity (NEC Table 310.16): values are transcribed from the table as reproduced in several secondary references and spot-checked, not read from the NEC itself.

Limits: AC results use DC resistance and ignore reactance and power factor. Ampacity is for not more than three current-carrying conductors at 30 C ambient unless you enter a factor. The tool does not size fuses, breakers, conduit, or grounding. Distances are limited to 10,000 ft and currents to 5,000 A. Verify all results with the code in force where you live.

This tool provides general information. It is not legal advice, an engineering or electrical design, a code inspection, or a contractor quote.