Voltage Drop Calculator
DC voltage drop for solar wiring by AWG and length.
Voltage Drop Calculator
Voltage drop measures how much electrical pressure is lost between the source and the load along a cable run. In solar and off-grid DC wiring, excessive drop means batteries and inverters see less voltage than you expect—chargers underperform, loads brown out, and efficiency falls.
This calculator estimates DC drop for copper wire using amp draw, system voltage, one-way cable length, and AWG size. It doubles the one-way length to account for the round trip (positive and negative conductors), then reports drop in volts, percent, and voltage at the load.
Use it to check an existing run or compare wire sizes before you buy cable. For automatic AWG selection against ampacity and a max drop target, use the wire size calculator. Many installers aim for 3% or less on critical DC runs.
How this calculator works
- Enter load current in amps—the steady DC draw on the circuit (array to controller, controller to battery, or battery to inverter).
- Enter system voltage at the source (e.g. 12, 24, or 48 V battery or array voltage).
- Enter one-way cable length in feet from source to load (not the round-trip total—the tool doubles it).
- Select wire AWG. Resistance values use standard DC copper ohms per 1,000 ft at about 20°C.
- Voltage drop = 2 × amps × resistance per foot × one-way feet. Drop % = (drop V ÷ system V) × 100.
Core voltage drop relationships
Drop V = 2 × I × R/ft × length (one-way) | Drop % = (Drop V ÷ system V) × 100 | V at load = system V − Drop V
Current through resistance creates a voltage loss. Because DC circuits need both outbound and return conductors, the effective length is twice the one-way distance. Percent drop normalizes the loss against system voltage—3% on a 12 V circuit is only 0.36 V, but 3% on 48 V is 1.44 V; the percent target is often similar while absolute volts differ.
Formulas used by this tool
- Uses DC resistance per 1000 ft for the selected AWG.
- Voltage drop = 2 × amps × resistance × one-way length (round trip).
- Drop % = (drop volts ÷ system volts) × 100.
Worked example: 10 A on 12 V, 25 ft
A 10 A load on a 12 V system uses 10 AWG copper with a 25 ft one-way run from battery to load.
- 10 AWG resistance ≈ 1.21 Ω per 1,000 ft → 0.00121 Ω per foot per conductor.
- Drop V = 2 × 10 A × 0.00121 × 25 ft ≈ 0.61 V round trip.
- Drop % = 0.61 ÷ 12 × 100 ≈ 5.1%—above a common 3% target; consider 8 AWG or shorter run.
- Voltage at load ≈ 12 − 0.61 = 11.39 V under full 10 A draw.
Practical tips
- Measure one-way length along the actual cable path, not straight-line distance.
- Higher system voltage (24 V or 48 V) reduces amps for the same power and usually reduces drop percentage.
- Terminal crimps, fuses, and undersized connectors add resistance not modeled here—keep runs neat and torqued.
- Solar arrays to MPPT runs often target ≤3% drop; some designers use ≤2% on long 12 V runs.
- If drop % is high, upsize AWG or move equipment closer before accepting power loss.
Frequently asked questions
Why multiply cable length by 2?
DC current travels out on the positive conductor and back on the negative. Resistance applies to both legs, so the calculator uses round-trip length: 2 × one-way feet. Enter only the distance from source to load once.
What voltage drop is acceptable?
Many solar and off-grid guides use 3% maximum on critical DC circuits. On 12 V systems some designers use 2% because absolute volts are small. National electrical codes for AC premises wiring differ—this tool is for DC planning; confirm local codes for installed systems.
Which AWG sizes are supported?
The calculator includes common copper AWG sizes from 14 through 4 with tabulated DC resistance per 1,000 ft. If you need larger cable, results extrapolate conceptually—consider manufacturer data for very high current installs.
Does this work for AC household wiring?
The resistance table and round-trip model target DC copper runs typical in solar, RV, and off-grid battery wiring. AC branch circuits use different code rules, power factor, and impedance—use a qualified method or electrician for premises AC design.
How do I get amps for the calculation?
Use measured draw, inverter input specs, or watts ÷ volts. For solar array to controller, use Isc or operating current from the string datasheet. Size for maximum expected current, not average load, so drop stays acceptable at peak draw.
What if drop is too high?
Upsize to a lower AWG number (heavier wire), shorten the run, raise system voltage, or split loads across shorter cables. The wire size calculator can recommend the smallest AWG that meets both ampacity and your max drop percent.
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