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DC Voltage Drop Calculator

DC voltage drop calculator for 12V, 24V, and 48V solar, battery, RV, marine, and low-voltage DC runs. For a 12V circuit at 20A over 10 ft one-way on #8 copper, the round-trip drop is about 0.31V, or 2.6%. Use the result to compare AWG sizes, then continue to solar combiner, ampacity, terminal-temperature, and equipment-listing checks when the run is part of a PV or battery system.

Updated June 21, 2026

For a PV cable screen, use module Isc or the documented DC current basis, one-way route length, conductor material, and a voltage-drop target, then carry the result into ampacity, terminal, derating, listed-equipment, utility, and AHJ review.

DC drop = I x R x 2 x one-way length / 1000 | PV handoff = DC drop + ampacity + terminal temperature + derating + equipment listing.

Enter DC voltage, current, one-way distance, conductor material, and selected size below for a PV conductor voltage-drop screen

Calculator Inputs

Quick Presets

DC amps being drawn

Distance from source to load (feet)

Calculation Results

Enter values above to see calculation results

Field kit

Tools for DC voltage checks

Use the voltage-drop result to plan a measurement point, then compare tools for checking DC voltage and current.

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Calculation history

Example Calculations

12V solar array to charge controller

Use the DC voltage drop workflow to compare #6 and #4 copper for a 25 ft one-way PV source-circuit planning run.

Inputs
  • System Voltage: 12
  • Current: 14
  • One-Way Distance: 25
  • Conductor Material: Copper
  • Target Voltage Drop Percent: 2
  • Candidate Wire Sizes: #6 AWG,#4 AWG

How to Use

DC Voltage Drop: Why It's 10× More Critical Than AC

In AC systems at 120V or 240V, a 3% voltage drop leaves 116.4V or 232.8V — still perfectly usable. In a 12V DC system, 3% is only 0.36V, leaving 11.64V. Many inverters shut down below 11.5V, and batteries charged below 14.0V never reach full charge. Getting DC wire sizing right is critical for system reliability.

DC Voltage Drop Formula

V_drop = I × R × 2 × L / 1000

Where:
 I = current in amps
 R = conductor resistance in Ω/1000ft (from NEC Chapter 9 Table 8)
 L = one-way distance in feet
 2 = round-trip factor (both positive and negative conductors)

For chassis/hull ground systems: Use 1× instead of 2× for the length factor, but add chassis resistance (typically 0.001–0.01Ω depending on connections and vehicle).

Voltage Sensitivity by System Voltage

System 2% Drop 3% Drop 5% Drop Risk at 5%
12V DC0.24V0.36V0.60VInverter fault, battery undercharge
24V DC0.48V0.72V1.20VReduced charging efficiency
48V DC0.96V1.44V2.40VModerate — more tolerant
120V AC2.40V3.60V6.00VEquipment operates normally
300–400V EV6–8V9–12V15–20VMinimal concern

Wire Sizing Quick Reference: Maximum Distance at 3% Drop

Wire AWG Ω/1000ft (Cu) 12V @ 10A 12V @ 20A 24V @ 20A 48V @ 30A
#14 (2.08mm²)3.145.7 ft2.9 ft5.7 ft7.6 ft
#12 (3.31mm²)1.989.1 ft4.5 ft9.1 ft12.1 ft
#10 (5.26mm²)1.2414.5 ft7.3 ft14.5 ft19.4 ft
#8 (8.37mm²)0.77823.1 ft11.6 ft23.1 ft30.8 ft
#6 (13.3mm²)0.49136.7 ft18.3 ft36.7 ft48.9 ft
#4 (21.2mm²)0.30858.4 ft29.2 ft58.4 ft77.9 ft
#2 (33.6mm²)0.19492.8 ft46.4 ft92.8 ft123.7 ft
#1/0 (53.5mm²)0.122147.5 ft73.8 ft147.5 ft196.7 ft

Worked Example: Solar Panel to Charge Controller

A 12V/400W solar array (Isc = 11.2A) is located 25 feet from the charge controller. For a planning screen, document the module Isc and the PV source-circuit current basis, then open the Solar Combiner Sizing Calculator and Solar Cable Voltage Drop Chart before carrying the result into ampacity, terminal, rooftop, conduit, equipment-listing, utility, and AHJ review.

  • Round-trip distance: 25 ft × 2 = 50 ft
  • Max voltage drop at 2%: 12V × 0.02 = 0.24V
  • Max resistance: 0.24V / 14.0A = 0.0171Ω
  • Cable resistance needed: 0.0171Ω / (50/1000) = 0.343 Ω/1000ft
  • Select wire: #6 AWG copper (0.491 Ω/1000ft) gives: 14.0 × 0.491 × 50/1000 = 0.344V (2.87%)
  • Better choice: #4 AWG copper (0.308 Ω/1000ft) gives: 14.0 × 0.308 × 50/1000 = 0.216V (1.8% — meets 2% target)

Key insight: Going from 12V to 24V halves the current, which halves the voltage drop and allows 2 AWG sizes smaller wire — significant cost savings on long runs.

ABYC Standards for Marine Wiring

Marine wiring follows ABYC E-11 with stricter requirements than land-based systems due to corrosion and safety concerns:

  • Critical circuits (bilge pumps, navigation): Maximum 3% voltage drop
  • Non-critical circuits (lighting, accessories): Maximum 10% voltage drop
  • Wire type: Tinned copper only (not bare copper) to prevent corrosion
  • Connections: Crimped with adhesive-lined heat shrink; no wire nuts allowed on boats
  • Overcurrent protection: Required within 7 inches of battery positive terminal

Common Applications

Solar PV wire sizing — panel to charge controller and charge controller to battery bank

RV/camper 12V distribution — size wires from battery to each circuit for reliable operation

Marine electrical systems — ABYC E-11 compliant wire sizing for boats and yachts

Battery bank interconnection — select welding cable size for battery-to-battery connections

Off-grid inverter wire sizing — calculate DC cable from battery bank to inverter input

EV charging infrastructure — size DC cables for high-current EV charging circuits

Automotive custom wiring — sound systems, winches, auxiliary lighting wire sizing

Telecommunications backup power — 48V DC distribution wire sizing for telecom shelters

Frequently Asked Questions

What is the maximum acceptable voltage drop for 12V DC systems?
For critical DC circuits such as inverters, charge controllers, navigation equipment, and PV source-circuit planning, many designers start with a 2% or 3% target and then adjust it to equipment instructions and project requirements. Voltage drop is a performance screen; final PV conductor choices still need ampacity, terminal temperature, derating, listed-equipment, adopted NEC, utility, and AHJ review.
How do I size wire for a solar panel to charge controller run?
Start with the module Isc and the PV source-circuit current basis documented from the module and combiner review. Apply the DC voltage drop formula: V_drop = I × R × 2L / 1000. For a 12V/400W array with Isc = 11.2A at 25 ft, a 2% planning target allows 0.24V of drop over the round-trip distance. The result should then be checked against the Solar Cable Voltage Drop Chart, ampacity, terminal temperature, rooftop or conduit derating, equipment listings, adopted NEC requirements, utility review, and AHJ expectations.
Why does doubling the system voltage reduce wire size requirements?
Power = Voltage × Current (P = V × I). For the same wattage, doubling voltage halves the current. Voltage drop is proportional to current (V_drop = I × R), so halving current halves the voltage drop. This means you can use wire with twice the resistance (2 AWG sizes smaller) and still meet the same percentage drop target. This is why 24V and 48V systems are preferred for larger solar arrays and long wire runs — a 48V system needs one-quarter the copper cross-section of a 12V system for the same power delivery. The wire cost savings often pay for the 48V charge controller premium.
Does temperature affect DC wire resistance and voltage drop?
Yes. Copper and aluminum resistance increase as conductor temperature rises, so hot rooftop conduit, attics, equipment areas, and battery compartments can raise voltage drop. For PV work, keep voltage-drop resistance correction separate from ampacity derating and terminal-temperature limits, then verify the actual conductor, cable, raceway, and equipment instructions.
How is DC voltage drop different from AC voltage drop calculation?
DC voltage drop uses only conductor resistance (R): V_drop = I × R × 2L. AC voltage drop also includes conductor reactance (X): V_drop = I × (R×cosθ + X×sinθ) × 2L for single-phase. In DC circuits there is no reactance, no power factor, and no skin effect (current distributes evenly through the conductor cross-section). DC resistance per foot is the same as AC resistance for small conductors, but for large conductors (#4/0 and above), AC resistance is higher due to skin effect. Another key difference: DC systems have no neutral conductor — the return path is either a dedicated negative conductor (factor of 2 in the formula) or the vehicle chassis/hull (factor of 1).

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