What Is Voltage Drop?
Learn how the formula works, what causes voltage drop, and when to use a calculator for harness wire sizing and long-run circuits.
What is Voltage Drop?
Voltage drop is the decrease in electrical voltage that occurs as current flows through the resistance of a conductor (wire). When electrons move through a wire, they encounter resistance, which converts some electrical energy into heat. This lost energy manifests as a reduction in voltage between the power source and the load.
In simple terms: if your power supply outputs 12V, but the load only receives 11.5V, the voltage drop is 0.5V. This 0.5V is lost as heat in the wiring. Every electrical circuit experiences some voltage drop—the goal is to keep it within acceptable limits.
Why Voltage Drop Matters
- • Lights dim or flicker at the end of long runs
- • Motors run slower and may overheat
- • Electronic equipment malfunctions or won't start
- • Energy is wasted as heat in the wiring
- • Can indicate undersized wiring (safety hazard)
Key Factors
- • Current (Amps): More current = more drop
- • Wire Length: Longer run = more drop
- • Wire Gauge: Thinner wire = more drop
- • Temperature: Higher temp = more resistance
- • Material: Aluminum has more resistance than copper
Voltage Drop Calculator
Skip the math—use our free online calculator to instantly check if your wire gauge is adequate for your circuit.
Supports DC and single-phase AC • AWG and metric wire sizes • Copper and aluminum
How to Calculate Voltage Drop
Calculating voltage drop requires knowing your circuit's current, wire length, and wire resistance. Here's a step-by-step guide with formulas for both DC and AC circuits.
Voltage Drop Formula (DC Circuits)
(for 2-wire DC or single-phase AC circuits)
Voltage Drop Equation (AC Circuits)
(for AC circuits where impedance matters)
In AC circuits, impedance (Z) replaces simple resistance. Impedance includes both resistance (R) and reactance (X): Z = √(R² + X²). For conductors smaller than AWG 2 and typical 60Hz installations, reactance is negligible and the DC formula works fine.
Copper Wire Resistance Reference Table
| AWG | mm² | Ohms/1000ft | Ohms/ft |
|---|---|---|---|
| 10 | 5.26 | 1.018 | 0.001018 |
| 12 | 3.31 | 1.619 | 0.001619 |
| 14 | 2.08 | 2.575 | 0.002575 |
| 16 | 1.31 | 4.094 | 0.004094 |
| 18 | 0.823 | 6.51 | 0.006510 |
| 20 | 0.518 | 10.35 | 0.010350 |
| 22 | 0.326 | 16.46 | 0.016460 |
Values for copper at 20°C (68°F). For 75°C operating temperature, multiply by 1.22. For aluminum, multiply by 1.6.
Step-by-Step Voltage Drop Calculation Example
Scenario: 12V automotive circuit, 10A load, 25 feet one-way distance, AWG 14 wire.
Real-World Examples
| System | Length | Current | AWG | Drop | Status |
|---|---|---|---|---|---|
| 12V Automotive | 10 ft | 10A | 14 | 0.26V (2.2%) | ✓ OK |
| 12V Automotive | 25 ft | 10A | 14 | 0.65V (5.4%) | ✗ Too high |
| 120V AC | 50 ft | 15A | 14 | 3.1V (2.6%) | ✓ OK |
| 24V DC | 100 ft | 5A | 18 | 3.2V (13%) | ✗ Too high |
NEC Voltage Drop Requirements
The National Electrical Code (NEC) provides voltage drop recommendations to ensure efficient and safe electrical installations. While these are technically informational notes (not mandatory requirements), they represent best practices that inspectors and engineers follow.
NEC Voltage Drop Limits by Application
| Application | NEC Reference | Limit | Notes |
|---|---|---|---|
| Branch circuits (general) | NEC 210.19(A) Informational Note No. 4 | 3% | Recommended, not mandatory |
| Feeder circuits | NEC 215.2(A) Informational Note No. 2 | 2% | Recommended for efficiency |
| Combined (feeder + branch) | NEC 210.19(A) | 5% | Total system maximum |
| Sensitive equipment | NEC 647.4(D) | 1.5% | Audio/recording studios |
| Fire pump circuits | NEC 695.7 | 15% | At motor terminals during starting |
NEC 210.19(A) - Branch Circuits
Informational Note No. 4 states that conductors should be sized to prevent voltage drop exceeding 3% at the farthest outlet for reasonable efficiency of operation.
NEC 215.2(A) - Feeder Circuits
Informational Note No. 2 recommends that feeder conductors be sized so voltage drop does not exceed 2%, allowing room for branch circuit drop.
Understanding the 5% Total Limit
The combined voltage drop from service entrance to the farthest outlet should not exceed 5%.
Automotive Standards
- • 0.5V max for critical circuits (ignition, fuel)
- • 3-4% typical guideline for 12V systems
- • 10% may be acceptable for heaters, fans
- • SAE J1128 specifies wire sizing standards
Low Voltage Lighting
- • 5% recommended for consistent brightness
- • 10% maximum before visible dimming
- • LED drivers often have wider input tolerance
- • Use larger gauge for longer landscape runs
Wire Sizing for Voltage Drop
Selecting the correct wire gauge involves balancing ampacity (current carrying capacity) with acceptable voltage drop. For long runs or low-voltage systems, voltage drop often requires upsizing beyond the minimum ampacity requirement.
When to Upsize Wire for Voltage Drop
- • Distance exceeds 50 feet (15m) for branch circuits
- • Low voltage systems (12V, 24V, 48V)
- • Motor loads (starting current is 6-8x running current)
- • Sensitive electronic equipment
- • Outdoor runs in high ambient temperatures
- • Circuits near maximum ampacity rating
- • When calculated drop exceeds 3%
- • Critical loads requiring voltage stability
AWG Selection Chart (12V DC, 3% Max Drop)
| Current | 10 ft | 25 ft | 50 ft | 75 ft | 100 ft |
|---|---|---|---|---|---|
| 5A | 18 | 16 | 14 | 12 | 10 |
| 10A | 16 | 14 | 12 | 10 | 8 |
| 15A | 14 | 12 | 10 | 8 | 6 |
| 20A | 14 | 10 | 8 | 6 | 4 |
| 30A | 12 | 8 | 6 | 4 | 2 |
| 50A | 10 | 6 | 4 | 2 | 1/0 |
AWG values shown are for copper wire at 20°C with 3% maximum voltage drop. For 24V systems, you can use one gauge smaller. For 120V AC, you can use 3-4 gauges smaller (but verify ampacity).
Upsizing Rules of Thumb
- • Double the distance = go up 3 AWG sizes
- • Double the current = go up 3 AWG sizes
- • Each 3 AWG drop = halves the resistance
- • Aluminum vs copper = go up 2 AWG sizes
Cost vs Performance
- • Larger wire costs more upfront
- • Lower drop = less wasted energy (ongoing savings)
- • Better performance and equipment longevity
- • Often cheaper than troubleshooting dim lights later
A Practical Voltage Drop Workflow
1. Start With the Explanation
Use this guide to understand the formulas, the NEC recommendations, and when voltage drop becomes the real sizing limit instead of pure ampacity.
2. Run the Exact Numbers
Move from theory to your actual one-way length, current, conductor material, and allowable percentage drop using the calculator.
Voltage Drop Calculator
Check the real circuit before locking the wire gauge.
AWG Converter
Translate AWG and metric sizes when standards or suppliers differ.
Bundle Diameter
Check whether the upsized wire still fits the packaging and routing envelope.
Release Workflow
See how sizing, review, and release documentation connect in the full design flow.
Frequently Asked Questions
Related Glossary Terms
Calculate Now
Check your wire sizing with our free Voltage Drop Calculator.