How to Calculate Voltage Drop in 12V and 24V Systems
You just installed a $400 LED light bar on your truck’s roof. You hit the switch and… it’s disappointingly dim. The wire gauge chart said 14 AWG was fine for 12 amps. What went wrong?
Voltage drop. It’s the most misunderstood concept in vehicle electrical systems, and it’s why your lights are dim, your winch pulls weak, and your fridge stops cooling even though everything is “wired correctly.”
What is Voltage Drop?
When electricity flows through a wire, the wire resists that flow. This resistance causes some voltage to be lost as heat before it reaches your device.
Think of it like water pressure in a hose. A short, thick hose delivers full pressure at the end. A long, thin hose? Reduced pressure. Same principle applies to electrical wires.
See It in Action First: The Light Bar Problem
Before we get into formulas, here’s what voltage drop looks like in real life.
Scenario:
- 150W LED light bar = 12.5A at 12V
- Mounted on roof, 20 feet from battery
- Using 12 AWG wire (seems reasonable for 12.5 amps, right?)
What happens:
- Total wire length: 20 ft to light bar + 20 ft back to battery = 40 ft
- 12 AWG wire has 0.00159 ohms resistance per foot
- Total resistance: 40 ft × 0.00159 = 0.0636 ohms
- Voltage drop: 12.5A × 0.0636Ω = 0.795V
- Percentage: 0.795V ÷ 12V = 6.6% voltage drop
Your light bar only gets 11.2 volts instead of 12. You’re losing about 7% of the brightness you paid for.
Use Wire Solved’s calculator to get the right wire gauge instantly.
The fix: Upgrade to 10 AWG wire and voltage drop falls to 4.2%. Your light bar performs like it should.
That’s voltage drop. Now let’s understand the math behind it.
The Math Behind Voltage Drop
Wire Solved’s calculator handles these calculations automatically, but understanding the formula helps you troubleshoot and verify results.
Voltage Drop = Current × Resistance × Length
Where:
- Current = Amps your device draws
- Resistance = Ohms per foot (specific to wire gauge)
- Length = TOTAL circuit length (power wire + ground wire combined)
Here’s the critical part most people miss: you must account for BOTH the power wire and ground wire. If you run 15 feet of wire to your device, the total circuit is 30 feet because electricity has to complete the round trip.
Important note: These calculations assume wire at room temperature. In hot environments like engine bays or roof-mounted wiring in summer, resistance increases by 10-15%. Add a safety margin for heat exposure.
Why 3-5% is the Standard
The automotive and marine industries recommend keeping voltage drop under 5% for most accessories, regardless of whether you’re running 12V or 24V. The percentage matters more than absolute voltage.
At 3% Voltage Drop:
- 12V system: You lose 0.36V (device gets 11.64V)
- 24V system: You lose 0.72V (device gets 23.28V)
- Effect: Barely noticeable to most devices
- Best for: Radios, GPS, sensitive electronics
At 5% Voltage Drop:
- 12V system: You lose 0.6V (device gets 11.4V)
- 24V system: You lose 1.2V (device gets 22.8V)
- Effect: Slightly dimmer lights, slightly slower motors
- Acceptable for: LED lights, accessories, most equipment
At 10% Voltage Drop:
- 12V system: You lose 1.2V (device gets 10.8V)
- 24V system: You lose 2.4V (device gets 21.6V)
- Effect: Noticeably dimmer lights, weak performance
- Sometimes okay for: Non-critical loads where performance loss is acceptable
Over 10% Voltage Drop:
- 12V system: Device gets less than 10.8V
- 24V system: Device gets less than 21.6V
- Effect: Severe performance degradation, potential damage
- Never acceptable
Need quick reference values? Skip to the voltage drop table.
More Real-World Examples
Example 2: Winch (High Current, Critical Performance)
Scenario:
- 9,000 lb winch = 300A peak draw
- 8 feet from battery (typical front bumper mount)
- Using 2 AWG wire
12V Calculation:
- Total length: 8 ft × 2 = 16 ft
- 2 AWG resistance: 0.000156 ohms/ft
- Total resistance: 16 ft × 0.000156 = 0.0025 ohms
- Voltage drop: 300A × 0.0025Ω = 0.75V
- Percentage: 0.75V ÷ 12V = 6.3% voltage drop
Your winch is weaker than rated. Under full load, you’re losing over half a volt.
24V version (same scenario):
- Same winch on 24V draws 150A (half the current for same power)
- Voltage drop: 150A × 0.0025Ω = 0.375V
- Percentage: 0.375V ÷ 24V = 1.6% voltage drop
This is why serious off-road rigs and commercial vehicles use 24V systems.
Use Wire Solved’s calculator to size your winch wiring correctly.
Solution for 12V: Upgrade to 1/0 AWG wire
- 1/0 resistance: 0.0000983 ohms/ft
- New voltage drop: 300A × (16 × 0.0000983) = 0.47V = 3.9%
- Much better pulling power, especially when you need it most.
Example 3: Fridge (Long Run, Moderate Current)
Scenario:
- 50W fridge = 4.2A at 12V
- Mounted in rear of vehicle, 25 feet from auxiliary battery
- Using 16 AWG wire
Calculation:
- Total length: 25 ft × 2 = 50 ft
- 16 AWG resistance: 0.00402 ohms/ft
- Total resistance: 50 ft × 0.00402 = 0.201 ohms
- Voltage drop: 4.2A × 0.201Ω = 0.84V
- Percentage: 0.84V ÷ 12V = 7% voltage drop
Your $300 Dometic fridge is struggling. Low voltage causes the compressor to run inefficiently or shut off entirely. That’s spoiled food on Saturday night.
Use Wire Solved’s calculator to prevent fridge failures.
Solution: Upgrade to 12 AWG wire
- 12 AWG resistance: 0.00159 ohms/ft
- New voltage drop: 4.2A × (50 × 0.00159) = 0.33V = 2.8%
- Your fridge gets proper voltage and runs efficiently all weekend.
Why 24V Systems Have Less Voltage Drop
For the same power delivery, 24V systems draw half the current.
Example: 240W Device
- 12V system: 240W ÷ 12V = 20A
- 24V system: 240W ÷ 24V = 10A
Since the voltage drop formula is Current × Resistance × Length, halving the current halves the voltage drop in volts. As a percentage, it’s even better because you’re dividing by a higher voltage.
This means 24V systems can:
- Use thinner wire for the same performance
- Run longer distances without voltage drop issues
- Waste less energy as heat in the wiring
- Operate more efficiently overall
This is why RVs, boats, and commercial vehicles often use 24V. It’s simply more efficient for high-power applications.
The Hidden Cost of Voltage Drop
Beyond poor performance, excessive voltage drop costs you in ways you might not notice immediately.
1. Energy Waste
Lost voltage becomes heat in the wire. You’re literally burning battery capacity before it reaches your device.
2. Shortened Equipment Life
Many devices are rated for 12V. Running them at 10.5V due to voltage drop puts stress on electronics and motors, shortening their lifespan.
3. Reduced Efficiency
Devices with voltage regulators work harder to maintain output. Resistive loads deliver less power. Either way, you’re not getting what you paid for.
4. False Troubleshooting
“My light bar is defective.” No, it’s just not getting enough voltage.
How to Measure Voltage Drop
Think your wire sizing is wrong? Here’s how to check before tearing apart your install.
You’ll need a multimeter and a helper:
- Turn on the device (light bar, winch, etc.) under typical load
- Measure voltage at battery terminals - should read around 12.6V
- Measure voltage at device terminals - should read close to battery voltage
- Calculate the difference - this is your voltage drop
- Divide by nominal voltage (12V or 24V) - this is your percentage
Example:
- Battery: 12.6V
- Device: 11.8V
- Drop: 0.8V
- Percentage: 0.8V ÷ 12V = 6.7% voltage drop
Note: Calculate percentage using the nominal voltage your system is designed for (12V or 24V), not the measured battery voltage which varies with charge state.
If you’re seeing over 5%, you need thicker wire or shorter runs.
Common Misconceptions
Does voltage drop only matter for lights?
No. Every electrical device suffers from voltage drop. Motors work harder, electronics get stressed, batteries drain faster. It affects everything.
If the wire doesn’t get hot, is it fine?
Not necessarily. Wire can stay within safe temperature limits (no fire hazard) but still lose too much voltage for proper equipment operation. Thermal safety and electrical performance are separate requirements. You must satisfy both.
Should I just use the biggest wire I can afford?
There are diminishing returns. Going from 12 AWG to 10 AWG on a long light bar run makes a big difference. Going from 4 AWG to 2 AWG on a 10A circuit is overkill because you’re already well below 3% voltage drop.
Note: Fuse size is based on wire ampacity at the source, not voltage drop. Upgrading to thicker wire for voltage drop reasons means the same fuse protects a more capable wire. That’s fine.
Do I only need to worry about the power wire?
False. The ground wire carries the same current and has the same resistance. Size both wires equally.
Quick Reference: Voltage Drop by Wire Gauge
Here’s how much voltage you lose per 100 feet ROUND TRIP (that’s 50 ft to device + 50 ft return ground) at various amperage levels:
| Wire Gauge | 10A | 20A | 30A | 50A | 100A |
|---|---|---|---|---|---|
| 22 AWG | 1.61V (13.4%) | 3.22V (26.8%) | - | - | - |
| 18 AWG | 0.64V (5.3%) | 1.28V (10.7%) | 1.92V (16%) | - | - |
| 14 AWG | 0.25V (2.1%) | 0.51V (4.2%) | 0.76V (6.3%) | 1.27V (10.6%) | - |
| 12 AWG | 0.16V (1.3%) | 0.32V (2.7%) | 0.48V (4%) | 0.80V (6.6%) | 1.59V (13.3%) |
| 10 AWG | 0.10V (0.8%) | 0.20V (1.7%) | 0.30V (2.5%) | 0.50V (4.2%) | 1.00V (8.3%) |
| 6 AWG | 0.04V (0.3%) | 0.08V (0.7%) | 0.12V (1%) | 0.20V (1.6%) | 0.40V (3.3%) |
| 2 AWG | 0.016V (0.1%) | 0.031V (0.3%) | 0.047V (0.4%) | 0.078V (0.7%) | 0.156V (1.3%) |
Values shown for 12V system. Highlighted cells show acceptable voltage drop (under 5%). For 24V systems, the voltage drop in volts is the same, but the percentage is cut in half. Example: 0.64V drop equals 5.3% at 12V but only 2.7% at 24V.
Note: Wire resistance values are for solid copper. Stranded wire (recommended for vehicle use due to flexibility and vibration resistance) has 2-5% higher resistance. Wire Solved’s calculator accounts for this.
Calculate Your Wire Size Now
Use Wire Solved’s calculator to factor in both amperage AND length. It automatically:
- Calculates total circuit resistance
- Computes voltage drop percentage
- Recommends proper wire gauge
- Explains why an upgrade is needed
Real-world examples where voltage drop is critical:
- Light bar installations - Long roof runs cause significant voltage drop
- Winch wiring - Voltage drop kills pulling power under load
- Solar charging - Even 3% voltage drop wastes solar harvest
What’s Next?
Now that you understand voltage drop:
- Fuse Sizing Guide - Protect your wires properly
- How to Use Wire Solved - Get accurate recommendations
- Wire Sizing 101 - Master the fundamentals