What is Voltage Drop?
Voltage drop is the decrease in electrical potential along the path of a current flowing through an electrical circuit. In simple terms, it's the loss of voltage that occurs as electricity travels through a wire.
This phenomenon occurs because all conductors have some resistance, and according to Ohm's Law (V = I × R), when current flows through a resistive element, a voltage drop develops across that element.
The Science Behind Voltage Drop
When current flows through a wire, the resistance of the wire causes some of the electrical energy to be converted to heat. This energy conversion results in a lower voltage at the end of the wire than at the beginning.
Voltage Drop Formula (DC Circuits):
Vdrop = 2 × L × R × I
Where:- Vdrop = Voltage drop (volts)
- L = One-way length of wire (feet)
- R = Wire resistance (ohms per foot)
- I = Current (amps)
- The factor of 2 accounts for both supply and return wires
For AC circuits, particularly three-phase systems, the calculation becomes more complex and must account for power factor and phase relationships.
Factors Affecting Voltage Drop
Several factors influence the amount of voltage drop in a circuit:
Wire Gauge (AWG)
Smaller gauge (larger AWG number) wires have higher resistance per foot, resulting in greater voltage drop.
For example, 18 AWG wire has approximately 4 times the resistance of 12 AWG wire, leading to 4 times the voltage drop for the same current and length.
Wire Length
Voltage drop increases linearly with wire length. Doubling the wire length doubles the voltage drop.
This makes voltage drop particularly significant in aerospace applications where weight constraints may favor longer, thinner wires.
Current Draw
Voltage drop increases linearly with current. Doubling the current doubles the voltage drop.
High-current applications such as starters, actuators, and high-power systems are particularly prone to voltage drop issues.
Temperature
Wire resistance increases with temperature, approximately 0.393% per °C for copper.
A wire at 100°C has about 30% higher resistance than at 20°C, resulting in 30% more voltage drop.
Why Voltage Drop Matters
Excessive voltage drop can have significant negative consequences in electrical systems:
| Issue | Effect | Critical in |
|---|---|---|
| Equipment Malfunction | Devices may operate erratically or not at all when voltage falls below design thresholds | Avionics, control systems, sensors |
| Reduced Motor Performance | Motors produce less torque and run slower with reduced voltage | Actuators, pumps, control surfaces |
| Increased Heat Generation | Lower voltage causes higher current draw, generating more heat in both wires and equipment | Power distribution systems, harnesses |
| Premature Component Failure | Consistent undervoltage can stress components and reduce service life | All systems, particularly in long-duration missions |
| System Inefficiency | Power wasted as heat in wires reduces overall system efficiency | Battery-powered systems, energy-limited applications |
Voltage Drop Standards and Recommendations
Industry standards have established maximum acceptable voltage drop limits to ensure reliable operation:
Ideal (Under 3%)
Less than 3% voltage drop is considered optimal for most applications.
This ensures equipment operates at near-nominal voltage levels with minimal performance impact.
Essential for sensitive equipment, critical systems, and applications requiring high precision.
Acceptable (3-6%)
Voltage drop between 3-6% is generally acceptable for non-critical applications.
May cause minor performance impacts but typically within operating tolerances for most equipment.
Not recommended for sensitive electronics or critical systems.
Excessive (>6%)
Voltage drop exceeding 6% is considered excessive and should be avoided.
Can cause significant performance degradation, unreliable operation, and potential equipment damage.
May lead to system failures and compromised safety in critical applications.
Industry-Specific Standards:
- Aerospace (MIL-STD-704): Often specifies tighter tolerances of 1-2% voltage drop for critical systems
- Military (MIL-HDBK-516): May require redundant systems to mitigate voltage drop risks
- Motorsport: Generally follows a 3% standard with higher tolerance for short-duration peak loads
- Marine (ABYC): Recommends 3% maximum for critical navigation and communication systems
Calculating Voltage Drop
To determine voltage drop in a DC circuit:
- Identify circuit parameters: Current (I), wire length (L), and wire gauge (AWG)
- Determine wire resistance: Look up resistance per foot for the selected AWG size
- Calculate voltage drop: Vdrop = 2 × L × R × I
- Calculate percentage: % Voltage Drop = (Vdrop ÷ System Voltage) × 100
Example Calculation:
For a 12V system with 10A current, 16 AWG wire (0.00402 Ω/ft), and 15-foot wire length:
- Vdrop = 2 × 15 × 0.00402 × 10 = 1.206V
- % Voltage Drop = (1.206 ÷ 12) × 100 = 10.05%
This exceeds the recommended 3% maximum and would require upgrading to a larger wire gauge.
Mitigating Voltage Drop
Several strategies can be employed to reduce voltage drop in electrical circuits:
Increase Wire Size
The most common solution is to use a larger wire gauge (smaller AWG number). This reduces resistance and therefore voltage drop.
Example: Upgrading from 16 AWG to 12 AWG can reduce voltage drop by approximately 60%.
Reduce Wire Length
Shortening wire runs by relocating components or rerouting wires can significantly reduce voltage drop.
Example: Relocating a power distribution node can halve wire lengths and therefore voltage drop.
Use Higher System Voltage
Higher voltage systems experience proportionally less voltage drop for the same power delivery.
Example: A 24V system experiences half the percentage voltage drop of a 12V system for the same power.
Parallel Conductors
Using multiple wires in parallel effectively reduces resistance and voltage drop.
Example: Two 16 AWG wires in parallel have equivalent resistance to one 13 AWG wire.
Voltage Drop in MIL-SPEC Applications
In aerospace and military applications, voltage drop considerations are particularly critical due to:
- Safety-critical systems: System failures can have catastrophic consequences
- Extended operational duration: Systems must maintain performance over long missions
- Environmental extremes: Temperature variations dramatically affect wire resistance
- Weight constraints: Weight optimization must be balanced against electrical performance
MIL-SPEC standards often implement more conservative voltage drop limits to ensure system reliability under worst-case conditions. These standards typically specify:
- Maximum 2% voltage drop for critical systems (flight controls, avionics, etc.)
- Analysis requirements for both normal and emergency operating modes
- Consideration of temperature extremes in voltage drop calculations
- Redundant power distribution for flight-critical systems
Conclusion
Understanding and managing voltage drop is essential for designing reliable electrical systems, particularly in aerospace, military, and high-performance applications. By selecting appropriate wire gauges, minimizing wire lengths, and following industry standards, you can ensure your electrical systems operate efficiently and reliably.
Our MIL-SPEC Wire Calculator automates voltage drop calculations, providing recommendations that account for your specific system parameters, environmental conditions, and industry standards. This ensures your wire selections meet both operational requirements and safety margins.
Important Reminder: Always verify voltage drop calculations for critical systems and consider worst-case scenarios including maximum current draw, elevated temperatures, and aging effects on conductors.