Voltage Drop Calculator (Advanced)
- Single-phase (2-wire): Vdrop = I × R_total (R_total includes round trip).
- Three-phase (balanced): Vdrop = √3 × I × R_phase (one-way conductor resistance).
- Resistance values shown are approximate standard values for copper (Ω per 1000 ft). Aluminum ~1.6× copper resistance.
- Recommended maximum voltage drop is often 3% for branch circuits.
Voltage drop is one of the most important concepts in electrical wiring. Our Voltage Drop Calculator helps you instantly calculate voltage drop, percentage drop, wire resistance, cable size requirements, and more. Whether you’re an electrician, engineer, or a DIY installer, this tool gives fast, accurate results based on length, wire size, phase type, and load.
Voltage Drop Recommended Limits by Application
| Application | Recommended Maximum Drop |
|---|---|
| Lighting Circuits | 3% |
| Motors / HVAC Units | 3–5% |
| General Branch Circuits | 3% |
| Feeder Circuits | 5% |
| Long-Distance Outdoor Wiring | 5–8% |
In this guide, we’ll explain how to calculate voltage drop, formulas, examples, and how voltage drop works across resistors and long cable runs.
What Is Voltage Drop?
Voltage drop refers to the reduction in voltage as electrical current flows through a conductor. Longer cables, smaller wire gauges, and higher loads all increase voltage drop.
Too much voltage drop can cause:
- Dim lights
- Overheating cables
- Inefficient equipment
- Motor failure
- Voltage instability
Most electrical codes recommend keeping voltage drop under 3 to 5% for safe operation.
How to Calculate Voltage Drop (Step-by-Step)
To calculate voltage drop, you need:
- Wire resistance
- Current (amps)
- Wire length
- Type of phase (single or three-phase)
Voltage Drop Formula (Single Phase)
Voltage Drop Formula (Three Phase)
Where:
- L = one-way length of cable
- I = current
- R = resistance per unit length
Or use our Voltage Drop Calculator to solve instantly.
How to Calculate Voltage Drop Across a Resistor
Voltage drop across a resistor is calculated using Ohm’s Law:
Example:
If current = 2A and resistor = 10Ω
Voltage Drop Example Calculation
Suppose you have:
- 120V system
- 10A load
- 100 ft wire length
- Copper 12 AWG wire
The voltage drop will be approximately 3.1V (≈2.6%), which is within the recommended range.
Voltage Drop Mistakes Beginners Make
Common Mistakes
- Measuring round-trip length incorrectly
- Using wrong wire type (copper vs aluminum)
- Ignoring temperature effects on resistance
- Not accounting for startup motor load
- Using cheap extension cords for high loads
Why Voltage Drop Matters
Proper voltage drop calculation ensures:
- Efficient performance
- Reduced power loss
- Protection of electrical equipment
- Compliance with electrical codes
Using the right wire size prevents overheating and system failures.
Try These Calculators Next
Frequently Asked Questions (FAQs)
How to calculate voltage drop?
Voltage drop is calculated using the formula:
Voltage Drop (V) = Current (I) × Resistance (R)
You simply multiply the current flowing through the wire by the wire’s resistance. For long circuits, multiply the total circuit length (there and back) by the resistance per unit length.
How to calculate voltage drop across a resistor?
To calculate voltage drop across a resistor, use Ohm’s Law:
V = I × R
V = voltage drop
I = current flowing through the resistor
R = resistor value (in ohms)
If multiple resistors are present, calculate each one separately based on the current in that branch.
How do I calculate voltage drop?
You calculate voltage drop by:
Finding the current (I) in the circuit
Finding the resistance (R) of the wire, cable, or resistor
Using V = I × R
For AC circuits, you also factor in conductor material (copper vs aluminum), temperature, and wire gauge.
How to calculate voltage drop in a series circuit?
In a series circuit, current stays the same through all components.
So:
Voltage Drop = I × R₁, I × R₂, I × R₃ …
Then add them:
Total Voltage Drop = V₁ + V₂ + V₃ …
Each resistor or wire section drops part of the voltage based on its resistance.
How to calculate voltage drop in a parallel circuit?
In a parallel circuit, voltage across each branch is the same, but the current splits.
To calculate voltage drop:
Find the current in each branch separately
Apply V = I × R for that branch
The supply voltage remains constant across all branches
Each branch may have different voltage drops inside wiring, but the load voltage is the same.
What is voltage drop calculation?
Voltage drop calculation is the process of determining how much voltage is lost as electricity travels through a wire or resistor. Wires have resistance, so some voltage is consumed along the path.
Voltage drop calculations help ensure:
Proper wire sizing
Less energy loss
Safe operation of motors, lights, and appliances