Voltage Drop Calculator: Formula & Sizing
Free online tool • Supports single-phase AC, three-phase AC, and DC calculations
Use this free voltage drop calculator to determine voltage loss in copper or aluminum conductors. Enter your system voltage, current, wire size, and one-way distance to see if your installation meets NEC recommendations (3% for branch circuits, 5% total).
Results
FAILExceeds NEC total recommendation
Circular Mils
6,530
Material
copper
Effective K
12.90
Suggestion: Upsize to 8 AWG conductors to keep voltage drop under the recommended 3% limit (estimated voltage drop: 2.60%).
What is Voltage Drop?
Voltage drop represents the amount of voltage lost between the source power supply and the end-use device. As electric current flows through a conductor (wire), it experiences resistance. This resistance naturally creates a drop in voltage. Over long distances, or when handling heavy currents, this drop can become significant enough to impair equipment performance, cause flickering lights, or generate excess heat in wires.
Voltage Drop Calculation Formula
How to calculate voltage drop: We use the standard circular mils voltage drop calculation formula recommended by electrical codes:
Single-Phase & DC: VD = (2 × K × I × D) ÷ CM
Three-Phase AC: VD = (1.732 × K × I × D) ÷ CM
• VD: Voltage drop in volts.
• K: Direct current resistivity constant (12.9 for copper, 21.2 for aluminum at 75°C).
• I: Current draw of the load in Amps.
• D: One-way distance of the wire run in feet.
• CM: Cross-sectional area of the conductor in circular mils.
NEC Voltage Drop Standards
- The National Electrical Code (NEC) recommends a maximum of 3% voltage drop for branch circuits to ensure proper efficiency.
- The total voltage drop from the main utility entrance to the furthest outlet should not exceed 5% overall (combining both feeder and branch circuit drops).
- Excessive drop causes electric motors to run hotter, reduces heating element output, and causes LED and incandescent bulbs to dim or flicker.
DC Voltage Drop Calculator
Our tool acts as a dedicated DC voltage drop calculator. Direct current (DC) circuits are highly sensitive to voltage drop due to the lower operating voltages typically used in low-voltage systems. Calculating voltage drop is essential when designing off-grid systems, battery setups, and low-voltage distribution runs.
How to Calculate Voltage Drop Across a Resistor
In electronics and basic DC circuits, you often need to find the voltage drop across a resistor instead of a long wire run. To calculate voltage drop across a resistor, you apply Ohm's Law:
Ohm's Law Formula: V = I × R
• V: The voltage drop across the resistor (in Volts).
• I: The current flowing through the resistor (in Amps).
• R: The resistance of the resistor (in Ohms).
For example, if a current of 2 Amps flows through a 5 Ohm resistor, the voltage drop across the resistor is 10 Volts (2A × 5Ω = 10V).
3 Phase Voltage Drop Calculator
For commercial and industrial sites, our 3 phase voltage drop calculator applies the 1.732 factor to calculate accurate voltage drops for three-phase motor feeds and distribution panels. This is crucial for verifying that heavy machines receive their rated voltages to operate efficiently without overheating.
12V Voltage Drop Calculator
Low voltage systems like 12V DC setups in RVs, solar arrays, marine crafts, and automotive installations require careful wire sizing. A drop of just 1 volt in a 12V system represents an 8.3% loss, which can cause low-voltage cutouts on inverters and charge controllers. Ensure your low voltage projects are safely sized.
Frequently Asked Questions
What is voltage drop and what causes it?
Voltage drop is the loss of electrical voltage that occurs when current flows through the resistance of a conductor (wire). It is caused by the physical resistance of the wire material, long circuit distances, or excessive current draw.
How much voltage drop is acceptable?
The National Electrical Code (NEC) recommends keeping voltage drop under 3% for branch circuits, and a total of 5% from the main service panel to the final outlet. Keeping voltage drop low ensures equipment runs safely and efficiently.
How do you calculate voltage drop in a circuit?
To calculate voltage drop, use the circular mils formula: VD = (2 × K × I × D) / CM (for single-phase/DC) or VD = (1.732 × K × I × D) / CM (for three-phase), where K is wire resistivity, I is current, D is distance in feet, and CM is circular mils wire area.
How do you reduce voltage drop?
You can reduce voltage drop by upgrading to a larger wire size (gauge), using copper instead of aluminum, shortening the length of the conductor run, or increasing the circuit's operating voltage.
How do you perform a voltage drop test?
To perform a voltage drop test, measure the voltage at the source (under load) and the voltage at the end of the run with a multimeter. The difference between these two readings is your voltage drop. You can also measure the resistance of the wire directly to calculate expected loss.