Precision Utility
Voltage Drop
Calculator
Standard
NEC
Limit
Max 3% Drop
Calculate voltage drop for any US electrical circuit instantly. Enter the current, wire length, gauge, supply voltage and conductor material — the calculator checks whether your wiring meets the NEC-recommended 3% branch circuit limit. Works for residential, commercial and industrial installations using standard AWG wire sizes.
Circuit Parameters
Voltage Drop
0.00 V
Voltage Drop
0.00 V
Drop %
0.00%
Voltage at Load
120.00 V
NEC Status
Pass
How the voltage drop calculator works
This calculator uses the standard NEC voltage drop formula: VD = (2 x K x I x L) / CM. You select a specific wire gauge and the calculator tells you exactly how much voltage is lost over the length of that circuit.
The factor of 2 accounts for the complete circuit — current flows out to the load and back. K is a resistivity constant that depends on the conductor material: 12.9 for copper, 21.2 for aluminum. I is the circuit current in amps and L is the one-way wire length in feet. CM is the circular mil area of the selected wire gauge.
The result card shows your voltage drop in volts, the percentage drop relative to supply voltage, and the actual voltage arriving at the load. The NEC status indicator turns green when the drop is within the recommended 3% limit, and red when it exceeds it.
Use this to verify your wire selection before installing, or to troubleshoot circuits where equipment is underperforming. If the drop is too high, try selecting a larger wire gauge or shortening the run.
Voltage drop limits and NEC guidelines
The National Electrical Code provides clear guidance on acceptable voltage drop levels for US electrical installations:
- 3% maximum — recommended for branch circuits (NEC 210.19(A)(1) Informational Note 4)
- 5% maximum — recommended for the combined feeder and branch circuit (NEC 215.2(A)(4) Informational Note 2)
- These are recommendations, not mandatory code requirements, but exceeding them can cause equipment problems
- Copper wire has lower resistivity (K=12.9) than aluminum (K=21.2), resulting in less voltage drop for the same gauge
- Longer wire runs always produce more voltage drop — doubling the length doubles the drop
Common consequences of excessive voltage drop include dimming lights, motors running hot, and electronic equipment malfunctioning. For sensitive equipment, keeping voltage drop under 2% is a best practice. Always verify your final installation with a licensed electrician.
Frequently asked questions
What is voltage drop and why does it matter?
Voltage drop is the reduction in voltage as electricity travels through a wire. Excessive voltage drop means equipment at the end of the circuit receives less power than intended, which can cause lights to dim, motors to overheat, and electronics to malfunction. The NEC recommends keeping voltage drop at or below 3% for branch circuits.
How do I calculate voltage drop for a circuit?
Use the formula VD = (2 x K x I x L) / CM. K is 12.9 for copper or 21.2 for aluminum, I is current in amps, L is the one-way wire length in feet, and CM is the circular mil area of the wire. Divide the result by your supply voltage and multiply by 100 to get the percentage drop.
What is the NEC maximum voltage drop for branch circuits?
The NEC recommends a maximum 3% voltage drop for branch circuits. For the combined feeder and branch circuit, the recommendation is 5% total. These are guidelines rather than hard code requirements, but exceeding them can cause performance problems and safety concerns.
How do I reduce voltage drop in a long wire run?
You can reduce voltage drop by using a larger wire gauge (lower AWG number), shortening the wire run, using copper instead of aluminum, or increasing the supply voltage. For very long runs, upsizing the wire by one or two gauges is the most common solution.
Does copper or aluminum have more voltage drop?
Aluminum has more voltage drop than copper for the same wire gauge. Aluminum's resistivity constant (K) is 21.2 compared to 12.9 for copper — roughly 64% higher. This means aluminum wire needs to be one or two sizes larger than copper to achieve similar voltage drop performance.
What voltage drop is acceptable for a 120V circuit?
For a 120V branch circuit, the NEC recommends no more than 3.6V of drop (3% of 120V). This ensures equipment receives at least 116.4V. For sensitive electronics or lighting, keeping the drop under 2% (2.4V) is even better for consistent performance.