Voltage Drop Calculator
Calculate the voltage drop across electrical wire based on gauge, length, current, and conductor material. Ensure your wiring meets NEC requirements for maximum allowable voltage drop. See also our Wire Size Calculator and Ohm's Law Calculator.
How to Calculate Voltage Drop
Voltage drop occurs because all conductors have resistance. As current flows through wire, some energy is lost as heat, reducing the voltage available at the load. The amount of voltage drop depends on four factors: wire gauge (resistance per foot), wire length (total resistance), current (I×R loss), and conductor material (copper vs aluminum).
The NEC recommends maximum 3% voltage drop for branch circuits and 5% total for the combination of feeder and branch circuits. Excessive voltage drop causes motors to run hot, lights to dim, and electronic equipment to malfunction. For sensitive equipment like computers and medical devices, even lower voltage drop (1-2%) may be required.
Voltage Drop Formula
Single Phase:
Vdrop = 2 × I × R × L / 1000
Three Phase:
Vdrop = √3 × I × R × L / 1000
Where:
I = current in amperes
R = wire resistance in Ω per 1000 feet
L = one-way length in feet
Percentage Drop:
%Vdrop = (Vdrop / Vsource) × 100
Power Loss:
Ploss = Vdrop × I (watts wasted as heat in wire)
Example Calculation
Calculate voltage drop for 12 AWG copper wire, 100 feet, carrying 15 amps on a 120V circuit:
Wire resistance: 12 AWG copper = 1.588 Ω/1000ft
Vdrop = 2 × 15A × 1.588 × 100 / 1000 = 4.764V
Percentage: 4.764 / 120 × 100 = 3.97%
Voltage at load: 120 - 4.764 = 115.236V
Power loss: 4.764 × 15 = 71.5W wasted in wire
⚠️ This exceeds 3% — consider upgrading to 10 AWG:
10 AWG: Vdrop = 2 × 15 × 0.999 × 100/1000 = 2.997V (2.50%) ✓
Voltage Drop Reference Table (Copper)
| AWG | Length (ft) | Current (A) | Voltage Drop (V) |
|---|---|---|---|
| 14 | 50 ft | 15 A | 3.79 V |
| 14 | 100 ft | 15 A | 7.58 V |
| 12 | 50 ft | 20 A | 3.18 V |
| 12 | 100 ft | 15 A | 4.76 V |
| 12 | 100 ft | 20 A | 6.35 V |
| 10 | 100 ft | 30 A | 5.99 V |
| 10 | 150 ft | 20 A | 5.99 V |
| 8 | 100 ft | 40 A | 5.02 V |
| 8 | 150 ft | 30 A | 5.65 V |
| 6 | 100 ft | 50 A | 3.95 V |
| 6 | 200 ft | 40 A | 6.32 V |
| 4 | 200 ft | 60 A | 5.98 V |
Frequently Asked Questions
What is acceptable voltage drop?
The NEC recommends no more than 3% for branch circuits and 5% total (feeder + branch). For a 120V circuit, 3% = 3.6V maximum drop. For a 240V circuit, 3% = 7.2V. Some applications (data centers, medical facilities) require stricter limits of 1-2%.
Why is the formula multiplied by 2?
The factor of 2 accounts for the round-trip distance — current must travel from the panel to the load through the hot wire AND return through the neutral wire. Both conductors have resistance, so the total wire length is twice the one-way distance. For three-phase circuits, the factor is √3 instead of 2.
How does voltage drop affect motors?
Motors are particularly sensitive to voltage drop. A motor receiving less than rated voltage draws more current to maintain torque, causing overheating. Starting current (5-7× running current) causes even greater voltage drop during startup, potentially preventing the motor from starting. Most motors tolerate ±10% voltage variation, but efficiency drops significantly below rated voltage.
Does voltage drop waste electricity?
Yes. Voltage drop represents real power loss (P = Vdrop × I) dissipated as heat in the wire. For example, 5V drop at 20A = 100W wasted continuously. Over a year, that is 876 kWh — roughly $100-150 in electricity costs. Proper wire sizing pays for itself through reduced energy waste.
How do I reduce voltage drop?
Options include: use larger wire gauge (lower resistance), shorten the wire run (relocate panel closer to load), increase system voltage (240V has half the drop of 120V for same power), reduce current (use more efficient equipment), or use parallel conductors. For long runs, increasing voltage is often the most cost-effective solution.
Is voltage drop different for AC vs DC?
For resistive loads, the calculation is the same for AC and DC. However, AC circuits also experience additional voltage drop from inductive reactance (skin effect and proximity effect), especially in larger conductors and at higher frequencies. For most residential and commercial 60Hz applications with wire sizes up to 4/0, the DC resistance calculation is sufficiently accurate.
NEC Voltage Drop Guidelines
While the NEC voltage drop recommendations (3% branch, 5% total) are advisory rather than mandatory in most jurisdictions, they represent good engineering practice. Some local codes have adopted these as requirements. The NEC informational notes in Articles 210.19(A) and 215.2(A) explain that these limits ensure reasonable efficiency of operation. For critical loads, design for lower voltage drop to provide margin for future load growth and conductor aging.
Practical Applications
- Residential wiring: Verify branch circuits meet 3% drop for outlets and lighting
- Well pumps: Long underground runs to well houses often exceed 3% without oversized wire
- Solar installations: Minimize losses between panels, inverter, and main panel
- EV chargers: High-current chargers in detached garages need careful voltage drop analysis
- Landscape lighting: Low-voltage (12V) systems are very sensitive to voltage drop
- Industrial feeders: Large motor loads require voltage drop analysis for starting and running conditions