Voltage Drop Calculator
Calculate voltage drop, determine proper wire size, and verify NEC compliance. Free online tool for electricians, engineers, and industrial installers.
| AWG / kcmil | Ω / 1000 ft (Copper) | Ω / 1000 ft (Aluminum) | Circular Mils |
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Voltage Drop Formula Explained
Voltage drop is the reduction in voltage in an electrical circuit between the source and the load. In industrial installations, excessive voltage drop can cause motors to overheat, lighting to dim, and sensitive equipment to malfunction. The NEC (National Electrical Code) recommends limiting voltage drop to 3% for branch circuits and 5% total (feeder + branch) for reasonable efficiency.
Calculation Formulas
This calculator uses the standard industry formulas based on Ohm's Law and conductor properties:
Vdrop = (2 × K × I × D) / CM
Vdrop = (1.732 × K × I × D) / CM
Where:
- K = Resistivity constant: 12.9 for copper, 21.2 for aluminum (at 75°C)
- I = Current in Amperes (Amps)
- D = One-way distance in feet
- CM = Circular Mils (cross-sectional area of the conductor)
- 1.732 = Square root of 3 (the three-phase factor)
NEC References
The NEC provides guidance on voltage drop through these key articles:
- NEC Article 210.19(A) Informational Note No. 4: Recommends that branch-circuit conductors be sized so that voltage drop does not exceed 3% at the farthest outlet.
- NEC Article 215.2(A) Informational Note No. 2: Recommends that feeder conductors be sized so that the combined voltage drop of feeder and branch circuit does not exceed 5%.
Practical Examples
Example 1: Motor at End of a Long Run
A 10 HP, 480V three-phase motor draws approximately 14A and is located 400 feet from the panel. Using AWG 12 copper wire, the voltage drop is approximately 11.1V (2.3%), which is within the NEC 3% recommendation. However, during motor startup the inrush current can be 6-8 times higher, so many engineers size for AWG 10 to provide extra margin.
Example 2: Parking Lot Lighting
A 120V single-phase lighting circuit feeds LED fixtures drawing 8A total, located 250 feet away. With AWG 14 copper, the voltage drop is 5.26V (4.4%), which exceeds the NEC 3% recommendation. Upsizing to AWG 12 reduces the drop to 3.16V (2.6%) -- a simple wire size change that ensures full brightness and fixture longevity.
Example 3: Subpanel Feed
A 240V single-phase subpanel with 100A load at 150 feet using AWG 2/0 copper has a voltage drop of 2.91V (1.2%). This is well within the 3% feeder recommendation, providing confidence that the subpanel will deliver stable voltage to all downstream circuits.
Frequently Asked Questions
When does voltage drop become a problem?
Voltage drop becomes a concern when it exceeds 3% on a branch circuit or 5% combined (feeder + branch). Symptoms include motors running hot, reduced torque, dim or flickering lights, and nuisance tripping of VFDs and PLCs. In industrial automation, sensitive electronics like sensors and controllers may malfunction when supply voltage drops below their rated input range.
How does distance affect wire size?
Voltage drop is directly proportional to distance. Doubling the distance doubles the voltage drop. To compensate, you must either increase the conductor size (which increases circular mils, reducing resistance) or increase the system voltage (higher voltage means lower current for the same power). For very long runs, it is often more economical to step up to a higher voltage than to use extremely large conductors.
Copper vs Aluminum wiring -- which is better?
Copper has lower resistivity (K = 12.9) and higher conductivity, meaning less voltage drop for the same wire size. It is also more corrosion-resistant and has better mechanical strength. Aluminum (K = 21.2) is lighter and less expensive, but requires a larger conductor size to achieve the same voltage drop. Aluminum is commonly used for service entrance cables and large feeder circuits where weight and cost savings matter. For industrial control panels and PLC cabinets, copper is the standard choice.