Voltage Drop Calculator

This is a calculator for the estimation of the voltage drop of an electrical circuit. The "NEC data" tab calculates based on the resistance and reactance data from the National Electrical Code (NEC). The "Estimated resistance" tab calculates based on the resistance data estimated from the wire size. Click the "Other" tab to use customized resistance or impedance data, such as data from other standards or wire manufacturers.


Modify the values and click the calculate button to use
Wire material
Wire size
Material of conduit
Power factor (PF)
Wire material
Wire size
Wire impendence
or resistance
Voltage
Phase
Number of conductors
Distance (one-way)
Load current Amps

Calculate Voltage Drop to Prevent Motor Burnout and Code Violations

Voltage drop determines if your equipment receives enough power to operate safely over distance. Ignoring this calculation leads to under-voltage conditions where motors draw higher amperage to compensate, causing insulation failure before breakers trip. Target a maximum 3% loss on branch circuits to ensure torque stability and longevity. Most assume voltage drop is an efficiency tax. It is actually a torque killer. A motor running at 95% voltage draws more current to maintain output, generating excess heat that degrades winding insulation. This calculator exists because circuit breakers protect wires from melting, not equipment from starving. You can have a perfectly safe circuit that destroys your compressor in six months.

The Relationship Between Length, Gauge, and Heat

Resistance is not static. It changes with temperature. The core formula for single-phase voltage drop is $V_d = \frac{2 \times L \times I \times R}{1000}$. Here, L represents length in feet, I is current in amps, and R is resistance per 1000 feet. The multiplier 2 accounts for the return path in single-phase systems. Many users input room temperature resistance values and ignore thermal coefficients. This creates a false sense of security. As current flows, conductors heat up. Resistance increases as temperature rises. A wire operating at 75°C has significantly higher resistance than one at 20°C.

Consider the feedback loop. High resistance causes voltage drop. Low voltage at the load causes motors to draw higher current to maintain torque. Higher current generates more heat. More heat increases resistance further. This cycle accelerates insulation breakdown. You might measure acceptable voltage at installation when the wire is cold. Under load, the numbers shift. The hidden variable is often the connection points, not the wire run itself. A loose termination adds resistance equivalent to hundreds of feet of cable. Heat concentrates at these points. This is where fires start, not in the middle of the run.

Length creates asymmetry in design choices. Doubling the distance doubles the voltage drop. You cannot simply accept this loss. You must increase conductor size to compensate. Moving from 12 AWG to 10 AWG reduces resistance by approximately 37%. This jump often matters more than tightening connections. If you choose to keep the smaller gauge to save cost, you lose equipment lifespan. The trade-off is immediate material savings versus long-term replacement costs. For runs over 100 feet, defaulting to the next wire size up is standard practice. Do not wait for the calculator to tell you the minimum code requirement. Calculate for performance, not just compliance.

Conductor Material and Installation Environment

Material selection dictates baseline resistance. Copper conducts better than aluminum. Aluminum requires larger gauges to carry the same current with equivalent loss. The table below lists standard DC resistance values at 75°C for uncoated conductors. These are physical constants derived from engineering handbooks.

Conductor Size (AWG/kcmil) Copper Resistance (Ohms/1000ft) Aluminum Resistance (Ohms/1000ft)
14 3.14 5.16
12 1.98 3.25
10 1.24 2.04
8 0.78 1.28
6 0.49 0.81

Aluminum offers cost savings but introduces mechanical risks. It expands and contracts more than copper during thermal cycling. This movement loosens terminations over time. Loose terminations increase resistance. Increased resistance creates heat. You must use anti-oxidant paste and torque wrenches specified for aluminum. Skipping these steps negates the cost benefit. Copper is more forgiving. It maintains connection integrity with less maintenance. If the installation is inaccessible after drywall closure, copper reduces future liability.

Installation method alters heat dissipation. Wire in free air cools faster than wire in conduit. Multiple conductors in a single conduit trap heat. This requires derating the ampacity. If you pack five current-carrying conductors into one pipe, you must reduce the allowable current. This reduction effectively increases resistance per amp. The voltage drop calculator must account for this ambient temperature adjustment. A run calculated for 30°C ambient will fail in a 50°C attic. Environmental factors act as multipliers on resistance.

Direct burial cables face different constraints. Soil thermal resistivity varies. Dry sand insulates heat; wet clay conducts it away. A cable buried in dry soil runs hotter than one in wet soil. Higher operating temperature means higher resistance. Always assume the worst-case thermal environment for your calculations. If you choose direct burial, verify soil conditions. If you choose conduit, verify fill ratios. Both decisions impact the final voltage at the load. Do not treat the wire as an isolated variable. It interacts with its surroundings.

Execute Oversizing for Long-Term Stability

Stop calculating for the minimum acceptable voltage. Calculate for the equipment manufacturer’s specification. Many motors tolerate +/- 10% voltage variation, but efficiency peaks at nominal voltage. Run the calculator with your actual load distance and current. If the result exceeds 3%, increase the wire gauge immediately. Do not accept 4% because the code allows it under certain conditions. Code minimums are safety floors, not performance ceilings. The one change you must make is to treat voltage drop as a design constraint equal to ampacity. Upsizing wire costs more initially but prevents costly service calls later.

Safety and Professional Verification Disclaimer

This tool provides estimates based on standard physical constants and mathematical formulas. It does not replace professional engineering judgment or local code enforcement. Electrical installations carry risk of fire, shock, and equipment damage. Always verify calculations with a licensed electrician or professional engineer. Local codes may supersede general industry guidelines. Ensure all work complies with the National Electrical Code or your local jurisdiction’s equivalent.