The motor on your shop air compressor hums, struggles, or trips the breaker. The welder in the barn barely strikes an arc. The grain dryer fan in the outbuilding runs slow and hot. Long feeder distance can be one contributor, but it is not the only possible cause. Nameplate data, connections, capacitors, controls, load torque, source impedance, conductor size, and breaker or overload settings all need qualified review.
Voltage drop is the loss of electrical potential that occurs when current flows through wire over distance. Every foot of wire has resistance, and that resistance converts some voltage into heat instead of delivering it to the load. This guide explains the planning math and field checks while keeping the limits clear: voltage-drop arithmetic does not select conductors, approve OCPD, diagnose a motor, or replace NEC, manufacturer, utility, AHJ, and qualified electrical review.
What Voltage Drop Actually Does to a Motor
Induction-motor torque is often discussed as roughly proportional to the square of applied voltage, so low voltage can reduce starting torque. That relationship is only one part of the starting problem. The actual outcome also depends on motor design, nameplate LRA, starting method, load inertia, source impedance, voltage unbalance, controls, overloads, and manufacturer limits.
A stalled motor can draw locked-rotor current and create heat quickly, but a field symptom alone does not prove the feeder is undersized. Measurements should compare source voltage and load voltage under the correct operating condition, then be reviewed with the motor manual, conductor installation, OCPD, starter, and AHJ requirements.
Voltage drop can be one contributor to repeated stalling or overheating, but it should not be treated as the primary cause until other electrical and mechanical causes are checked. Replacing conductors, motors, capacitors, breakers, starters, or controls should be based on measurements and qualified review.
NEC voltage-drop language is informational-note context, not a complete pass/fail rule in this guide. Adopted code, local amendments, AHJ expectations, product data, ampacity, derating, terminal ratings, and safe-work procedures still control the formal design.
Long-Run Voltage Drop Calculator
Calculate voltage drop for long wire runs to detached shops, barns, garages, and outbuildings. Compares copper vs aluminum, shows motor starting voltage impact, and recommends the right wire size for your distance and load.
How to Calculate Voltage Drop for Your Wire Run
The voltage drop formula for single-phase circuits is: Vd = (2 × L × I × R) / 1000, where Vd is the voltage drop in volts, L is the one-way distance in feet, I is the current in amps, and R is the wire resistance in ohms per 1000 feet. For copper wire, common resistance values are: 14 AWG = 3.14 Ω/1000ft, 12 AWG = 1.98, 10 AWG = 1.24, 8 AWG = 0.778, 6 AWG = 0.491, 4 AWG = 0.308, 2 AWG = 0.194, 1/0 AWG = 0.122.
Example: if a source-verified motor current is 28 amps on a 240-volt circuit and the one-way distance is 200 feet, a local 10 AWG copper row gives Vd = (2 × 200 × 28 × 1.24) / 1000 = 13.9 volts, or about 5.8% drop. During starting, the current can be much higher than full-load current, but the correct value comes from nameplate LRA, motor data, starter/VFD behavior, and source impedance rather than a universal multiplier.
Working backward from a 3% local review prompt can show what resistance row would reduce steady-state drop, but that is still not a conductor selection. Final wire choice must also satisfy ampacity, insulation type, terminal temperature, raceway or cable method, burial, OCPD, grounding, neutral, fault-current, product listing, permit, and AHJ requirements.
For three-phase circuits, the formula changes to: Vd = (1.732 × L × I × R) / 1000. Three-phase voltage-drop arithmetic is different, but service availability, equipment, phase balance, motor design, utility rules, and installation requirements still need separate review.
Vd = (2 × L × I × R) / 1000
L = one-way distance (feet)
I = current (amps)
R = wire resistance (Ω/1000 ft)
% drop = (Vd / source voltage) × 100
Treat 3% and 5% as review prompts, not final compliance decisions.
Sizing Wire for Distance: What the Screen Can and Cannot Do
One way to reduce voltage drop is to use a lower-resistance conductor row, but that is not the same as final wire sizing. A conductor that looks acceptable in voltage-drop arithmetic still needs ampacity, insulation, terminal temperature, installation method, OCPD, neutral, grounding, derating, and AHJ review.
Local planning tables can help compare rows, but they should not be copied into a permit set without source reconciliation. Use the calculator to identify rows worth discussing with an electrician, then verify against the adopted NEC edition, manufacturer instructions, product listings, and site conditions.
Aluminum conductors may be appropriate for feeders when the selected product, terminals, connector ratings, preparation, torque, installation method, and AHJ review support them. This guide does not approve aluminum terminations or declare any feeder code-compliant.
If existing conduit or direct-burial cable is involved, verify fill, conductor type, insulation, burial depth, damage, corrosion, pulling tension, grounding, and local inspection rules before replacing anything. Trenching and rewiring decisions should be made from field conditions and qualified review.
Wire Sizing Calculator
Find the right AWG wire gauge for any electrical run. Enter amps, distance, and voltage to get NEC-compliant sizing with derating, voltage drop, and copper vs aluminum cost comparison.
Symptoms: When Voltage Drop Deserves a Measurement
Not every motor problem is voltage drop, but some symptoms justify measurement: humming or slow starting under load, visible light dimming during start, repeated overload or breaker trips, or high motor temperature after short operation. These symptoms can also come from mechanical load, controls, capacitors, weak connections, overload settings, or failing equipment.
To evaluate voltage drop, measure voltage with suitable instruments and safe-work controls at the source and at the motor or load under the relevant operating condition. A large difference is a review prompt, not a complete diagnosis, because power factor, starting current, source impedance, terminal condition, and motor data still matter.
Capacitors, starters, overloads, contacts, bearings, load torque, and supply quality can produce similar symptoms. Use manufacturer troubleshooting procedures and qualified electrical/mechanical review before replacing parts or declaring the conductor as the cause.
Breaker or fuse changes are not a voltage-drop repair. OCPD changes must protect the conductor and equipment under the adopted NEC and product instructions, with fault-current, coordination, listing, and AHJ review.
Other Solutions: Soft Starters, Phase Converters, and Subpanels
If replacing conductors is impractical, a qualified reviewer may consider alternatives such as a listed soft starter, VFD, different motor, load changes, source changes, or feeder redesign. Each option has product instructions, listing, protection, grounding, harmonic, ventilation, and AHJ implications.
Phase converters and three-phase equipment can change current and voltage-drop behavior, but they also introduce product-specific sizing, balance, protection, enclosure, grounding, and utility questions. They should not be selected from voltage-drop arithmetic alone.
A subpanel or new feeder may be appropriate when multiple circuits are needed, but it is a service/load calculation, conductor/OCPD, grounding, bonding, permit, inspection, and utility/AHJ discussion. Use the voltage-drop screen as one input to that discussion, not the design decision.