The motor on your shop air compressor hums, struggles, and trips the breaker. The welder in the barn barely strikes an arc. The grain dryer fan in the outbuilding runs slow and hot. You replace the motor, and the new one does the same thing. You call an electrician, and he says the motor is fine. The problem is not the motor. The problem is 200 feet of undersized wire between the main panel and the outbuilding.
Voltage drop is the loss of electrical pressure that occurs when current flows through wire over distance. Every foot of wire has resistance, and that resistance converts some of the voltage into waste heat instead of delivering it to the motor. The longer the wire run and the smaller the wire gauge, the more voltage you lose. A motor that needs 240 volts to start might only be getting 210 volts at the end of a long run, and that 12% drop is enough to prevent the motor from developing starting torque. This guide explains why it happens, how to calculate it, and what to do about it.
What Voltage Drop Actually Does to a Motor
An induction motor produces torque in proportion to the square of the applied voltage. If voltage drops by 10%, torque drops by roughly 19%. If voltage drops by 15%, torque drops by roughly 28%. Starting torque, which is the torque the motor produces to get the shaft turning from a dead stop, is already the hardest demand on the electrical system. A motor that needs 100% starting torque at 240 volts can only produce 81% at 216 volts. If the connected load (compressor, saw, grinder) requires more than 81% of rated starting torque, the motor stalls.
A stalled motor is not just an inconvenience. It draws locked-rotor current, which is 5 to 7 times the normal running current, and it draws that current continuously instead of for the brief 1 to 2 second starting interval. The wire run that was already dropping voltage under normal load now drops even more under locked-rotor current, creating a feedback loop: low voltage causes a stall, the stall draws more current, more current causes more voltage drop, and the voltage drops further. The thermal overload or breaker eventually trips, but not before the motor windings get hot.
Repeated stalling and overheating is the primary cause of premature motor failure in outbuildings. The motor is not defective. The wire feeding it is too small for the distance. Replacing the motor without fixing the voltage drop guarantees the new motor will fail the same way.
The NEC (National Electrical Code) recommends a maximum of 3% voltage drop for branch circuits and 5% total from the service entrance to the final outlet. For a 240-volt circuit, 5% is 12 volts, meaning the motor should see at least 228 volts. Many outbuilding installations violate this guideline because the wire was sized for current capacity (ampacity) without accounting for the distance. A 10-gauge wire is rated for 30 amps, which is plenty for a 5 HP motor. But at 200 feet, the voltage drop at full load is over 7%, well past the point where motors struggle.
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: A 5 HP single-phase motor draws about 28 amps at full load on a 240-volt circuit. The outbuilding is 200 feet from the main panel. Using 10 AWG copper wire: Vd = (2 × 200 × 28 × 1.24) / 1000 = 13.9 volts. That is a 5.8% drop, leaving only 226 volts at the motor. During startup, the motor draws 140 to 196 amps (5-7 times full load), and the momentary voltage drop is catastrophic: (2 × 200 × 168 × 1.24) / 1000 = 83 volts dropped, leaving only 157 volts at the motor. The motor cannot start.
To keep the drop under 3% at full load current for the same 200-foot run: you need Vd ≤ 7.2 volts. Working backward: R ≤ (7.2 × 1000) / (2 × 200 × 28) = 0.643 Ω/1000ft. That requires 6 AWG wire (0.491 Ω/1000ft). Jumping from 10 AWG to 6 AWG costs more in wire, but it is the only fix that actually solves the problem.
For three-phase circuits, the formula changes to: Vd = (1.732 × L × I × R) / 1000. Three-phase has a natural advantage because the current per conductor is lower for the same horsepower, and the multiplier is 1.732 instead of 2. A 5 HP three-phase motor draws about 15 amps at 240 volts, less than half the single-phase current. This is why electricians recommend three-phase power for outbuildings with significant motor loads whenever it is available.
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
Keep below 3% for branch circuits, 5% total.
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.
Sizing Wire for Distance: The Fix That Actually Works
The solution to voltage drop is straightforward: use larger wire. The wire size that meets ampacity requirements (current-carrying capacity based on insulation temperature rating) is a minimum, not a recommendation. For outbuilding runs over 100 feet, you almost always need to go up one or two wire sizes beyond the ampacity minimum to keep voltage drop within acceptable limits.
A practical sizing table for single-phase 240V circuits to outbuildings at 3% maximum drop: a 20-amp circuit at 100 feet needs 10 AWG (ampacity minimum); at 200 feet it needs 6 AWG; at 300 feet it needs 4 AWG. A 30-amp circuit at 100 feet needs 8 AWG; at 200 feet it needs 4 AWG; at 300 feet it needs 2 AWG. A 50-amp circuit at 100 feet needs 6 AWG; at 200 feet it needs 2 AWG; at 300 feet it needs 1/0 AWG.
Aluminum wire is a cost-effective alternative for long runs. Aluminum costs about 40% less per foot than copper for the same ampacity, and the weight savings make it easier to pull through conduit. The tradeoff is that aluminum has higher resistance per foot, so you need to go up one size compared to copper (for example, 4 AWG aluminum instead of 6 AWG copper). Aluminum also requires anti-oxidant compound at every connection and compatible lugs rated for aluminum. Done correctly, aluminum feeders are the standard choice for outbuilding runs of 100 feet or more.
If the outbuilding already has undersized wire in conduit, you may be able to pull new wire without trenching. If the existing conduit is large enough for the new wire (check conduit fill calculations), you can pull the old wire out and pull the new wire in. If the conduit is too small, you need a new conduit run or direct-burial cable. Direct-burial UF cable is allowed for underground runs but must be buried at least 24 inches deep (12 inches if in rigid conduit). The trenching cost is typically $3 to $8 per linear foot, which is often the largest expense in an outbuilding electrical upgrade.
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: How to Tell If Voltage Drop Is Your Problem
Not every motor problem is voltage drop, but certain symptoms are strong indicators. The motor hums loudly but turns slowly or does not turn at all when started under load. The motor starts fine when unloaded (no belt, no compressor head) but stalls under load. Lights in the outbuilding dim noticeably when the motor starts. The breaker trips after 5 to 15 seconds, which is the thermal overload time for locked-rotor current. The motor runs but is noticeably hot to the touch after even short operation.
To confirm voltage drop, measure voltage at the motor terminals while the motor is running at full load. Compare this to the voltage at the main panel. If the difference is more than 5% of the panel voltage, voltage drop is the problem. For example, if the panel reads 243 volts and the motor terminals read 224 volts, the drop is 19 volts, or 7.8%. That is enough to cause problems with starting torque and motor overheating.
A common misdiagnosis is a bad capacitor. Start capacitors boost starting torque on single-phase motors, and a weak or failed capacitor produces symptoms similar to voltage drop: slow starting, humming, and tripping. The difference is that a capacitor problem shows up even with good voltage at the motor terminals. If you measure 235+ volts at the motor and it still will not start, check the capacitor. If you measure 215 volts or less, the wire is the problem regardless of capacitor condition.
Another misdiagnosis is a weak breaker. Some electricians will upsize the breaker when a motor trips repeatedly. This is dangerous. The breaker is sized to protect the wire, not the motor. If the motor is stalling due to voltage drop and drawing locked-rotor current, a larger breaker allows that current to flow longer, overheating the wire and creating a fire hazard. The correct fix is always larger wire, never a larger breaker on the same wire.
Other Solutions: Soft Starters, Phase Converters, and Subpanels
If replacing the wire is impractical or too expensive, several alternatives can mitigate voltage drop problems. A soft starter reduces the inrush current during motor startup by ramping the voltage up gradually over 2 to 10 seconds. Instead of hitting the wire with 6 times full-load current instantaneously, the soft starter limits inrush to 2 to 3 times full-load current. The reduced inrush causes less voltage drop, and the motor accelerates smoothly instead of slamming into a stall. Soft starters cost $200 to $600 for motors up to 10 HP and are a practical retrofit when rewiring is not feasible.
A static phase converter generates a third phase from single-phase power, allowing the use of three-phase motors. Three-phase motors draw less current per leg than equivalent single-phase motors, which reduces voltage drop by 40 to 50 percent on the same wire. A 5 HP three-phase motor on a static phase converter draws about 15 amps per leg instead of 28 amps on single phase. The voltage drop on the same 200-foot, 10 AWG run drops from 5.8% to about 2.5%. The converter costs $400 to $800, and the three-phase motor may cost slightly more than a single-phase equivalent, but the total investment is often less than rewiring.
A subpanel at the outbuilding with its own feeder from the meter is another option if multiple circuits are needed. Instead of running separate circuits for each load, you run one properly sized feeder to a small panel in the outbuilding and branch from there. This concentrates the cost in one large wire run instead of multiple smaller ones. A 100-amp subpanel with a 200-foot feeder in 2 AWG aluminum costs about the same as running two separate 30-amp circuits in 6 AWG copper, but it gives you much more capacity and flexibility for future loads.