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Calculate voltage drop in electrical wires based on wire gauge, length, current, and material.
Distance from panel to load (not round trip)
Single Phase/DC: VD = (2 × L × R × I) ÷ 1000
Three Phase: VD = (√3 × L × R × I) ÷ 1000
Where:
These are recommendations, not requirements. Some applications may need stricter limits.
Maximum one-way distance for 3% voltage drop at rated ampacity:
| AWG | Copper Amps | Max @ 120V | Max @ 240V |
|---|---|---|---|
| 14 | 15A | 50 ft | 100 ft |
| 12 | 20A | 79 ft | 158 ft |
| 10 | 30A | 100 ft | 200 ft |
| 8 | 40A | 152 ft | 304 ft |
| 6 | 55A | 202 ft | 404 ft |
| 4 | 70A | 253 ft | 506 ft |
| 3 | 85A | 300 ft | 600 ft |
| 2 | 95A | 340 ft | 680 ft |
| 1 | 110A | 380 ft | 760 ft |
| 1/0 | 125A | 430 ft | 860 ft |
| 2/0 | 145A | 480 ft | 960 ft |
| 3/0 | 165A | 540 ft | 1080 ft |
| 4/0 | 195A | 600 ft | 1200 ft |
Voltage drop is the reduction in voltage that occurs as electrical current flows through the resistance of a conductor. Every wire, regardless of its size, has some inherent electrical resistance that opposes current flow and converts a portion of the electrical energy into heat. This means the voltage measured at the load end of a circuit is always lower than the voltage at the source. The amount of voltage lost depends on four key factors: the current flowing through the wire, the wire's resistance per unit length (determined by its gauge and material), the total length of the wire run (both the supply and return conductors), and whether the circuit is single-phase or three-phase. The National Electrical Code (NEC) recommends that voltage drop should not exceed 3% for branch circuits or 5% total for the combined feeder and branch circuit. Excessive voltage drop wastes energy, reduces equipment performance, and can prevent motors and other devices from starting or operating correctly.
You need four values: the load current in amps (I), the one-way wire distance in feet (L), the wire gauge (AWG), and the conductor material (copper or aluminum). Also note whether the circuit is single-phase/DC (multiplier of 2) or three-phase (multiplier of 1.732).
Find the resistance per 1,000 feet for your wire gauge from NEC Chapter 9, Table 8. For example, 12 AWG copper has a resistance of 1.98 ohms per 1,000 feet, while 10 AWG copper is 1.24 ohms per 1,000 feet. Aluminum wire has approximately 1.6 times the resistance of copper for the same gauge.
For single-phase circuits: Vdrop = (2 x L x I x R) / 1000. For three-phase circuits: Vdrop = (1.732 x L x I x R) / 1000. The factor of 2 accounts for the round-trip distance (supply and return conductors). Example: 20A through 100 feet of 12 AWG copper = (2 x 100 x 20 x 1.98) / 1000 = 7.92 volts.
Divide the voltage drop by the source voltage and multiply by 100: VD% = (Vdrop / Vsource) x 100. From our example: (7.92 / 120) x 100 = 6.6%, which exceeds the recommended 3%. You would need to upgrade to 10 AWG (yielding 4.96V drop = 4.1%) or 8 AWG (yielding 3.11V = 2.6%) to meet the recommendation.
Electric motors are particularly sensitive to low voltage. A motor receiving 10% less than its rated voltage produces about 19% less torque and draws higher current to compensate, running hotter and wearing out faster. Air conditioner compressors may fail to start if voltage drop is excessive, leading to locked-rotor conditions that can trip breakers or damage windings.
Incandescent and halogen lights are highly sensitive to voltage -- a 10% voltage reduction causes a 30% drop in light output. LED drivers may flicker or produce uneven brightness when input voltage falls below their minimum operating range. Long runs to outdoor or landscape lighting are especially prone to noticeable dimming at the end of the circuit.
Voltage drop represents real power being wasted as heat in the wiring. In a circuit with 5% voltage drop, approximately 5% of the energy is lost before reaching the load. For a 30A, 240V circuit running 8 hours daily, 5% loss equals about 2.88 kWh per day or $126 per year at $0.12/kWh -- energy that produces nothing but warm walls and wasted money.
| Application | Max VD % | Standard/Source | Notes |
|---|---|---|---|
| NEC Branch Circuit | 3% | NEC 210.19(A) FPN | Recommendation, not a requirement |
| NEC Feeder | 3% | NEC 215.2(A) FPN | Recommendation, not a requirement |
| NEC Total (Feeder + Branch) | 5% | NEC 210.19(A) FPN | Combined feeder and branch circuit |
| Sensitive Electronics | 2% | Industry Practice | Computers, servers, medical equipment |
| Motor Circuits | 3% | NEC / IEEE | Critical for motor starting torque |
| Fire Alarm Circuits | 5% | NFPA 72 | End-of-line device must receive adequate voltage |
| Landscape Lighting (12V) | 5% | Industry Practice | 0.6V max at 12V -- very sensitive to run length |
| Solar PV (DC side) | 1.5 - 2% | Industry / IEC | Minimizes energy harvest loss |
| EV Charger Circuits | 3% | NEC / SAE | High current, long runs common |
| Welding Equipment | 4% | AWS / Industry | High current, intermittent duty |
NEC voltage drop percentages are recommendations in fine print notes (FPN), not enforceable code requirements. However, many jurisdictions adopt them as local requirements.
Yes, voltage drop is directly proportional to current. At half the rated load, the voltage drop will be half as well. This is why circuits that are only partially loaded may show acceptable voltage at the outlet but become problematic when the full load is applied, such as when an air conditioner compressor kicks on.
Using 240V instead of 120V for the same wattage load cuts the current in half (since P = V x I). With half the current, the voltage drop in volts is halved. Additionally, the percentage drop is calculated against a higher base voltage, so it is quartered. This is why 240V is preferred for high-power, long-distance circuits such as well pumps and outbuildings.
Aluminum has about 1.6 times the resistance of copper for the same wire gauge, resulting in 60% more voltage drop. To achieve equivalent voltage drop performance, you typically need to increase the aluminum wire by 2 AWG sizes. For example, where 10 AWG copper works, you would need 8 AWG aluminum. Despite this, aluminum is often more cost-effective for large feeders.
For three-phase circuits, the round-trip multiplier changes from 2 to 1.732 (the square root of 3). This means three-phase circuits have about 13% less voltage drop than single-phase circuits for the same current and distance. This is one reason three-phase power is preferred for long-distance power distribution and large motor loads.
Yes, running two conductors in parallel for each phase effectively halves the resistance and voltage drop. NEC 310.10(G) permits paralleling conductors 1/0 AWG and larger. Each parallel set must use the same conductor material, size, length, and insulation type. Parallel conductors must be routed in the same raceway or grouped together to maintain equal impedance.
Determine the correct AWG wire gauge for your circuit based on current, voltage drop limits, and distance. Considers both ampacity and voltage drop criteria.
Look up the maximum current capacity of any wire gauge based on NEC Table 310.16, with adjustments for insulation temperature rating and installation conditions.
Calculate the total resistance of a wire run based on gauge, material, and length. Understanding wire resistance is the foundation of accurate voltage drop calculations.