Voltage Drop Calculator
// CALCULATE VOLTAGE DROP AND SELECT OPTIMAL WIRE SIZE //
Consider using a larger wire gauge or shorter run to reduce voltage drop below 3%.
Wire Resistance Reference (NEC Chapter 9 Table 8)
Resistance values in ohms per 1,000 feet at 75°C operating temperature. These values are essential for accurate voltage drop calculations in electrical circuit design.
| AWG Size | Copper (Ω/1000ft) | Aluminum (Ω/1000ft) | Typical Application |
|---|---|---|---|
| 14 AWG | 3.07 | 5.06 | Lighting circuits (15A) |
| 12 AWG | 1.93 | 3.18 | General outlets (20A) |
| 10 AWG | 1.21 | 1.99 | Dryers, water heaters (30A) |
| 8 AWG | 0.764 | 1.26 | Ranges, EV chargers (40-50A) |
| 6 AWG | 0.491 | 0.808 | Subpanels, HVAC (55-65A) |
| 4 AWG | 0.308 | 0.508 | Large appliances (70-85A) |
| 2 AWG | 0.194 | 0.319 | Service entrance (95-115A) |
| 1/0 AWG | 0.122 | 0.201 | Main service (125-150A) |
| 4/0 AWG | 0.0608 | 0.100 | Large service (180-230A) |
Understanding Voltage Drop in Electrical Circuits
Voltage drop is one of the most critical factors in electrical circuit design and is essential knowledge for electricians, engineers, and anyone working with electrical installations. When electrical current flows through a conductor, some of the electrical energy is converted to heat due to the conductor's resistance. This energy loss manifests as a reduction in voltage between the source and the load.
Motor Failure
Overheating & stalling
Light Dimming
Flickering & reduced output
Energy Waste
Heat loss in wires
Code Violation
Failed inspections
National Electrical Code (NEC) Requirements
The NFPA 70 National Electrical Code provides recommendations for acceptable voltage drop levels per NEC 210.19(A) and 215.2(A) NFPA 70 National Electrical Code:
3%
Branch Circuits
From panel to outlets, lights, and appliances
3%
Feeder Circuits
From service entrance to panelboard
5%
Combined Total
Feeder + branch from source to load
Note: While these are recommendations rather than mandatory requirements, they represent industry best practices. Many local jurisdictions enforce these limits, and inspectors commonly flag installations that exceed them.
Voltage Drop Calculation Formulas
Single-Phase Circuit
VD = 2 × I × R × L ÷ 1000
• Factor of 2 = round-trip distance
• Used for 120V and 240V single-phase
Three-Phase Circuit
VD = √3 × I × R × L ÷ 1000
• √3 ≈ 1.732 phase factor
• Used for 208V, 480V three-phase
| Variable | Description | Unit |
|---|---|---|
| VD | Voltage Drop | Volts (V) |
| I | Current (load amperage) | Amperes (A) |
| R | Wire resistance (from NEC Table 8) | Ω per 1000 ft |
| L | One-way wire length | Feet (ft) |
Key Factors That Affect Voltage Drop
Wire Gauge (AWG)
Larger wires (lower AWG numbers) have lower resistance. 10 AWG has ~40% less resistance than 12 AWG. Increasing wire gauge is the most common solution for excessive voltage drop.
Wire Length
Voltage drop increases proportionally with length. A 100-foot run has twice the voltage drop of a 50-foot run. Long runs to outbuildings require careful calculation.
Current Load
Higher current = greater voltage drop. High-current applications like EV chargers and welders are particularly sensitive to voltage drop issues.
Conductor Material
Copper has ~61% the resistance of aluminum. Aluminum requires 2 AWG sizes larger for equivalent performance but costs less for large feeders.
Practical Strategies to Reduce Voltage Drop
| Strategy | Effect | Best For |
|---|---|---|
| Increase Wire Gauge | ~26% reduction per size increase | Long runs, high current loads |
| Shorten Wire Runs | Proportional reduction | New construction, layout planning |
| Use Higher Voltage | 240V = half the % drop of 120V | High-power equipment |
| Add Subpanel | Reduces branch circuit lengths | Garages, workshops, outbuildings |
| Use Copper (vs Aluminum) | ~39% lower resistance | Retrofit, limited conduit space |
| Parallel Conductors | Halves resistance (2 conductors) | Large service entrances |
Common Voltage Drop Scenarios
Detached Garage Workshop
100 ft
Distance
50A
Subpanel
240V
Voltage
4 AWG
Recommended
6 AWG gives 3.4% drop (fails). 4 AWG reduces to 2.1% (compliant). Suitable for power tools and welding.
Level 2 EV Charger
80 ft
Distance
40A
Load
240V
Voltage
8 AWG
Minimum
8 AWG at 80 ft = 2.9% (acceptable). At 120 ft, upgrade to 6 AWG required. See our EV charger wiring guide.
Farm Irrigation Pump
500 ft
Distance
20A
Load
240V
Voltage
4 AWG
Minimum
Long agricultural runs require significant wire sizing. 4 AWG keeps drop under 5%. Consider solar system sizing needs as well.
Why Accurate Voltage Drop Calculations Matter
Equipment Performance
Motors and equipment operate within specified voltage ranges. Low voltage causes inefficiency, overheating, and premature failure.
Energy Efficiency
Voltage drop = wasted energy as heat. Higher drop means higher electricity bills and increased carbon footprint.
Code Compliance
Many jurisdictions enforce NEC recommendations. Failed inspections delay projects and require costly corrections.
What is an acceptable voltage drop percentage?
The NEC recommends a maximum of 3% voltage drop for branch circuits and 5% total for combined feeder and branch circuits. However, for sensitive electronic equipment, a lower voltage drop of 2% or less may be desirable.
How does voltage drop affect LED lights?
LED lights are generally more tolerant of voltage variations than incandescent bulbs. However, significant voltage drop can still cause dimming, color temperature shifts, and reduced lifespan. Keep voltage drop below 3% for lighting circuits.
Should I use copper or aluminum wire?
Copper has lower resistance and is preferred for most applications. Aluminum is cost-effective for large feeders and service entrances but requires larger wire sizes (typically 2 AWG numbers larger than copper for the same ampacity).
How do I calculate voltage drop for a motor circuit?
For motor circuits, use the full-load current rating from the motor nameplate. For motor starting, voltage drop can be higher temporarily, but the running voltage should remain within 3% of rated voltage.
Does conduit type affect voltage drop?
The conduit type (EMT, PVC, rigid) does not directly affect voltage drop. However, it can affect heat dissipation, which indirectly impacts conductor resistance. Steel conduits may also introduce additional impedance in AC circuits due to magnetic effects.
For official standards and additional technical information, consult these authoritative sources: