Voltage Drop Calculator
A voltage drop calculator determines how much voltage is lost in an electrical wire run due to conductor resistance. Enter your source voltage, load current, wire gauge (AWG), and one-way run distance to instantly calculate the voltage drop in volts, percentage drop, voltage at the load, and whether your installation meets NEC guidelines (3% for branch circuits, 5% combined maximum).
Voltage Drop Calculator
Calculate voltage drop, voltage at load, and NEC compliance for electrical wiring.
Enter the one-way wire run distance. The formula accounts for the return conductor automatically.
Frequently Asked Questions
What is voltage drop in electrical wiring?
Voltage drop is the reduction in voltage that occurs when electrical current flows through a conductor due to the conductor's resistance. Ohm's Law states V = I × R: the longer or thinner the wire, the higher its resistance, and the more voltage is lost before reaching the load. For example, a 100-foot run of 12 AWG wire carrying 20 A at 120 V drops about 7.92 V, leaving only 112 V at the device.
What is the NEC voltage drop recommendation?
The National Electrical Code (NEC) Article 210.19(A) Informational Note No. 4 recommends that voltage drop on branch circuits not exceed 3%, and that the combined voltage drop for both the feeder and branch circuit not exceed 5%. These are recommendations, not mandatory code requirements, but exceeding them can cause overheating, equipment malfunction, and reduced lifespan of appliances.
How do you calculate voltage drop for a single-phase circuit?
For a single-phase circuit, use the formula: V_drop = 2 × I × R × (L / 1000), where I is the current in amperes, R is the wire resistance in ohms per 1,000 feet (from NEC Chapter 9 Table 9), and L is the one-way run length in feet. The factor of 2 accounts for both the outgoing and return conductors. Then calculate the percentage: % drop = (V_drop / V_source) × 100.
How do you calculate voltage drop for a three-phase circuit?
For a three-phase circuit, use: V_drop = √3 × I × R × (L / 1000). The √3 factor (approximately 1.732) replaces the factor of 2 used in single-phase calculations. Three-phase distribution is more efficient — the same wire and current results in about 13% less voltage drop than single-phase because the three conductors share the load more efficiently.
What wire gauge should I use to stay within the NEC 3% limit?
The required wire gauge depends on current, distance, and voltage. As a general rule: for a 20 A circuit at 120 V, use 12 AWG for runs up to about 50 feet, 10 AWG for up to 80 feet, and 8 AWG for up to 125 feet to stay within 3%. For 240 V circuits the same wire can run twice as far for the same percentage drop. Use the calculator above to find the exact crossover point for your specific situation.
Does wire gauge affect voltage drop?
Yes, wire gauge (AWG) directly controls resistance and therefore voltage drop. Lower AWG numbers mean larger conductors with lower resistance. For example, 12 AWG copper has 1.98 Ω/1000ft while 10 AWG has only 1.24 Ω/1000ft — a 37% reduction. Going from 12 AWG to 8 AWG reduces resistance by 61%, which cuts voltage drop by the same amount. Always size wire to meet both ampacity and voltage drop requirements.
Why does voltage drop matter for motors and HVAC equipment?
Motors and HVAC compressors are particularly sensitive to low voltage. A motor operating at 90% of rated voltage draws approximately 110% of rated current (since power = voltage × current), causing increased heat, reduced torque, and premature insulation failure. Most motor manufacturers specify ±10% voltage tolerance. For a 240 V motor, that means the voltage at the motor terminals must stay between 216 V and 264 V.
What is the difference between voltage drop and voltage loss?
These terms are often used interchangeably. Voltage drop refers to the reduction in voltage measured across a length of conductor under load. Voltage loss emphasizes that this energy is dissipated as heat in the wire (P = I² × R watts). Excessive voltage drop wastes energy and can add significantly to operating costs in high-current, long-run applications such as industrial motors, outdoor lighting, and electric vehicle charging stations.
Voltage Drop Calculator: Complete Guide to NEC Compliance & Wire Sizing
The Voltage Drop Calculator determines how much voltage is lost in a wire run due to conductor resistance. It calculates voltage drop in volts, percentage drop, voltage available at the load, and whether the installation meets NEC guidelines. Use it to select the correct wire gauge for circuits in homes, commercial buildings, and industrial facilities.
Table of Contents
Voltage Drop Formula
The voltage drop formula used by this calculator is based on Ohm's Law and accounts for conductor resistance:
Single-Phase (2-wire):
Vdrop = 2 × I × R × (L / 1000)Three-Phase (3-wire):
Vdrop = √3 × I × R × (L / 1000)- I — load current in amperes (A)
- R — conductor resistance in ohms per 1,000 feet (Ω/1000ft)
- L — one-way wire run length in feet
- 2 — accounts for both outgoing and return conductors (single-phase)
- √3 ≈ 1.732 — three-phase power factor
Drop Percentage:
% Drop = (Vdrop / Vsource) × 100NEC Guidelines (3% / 5%)
The National Electrical Code (NEC) Article 210.19(A) Informational Note No. 4 recommends limiting voltage drop to maintain efficient and safe equipment operation:
| Circuit Type | NEC Recommended Max | Notes |
|---|---|---|
| Branch circuit | 3% | Outlet to equipment |
| Feeder circuit | 3% | Panel to subpanel or outlet |
| Combined (feeder + branch) | 5% | Total from service entrance to load |
Note: These are NEC recommendations, not mandatory code requirements. Some applications — such as motors, medical equipment, and sensitive electronics — may require tighter limits (1–2%). Always verify with local codes and equipment specifications.
Wire Gauge Resistance Table (Copper, 75°C)
Resistance values are in ohms per 1,000 feet (Ω/1000ft) for copper conductors at 75°C, per NEC Chapter 9 Table 9.
| AWG / kcmil | Resistance (Ω/1000ft) | Typical Ampacity (75°C) | Common Use |
|---|---|---|---|
| 14 AWG | 3.14 | 15 A | Lighting, small outlets |
| 12 AWG | 1.98 | 20 A | General outlets, kitchen |
| 10 AWG | 1.24 | 30 A | Dryers, A/C, EV chargers |
| 8 AWG | 0.778 | 50 A | Ranges, large A/C |
| 6 AWG | 0.491 | 65 A | Sub-panels, service |
| 4 AWG | 0.308 | 85 A | Large motors, service |
| 3 AWG | 0.245 | 100 A | Service entrance |
| 2 AWG | 0.194 | 115 A | Service entrance |
| 1 AWG | 0.154 | 130 A | Service entrance |
| 1/0 AWG | 0.122 | 150 A | Service feeder |
| 2/0 AWG | 0.0967 | 175 A | Service feeder |
| 3/0 AWG | 0.0766 | 200 A | Service feeder |
| 4/0 AWG | 0.0608 | 230 A | Large service |
Ampacity values per NEC Table 310.15(B)(16) for copper conductors with THHN/THWN-2 insulation in conduit at 75°C. Derate for bundling, ambient temperature, and conduit fill.
How to Use the Voltage Drop Calculator
- Select phase type — single-phase for most residential, three-phase for commercial/industrial.
- Enter source voltage — use presets (120V, 208V, 240V, 480V) or type your own value.
- Enter load current — the maximum continuous current draw of the load in amperes.
- Select wire gauge — choose the AWG size of the conductors being used.
- Enter one-way distance — the run length from the panel to the load in feet (the calculator doubles this for single-phase).
- Click Calculate — view voltage drop, voltage at load, percentage, and NEC compliance.
How to Reduce Voltage Drop
If your calculation shows excessive voltage drop, use one or more of these strategies:
1. Increase Wire Size (Most Effective)
Going from 12 AWG to 10 AWG reduces resistance by 37%. Each gauge step (e.g., 12→10→8) roughly halves the resistance and voltage drop. Use the calculator to test different gauges until the drop percentage is acceptable.
2. Reduce Run Length
Relocate the panel closer to the load, or add a sub-panel in a distant building. Voltage drop is directly proportional to distance — cutting the run in half halves the drop.
3. Use Higher Voltage
Running 240V instead of 120V delivers the same power at half the current, reducing voltage drop by 50% (since drop is proportional to current). The percentage drop is also halved.
4. Use Three-Phase Power
Three-phase distribution uses √3 ≈ 1.73 instead of 2 as the multiplier, which results in ~13% less voltage drop versus single-phase for the same wire and current. Ideal for long commercial runs.
Worked Examples
Example 1: Outlet 100ft from Panel (120V, 20A, 12 AWG)
- Vdrop = 2 × 20 × 1.98 × (100/1000) = 7.92 V
- Vload = 120 − 7.92 = 112.08 V
- % drop = (7.92 / 120) × 100 = 6.6% — exceeds NEC 3% and 5%
- Solution: Upgrade to 10 AWG → 2 × 20 × 1.24 × 0.1 = 4.96V (4.1%) or 8 AWG → 3.11V (2.6% ✓)
Example 2: HVAC Unit 50ft from Panel (240V, 30A, 10 AWG)
- Vdrop = 2 × 30 × 1.24 × (50/1000) = 3.72 V
- Vload = 240 − 3.72 = 236.28 V
- % drop = (3.72 / 240) × 100 = 1.55% — within NEC 3% ✓
Example 3: Three-Phase Motor 150ft Away (208V, 40A, 8 AWG)
- Vdrop = √3 × 40 × 0.778 × (150/1000) = 1.732 × 40 × 0.778 × 0.15 = 8.09 V
- Vload = 208 − 8.09 = 199.91 V
- % drop = (8.09 / 208) × 100 = 3.89% — within NEC 5% combined limit