What is acceptable voltage drop?

The NEC recommends no more than 3% voltage drop for branch circuits and 5% total from service entrance to the farthest outlet. Going below this ensures equipment operates properly. Some sensitive applications require under 2%; some less critical may tolerate up to 10%.

Does wire length count round-trip?

Yes. Voltage drop depends on the total wire length (supply and return), which is double the one-way distance. A 100 ft run from panel to outlet has 200 ft of total wire carrying current. Always use 2× one-way distance in calculations.

Copper vs aluminum wire?

Copper has ~62% lower resistance than aluminum for the same gauge (or aluminum has ~1.6× higher resistance). For the same voltage drop, aluminum needs 2-3 AWG sizes thicker than copper. Aluminum is cheaper per pound but requires special terminations and antioxidant grease.

How do I read wire gauge (AWG)?

Smaller AWG number = thicker wire (counterintuitive). 14 AWG = small (15A circuits). 6 AWG = thicker (60A circuits). 4/0 AWG = very thick (large feeders). Each step approximately doubles the cross-sectional area. mm² makes more physical sense: 14 AWG = 2.08 mm², 6 AWG = 13.3 mm².

What size wire for 20 amps over 100 feet?

12 AWG copper meets ampacity for 20A but voltage drop at 100 ft one-way (200 ft round-trip) is ~4% — slightly over 3% recommendation. 10 AWG copper: drop ~2.5%, well within recommendation. For long runs at full load, upsize one AWG size for margin.

Why does voltage drop matter?

Excess drop causes equipment problems: motors fail to start or run hot, LED lights flicker, heaters underperform (P = V²/R), electronics may fault. Plus the dropped voltage is wasted as heat in the wire — over time this energy loss adds up to significant cost.

How do I calculate voltage drop in three-phase?

Use √3 factor: V_drop = (√3 × L × R_per_kft × I) / 1000. Three-phase circuits typically have lower voltage drop than single-phase for the same load because current is distributed across three conductors with phase advantage.

Does voltage drop affect motors more than lights?

Yes. Motors are sensitive to low voltage — typically need 90%+ of rated voltage to start reliably and run cool. Heating elements work at any voltage but lose power (V² dependence). LEDs vary by driver; some tolerate wide range, others are very sensitive. Always size for the most sensitive load.

Voltage Drop Calculator

Enter wire gauge, one-way distance, current, and system voltage to compute voltage drop percentage. Helps ensure electrical circuits stay within the recommended 3-5% voltage drop for branch circuits.

Voltage drop is the loss of voltage along an electrical wire as current flows through it. All real wires have small but non-zero resistance, so the voltage at the load is slightly less than at the source. For short runs at modest current, the drop is negligible. For long runs or high current, drop becomes significant — equipment may underperform, fail to start, or operate erratically.

The National Electrical Code (NEC) recommends keeping voltage drop under 3% on branch circuits and 5% total from service entrance to farthest outlet. Inside this range, motors start reliably, lights illuminate fully, electronics operate within spec, and heaters reach rated output. Exceeding these limits causes practical problems and wastes energy as heat in the wires.

The drop depends on four things: current (more current = more drop), wire length (longer = more drop, counting round trip), wire size (thicker = less resistance), and material (copper better than aluminum for same gauge). The formula uses Ohm's Law: V_drop = I × R_total, where R_total is the wire's total round-trip resistance.

Wire gauge (AWG) standardizes wire sizes. Smaller AWG numbers = thicker wire (counterintuitive but true). 14 AWG = ~1.6 mm diameter (typical 15A branch circuit). 6 AWG = ~4.1 mm. 4/0 AWG = ~11.7 mm (large feeder cables). Each step in AWG roughly doubles cross-sectional area.

Common applications: residential and commercial wiring design, long agricultural runs (well pumps, barns), solar PV systems, EV charging installations, industrial machinery wiring, marine electrical, and any electrical run longer than ~50 feet at significant current.

Inputs

System Voltage (V)

Current (Amps)

One-Way Distance (ft)

Wire Gauge (AWG)

Wire Material

Results

Voltage Drop

5.94 V

Drop %

4.95%

End Voltage

114.06 V

Assessment

Acceptable (3-5%)

Voltage Drop Details

Parameter	Value
System Voltage	120 V
Current	15 A
One-Way Distance	100 ft
Round-Trip Wire	200 ft
Wire Gauge	12 AWG (copper)
Wire Resistance	0.3960 ohms
Voltage Drop	5.94 V
Drop Percentage	4.95%
End Voltage	114.06 V
Assessment	Acceptable (3-5%)

Last updated: May 29, 2026

Formula

**Voltage drop (basic):** V_drop = I × R_total Where: - I = current (A) - R_total = total round-trip resistance (Ω) **Round-trip resistance:** R_total = R_per_foot × 2 × length_one_way Multiply by 2 because current goes out AND comes back. **For DC or single-phase AC:** V_drop = (2 × L × R_per_kft × I) / 1000 Where L = one-way length in feet, R_per_kft = ohms per 1000 feet. **For three-phase:** V_drop = (√3 × L × R_per_kft × I) / 1000 **Resistance per 1,000 feet (copper):** | AWG | mm² | Ω/kft | |---|---|---| | 14 | 2.08 | 2.525 | | 12 | 3.31 | 1.588 | | 10 | 5.26 | 0.999 | | 8 | 8.37 | 0.628 | | 6 | 13.3 | 0.395 | | 4 | 21.2 | 0.249 | | 2 | 33.6 | 0.156 | | 1 | 42.4 | 0.124 | | 1/0 | 53.5 | 0.098 | | 2/0 | 67.4 | 0.078 | | 3/0 | 85.0 | 0.062 | | 4/0 | 107 | 0.049 | Aluminum is ~1.6× higher (uses bigger gauge for same drop). **Worked example: 100-foot circuit** Setup: 15A, 120V, 100 ft, 12 AWG copper. R per ft (12 AWG copper) = 0.00159 Ω/ft. Round-trip: 2 × 100 = 200 ft. R_total = 200 × 0.00159 = 0.318 Ω. V_drop = 15 × 0.318 = 4.77 V. Drop %: 4.77/120 = 3.98% ✓ (just over 3% NEC recommended). Solution: upgrade to 10 AWG. R per ft = 0.000999. R_total = 0.200 Ω. V_drop = 15 × 0.200 = 3 V. Drop %: 2.5% ✓ (within NEC). **Wire size recommendations:** NEC ampacity (max current) and voltage drop both must be satisfied. | Wire | NEC ampacity (75°C) | Typical use | |---|---|---| | 14 AWG | 15 A | 15 A circuits (lighting) | | 12 AWG | 20 A | 20 A circuits (outlets) | | 10 AWG | 30 A | 30 A circuits (dryer, AC) | | 8 AWG | 50 A | 50 A circuits (range, EV charger) | | 6 AWG | 65 A | 60 A subpanel feed | | 4 AWG | 85 A | Larger subpanel | | 2 AWG | 115 A | Main feeder for small home | | 1/0 AWG | 150 A | Main feeder for home | | 2/0 AWG | 175 A | 200A home service | | 4/0 AWG | 230 A | 200A+ service, large building | **Voltage drop guidelines:** | Limit | Application | |---|---| | 1-2% | Sensitive electronics | | 3% | Branch circuit (NEC recommend) | | 5% | Total from service entrance | | 10% | Maximum tolerable (some lights/motors) | | 15%+ | Equipment may fail to start/operate | **Quick formula (copper, 1% drop):** For 120V single-phase, max one-way distance: L_max = (0.01 × 120) / (2 × R_per_ft × I) For 12 AWG copper @ 15A: L_max = 1.2 / (2 × 0.00159 × 15) = 25 ft for 1% drop. For 3% drop: L_max ≈ 75 ft. For 5% drop: L_max ≈ 125 ft. Long runs need upsized wire. **Aluminum vs copper:** Aluminum: 1.6× resistance of copper for same gauge. - For same drop: aluminum needs 2-3 sizes thicker. - Or copper at smaller (and cheaper) size. | Equivalent | Copper | Aluminum | |---|---|---| | Same ampacity | 12 AWG | 10 AWG | | Same ampacity | 6 AWG | 4 AWG | | Same ampacity | 1/0 AWG | 2/0 AWG | | Same ampacity | 4/0 AWG | 250 kcmil | Aluminum is cheaper but requires antioxidant compound and proper terminations. **Temperature effect:** Wire resistance increases ~0.4% per °C above 20°C. At 75°C (max operating temp): ~22% higher than 20°C. NEC tables typically use 75°C ratings; cold-weather actual drop is slightly lower. **Worked example: solar PV system** 200 ft from inverter to combiner box. 30A DC current at 600V. Allowable drop: 2% = 12V. R_total max = 12/30 = 0.4 Ω. R_per_kft max = 0.4 / (2 × 200/1000) = 1.0 Ω/kft. Use 10 AWG copper (0.999 Ω/kft) ✓. **Special considerations:** **Motor starting:** during start, motors draw 5-7× running current. Voltage drop must be acceptable at start surge. Often requires larger wire than steady-state suggests. **LED lights:** very sensitive to voltage. 10% low can cause flicker, dimming, or driver failure. **Electric motors:** below 90% rated voltage, motors run hot and may fail to start. Cumulative damage with sustained undervoltage. **Heaters:** voltage drop reduces output (P = V²/R). 10% voltage drop = 19% power drop. **Electronics:** most modern equipment has wide tolerance (90-110%) but precision instruments may need tighter. **Common applications:** - **Long branch circuits**: kitchen, basement extensions. - **Outbuildings**: detached garage, shed, barn. - **Well pumps**: often 300+ ft from house. - **EV chargers**: 50A circuits with significant runs. - **Pool/spa equipment**: typically far from main panel. - **Subpanels**: feeder calculations. - **Solar PV**: DC and AC runs. - **Marine wiring**: salt corrosion increases resistance over time. **Voltage drop and power loss:** Power lost in wire = I² × R = I × V_drop. For our example: 15A × 4.77V = 71.5 W lost as heat in wire. Over a year of continuous use: 626 kWh wasted (~$94 at $0.15/kWh). Larger wire pays back over time via reduced waste.

How to use this calculator

Enter system voltage (typically 120V or 240V residential US).
Enter circuit current in amps (look at breaker rating or appliance label).
Enter one-way distance in feet (calculator handles round-trip).
Choose wire gauge (AWG); smaller number = thicker wire.
Choose copper or aluminum material.
Calculator returns voltage drop in volts and percentage.
Target: < 3% drop on branch circuits per NEC recommendation.

Worked examples

Kitchen outlet circuit

**Scenario:** 20A circuit, 12 AWG copper, 75 ft one-way (140 ft round-trip in real wire). 120V supply. **Calculation:** R_total = 2 × 75 × 0.00159 = 0.239 Ω. V_drop = 20 × 0.239 = 4.77 V. Drop %: 3.98% — slightly over NEC 3% recommendation. **Result:** Upgrading to 10 AWG: V_drop = 20 × 2 × 75 × 0.000999 = 3.0 V = 2.5% ✓. For long runs, oversize wire by one or two AWG sizes for safety margin and future-proofing.

EV charger installation

**Scenario:** Level 2 EV charger, 40A continuous (50A circuit). 240V split phase. 80 ft from panel to garage. **Calculation:** 8 AWG copper @ 40A: R_total = 2 × 80 × 0.000628 = 0.100 Ω. V_drop = 40 × 0.100 = 4 V. Drop %: 1.67% ✓. **Result:** 8 AWG is fine for both ampacity (50A) and voltage drop (1.67%). Use 6 AWG for greater margin or planned future expansion. Voltage drop matters less here than for sensitive equipment, but still keep under 3%.

Detached garage feeder

**Scenario:** 50A feeder to detached garage, 150 ft from main panel, 240V. **Calculation:** 6 AWG copper: R = 2 × 150 × 0.000395 = 0.119 Ω. V_drop = 50 × 0.119 = 5.93 V. Drop %: 2.47% ✓. Aluminum equivalent: 4 AWG. **Result:** 6 AWG copper or 4 AWG aluminum. For 200 ft, upgrade to 4 AWG copper or 2 AWG aluminum. Larger gauges save money long-term through reduced losses, especially if running power-hungry equipment continuously.

When to use this calculator

**Use voltage drop calculations for:**

- **Residential branch circuits** longer than ~50 ft. - **Subpanels and feeders** in any building. - **Outbuildings** (garages, sheds, barns). - **Solar PV systems**: DC and AC sides. - **EV charging installations**: high-current dedicated circuits. - **Industrial machinery**: motor and equipment feeds. - **Long agricultural runs**: well pumps, irrigation. - **Marine electrical**: harsh environments worsen connections.

**NEC requirements:**

NEC doesn't *require* meeting voltage drop limits (only ampacity), but Articles 210.19 and 215.2 recommend: - Branch circuit: 3% maximum. - Combined feeder + branch: 5% maximum.

State and local codes may make these mandatory.

**Why exceeding drop limits matters:**

- **Lighting**: dim or flickers (especially LEDs). - **Motors**: starting issues, overheating, premature failure. - **Heaters**: reduced output (V² dependence in P = V²/R). - **Electronics**: erratic operation or damage. - **Energy waste**: lost as heat in wire.

**Sizing wire for long runs:**

1. Start with NEC-required ampacity. 2. Calculate voltage drop for that size. 3. If drop > 3%, increase wire size. 4. Recalculate until both ampacity AND drop OK.

Often the controlling factor for long runs is voltage drop, not ampacity.

**Common applications:**

- **EV chargers** (Tesla Wall Connector, ChargePoint): 40-80A circuits, long runs. - **Wells** (submersible pump): 200-500 ft from house, 240V single phase. - **HVAC** (mini-split, heat pump): 30-50A circuits. - **Solar inverters**: DC string lines, AC feeders. - **Welders** (industrial): 50-100A dedicated circuits. - **Workshop machines** (table saw, compressor): 20-50A circuits. - **Pool/spa equipment**: heaters, pumps.

**Three-phase calculations:**

For 3-phase: V_drop = √3 × L × R_per_kft × I / 1000

Where V_drop is line-to-line drop.

Three-phase systems typically distribute power more efficiently than single-phase for same total power.

**Aluminum cable considerations:**

Aluminum is cheaper but requires: - Antioxidant grease at terminations. - Listed connectors (most are stricter about aluminum). - Routine retorquing (aluminum creeps). - Larger conduit fill due to bigger cable.

Modern aluminum cable (AA-8000 series) is much improved over older versions and is used commonly for service entrances.

**Software:**

- **Online voltage drop calculators**: many free, often state-code specific. - **AGI32**: commercial lighting design with VD calculations. - **ETAP, SKM**: industrial power design. - **Solar PV design**: PVsyst, Aurora, Helioscope.

**Pitfalls:**

- **Forgetting round-trip**: voltage drop uses 2× one-way distance. - **Using ampacity-only sizing**: drop may dictate larger wire. - **Ignoring starting surge**: motors need larger wire for inrush. - **Mixing AWG and mm² confusion**: AWG decreases as size increases. - **Aluminum vs copper**: aluminum needs ~2 sizes larger. - **Temperature derating**: hot conduits reduce ampacity and increase resistance. - **Voltage drop on neutral**: 240V circuits with imbalanced loads see drop on neutral too. - **Forgetting conduit fill**: code limits cables per conduit.

Common mistakes to avoid

Forgetting voltage drop uses round-trip distance (2× one-way).
Sizing wire for ampacity only, ignoring voltage drop for long runs.
Using aluminum tables when copper is installed (or vice versa).
Forgetting motor starting current (5-7× running) when sizing.
Mixing AWG and mm² (AWG decreases as wire gets thicker).
Treating residential 120V same as commercial 277V (different drop %).
Ignoring temperature derating for hot conduits.
Using 60°C wire ratings when 75°C or 90°C is appropriate.

Frequently Asked Questions

Sources & further reading

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