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Voltage Drop Calculator

Calculate voltage loss in electrical wires

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Formulas

1-PhaseVd = 2 ร— I ร— R ร— L รท 1000
3-PhaseVd = โˆš3 ร— I ร— R ร— L รท 1000

NEC Guidelines

Circuit TypeMax Drop
Branch Circuits3%
Feeders2%
Total Combined5%

Wire Resistance (ฮฉ/1000ft)

AWGCopperAluminum
142.5254.04
121.5882.54
100.9991.60
80.6281.00

Related

Wire GaugeOhm's Law

Understanding Voltage Drop in Electrical Circuits

Voltage drop is the decrease in electrical voltage that occurs when current flows through a conductor due to its inherent resistance. Every wire has some resistance that causes a portion of electrical energy to be converted into heat, resulting in lower voltage at the load end.

Why Voltage Drop Matters

Excessive voltage drop can cause: dimming lights, motor overheating, equipment malfunction, reduced efficiency, and potential safety hazards. Motors are particularly sensitive - running at low voltage causes them to draw more current, leading to overheating and premature failure.

Factors Affecting Voltage Drop

Several factors influence voltage drop: Wire length (longer runs = more drop), Wire size (smaller gauge = more resistance), Current draw (higher amperage = more drop), Wire material (aluminum has ~60% more resistance than copper), and Temperature (higher temps increase resistance).

How to Reduce Voltage Drop

If your calculated voltage drop exceeds acceptable limits: increase wire gauge, shorten wire runs, use copper instead of aluminum, increase system voltage (240V has half the drop of 120V for same power), or run parallel conductors.

Frequently Asked Questions

Voltage drop is the reduction in electrical voltage as current flows through a wire's resistance. It matters because excessive drop causes lights to dim, motors to overheat, equipment to malfunction, and energy waste. The NEC recommends keeping voltage drop below 3% for branch circuits and 5% total.

Per NEC recommendations: Branch circuits should have no more than 3% voltage drop, feeders no more than 2%, and the total combined drop should not exceed 5%. For sensitive electronic equipment, aim for less than 2% voltage drop.

For single-phase: Vdrop = (2 ร— K ร— I ร— L) / CM, where K is resistivity constant (12.9 copper, 21.2 aluminum), I is current, L is one-way length, CM is circular mils. For three-phase, replace 2 with 1.732 (โˆš3). Alternatively: Vdrop = (2 ร— I ร— R ร— L) / 1000 using resistance per 1000 feet.

The formula already accounts for round-trip by multiplying by 2 (single-phase) or โˆš3 (three-phase). Enter only one-way distance from source to load. The calculation automatically doubles this to account for current flowing through both hot and neutral conductors.

Copper has about 40% less resistance than aluminum, making it better for minimizing voltage drop at the same wire gauge. However, aluminum is lighter and cheaper. To achieve equivalent performance, use aluminum wire approximately 2 AWG sizes larger than copper (e.g., #4 aluminum instead of #6 copper).

Yes, voltage drop represents energy lost as heat in the wiring, which you pay for but don't use productively. Additionally, motors may draw more current to compensate for low voltage, further increasing energy consumption. Minimizing voltage drop improves efficiency and reduces operating costs.

Higher voltage systems have lower percentage voltage drop for the same power delivery. A 240V circuit delivering the same wattage as a 120V circuit has half the current and therefore half the voltage drop. This is why long runs often use 240V and industrial facilities use 480V or higher.

Use the Wire Gauge Calculator, or calculate: CM = (2 ร— K ร— I ร— L) / Vdrop, where K is 12.9 for copper or 21.2 for aluminum, I is current, L is one-way length, and Vdrop = voltage ร— 0.03. Match the calculated circular mils to standard wire sizes.