Voltage Drop Calculator

Understanding Voltage Drop

When electric current travels through a wire, it faces opposition caused by the wire’s properties. This opposition leads to a reduction in voltage, known as voltage drop. In the case of alternating current (AC), this opposition is called impedance, which includes both resistance and reactance (the response of an electric field to changes in current). For direct current (DC), only resistance matters.

If voltage drop is too high, it can cause lights to flicker or dim, heaters to perform poorly, and motors to overheat and fail. To avoid these problems, it’s recommended that voltage drop remains under 5% under full load conditions. Using the correct wire size and minimizing the use of extension cords are key ways to control voltage drop.

Main Factors Causing Voltage Drop

1. Wire Material

The type of material used affects conductivity. Metals like silver, copper, gold, and aluminium offer high conductivity, but copper and aluminium are the most widely used due to cost-effectiveness. Copper is a better conductor than aluminium, resulting in lower voltage drop for the same size and length of wire.

2. Wire Size

Wire diameter significantly impacts voltage drop. Thicker wires (larger gauge sizes) have lower resistance and, therefore, less voltage drop.

• In American Wire Gauge (AWG), every 6-gauge decrease doubles the diameter, and every 3-gauge decrease doubles the cross-sectional area.

• In the Metric Gauge system, gauge equals 10 times the wire’s diameter in millimeters (e.g., a 50-gauge wire is 5 mm in diameter).

3. Wire Length

Longer wires create more voltage drop. While this usually isn’t a problem inside homes, it can be significant when running cables to outdoor structures like sheds, well pumps, or detached garages.

4. Amount of Current (Ampacity)

The higher the current flowing through a wire, the greater the voltage drop.
Ampacity — short for ampere capacity — defines how much current a wire can carry without overheating. Factors affecting ampacity include:

• Wire material

• Alternating current frequency

• Ambient temperature

• Cable bundling (grouped cables generate more heat)

Strict guidelines exist for bundling cables to manage heat and maintain safety.

Choosing the Right Cable

When selecting cables:

1. Current Load: Ensure the cable can handle the maximum expected current even under the highest temperature conditions.

2. Safety: The cable should provide reliable grounding to limit exposure to dangerous voltages and ensure fast circuit breaker operation in case of faults.

Voltage Drop Calculation

The basic voltage drop formula is derived from Ohm’s Law:

Vdrop = I × R

where:

• I = current in amperes

• R = wire resistance in ohms

Resistance is often given per unit length (e.g., ohms per kilometer or per 1000 feet). Because wires usually carry current both to and from a load, the formula for DC or single-phase circuits becomes:

Vdrop = 2 × I × R × L

For three-phase circuits, the voltage drop is calculated using:

Vdrop = √3 × I × R × L

where:

• I = current in amperes

• R = resistance per unit length

• L = one-way distance (length) in the circuit

Typical American Wire Gauge (AWG) Sizes

The AWG system is mainly used in North America to specify wire diameters for solid, non-ferrous, electrically conductive wires. Here’s a reference for common AWG sizes: