How to Calculate Voltage Drop for Commercial Buildings

When designing electrical systems for commercial buildings, it is crucial to calculate voltage drop to ensure efficient and safe operation of lighting, equipment, and appliances. Voltage drop is the reduction in voltage in an electrical circuit between the power source and load. Excessive voltage drop can cause motors and lighting to operate inefficiently. I'll explain the key factors to consider when performing voltage drop calculations for commercial buildings.

Why Voltage Drop Calculations Are Important

Performing accurate voltage drop calculations helps prevent:

Voltage drop calculations ensure electrical systems function as intended. Correct voltage allows lighting, equipment, and appliances to operate efficiently and safely.

Key Factors That Impact Voltage Drop

Several key factors determine voltage drop in an electrical circuit:

Length of Wire Runs

Wire Size (Ampacity)

Current Draw of Loads

Power Source Impedance

Power Factor

3% Voltage Drop Limit

Understanding these factors allows proper wire sizing and voltage drop control. Next I'll go over the step-by-step calculation process.

Step-by-Step Voltage Drop Calculation

Calculating voltage drop involves just a few steps:

1. Determine Design Current

2. Determine Resistance of Wire Run

Total resistance (Ω) = Resistance per 1000 feet (Ω/1000 ft) x Total wire run length (ft) / 1000

3. Use the Voltage Drop Formula

Percentage voltage drop = (Current (amps) x Total resistance (Ω) / Voltage (volts)) x 100

4. Check Voltage Drop to Farthest Load

Following these steps will ensure voltage drop is within acceptable limits. Let's look at a quick example calculation.

Voltage Drop Example

Let's calculate voltage drop for a 120V, 20A branch circuit supplying receptacles in an office:

Load Details

Wire Run Details

Resistance per 1000ft for #12 AWG wire is 1.588Ω (from table)

Voltage drop formula:

Percentage drop = (Current x Resistance / Voltage) x 100

Voltage drop = (12A x (120ft x 1.588Ω / 1000ft) / 120V) x 100

Voltage drop = (12 x 0.19Ω / 120V) x 100 = 1.92%

The 1.92% drop is less than 3% so is acceptable. A #12 AWG wire size is adequate for this circuit.

This example demonstrates how a few simple calculations can prevent problems from excessive voltage drop.

Additional Recommendations

Here are some additional tips for controlling voltage drop in commercial facilities:

Properly accounting for voltage drop during design will prevent problems and provide quality, reliable electrical power. Let me know if you need any clarification on these voltage drop calculation procedures. I'm happy to provide additional examples or explanations.