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:

Dimming of lights  If voltage drops below design levels, lighting will dim. This degrades lighting quality.

Overheating of motors  Motors rely on rated voltage. If voltage drops, motors draw more current to produce needed power. This causes overheating and reduces motor life.

Nuisance tripping  Components like contactors and relays may unexpectedly trip if voltage drops too far below ratings.

Fire hazards  Wires and connections heat up when excessive current flows due to low voltage. This creates fire risks.
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
 Longer wire runs increase resistance and voltage drop. For long runs, larger wire sizes are needed.
Wire Size (Ampacity)
 Smaller wires have higher resistance. Choose larger wires to minimize voltage drop.
Current Draw of Loads
 Higher load currents cause greater voltage drops across wires. Size wires based on expected load current.
Power Source Impedance
 Generators, transformers, and utilities add impedance that contributes to voltage drop. Account for this impedance.
Power Factor
 Loads like motors have inductive reactance that increases impedance and voltage drop. Consider power factor.
3% Voltage Drop Limit
 Most electrical codes limit voltage drop to 3% to ensure proper equipment operation.
Understanding these factors allows proper wire sizing and voltage drop control. Next I'll go over the stepbystep calculation process.
StepbyStep Voltage Drop Calculation
Calculating voltage drop involves just a few steps:
1. Determine Design Current

For each circuit, calculate expected load current based on connected lighting, motors, appliances, etc.

For continuous loads like lighting, use average expected current. For motors, use coderequired motor load currents.

Add together currents of all loads on a circuit to obtain the design current.
2. Determine Resistance of Wire Run

Find the total length of the wire run from the power source to the load in feet. Include length going to and returning from the load.

Determine the wire size you plan to use for the run.

Look up the resistance per 1000 feet for the wire size from wire resistance tables.

Calculate the total resistance of the run:
Total resistance (Ω) = Resistance per 1000 feet (Ω/1000 ft) x Total wire run length (ft) / 1000
3. Use the Voltage Drop Formula
 Use the voltage drop formula:
Percentage voltage drop = (Current (amps) x Total resistance (Ω) / Voltage (volts)) x 100

For 120V circuits, use 120 volts in the formula. For 208V, 240V, 277V, or 480V circuits, use the applicable system voltage.

The calculated voltage drop percentage must not exceed 3%. If it does, increase the wire size and recalculate.
4. Check Voltage Drop to Farthest Load
 For branch circuits, check voltage drop from the panelboard to the load farthest from the source. Voltage drop to the worstcase load must not exceed 3%.
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

8 total receptacles with computers, monitors, printers etc.

Estimated total load current is 12A
Wire Run Details

120 feet total distance from panel to farthest receptacle

12 AWG copper wire will be used
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:

For feeders and branch circuits over 100 feet long, consider going to the next larger standard wire size.

Use copper wiring rather than aluminum, since copper has lower resistance per foot.

Use 120V distribution for lighting circuits rather than 277V to minimize voltage drop.

Install panelboards and disconnect switches close to loads to minimize wire runs.

Use lower‐voltage systems like 12V or 24V DC for critical equipment sensitive to voltage drop.
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.