Voltage drop is an important concept for anyone working with electrical systems. It refers to the reduction in voltage that occurs as electricity flows through wires, cables, and other conductors. Without accounting for voltage drop, your electrical system may not function properly. In this comprehensive guide, I will explain what voltage drop is, why it matters, and how to calculate it for your entire electrical system.
What is Voltage Drop?
Voltage drop occurs due to resistance in conductors like wires and cables. As electric current flows through a conductor, it encounters resistance. This resistance restricts the flow of electrons, dissipating some energy as heat. The effect is a reduction in voltage along the conductor. The longer the conductor, the higher its resistance, and the greater the voltage drop.
In electrical terms, voltage drop is calculated as:
Voltage Drop = Current (amps) x Resistance (ohms)
The amount of voltage drop depends on:
 Length of the conductor  Longer cables or wires have higher resistance.
 Crosssectional area  Thicker wires can carry more current and have lower resistance.
 Temperature  Resistance increases with heat, which increases voltage drop.
 Material  Some materials like copper have lower resistance than others.
Why Voltage Drop Matters
Voltage drop matters because your electrical system components require steady, optimum voltage to operate correctly.
Excessive voltage drop can lead to:
 Dim lights and motors running slower due to reduced voltage.
 Overheated conductors as they carry more current to compensate.
 Circuit breakers tripping or fuses blowing.
 Damage to sensitive electronics and appliances.
To avoid these issues, you need to account for voltage drop when designing and installing electrical systems. The key is to size conductors appropriately and use voltage drop calculations to ensure optimum voltage at each device.
StepbyStep Voltage Drop Calculation
To calculate voltage drop for an entire electrical system:
1. Understand the System Layout
 Identify all circuits, wire runs, conductors, and connected devices.
 Note the length of each wire run and gauge of conductors used.
 Find the wattage or amp rating for all lamps, appliances, motors etc.
 Identify the main service panel voltage. A common value is 120/240V.
This provides the information needed for voltage drop calculations.
2. Calculate Voltage Drop for Individual Circuits
For each circuit:
 Find the load current by dividing the total watts by the voltage. For example, a 1200W 120V appliance would draw 10A (1200/120 = 10A).
 Look up the resistance per unit length for the gauge of wire used. Resistance tables are readily available online.
 Multiply the resistance/length by total wire run length to get total resistance.
 Use the voltage drop formula: Voltage Drop = Current (amps) x Resistance (ohms).
Do this calculation for every wire run on each circuit. Sum the voltage drops for all wire runs on a circuit to get the total circuit voltage drop.
3. Analyze Voltage Drop Impact
Compare the calculated voltage drops to allowable limits:
 Branch circuits  3% max voltage drop is ideal, 5% acceptable.
 Feeders  2% max voltage drop is preferable, 3% acceptable.
If any circuit exceeds limits, use thicker wire or shorter runs to reduce voltage drop.
4. Calculate Voltage Drop to Farthest Outlet
Determine the outlet/device farthest from the main panel on each circuit. Sum the voltage drops of all conductors between the main panel and the farthest outlet. This provides the worstcase voltage drop to ensure adequate voltage.
5. Consider Simultaneous Loads
The above calculations assume only one device operates at a time. For multiple loads used simultaneously, sum their amperages and recalculate voltage drops. This gives a more realistic voltage drop with everything turned on.
6. Select Appropriate Wire Size
Use the results to select appropriate wire sizes for your system. Choose wire gauges that keep voltage drop within acceptable limits.
Using these steps provides insight into voltage drop in your specific electrical system layout. Running the calculations during design helps size conductors appropriately before installation.
RealWorld Considerations
A few additional tips for voltage drop calculations:

Measuring actual current draw and using multimeters to check voltage drops validates the theoretical calculations.

Long motor runs have additional voltage drop when starting due to high startup current. Account for inrush current in voltage drop calculations.

Lighting circuits have different voltage drops with all lamps on vs just a few. Calculate both scenarios.

Wiring insulation quality affects conductor resistance. Consult wire manufacturers for actual resistance values.

Power factor, harmonics, and other variables can impact voltage drop. Use appropriate engineering factors for precise results.
Conclusion
Voltage drop is a critical electrical design consideration, but it is easy to calculate with some basic information about your system. Following the stepbystep process outlined helps ensure adequate voltage for reliable device operation and prevents issues like overheating or premature failures. Proper conductor sizing using voltage drop calculations is key for any new or upgraded electrical installation.