Voltage Drop 101: When and How to Account for It

Introduction

Voltage drop is a common but often overlooked issue in electrical design. When conductors are too small or runs are too long, resistance causes a reduction in voltage at the load end of a circuit—leading to performance issues, overheating, and code violations.

This article explains what voltage drop is, when it becomes a concern, how to calculate it, and how the NEC addresses it.

What Is Voltage Drop?

• Voltage drop refers to the reduction in voltage as electrical current travels through wire.

• All conductors have resistance; the longer or smaller the wire, the greater the voltage drop.

• This can lead to lights dimming, motors overheating, or sensitive equipment malfunctioning.

Key Variables That Affect Voltage Drop:

• Conductor size (AWG)

• Conductor material (copper vs. aluminum)

• Current (amps)

• Length of run (one-way or round trip)

• Voltage level (120V, 208V, 240V, 480V, etc.)

• Power factor for AC circuits

When Does Voltage Drop Become a Problem?

NEC Guidance (Informational, Not Enforced)

• NEC Informational Note in 210.19(A)(1) and 215.2(A)(1) recommends limiting total voltage drop to:

  • 3% on branch circuits

  • 5% total for feeder + branch

• While these are not enforceable code requirements, many jurisdictions adopt them as best practice or mandate them under local rules.

Practical Guidelines:

• Any feeder or branch over 100 feet should be reviewed for voltage drop.

• Sensitive loads like motors, HVAC, and electronics require tighter voltage tolerances.

• Utility voltage fluctuations may combine with drop, pushing total variation over safe thresholds.

How to Calculate Voltage Drop

Basic Formula (Single-phase AC):
Voltage Drop (V) = (2 × K × I × L) ÷ CM
Where:

• K = Resistance constant (12.9 for copper, 21.2 for aluminum)

• I = Current in amperes

• L = One-way length in feet

• CM = Circular mil area of the conductor (e.g., #6 AWG = 26240)

Three-phase AC circuits:
Voltage Drop (V) = (√3 × K × I × L) ÷ CM

You can also use NEC tables, calculator apps, or manufacturer tools for quick reference.

Real-World Example

Load: 40A EV charger
Voltage: 240V single-phase
Distance: 120 feet one way
Conductor: #8 AWG copper (16510 CM)
Voltage Drop = (2 × 12.9 × 40 × 120) ÷ 16510 ≈ 7.5 volts
Drop Percentage = (7.5 ÷ 240) × 100 ≈ 3.1%

➡ Result: Voltage drop exceeds the recommended 3% threshold. Upsize to #6 copper or shorten the run.

How to Minimize Voltage Drop

1. Upsize the Conductor

   • The most common fix; larger wires have less resistance.

2. Reduce Run Length

   • Reposition panels or equipment to shorten the circuit.

3. Use Copper Instead of Aluminum

   • Copper has lower resistance per foot.

4. Balance Loads Across Phases

   • Reduces current on each conductor.

5. Use Higher Voltage

   • Higher voltage = lower current = lower voltage drop (e.g., 277V instead of 120V).

6. Parallel Conductors

   • Splitting current across multiple conductors reduces drop and heat buildup.

NEC Considerations

• NEC doesn’t enforce voltage drop limits but includes them as informational notes in:

  • 210.19(A)(1) – Branch circuit conductors

  • 215.2(A)(1) – Feeder conductors

  • 310.15 – Ampacity adjustment

• Energy code, engineering specs, or inspector requirements may enforce stricter limits.

• Drop must not cause total voltage at load to fall below equipment minimum (e.g., motor nameplate).

Common Pitfalls

• Failing to check drop in long rural runs or garages.

• Using voltage drop as an afterthought instead of designing for it up front.

• Undersizing conductors to save cost—leading to callbacks, failures, or overheating.

• Assuming the NEC's 3% and 5% limits are optional everywhere (some AHJs adopt them as enforceable rules).

Conclusion

Voltage drop isn’t just a theoretical issue—it’s a real-world performance and safety concern. Understanding how to identify, calculate, and mitigate it is key for electricians, designers, and contractors. Even though the NEC treats voltage drop as a recommendation, smart professionals treat it as essential.

References

• NEC 2023:

  • 210.19(A)(1) – Branch circuit conductor sizing

  • 215.2(A)(1) – Feeder conductor sizing

  • 310.15 – Ampacity and conductor sizing

• IEEE Std 141 (Red Book) – Voltage drop in distribution systems

• UL White Book – Equipment voltage tolerances

• IAEI – Inspector guidance on conductor sizing and voltage drop

Disclaimer:
The information, tools, and resources provided on this website are for general informational purposes only. While Empower Engineering strives to ensure accuracy and relevance, no guarantee is made regarding the completeness or applicability of the content to specific projects. Users are responsible for verifying all calculations, code interpretations, and design decisions with licensed professionals and local authorities. Empower Engineering assumes no liability for any loss, damage, or code violation resulting from the use of this website’s content.

Empower Engineering | July 15, 2025