TradeCalc

Voltage Drop Calculator

Calculate voltage drop for copper and aluminum wire based on wire size, length, and load current. Uses NEC resistance tables for accurate results.

How to Calculate Voltage Drop

What Is Voltage Drop?

Voltage drop is the reduction in voltage that occurs as electrical current flows through a wire due to the wire's inherent resistance. Every conductor — whether copper or aluminum — opposes the flow of current, and this opposition converts some electrical energy into heat. The longer the wire run and the higher the current, the greater the voltage drop.

Excessive voltage drop causes motors to overheat, lights to dim, and equipment to malfunction. The National Electrical Code (NEC) recommends keeping voltage drop within acceptable limits to ensure safe and efficient operation.

The Voltage Drop Formula

Single-phase: VD = (2 × L × R × I) / 1000

Three-phase: VD = (√3 × L × R × I) / 1000

VD%: (VD / Voltage) × 100

Where L = one-way distance (ft), R = resistance per 1000 ft (Ω/kft) from NEC Chapter 9 Table 8, I = load current (A)

The NEC recommends a maximum of 3% voltage drop for branch circuits and 5% total (feeder + branch circuit) per NEC 210.19 and 215.2. While these are fine print notes (FPN), most jurisdictions treat them as mandatory.

Worked Example

Scenario: A 20A, 240V single-phase load is located 150 ft from the panel. The wire is 12 AWG copper.

Resistance of 12 AWG copper = 1.98 Ω/1000 ft (NEC Table 8)
VD = (2 × 150 × 1.98 × 20) / 1000 = 11.88V
VD% = (11.88 / 240) × 100 = 4.95%
This exceeds the 3% limit — upsize to 10 AWG (R = 1.24):
VD = (2 × 150 × 1.24 × 20) / 1000 = 7.44V → 3.10% (still over!)
Upsize to 8 AWG (R = 0.778): VD = 4.67V → 1.95% ✓

Practical Tips

Always calculate voltage drop before pulling wire — upsizing after the fact is expensive.
Aluminum wire has ~64% more resistance than copper for the same size. Upsize 1–2 AWG sizes when using aluminum.
For three-phase systems, the √3 multiplier (1.732) is smaller than the single-phase multiplier of 2, so three-phase runs typically have less voltage drop.
Long runs to outbuildings, well pumps, or remote equipment are the most common source of excessive voltage drop.

Code References

NEC 210.19, NEC 215.2

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Frequently Asked Questions

What is the maximum allowable voltage drop according to the NEC?

The NEC recommends a maximum voltage drop of 3% for branch circuits and 5% total for feeders plus branch circuits (NEC 210.19 and 215.2). While these are recommendations (fine print notes), not mandatory requirements, most jurisdictions enforce them as code.

Does voltage drop differ between single-phase and three-phase systems?

Yes. For single-phase systems, the multiplier is 2 (current travels out and back). For three-phase systems, the multiplier is √3 (approximately 1.732) because the voltage between phases is lower relative to the line-to-neutral voltage.

Why does aluminum wire have more voltage drop than copper?

Aluminum has approximately 61% of the conductivity of copper, meaning it has higher resistance per unit length. For the same wire size, aluminum will produce roughly 64% more voltage drop than copper. This is why aluminum wire must typically be sized one or two sizes larger than copper for the same application.