Voltage Drop Calculator — NEC (Copper)

Calculate

System Voltage

Select the nominal system voltage for the circuit

Phase Configuration

Select single-phase or three-phase circuit configuration

Load Current(A)

Enter the expected load current in amperes

One-Way Wire Length(ft)

Enter the one-way distance from the panel to the load

Wire Size (AWG/kcmil)

Select wire gauge — resistance values from NEC Ch.9 Table 8 (uncoated copper, Ω/km)

Conductors per Phase

Number of conductors per phase (parallel runs reduce effective resistance)

Overview

A Voltage Drop Calculator helps determine the voltage lost across a conductor due to its resistance when carrying current over a given distance. This calculator uses NEC Chapter 9 Table 8 DC resistance values for uncoated copper conductors to estimate voltage drop for both single-phase and three-phase circuits.

Voltage drop is a critical consideration in electrical design because excessive drop can cause equipment malfunction, reduced efficiency, and code compliance issues. The NEC recommends — though does not mandate — that voltage drop on branch circuits not exceed 3%, and that total voltage drop from service entrance to the farthest outlet not exceed 5%.

This calculator is intended as a practical estimation tool for circuit planning, wire sizing verification, and preliminary electrical design. It does not replace detailed engineering analysis that accounts for conductor temperature, conduit fill, ambient conditions, or AC impedance effects.

How to Use This Calculator

Select system voltage — choose from 120 V, 208 V, 240 V.
Select phase configuration — choose from Single-Phase, Three-Phase.
Enter load current — in A.
Enter one-way wire length — in m or ft.
Select wire size (awg/kcmil) — choose from 14 AWG, 12 AWG, 10 AWG.
Select conductors per phase — choose from 1, 2, 3.
Click "Calculate" — get voltage drop, voltage drop percentage, voltage at load.

Use the result to support your engineering design and analysis decisions.

Inputs & Outputs

Inputs

•System Voltage — Options: 120 V, 208 V, 240 V, 277 V, 480 V, 600 V
•Phase Configuration — Options: Single-Phase, Three-Phase
•Load Current (A)
•One-Way Wire Length (m / ft)
•Wire Size (AWG/kcmil) — Options: 14 AWG, 12 AWG, 10 AWG, 8 AWG, 6 AWG, 4 AWG, 3 AWG, 2 AWG, 1 AWG, 1/0 AWG, 2/0 AWG, 3/0 AWG, 4/0 AWG, 250 kcmil, 300 kcmil, 350 kcmil, 500 kcmil, 600 kcmil, 750 kcmil
•Conductors per Phase — Options: 1 (Standard), 2 (Parallel), 3 (Parallel), 4 (Parallel)

Outputs

•Voltage Drop (V)
•Voltage Drop Percentage (%)
•Voltage at Load (V)

Formula

Calculator Formula

This calculator estimates voltage drop using DC resistance values from NEC Chapter 9 Table 8 for uncoated copper conductors.

Step 1: Effective Resistance

R_effective = R_wire / N_conductors

Where R_wire is the resistance per 1000 m (Ω/km) for the selected wire gauge, and N_conductors is the number of parallel conductors per phase.

Step 2: Phase Factor

Single-Phase: Factor = 2
Three-Phase:  Factor = √3 ≈ 1.732

Step 3: Voltage Drop

Vd = Factor × I × L × R_effective / 1000

Step 4: Voltage Drop Percentage

Vd% = (Vd / V_system) × 100

Step 5: Voltage at Load

V_load = V_system − Vd

Variables

Variable	Meaning	Units
V_system	Nominal system voltage	V
I	Load current	A
L	One-way wire length	m (ft converted internally)
R_wire	Wire resistance per NEC Ch.9 Table 8	Ω/km
N_conductors	Conductors per phase (parallel runs)	—
Factor	2 for single-phase, √3 for three-phase	—
Vd	Calculated voltage drop	V
Vd%	Voltage drop as percentage of system voltage	%
V_load	Voltage delivered at the load	V

Key Facts

Voltage drop is caused by the resistance of the conductor — longer runs and smaller wire sizes produce more drop.
The NEC recommends (but does not require) a maximum 3% voltage drop on branch circuits and 5% total from service entrance to farthest outlet.
Using parallel conductors per phase effectively reduces the resistance and voltage drop proportionally.
Copper conductors have lower resistance than aluminum for the same wire gauge, resulting in less voltage drop.
The factor of 2 for single-phase accounts for current flowing through both the hot and neutral conductors.
For three-phase circuits, the √3 (1.732) factor replaces the factor of 2 used in single-phase calculations.
Wire resistance increases with temperature — NEC Table 8 values are based on 75°C conductor temperature.

Applications

Branch circuit wire sizing verification for residential and commercial installations.
Feeder circuit voltage drop estimation for panel-to-panel runs.
Long-run circuit planning where voltage drop is a primary design concern.
Comparing wire gauge options to find the most cost-effective size that meets voltage drop targets.
Preliminary electrical design for new construction and renovation projects.
Evaluating whether parallel conductors are needed to reduce voltage drop on high-current circuits.
Educational reference for electricians, contractors, and engineering students.

Example Calculation

Example Calculation — Single-Phase

Given:

System voltage = 240 V
Phase = Single-phase
Load current = 40 A
One-way wire length = 30 m (≈ 100 ft)
Wire size = 8 AWG (R = 3.18 Ω/km)
Conductors per phase = 1

Step 1: Effective Resistance

R_effective = 3.18 / 1 = 3.18 Ω/km

Step 2: Phase Factor

Factor = 2 (single-phase)

Step 3: Voltage Drop

Vd = 2 × 40 × 30 × 3.18 / 1000 = 7.63 V

Step 4: Voltage Drop Percentage

Vd% = (7.63 / 240) × 100 = 3.18%

Step 5: Voltage at Load

V_load = 240 − 7.63 = 232.37 V

Interpretation: The voltage drop is 3.18%, which slightly exceeds the NEC-recommended 3% guideline for branch circuits. Consider upsizing to 6 AWG to reduce the drop below 3%.

Example 2 — Three-Phase

Given:

System voltage = 480 V
Phase = Three-phase
Load current = 100 A
One-way wire length = 60 m (≈ 200 ft)
Wire size = 1/0 AWG (R = 0.319 Ω/km)
Conductors per phase = 1

Step 1: Effective Resistance

R_effective = 0.319 / 1 = 0.319 Ω/km

Step 2: Phase Factor

Factor = 1.732 (three-phase)

Step 3: Voltage Drop

Vd = 1.732 × 100 × 60 × 0.319 / 1000 = 3.31 V

Step 4: Voltage Drop Percentage

Vd% = (3.31 / 480) × 100 = 0.69%

Step 5: Voltage at Load

V_load = 480 − 3.31 = 476.69 V

Interpretation: The voltage drop is only 0.69%, well within the NEC-recommended 3% guideline. The 1/0 AWG copper conductor is well-suited for this circuit.

Standards & References

NEC Chapter 9, Table 8 — DC resistance values for conductors (uncoated copper, 75°C)
NEC 210.19(A) Informational Note No. 4 — Recommends branch circuit voltage drop not exceed 3%
NEC 215.2(A) Informational Note No. 2 — Recommends feeder voltage drop not exceed 3%, with total not exceeding 5%
NEC 310.16 — Allowable ampacities for insulated conductors
IEEE Std 141 (Red Book) — Recommended practice for electric power distribution, including voltage drop considerations

Limitations

This calculator uses DC resistance values from NEC Chapter 9 Table 8 — actual AC impedance may differ due to skin effect and conduit material.
It does not account for power factor, which affects voltage drop in AC circuits with reactive loads.
Conductor temperature is assumed at 75°C per NEC Table 8 — actual resistance varies with operating temperature.
The calculator does not verify conductor ampacity — wire sizing for ampacity must be checked separately.
It does not account for voltage drop in the neutral conductor for unbalanced three-phase loads.
Results are estimates for planning purposes — final design should consider all installation-specific factors.

Common Mistakes to Avoid

Using round-trip distance instead of one-way distance (the formula already accounts for the return path with the factor of 2 or √3).
Confusing wire gauge resistance values between copper and aluminum conductors.
Forgetting to account for parallel conductors when multiple conductors per phase are installed.
Using AC impedance values when DC resistance values are more appropriate for simple voltage drop estimates, or vice versa.
Ignoring voltage drop entirely and sizing wire only for ampacity, which can result in poor equipment performance.
Applying the 3% voltage drop guideline as a mandatory code requirement rather than an informational recommendation.
Not verifying that the selected wire size also meets ampacity requirements — voltage drop and ampacity are separate checks.

Frequently Asked Questions

What does this Voltage Drop Calculator calculate?

It calculates the estimated voltage drop, voltage drop percentage, and voltage at the load for copper conductors using NEC Chapter 9 Table 8 DC resistance values.

What is an acceptable voltage drop?

The NEC recommends (informational note, not a requirement) that branch circuit voltage drop not exceed 3%, and that total voltage drop from service entrance to the farthest outlet not exceed 5%.

Why does the calculator use a factor of 2 for single-phase?

Because current flows through both the hot conductor to the load and returns through the neutral conductor, so the total conductor length carrying current is twice the one-way distance.

Why does three-phase use √3 instead of 2?

In a balanced three-phase system, the phase relationship between conductors means the effective voltage drop factor is √3 (approximately 1.732) rather than 2.

Does this calculator account for AC impedance?

No. It uses DC resistance values from NEC Chapter 9 Table 8. For circuits where AC impedance effects are significant (large conductors in steel conduit), a more detailed analysis may be needed.

Can I use this for aluminum conductors?

No. This calculator is specifically for copper conductors. Aluminum conductors have different resistance values and would require a separate calculation.

What does 'conductors per phase' mean?

When a single wire size cannot carry the required current or produces too much voltage drop, multiple conductors can be run in parallel for each phase. This effectively divides the resistance.

Should I enter round-trip or one-way distance?

Enter the one-way distance from the panel to the load. The formula already accounts for the return path using the phase factor (2 for single-phase, √3 for three-phase).

Frequently Used Together

Engineers often use these calculators in combination for complete project workflows:

Voltage Drop Calculator Nec Aluminum