Voltage Drop Calculator

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

Conductor Resistance

Select conductor size and material — resistance values in ohms per 1000 ft (NEC Chapter 9 Table 8, 75°C)

Conductors per Phase

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

Overview

A Voltage Drop Calculator estimates how much voltage is lost along a conductor run under load. This calculator takes the selected conductor resistance, applies the correct system multiplier for the circuit type, calculates voltage drop in volts, converts that result into a percentage of system voltage, and calculates the voltage available at the load. It is useful for branch-circuit planning, feeder review, and conductor comparisons.

NEC-oriented design guidance commonly references about 3% voltage drop on a branch circuit and 5% maximum total including feeder plus branch circuit as a practical target for reasonable efficiency of operation.

This calculator is intended as a practical estimation tool for circuit planning, conductor 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, 277 V, 480 V, or 600 V.
Select phase configuration — choose from Single-Phase, Three-Phase.
Enter load current — in A.
Enter one-way wire length — in m (Metric) or ft (Imperial).
Select conductor size and material — resistance values are shown in Ω/1000 ft.
Select conductors per phase — choose from 1, 2, 3, or 4.
Click "Calculate" — get voltage drop, voltage drop percentage, voltage at load.

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)
•Conductor Resistance — Options: 14 AWG Copper — 3.14 Ω/1000 ft, 12 AWG Copper — 1.98 Ω/1000 ft, 10 AWG Copper — 1.24 Ω/1000 ft, 8 AWG Copper — 0.778 Ω/1000 ft, 6 AWG Copper — 0.491 Ω/1000 ft, 4 AWG Copper — 0.308 Ω/1000 ft, 3 AWG Copper — 0.245 Ω/1000 ft, 2 AWG Copper — 0.194 Ω/1000 ft, 1 AWG Copper — 0.154 Ω/1000 ft, 1/0 AWG Copper — 0.122 Ω/1000 ft, 2/0 AWG Copper — 0.0967 Ω/1000 ft, 3/0 AWG Copper — 0.0766 Ω/1000 ft, 4/0 AWG Copper — 0.0608 Ω/1000 ft, 250 kcmil Copper — 0.0515 Ω/1000 ft, 300 kcmil Copper — 0.0429 Ω/1000 ft, 350 kcmil Copper — 0.0367 Ω/1000 ft, 500 kcmil Copper — 0.0258 Ω/1000 ft, 12 AWG Aluminum — 5.17 Ω/1000 ft, 10 AWG Aluminum — 3.25 Ω/1000 ft, 8 AWG Aluminum — 2.04 Ω/1000 ft, 6 AWG Aluminum — 1.29 Ω/1000 ft, 4 AWG Aluminum — 0.808 Ω/1000 ft, 3 AWG Aluminum — 0.642 Ω/1000 ft, 2 AWG Aluminum — 0.510 Ω/1000 ft, 1 AWG Aluminum — 0.404 Ω/1000 ft, 1/0 AWG Aluminum — 0.321 Ω/1000 ft, 2/0 AWG Aluminum — 0.254 Ω/1000 ft, 3/0 AWG Aluminum — 0.201 Ω/1000 ft, 4/0 AWG Aluminum — 0.160 Ω/1000 ft, 250 kcmil Aluminum — 0.135 Ω/1000 ft, 300 kcmil Aluminum — 0.113 Ω/1000 ft, 350 kcmil Aluminum — 0.0967 Ω/1000 ft, 500 kcmil Aluminum — 0.0678 Ω/1000 ft
•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 a resistance-based circuit model. The conductor resistance is expressed in ohms per 1000 ft.

Step 1: Effective Resistance

R_effective = R_conductor / N_conductors

Where R_conductor is the resistance per 1000 ft (Ω/1000 ft) for the selected conductor, 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 × R_effective × L) / 1000

Where I = load current in amperes, and L = one-way length of run in feet.

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	ft
R_conductor	Conductor resistance per 1000 ft	Ω/1000 ft
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

Unit Handling

One-way wire length can be entered in feet (Imperial mode) or meters (Metric mode). When meters are entered, the value is converted to feet internally before applying the formula, since conductor resistance values are expressed in Ω/1000 ft per NEC Chapter 9 Table 8. All other inputs and outputs — voltage, current, resistance, and percentage — remain the same regardless of unit mode.

Key Facts

This calculator uses a resistance-based method — it starts with the selected conductor resistance and the entered run conditions, then calculates actual dropped voltage, percentage drop, and final load voltage.
Percentage voltage drop is usually the primary result, while voltage drop in volts and final load voltage are supporting outputs.
The NEC recommends (but does not require) a maximum 3% voltage drop on branch circuits and 5% total from service entrance to farthest outlet.
One-way length is used because the system multiplier (2 for single-phase, √3 for three-phase) already accounts for the circuit path.
Lower voltage drop is generally better, but the result should be read as design guidance, not automatic proof of compliance or violation.
Using parallel conductors per phase effectively reduces the resistance and voltage drop proportionally.
Wire resistance increases with temperature — the resistance values used here are based on 75°C conductor temperature per NEC Chapter 9 Table 8.

Applications

Branch-circuit voltage drop review for residential and commercial installations.
Feeder voltage drop review for panel-to-panel runs.
Comparing conductor options across copper and aluminum materials side by side.
Long-run circuit planning where voltage drop is a primary design concern.
Checking whether voltage drop is likely acceptable before installation.
Reviewing low-voltage conditions at the load.
Conductor upsizing decisions based on voltage drop results.
Preliminary circuit design screening for new construction and renovation projects.

Example Calculation

Example Calculation — Single-Phase

Given:

System voltage = 120 V
Phase = Single-phase
Load current = 20 A
One-way wire length = 150 ft
Conductor resistance = 2.0 Ω/1000 ft
Conductors per phase = 1

Step 1: Effective Resistance

R_effective = 2.0 / 1 = 2.0 Ω/1000 ft

Step 2: Phase Factor

Factor = 2 (single-phase)

Step 3: Voltage Drop

VD = (2 × 20 × 2.0 × 150) / 1000 = 12.0 V

Step 4: Voltage Drop Percentage

VD% = (12.0 / 120) × 100 = 10.0%

Step 5: Voltage at Load

V_load = 120 − 12.0 = 108.0 V

Interpretation: A voltage drop of 10.0% is well above common practical design guidance. In a case like this, the circuit would usually benefit from a larger conductor, a shorter run, or further design review rather than being accepted as-is. This example also shows why the general calculator is useful: even without locking the page to a specific copper-only or aluminum-only method, the resistance-based result still makes the design issue obvious. Practical NEC-style guidance commonly points to about 3% branch-circuit drop and 5% total system drop as a reasonable target for efficient operation.

Example 2 — Three-Phase

Given:

System voltage = 480 V
Phase = Three-phase
Load current = 100 A
One-way wire length = 200 ft
Conductor resistance = 0.122 Ω/1000 ft (1/0 AWG Copper)
Conductors per phase = 1

Step 1: Effective Resistance

R_effective = 0.122 / 1 = 0.122 Ω/1000 ft

Step 2: Phase Factor

Factor = 1.732 (three-phase)

Step 3: Voltage Drop

VD = (1.732 × 100 × 0.122 × 200) / 1000 = 4.23 V

Step 4: Voltage Drop Percentage

VD% = (4.23 / 480) × 100 = 0.88%

Step 5: Voltage at Load

V_load = 480 − 4.23 = 475.77 V

Interpretation: The voltage drop is only 0.88%, well within the NEC-recommended 3% guideline. This result confirms the conductor selection provides comfortable margin for this 480 V feeder run.

Standards & References

NEC Chapter 9, Table 8 — DC resistance values for conductors (copper and aluminum, 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
IAEI Voltage Drop Reference — Identifies resistance-based methods as generally acceptable for voltage-drop calculations
IEEE Std 141 (Red Book) — Recommended practice for electric power distribution, including voltage drop considerations

Limitations

This calculator is a practical voltage-drop tool, not a full conductor-design or installation-approval system.
It does not replace conductor ampacity review, insulation temperature limits, equipment nameplate requirements, or local code enforcement.
It assumes the selected conductor resistance value is appropriate for the scenario being evaluated.
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 Chapter 9 Table 8 — actual resistance varies with operating temperature.
Results are estimates for planning purposes — final design should consider all installation-specific factors including harmonics, conduit fill, and ambient temperature.

Common Mistakes to Avoid

Entering round-trip length instead of one-way length — the formula already accounts for the return path with the factor of 2 or √3.
Confusing ampacity compliance with voltage-drop quality — a conductor can still be acceptable for ampacity yet produce too much voltage drop for good system performance.
Ignoring whether the review covers only the branch circuit or the combined feeder-plus-branch total.
Assuming every voltage-drop calculator uses the same material constants or conductor assumptions — this page uses a general resistance-based model distinct from the NEC Copper and NEC Aluminum K-factor pages.
Forgetting to account for parallel conductors when multiple conductors per phase are installed.
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 voltage drop in volts, percentage voltage drop, and final load voltage using a resistance-based circuit model. Enter the system voltage, phase type, load current, conductor length, and conductor resistance to get all three results.

What formula does this calculator use?

It uses VD = (2 × I × R × L) / 1000 for single-phase circuits and VD = (1.732 × I × R × L) / 1000 for three-phase circuits, then converts the result to percentage drop and final load voltage.

Why does voltage drop matter if ampacity is already acceptable?

Because a conductor can still carry the load without overheating and yet still deliver poor voltage quality at the load if the run is long or resistance is too high. Ampacity and voltage drop are separate design checks that must both be satisfied.

Does NEC require exactly 3% voltage drop?

No. NEC informational notes reference 3% on branch circuits and 5% total including feeders as practical design guidance for reasonable efficiency of operation. These are recommendations, not mandatory code requirements.

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

Enter the one-way distance from the panel to the load. The formula multiplier (2 for single-phase, 1.732 for three-phase) already accounts for the return path of the circuit.

Can this calculator replace conductor ampacity review?

No. Voltage drop and ampacity are related but different design issues. A conductor that meets the voltage drop target must still be verified against NEC 310.16 ampacity tables separately.

What should I do if the voltage drop is too high?

The most common next steps are to increase conductor size, add parallel conductors per phase, reduce run length if possible, or relocate the panel closer to the load. Each option reduces effective resistance and improves voltage quality.

Frequently Used Together

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