SpecCalc Hub · Medium/High
Voltage Drop Calculator
Estimate DC, single-phase and three-phase voltage drop, percent loss and load voltage from cable size, length, current and material in simplified PF=1 mode.
Preliminary voltage-drop estimate only. Not final cable selection. Ampacity, protection, installation method, temperature, insulation, local code and safety must be verified by a qualified professional.
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Formula
R = ρ * length / area DC/single-phase drop = 2 * I * R three-phase drop = √3 * I * R voltage_drop_percent = drop / voltage * 100
Assumptions
- Inputs are user-provided.
- Results are preliminary estimates.
- Simplified resistance-only calculation with PF fixed at 1.
Limitations
- Does not check ampacity, protection, installation method, grouping, insulation, temperature, fault conditions, or local code.
- AC reactance and low power factor are not modeled in this simplified estimate.
Sources / methodology
Uses standard simplified resistivity-based voltage drop equations.
- Formula version
- 1.0.7
- Formula review date
- 2026-06-20
Source links
- NFPA NEC 2025 public input response PDF
- NFPA NEC 2020 first draft report PDF
- Schneider Electric - Determination of voltage drop
- Schneider Electric - Calculation of voltage drop in steady load conditions
- Schneider Electric - Maximum voltage drop limit
- Schneider Electric - Sizing and protection of conductors
- Schneider Electric - General method for cable sizing
- Copper Development Association - Copper building wire practices
- Cerrowire - Voltage Drop Calculator
- Southwire - Voltage Drop Calculator
Quick answer: This voltage drop calculator estimates voltage loss, percent drop and load voltage for simplified DC, single-phase and three-phase circuits from conductor material, cross-section, cable length, current and supply voltage. It is a preliminary estimate only and cannot replace final cable selection or code checks.
Input parameters
- System type: Single-phase AC
- Voltage (V): 230 V
- Current (A): 10 A
- One-way length (m): 20 m
- Conductor material: Copper
- Wire standard: Metric (mm²)
- Conductor area (mm²): 2.5 mm²
- Conductor size (AWG): 14 AWG
Output values
- Voltage drop (V)
- Voltage drop (%)
- Load voltage (V)
- One-way resistance (Ω)
Step-by-step example
- Enter values into the calculator fields.
- Run the calculation and review the result values.
- Compare the result with the assumptions, limitations and sources below.
Choose the voltage-drop task
Use this page to calculate voltage drop for a selected conductor, or continue to the dedicated planning tools for the inverse questions.
Single phase voltage drop calculator
The page already supports system type selection, so you can compare DC, single-phase AC and three-phase AC in one place. For a single-phase circuit, the simplified drop uses the round-trip resistive path, which is why long runs and smaller conductors quickly increase voltage loss.
3 phase voltage drop calculation formula
The simplified three-phase branch uses sqrt(3) x I x R after estimating one-way conductor resistance from material, length and cross-section. The page keeps this visible so you can compare the three-phase result with a single-phase or DC assumption instead of treating the output as a black box.
DC vs single-phase vs three-phase voltage drop
DC and simplified single-phase calculations use 2 x I x R because current travels out and back along the conductor path. The simplified three-phase model uses sqrt(3) x I x R. These are preliminary resistive estimates only and do not model AC reactance or low power factor.
Voltage drop formula
The page first estimates one-way resistance from material resistivity, one-way length and cross-section. Then it applies the simplified system factor for DC, single-phase or three-phase service and converts the voltage loss into both volts and percent of supply voltage.
Worked example: 230 V single-phase circuit
A 230 V single-phase copper circuit at 10 A over 20 m with 2.5 mm2 cross-section gives a small but visible voltage loss. If the run becomes longer or the load current increases, the same cable can cross a common planning threshold quickly, so the calculator is useful for comparing options before deeper cable review.
Worked example: 12 V DC cable
Low-voltage DC systems are more sensitive because the same voltage loss consumes a larger share of the supply. A 0.5 V drop on a 12 V circuit is already about 4.2%, which is why short runs and larger conductors become important for batteries, inverters and off-grid loads.
What changes the result most?
Length and current push voltage drop upward, while larger cross-section pulls it downward. Material matters too, but the biggest mistakes usually come from underestimating route length, ignoring the return path on DC or single-phase circuits, or selecting a conductor area that is too small for the actual run.
| Change | Typical effect on drop |
|---|---|
| Longer cable run | Higher drop |
| Higher load current | Higher drop |
| Larger conductor area | Lower drop |
| Aluminum instead of copper | Higher drop for the same size |
Worked examples
230 V single-phase example
Use the single-phase mode when checking a typical AC branch circuit. If current rises or length increases while conductor area stays fixed, the percent voltage drop rises quickly.
12 V DC example
Use the DC mode for batteries, low-voltage lighting or mobile systems. Small voltage losses matter much more on a 12 V system than on a 230 V circuit.
Cable-selection caution
A voltage-drop result can still look acceptable while ampacity, protection, installation method or local code remain unacceptable. Use the result only as a preliminary comparison.
400 V three-phase feeder example
Use the three-phase mode for motor feeders or distribution runs where the simplified sqrt(3) branch is more appropriate than the round-trip single-phase path. Then compare the result with Maximum Cable Length or Wire Size Estimate before any real cable decision.
Common mistakes
- Enter one-way route length, not the already doubled conductor length.
- Do not use voltage drop as an ampacity or protection check.
- Verify installation method, temperature, grouping, insulation and local rules separately.
What to check next
Check the inputs, limitations, sources and related calculators before using the estimate in a real decision.
FAQ
How do you calculate voltage drop?
A simplified estimate starts from conductor resistance, then applies the system factor for DC, single-phase or three-phase current and converts the result to volts and percent.
What is a single phase voltage drop calculator?
It is a tool that estimates voltage loss for a single-phase circuit using current, conductor size, material, length and supply voltage assumptions.
What is the three-phase voltage drop formula here?
The simplified three-phase branch uses sqrt(3) x I x R after the page estimates one-way conductor resistance from material, length and cross-section.
Why is voltage drop worse at low voltage?
The same absolute voltage loss consumes a larger percentage of a low-voltage system. That is why 12 V DC systems become sensitive quickly.
Can this replace cable selection?
No. The result is a preliminary voltage-drop estimate and does not verify ampacity, protection, installation method, insulation, temperature or local code.
What limits should I use?
Use only the planning limits required by your project or local practice, then verify the full design separately. The page does not decide the correct limit for your installation.
Does this page support AWG or feet directly?
AWG is supported through the wire-standard selector. Feet are not entered directly, so convert feet to meters before entering the one-way cable length.
Related calculators
This calculator provides an estimate for informational purposes only. It is not a certified engineering design, electrical safety approval, or professional installation recommendation. Always verify final decisions with a qualified professional and applicable local codes.