Cable Sizing Calculator
Calculate recommended cable size from amps, voltage, phase, one-way cable length, conductor material, voltage drop, and ampacity.
Calculator is for informational purposes only. Terms and Conditions
Choose the cable sizing setup
Select the calculation mode, unit layout, circuit type, and load input method.
Enter the known values
Fill in the visible fields. The calculator updates automatically.
Cable run visual
The length input is one-way distance. The calculator applies the proper voltage-drop path.
Solution
Recommended size, voltage drop, ampacity check, and practical design notes.
Quick checks
- Load current—
- Design current—
- Voltage drop—
- Voltage at load—
- Adjusted ampacity—
- Power loss—
Nearby cable comparison
| Size | Ampacity | Voltage Drop | Status |
|---|---|---|---|
| Enter values to compare nearby cable sizes. | |||
Source, standards, and assumptions
Educational NEC-style estimate
- Uses common voltage drop equations for DC, single-phase AC, and three-phase AC circuits.
- Uses built-in conductor resistance and approximate ampacity tables for educational sizing only.
- Uses a resistance-only voltage-drop estimate. AC reactance is not included.
- Does not claim final NEC, IEC, BS 7671, or local-code compliance.
- Final conductor, insulation, terminal temperature, conduit fill, grounding, and overcurrent protection must be verified by a qualified professional.
Show solution steps See load conversion, voltage drop, ampacity, and selected cable reasoning
- Enter values to see the full solution steps and checks.
How to Use a Cable Sizing Calculator Correctly
A cable sizing calculator helps answer one practical question: what conductor size do I need for this load, voltage, cable length, and installation condition? The answer should not be based on current alone. A useful result checks both ampacity, which is the conductor’s current-carrying capacity, and voltage drop, which is the voltage lost over the cable run.
That is why this calculator is designed to show more than a single wire size. It estimates the recommended conductor, voltage drop in volts and percent, voltage at the load, adjusted ampacity, power loss, and the reason the size was selected. For final work, always verify the result against the adopted electrical code, equipment terminations, conductor insulation, raceway conditions, and local requirements.
Cable Sizing Formulas
Cable sizing usually starts with two checks. First, the conductor must carry the required design current after any derating. Second, the same conductor must keep voltage drop within the selected limit. A short run may be controlled by ampacity. A long run may require a larger conductor because voltage drop becomes the limiting factor.
DC and Single-Phase Voltage Drop
The factor of 2 accounts for the out-and-back current path. The calculator asks for one-way length and applies the return path internally.
Three-Phase Voltage Drop
Three-phase circuits use the square-root-of-three relationship. This calculator uses a simplified resistance-only voltage-drop estimate and does not include AC reactance.
Voltage Drop Percentage
Voltage drop percent compares the lost voltage to the source voltage. This is especially important for low-voltage systems because a small loss in volts can be a large percentage loss.
Design Current and Ampacity Check
Voltage drop is normally checked using the actual load current. Ampacity is checked using design current after any continuous-load or project-specific factor.
Important calculation note
This calculator is an educational sizing tool. It uses built-in resistance and approximate ampacity data to estimate a practical conductor size. It does not replace final code review, equipment documentation, or a full engineered design.
Inputs That Change Cable Size the Most
The best cable sizing calculator should not overwhelm users with unnecessary fields, but it must include the inputs that actually change the result. The table below explains the main values users should understand before trusting the recommendation.
| Input | What to Enter | How It Changes the Result |
|---|---|---|
| Load Current | The actual operating current in amps | Higher current increases both voltage drop and ampacity demand |
| Power | Watts, kW, VA, kVA, or horsepower if current is unknown | The calculator converts power to current using voltage, phase, power factor, and efficiency |
| System Voltage | Nominal source voltage; line-to-line voltage for three-phase | Lower voltage systems are more sensitive to voltage drop |
| One-Way Length | Distance from source to load, not the round-trip path | Longer runs often require larger conductors for voltage-drop control |
| Phase Type | DC, single-phase AC, or three-phase AC | Changes the voltage-drop equation used by the calculator |
| Conductor Material | Copper or aluminum | Aluminum usually needs a larger size than copper for the same voltage-drop target |
| Voltage-Drop Limit | Common targets include 2%, 3%, or 5% | A tighter limit usually increases conductor size |
| Derating Factor | Adjustment for installation conditions | Lower derating factors reduce usable ampacity and may force a larger conductor |
| Parallel Conductors | Number of parallel runs per phase or polarity | Splits current between conductors and can reduce drop per run, but must be code-verified |
Ampacity vs. Voltage Drop
The most important idea in cable sizing is that ampacity and voltage drop are not the same check. A conductor may be large enough to safely carry the current but still be too small for a long run because the voltage at the load becomes too low.
When Ampacity Governs
- The load current is high relative to the conductor size
- A continuous-load factor is applied
- Temperature or conductor grouping reduces usable ampacity
- The termination rating limits the usable ampacity basis
When Voltage Drop Governs
- The cable run is long
- The circuit is 12 V, 24 V, or another low-voltage system
- The selected voltage-drop limit is tight
- The load is sensitive to low delivered voltage
How to use the governing criterion
If the calculator says voltage drop controls, the conductor was likely upsized for performance over distance. If it says ampacity controls, the current-carrying requirement is the limiting check.
What Is an Acceptable Voltage Drop?
A common design target is about 3% for a branch circuit and about 5% total for feeder plus branch circuit voltage drop. These are commonly used educational design targets, not a substitute for the final requirements that apply to your specific installation.
1% to 2%
Useful for sensitive electronics, low-voltage controls, instrumentation, and performance-critical DC circuits.
3%
A common branch-circuit design target for keeping delivered voltage close to source voltage.
5%
A common overall feeder-plus-branch target when evaluating the full circuit path.
Voltage drop is especially important on 12 V and 24 V circuits. A 2 V drop is less than 1% on a 240 V circuit, but more than 16% on a 12 V circuit. That is why low-voltage DC cable sizing often requires much larger conductors than users expect.
AWG and mm² Cable Size Chart
Users often search for cable size in either AWG/kcmil or mm². These systems are not exact one-to-one matches, so the calculator should treat metric values as nearest standard sizes rather than perfect equivalents.
| AWG / kcmil | Approx. Area | Nearest Common Metric Size | Typical User Context |
|---|---|---|---|
| 14 AWG | 2.08 mm² | 2.5 mm² | Small branch circuits and light loads, subject to local code limits |
| 12 AWG | 3.31 mm² | 4 mm² | Common small branch circuit comparison size |
| 10 AWG | 5.26 mm² | 6 mm² | Higher current branch circuits and longer small-load runs |
| 8 AWG | 8.37 mm² | 10 mm² | Longer 30–50 A style runs, depending on conditions |
| 6 AWG | 13.3 mm² | 16 mm² | Feeders, EV-style loads, and longer circuits |
| 4 AWG | 21.2 mm² | 25 mm² | Larger feeders and voltage-drop-controlled designs |
| 2 AWG | 33.6 mm² | 35 mm² | Large branch circuits and feeder comparisons |
| 1/0 AWG | 53.5 mm² | 70 mm² | Larger feeders and service-style comparisons |
| 4/0 AWG | 107.2 mm² | 120 mm² | Large feeders where voltage drop and ampacity both matter |
| 500 kcmil | 253 mm² | 300 mm² | Large equipment feeders, often requiring full engineering review |
Do not treat AWG and mm² as exact replacements
AWG and metric conductor sizes come from different sizing systems. When switching between them, use the next suitable standard size and verify final ampacity, voltage drop, insulation, and termination requirements.
Copper vs. Aluminum Cable Sizing
Copper and aluminum can both be valid conductor materials, but they do not size the same way. Copper has lower electrical resistance, so it usually meets the same voltage-drop target with a smaller conductor. Aluminum can still be economical for larger feeders, but it requires compatible terminations, connectors, torque requirements, and installation practices.
| Factor | Copper | Aluminum |
|---|---|---|
| Resistance | Lower | Higher |
| Typical conductor size for same drop | Usually smaller | Usually larger |
| Common use case | Branch circuits, compact spaces, smaller equipment runs | Larger feeders and cost-sensitive larger installations |
| Main caution | Higher material cost | Requires aluminum-rated terminations and correct installation practice |
Important aluminum note
A calculator may show aluminum as a workable option, but the final installation must still confirm equipment lugs, connectors, torque values, conductor type, and local code requirements.
Single-Phase, Three-Phase, and DC Cable Sizing
The same current, voltage, and length can produce a different result depending on whether the system is DC, single-phase AC, or three-phase AC. This is why the calculator asks for circuit type before recommending a conductor.
DC Circuits
DC systems use an out-and-back conductor path. Low-voltage DC systems, such as 12 V and 24 V, are often voltage-drop controlled.
Single-Phase AC
Single-phase circuits also use a two-conductor voltage-drop path in this simplified calculator. Long runs can quickly require larger conductors.
Three-Phase AC
Three-phase voltage drop uses the square-root-of-three relationship. This is common for commercial, industrial, and larger equipment feeders.
Reactance Limitation
This calculator uses a resistance-only estimate. For large AC conductors, long motor feeders, or power-factor-sensitive designs, a full impedance-based calculation may be required.
Common Cable Sizing Use Cases
The same calculator can support many search intents, but users should interpret the result differently depending on the application. A cable size that is reasonable for a low-voltage control circuit may not be appropriate for building branch-circuit wiring.
| Use Case | Most Important Check | Practical Warning |
|---|---|---|
| 12 V / 24 V DC | Voltage drop percentage | Even small voltage losses can be severe at low voltage |
| 120 V / 240 V branch circuits | Ampacity, voltage drop, and overcurrent protection | Do not use the calculator result as final code approval |
| 480 V three-phase feeders | Ampacity, voltage drop, and installation derating | Large AC feeders may require impedance-based voltage drop |
| Solar / battery / inverter wiring | Voltage drop and continuous current | DC current can be high, and conductor heating must be reviewed carefully |
| Motor circuits | Starting performance, voltage drop, and code-specific motor rules | Motor sizing may require dedicated rules beyond a simple calculator |
| EV charging circuits | Continuous load factor and ampacity | Long-duration loading and equipment instructions matter |
How to Use the Cable Sizing Calculator
Use the calculator like a structured design check. Start with the load, voltage, and one-way length. Then review whether the selected conductor passes both ampacity and voltage-drop limits.
Choose size mode or check mode
Use Size a new cable when you need a recommendation. Use Check an existing cable if you already have an AWG, kcmil, or mm² conductor size.
Enter current or power
If you know amps, enter current directly. If you know watts, kW, VA, kVA, or horsepower, choose the power input option so the calculator can estimate current first.
Enter one-way cable length
Do not double the cable length manually. Enter the one-way distance from source to load. The calculator applies the appropriate path relationship internally.
Select material and voltage-drop target
Choose copper or aluminum, then set the maximum voltage drop. For many preliminary branch-circuit checks, 3% is a common starting point.
Use Advanced Options when conditions are not standard
Set continuous-load factor, derating factor, conductor temperature, application type, and parallel conductors when those conditions affect the result.
Step-by-Step Worked Example
A worked example helps explain why the calculator may recommend a larger conductor than expected. In many real circuits, voltage drop rather than ampacity controls the final size.
Design Current
Voltage-Drop Limit
Interpretation
The calculator checks the conductor against the 60 A design current, but voltage drop is based on the actual 48 A load current. If a smaller conductor can carry the current but exceeds 7.2 V of drop, the calculator must size up for voltage performance.
What this example teaches
Cable sizing is not just about avoiding overheating. It is also about delivering enough voltage at the load. This is why long circuits, low-voltage systems, and sensitive equipment often require larger conductors.
How to Read the Calculator Results
The calculator output is designed to explain the recommendation, not just show a wire size. Use the table below to understand each result.
| Result | What It Means | What to Check Next |
|---|---|---|
| Recommended cable size | The smallest built-in conductor that passes ampacity and voltage-drop checks | Verify it against local code, equipment ratings, and installation method |
| Existing cable passes/fails | Whether the cable you selected in check mode passes the calculator’s checks | If it fails, review whether ampacity or voltage drop caused the failure |
| Load current | The current used for voltage-drop calculation | Confirm the actual load estimate is realistic |
| Design current | The current used for ampacity after load factor | Check whether a continuous-load factor or equipment-specific rule applies |
| Voltage drop | Estimated volts and percent lost in the conductor run | Compare against the selected maximum voltage-drop limit |
| Voltage at load | Estimated delivered voltage after conductor drop | Confirm the load can operate properly at that voltage |
| Adjusted ampacity | Base ampacity after derating and parallel-run adjustment | Verify final ampacity using the applicable code table and installation conditions |
| Power loss | Estimated conductor loss as heat | Large losses may justify upsizing for efficiency |
Temperature and Grouping Derating
Derating is one of the most important reasons a simple wire-size answer can be wrong. Conductors may need to be derated for ambient temperature, number of current-carrying conductors, raceway conditions, insulation type, and equipment terminations.
Why derating matters
A conductor that appears acceptable from a basic ampacity table may no longer pass once correction and adjustment factors are applied. When in doubt, use a conservative derating factor and verify the final value from the governing code.
Ambient Temperature
Higher ambient temperature can reduce conductor ampacity.
Conductor Grouping
Multiple current-carrying conductors in the same raceway can require ampacity adjustment.
Termination Temperature
Equipment terminals may limit the ampacity column that can be used.
Common Cable Sizing Mistakes That Cause Wrong Answers
These mistakes are common because cable sizing looks simple until installation conditions, voltage drop, and code limitations are included.
Common Don’ts
- Size by ampacity only and ignore voltage drop
- Enter round-trip length when the calculator asks for one-way length
- Use copper results for aluminum conductors
- Ignore continuous-load requirements
- Assume AWG and mm² sizes are exact equivalents
- Ignore temperature, grouping, raceway, and termination effects
- Treat a preliminary calculator result as final code approval
Better Checks
- Check both ampacity and voltage drop
- Use one-way source-to-load length
- Select the correct phase type
- Use a realistic voltage-drop target
- Review the nearby cable comparison table
- Use advanced options when installation conditions are not standard
- Verify final design against the adopted electrical code
When It Makes Sense to Size Up
The smallest passing conductor is not always the best practical choice. If the result is close to the voltage-drop limit or ampacity margin is thin, going one size larger can improve performance, reduce losses, and provide future flexibility.
Long Cable Runs
If voltage drop is close to the limit, upsizing may provide better end-of-line voltage and lower power loss.
Low-Voltage DC
12 V and 24 V circuits are often worth upsizing because small voltage losses become large percentage losses.
Motor or Compressor Loads
Reduced voltage can affect starting and performance, so additional voltage-drop margin may be useful.
Future Load Growth
A slightly larger conductor may avoid expensive rework if the equipment load increases later.
Limitations of This Cable Sizing Calculator
This calculator is intended for educational estimating and preliminary design review. It gives a strong starting point, but it does not model every condition that may govern final conductor selection.
Code Compliance
The calculator does not guarantee NEC, IEC, BS 7671, AS/NZS, or local-code compliance.
AC Reactance
Large AC conductors and long feeders may need impedance-based voltage-drop calculations.
Fault Current
Short-circuit withstand, fault loop impedance, and protective-device coordination are not fully modeled.
Installation Details
Conduit fill, insulation type, terminals, wet locations, burial conditions, and manufacturer requirements must be checked separately.
High-quality external references
For additional verification, compare the result against manufacturer and standards-based resources such as the Southwire Voltage Drop Calculator, the Encore Wire Size and Voltage Drop Calculator, or an IEC/BS-style cable sizing method when your project is outside a U.S. NEC-style context.
Frequently Asked Questions
What does a cable sizing calculator actually calculate?
It estimates the conductor size needed to satisfy current-carrying capacity and voltage-drop limits for the load, voltage, length, phase type, and conductor material you enter.
Is cable sizing based on amps or voltage drop?
It should be based on both. Ampacity checks whether the conductor can safely carry the design current, while voltage drop checks whether the load receives enough voltage over the run.
Do I enter one-way cable length or total round-trip length?
Enter one-way length from the source to the load. The calculator applies the correct voltage-drop path internally for DC, single-phase AC, or three-phase AC.
Why is my recommended cable size larger than expected?
The most common reasons are long distance, low system voltage, a tight voltage-drop limit, continuous-load adjustment, aluminum conductors, or derating for installation conditions.
Why does low-voltage DC often need larger cable?
Low-voltage systems are very sensitive to percentage voltage drop. A small loss in volts can be a large percentage of a 12 V or 24 V supply.
Should I use copper or aluminum cable?
Copper usually sizes smaller because it has lower resistance. Aluminum can be practical for larger feeders, but the final design must confirm compatible equipment terminations and installation requirements.
What does “governing criterion” mean?
It tells you whether the selected conductor size was controlled mainly by ampacity or by voltage drop. This explains why the calculator did not choose a smaller conductor.
Can I use this calculator for final electrical design?
Use it as a preliminary educational tool. Final design must be checked against the adopted electrical code, equipment manufacturer requirements, conductor insulation, temperature ratings, installation method, and local authority requirements.