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.
On this page
Calculator Guide
How to Use the Cable Sizing Calculator
The Cable Sizing Calculator above estimates the recommended conductor size from current, voltage, phase, one-way cable length, conductor material, voltage drop limit, and ampacity. Use it when you need a quick copper or aluminum cable size estimate and want to check both current-carrying capacity and voltage drop in the same workflow.
Cable sizing is not just an amp chart lookup. A conductor must normally pass two checks: it needs enough adjusted ampacity for the design current, and it needs low enough resistance to keep voltage drop within the selected limit.
Quick Answer
To estimate cable size, enter the load current or power, system voltage, circuit type, one-way length, conductor material, and maximum voltage drop. The calculator checks ampacity and voltage drop, then selects the smallest listed cable size that satisfies both conditions.
Important safety and code note
Use this tool for preliminary sizing and education only. Final conductor selection must be verified against the applicable electrical code, insulation rating, terminal temperature limits, ambient correction, number of current-carrying conductors, conduit fill, grounding, overcurrent protection, manufacturer data, and qualified professional judgment.
Cable Sizing Calculator Inputs and Outputs
The calculator uses electrical load, circuit, conductor, and design-limit inputs to estimate a cable size. The most common mistake is entering the correct number in the wrong unit, especially length, current, voltage, or power.
| Type | Value | What It Means | Common Unit |
|---|---|---|---|
| Input | Load current | Current the conductor must carry before any optional continuous-load multiplier. | A |
| Input | Load power | Watts, kW, VA, kVA, or horsepower converted into current before sizing. | W, kW, VA, kVA, hp |
| Input | System voltage | Nominal circuit voltage. For three-phase, use line-to-line voltage unless the tool states otherwise. | V |
| Input | One-way cable length | Physical distance from source to load. Do not double the length for normal use. | ft or m |
| Input | Conductor material | Copper or aluminum. Material changes resistance, voltage drop, and practical conductor size. | copper or aluminum |
| Input | Voltage drop limit | Maximum allowed voltage loss as a percentage of source voltage. | % |
| Input | Derating and load factor | Adjustment factors for continuous loads, installation conditions, or conservative sizing. | multiplier |
| Output | Recommended cable size | Smallest listed conductor size that passes ampacity and voltage drop checks. | AWG, kcmil, or mm² |
| Output | Voltage drop and load voltage | Estimated voltage lost in the conductor and voltage remaining at the load. | V and % |
| Output | Adjusted ampacity | Estimated ampacity after derating and parallel conductor assumptions. | A |
Cable Sizing Formula
Cable sizing combines an ampacity check with a voltage drop check. Ampacity checks current capacity, while voltage drop checks conductor resistance over distance.
Design Current
Use \(F_{\text{load}}=1.25\) for a 125% continuous-load estimate, or \(F_{\text{load}}=1.00\) when no additional load factor is applied.
Power-to-Current Formulas
Use these formulas when the load is entered as watts, kW, or horsepower. For horsepower, first convert mechanical horsepower to watts using \(P \approx hp \times 746\), then account for efficiency.
Apparent Power Current
Use apparent power formulas for VA or kVA inputs. Do not apply power factor again when the load is already entered as apparent power.
Adjusted Ampacity Check
The conductor passes the ampacity check when \(A_{\text{adjusted}} \ge I_{\text{design}}\).
DC and Single-Phase Voltage Drop
The factor of 2 represents the outgoing and return current path when \(L\) is entered as one-way length.
Three-Phase Voltage Drop
Use this simplified form for balanced three-phase estimates using line-to-line voltage.
AWG/kcmil Circular Mil Method
For DC or single-phase, \(F=2\). For three-phase, \(F=\sqrt{3}\). \(K\) is the conductor material constant and \(CM\) is conductor area in circular mils. This form is useful when working with AWG and kcmil conductor sizes.
Minimum Circular Mils from Voltage Drop
This rearranged formula estimates the minimum conductor area needed to stay within a voltage drop target before rounding up to a real AWG or kcmil size.
Percent Voltage Drop and Load Voltage
The cable passes the voltage drop check when \(\%V_d\) is less than or equal to the selected voltage drop limit.
Which current should be used for voltage drop?
Voltage drop is commonly checked using the expected operating load current. Some conservative workflows also check voltage drop using design current, especially where a continuous load may operate near full current for long periods.
What the Cable Sizing Variables Mean
Each variable affects the recommended cable size. Current and length increase voltage drop, while larger conductor area and lower resistance reduce voltage drop.
\(I_{\text{load}}\)
Actual load current in amps before optional continuous-load adjustment. If power is entered instead, the calculator converts power to current first.
\(I_{\text{design}}\)
Current used for the ampacity check after applying the selected continuous-load factor or custom load factor.
\(V_s\) or \(V_{LL}\)
Source or nominal system voltage. For three-phase systems, \(V_{LL}\) is line-to-line voltage.
\(L\)
One-way cable length from the source to the load. The voltage drop formula accounts for the circuit path.
\(R\)
Conductor resistance per unit length. It depends on cable size, material, conductor temperature, and whether parallel conductors are used.
\(V_d\)
Voltage drop in volts. Percent voltage drop compares this value to the source voltage.
\(PF\)
Power factor for AC real-power calculations. It is used when converting watts or kW into current.
\(\eta\)
Efficiency as a decimal. For example, \(90\%\) efficiency is entered into the formula as \(0.90\).
\(CM\)
Conductor area in circular mils. This is commonly used for AWG and kcmil voltage drop calculations.
\(K\)
Material constant used in the circular-mil method. Copper and aluminum have different resistance characteristics, so they use different practical constants.
How to Use the Cable Sizing Calculator
Use the calculator by matching the setup to the circuit, entering one-way length, and checking both the recommended cable size and the quick-check outputs.
Select the calculation mode
Choose whether you want to size a new cable or check an existing cable. Check mode is useful when you already know the installed AWG, kcmil, or mm² size.
Choose circuit type and units
Select DC, single-phase AC, or three-phase AC. Then choose U.S. units, metric units, or both depending on whether you want AWG/kcmil, mm², or a comparison.
Enter current or power
If you know amps, enter current directly. If you know watts, kW, VA, kVA, or horsepower, enter power and confirm power factor and efficiency where applicable.
Enter voltage, length, material, and drop limit
Use nominal voltage, one-way distance, copper or aluminum, and the maximum voltage drop percentage you want to allow.
Review the pass/fail logic
Do not look only at the cable size. Review load current, design current, adjusted ampacity, voltage drop, load voltage, power loss, and nearby cable comparison results.
How to Interpret Cable Sizing Results
The recommended cable size is the smallest listed size that passes the selected checks. A larger cable may still be appropriate if voltage drop, future load growth, temperature, conduit fill, or project standards require a more conservative design.
What to do with the result
Use the recommended size as a preliminary selection, then verify the final conductor against code, installation conditions, and equipment requirements.
What changes the result most?
Current, one-way length, voltage drop limit, conductor material, and voltage level usually dominate the result.
Practical sanity check
If doubling the length does not increase the required conductor size for a long run, recheck the voltage drop settings and units.
| Result Pattern | What It May Mean | What to Check Next |
|---|---|---|
| Ampacity passes, voltage drop fails | The conductor can carry the current, but the run is too long or resistance is too high for the selected drop limit. | Increase conductor size, reduce run length, increase voltage, or review the voltage drop target. |
| Voltage drop passes, ampacity fails | The conductor resistance may be acceptable, but the conductor is not rated for enough current after adjustment. | Use a larger conductor or revisit derating, continuous load, and parallel conductor assumptions. |
| Very large cable size | The voltage is low, current is high, the run is long, or the voltage drop target is very strict. | Check whether the length was accidentally entered as round-trip distance or whether power was entered in the wrong unit. |
| No listed size passes | The load and distance combination may exceed the calculator table or simplified assumptions. | Consider a higher distribution voltage, parallel runs, engineering review, or a more detailed design method. |
Input Checklist Before You Trust the Answer
Most cable sizing errors come from length, phase, voltage, and unit mistakes. Review these items before using the result in a design workflow.
Length
Enter source-to-load distance, not total out-and-back distance, unless you are intentionally using a custom method.
Voltage
Use the correct nominal voltage. For three-phase systems, use line-to-line voltage when using the standard three-phase formula.
Power input
Do not mix watts and kilowatts. For AC real power, check power factor. For motor horsepower, check efficiency.
Material
Confirm copper versus aluminum. Aluminum usually needs a larger conductor for the same current and voltage drop limit.
Derating
Use derating only when you understand the installation condition it represents. Guessing can make the result misleading.
Continuous load
Apply the continuous-load factor when the load may run for long periods and the applicable design rules require it.
Worked Cable Sizing Example
This example shows how a cable can be checked for voltage drop before the calculator rounds up to a real conductor size and checks ampacity.
Formula
Substitution
Percent voltage drop
Final answer
The estimated voltage drop is \(6 \, V\), or \(2.5\%\). This passes a \(3\%\) voltage drop target, but the conductor must still pass the ampacity check before it should be considered acceptable.
Why this example uses trial resistance
A real calculator checks many listed conductors. This hand example uses a trial resistance to show the voltage drop process. In practice, the calculator compares available AWG, kcmil, or mm² sizes and chooses the smallest size that passes both ampacity and voltage drop.
How to Visualize Cable Sizing
Cable sizing is easiest to understand as a three-part workflow: determine the electrical load, check ampacity and voltage drop, then choose the smallest real conductor size that passes both checks.
This visual uses short labels only so it remains readable on mobile and does not place text over arrows, lines, or dark backgrounds.
Reference Checks for Cable Sizing
Reference values are useful for catching unrealistic results, but they are not a substitute for the applicable code or project standard.
Voltage drop targets
Common preliminary targets are around \(3\%\) for a branch-circuit-style check and around \(5\%\) total for feeder plus branch. Treat these as design guidance, not a universal legal rule.
Low-voltage DC circuits
Low-voltage systems are very sensitive to voltage drop. A small voltage loss can be a large percentage of a 12 V, 24 V, or 48 V system.
Copper versus aluminum
For the same physical size, copper usually has lower resistance. Aluminum can be practical, but it often requires a larger conductor and correct terminations.
Long runs
As length increases, voltage drop increases directly. Long runs are often controlled by voltage drop rather than ampacity.
Design Notes and Practical Ranges
For practical design, cable size should be checked against both the calculator result and the real installation. A conductor that looks acceptable in a simplified estimate may still fail when installation method, ambient temperature, conductor count, insulation type, or terminal temperature is considered.
Good engineering workflow
Start with load current, estimate the conductor size, review voltage drop, check ampacity adjustment, verify conduit fill with a Conduit Fill Calculator, and then compare the final selection with applicable code and equipment data.
Branch circuits
Often controlled by ampacity for short runs and by voltage drop for longer runs.
Feeders
Should be evaluated with downstream branch-circuit voltage drop so the total path remains acceptable.
Motors and EV loads
May require continuous-load, starting-current, nameplate, or manufacturer-specific checks beyond a simplified cable estimate.
Solar and battery circuits
Often combine high current with low voltage, so voltage drop and conductor heating both matter.
Units and Conversions for Cable Sizing
Unit consistency is critical. A cable size estimate can be off by a large amount if feet are entered as meters, kW is entered as W, or line-to-neutral voltage is used in a line-to-line three-phase formula.
Current
\(1 \, kA = 1000 \, A\) and \(1 \, A = 1000 \, mA\). Most building and equipment cable sizing uses amps.
Power
\(1 \, kW = 1000 \, W\), \(1 \, kVA = 1000 \, VA\), and \(1 \, hp \approx 746 \, W\) before efficiency is considered.
Length
\(1 \, m \approx 3.28084 \, ft\). Confirm that the calculator converts length when switching between U.S. and metric layouts.
Conductor size
AWG/kcmil and mm² are not just formatting choices. Code tables, insulation types, and installation rules may be different by region.
Cable Sizing vs Related Electrical Calculations
Cable sizing is connected to several other electrical calculations. Use the right tool depending on whether you are finding current, checking voltage drop, sizing overcurrent protection, or evaluating power factor.
Cable sizing calculator
- Best when you need a recommended conductor size.
- Checks ampacity and voltage drop together.
- Useful for comparing copper, aluminum, AWG, kcmil, and mm² options.
Voltage drop calculator
- Best when you already know the conductor size and want to estimate voltage loss.
- Useful for long cable runs and load-voltage checks.
- Helpful after using the Voltage Drop Calculator for a more focused conductor-run comparison.
Common Cable Sizing Mistakes
The most common cable sizing mistakes come from using only one check, entering the wrong length, or assuming the calculator result is the final code-approved conductor.
Do
- Use one-way length when the calculator asks for one-way length.
- Check both ampacity and voltage drop.
- Use line-to-line voltage for standard three-phase power calculations.
- Apply realistic power factor and efficiency values for motor and AC power inputs.
- Use the Electrical Load Calculator first when the load current is not yet known.
Don’t
- Do not size a long run from ampacity alone.
- Do not enter round-trip length unless the method specifically asks for it.
- Do not assume copper and aluminum are interchangeable at the same size.
- Do not ignore derating for grouped conductors, ambient temperature, or installation conditions.
- Do not treat a simplified calculator as final code approval.
Troubleshooting Unrealistic Cable Size Results
If the recommended cable seems too large, too small, or impossible, check the basic inputs before changing the design. Most suspicious results come from unit scale, voltage, length, or load-conversion errors.
Result is much too large
Check whether length was entered as round-trip distance, power was entered in kW when W was selected, or a very low voltage was selected by mistake.
Voltage drop is too high
Increase conductor size, reduce length, reduce current, increase system voltage, or review whether the selected voltage drop target is appropriate.
Ampacity fails
Review continuous-load factor, derating factor, parallel runs, and the base ampacity assumption. A larger conductor may be required.
Three-phase result looks wrong
Confirm that the voltage is line-to-line and the circuit type is set to three-phase. Using single-phase by accident can shift the result.
Motor load seems wrong
Check horsepower, efficiency, power factor, voltage, and whether the calculator is estimating running current rather than starting current.
No cable size passes
The design may need parallel conductors, a different voltage level, shorter run, more detailed engineering review, or a conductor size outside the simplified table.
Assumptions and Limitations
This calculator is intended for educational and preliminary cable sizing. It uses simplified conductor resistance, ampacity, phase, and voltage drop assumptions that are useful for estimating, but not enough for final electrical design by themselves.
Resistance-only voltage drop
A simplified estimate may ignore AC reactance, cable spacing, power factor angle effects, harmonics, and detailed impedance data.
Approximate ampacity
Real ampacity depends on insulation rating, terminal temperature, ambient temperature, raceway, conductor count, and installation method.
Parallel conductors
Parallel runs require code-compliant installation, matching conductor characteristics, and correct terminations. Do not assume any conductor can be paralleled in any situation.
Regional standards
AWG/kcmil and mm² sizing practices may follow different standards. Final design must follow the local electrical code and project specifications.
Equipment protection
Cable sizing does not automatically size breakers, fuses, grounding conductors, or equipment protection. Use a separate review for overcurrent protection.
Professional review
When safety, cost, code compliance, or critical equipment is involved, have the result reviewed by a qualified electrical professional.
Key Cable Sizing Terms
These terms help connect the calculator inputs, formulas, and result interpretation.
Ampacity
The current a conductor can carry under stated conditions without exceeding its temperature limit.
Voltage drop
The voltage lost across a conductor because of resistance and current flow.
AWG
American Wire Gauge, a common U.S. conductor size system where smaller gauge numbers generally mean larger conductors.
kcmil
Thousand circular mils, commonly used for larger conductor sizes.
mm²
Square millimeters of conductor cross-sectional area, commonly used in metric cable sizing.
Derating
An adjustment that reduces allowable ampacity for installation conditions such as heat, grouping, or other constraints.
Cable Sizing Calculator FAQ
How do you calculate cable size?
Cable size is estimated by checking both ampacity and voltage drop. The selected conductor should have enough adjusted ampacity for the design current and low enough resistance to keep voltage drop within the chosen limit.
What size cable do I need for 30 amps?
The cable size for 30 amps depends on voltage, one-way length, conductor material, circuit type, voltage drop limit, insulation and installation conditions, and applicable code requirements. Short runs may be controlled by ampacity, while long runs may require a larger conductor to limit voltage drop.
Do I enter one-way cable length or round-trip length?
Enter one-way cable length from the source to the load. For DC and single-phase circuits, the return path is handled by the voltage drop factor. For balanced three-phase circuits, the square-root-of-three relationship is used.
What is the difference between ampacity and voltage drop?
Ampacity checks whether the conductor can safely carry current without overheating. Voltage drop checks how much voltage is lost along the run. A conductor can pass ampacity and still be too small for a long run because voltage drop increases with length and current.
Can this cable sizing calculator be used for final code compliance?
No. Use it for preliminary sizing, learning, and comparison. Final conductor selection should be checked against the applicable electrical code, installation method, insulation rating, terminal temperature limits, conduit fill, ambient temperature, overcurrent protection, and professional judgment.