Cable Size Calculator
Screen standard mm² from design amps (with voltage, phase, and Cu / Al context) for branch and feeder planning—then check derating, voltage drop, and protection coordination on real tables.
Calculator
Quick: design amps and material. Advanced: voltage and phase gate the mm² ladder before you apply installation derating in code tables.
Inputs (amps, voltage, phase, material)
Used for quick conductor screening in industrial and commercial panels. The stepped IEC-style ladder and worked examples are in the quick reference and typical circuit examples sections below; after you pick a candidate size, validate long runs with the Voltage drop calculator.
Coming from power calculations?
First calculate your current:
kW → kVA → kVA → Amps · or use 3-Phase PowerTip: Change current, voltage, phase, or material to refresh the screening mm² instantly.
About this calculator
Turn design amps into the next standard mm² from a fixed lookup table before you lock BOMs or panel schedules—not a substitute for installation derating, grouping, or fault-level proofs. For the same protection workflow lane, use the protection calculators hub; for long feeders after sizing, use the voltage drop calculator. Deeper narrative: Voltage Drop Calculation Guide and Motor Starting Current & Protection.
If your load is given in kW or kVA first, obtain design current with kW to kVA, 3-phase power, or kVA to amps (use your voltage and phase), then screen mm² here.
Calculation Results
Calculation Results
System: Single Phase, 230 V, Copper conductor
Recommended Cable Size: 2.5 mm²
Based on a simplified current-to-mm² step table for industrial screening; harmonics, demand, ambient, grouping, and short-circuit limits are not modeled here.
Engineering disclaimer
This calculator provides cable screening estimates only. For final electrical system design, conductor selection, and compliance with local electrical codes, consult a licensed electrical engineer or certified professional. Actual requirements may vary based on installation method, ambient conditions, harmonics, and specific application requirements.
Common screening ladder — quick reference (Cu)
Illustrative pairs from the same IEC 60228-style step table used in the calculator (typical copper screening at moderate conditions). Enter your design current above; verify tabulated ampacity with manufacturer data after derating and voltage drop. AWG column is approximate for NEC cross-check only.
| Design current (A) | Next standard Cu mm² (screening) | Approx. AWG (NEC ref.) |
|---|---|---|
| 15 | 1.5 | 16 AWG |
| 20 | 2.5 | 14 AWG |
| 30 | 4 | 12 AWG |
| 40 | 6 | 10 AWG |
| 55 | 10 | 8 AWG |
| 75 | 16 | 6 AWG |
| 100 | 25 | 4 AWG |
| 125 | 35 | 2 AWG |
| 150 | 50 | 1 AWG |
| 200 | 70 | 2/0 AWG |
Cable sizing method & formula context
- I
- Design load current in amperes (RMS, steady-state basis you are sizing for).
- V
- Nominal system voltage in volts (line-to-neutral for single-phase, line-to-line for three-phase).
- mm²
- Conductor cross-sectional area per IEC 60228 standard sizes; output here is always an integer mm² step.
- Ampacity
- Tabulated allowable current for a conductor under stated conditions; must be derated for grouping, ambient, and installation method.
- PF (cos φ)
- Power factor when converting from kW to kVA before finding line current.
Obtain design current: single-phase I = kVA × 1000 ÷ V; balanced three-phase I = kVA × 1000 ÷ (√3 × V) with √3 ≈ 1.732. From kW: kVA = kW ÷ PF, then use the line-current formula for your phase.
Screening rule (this tool): pick the smallest standard mm² whose ladder step current is ≥ I (next standard size up). This is not a substitute for installation derating, grouping, harmonics, insulation temperature rating (for example XLPE 90 °C), or short-circuit/adiabatic withstand proofs.
Worked example (with numbers)
Given: 37 kW production load, PF 0.85, 400 V three-phase copper feeder.
Step 1 — kVA: 37 ÷ 0.85 ≈ 43.5 kVA.
Step 2 — line current: 43.5 × 1000 ÷ (1.732 × 400) ≈ 62.8 A → use 63 A design current.
Step 3 — screening mm²: 63 A maps to the next ladder step → 16 mm² Cu (integer output).
Step 4 — verify: apply grouping/ambient derating on project tables; check route length with the voltage drop calculator; confirm OCPD and fault current withstand with the breaker size calculator.
Use the suggested mm² as an early BOM or panel schedule checkpoint, then validate against detailed manufacturer tables including ambient, installation method, and voltage drop along the route.
Next step after screening: combine your candidate size with breaker frames and source ratings using the breaker size calculator, transformer size calculator, and generator size calculator, then confirm against IEC/NEC tables. Browse the protection calculators hub for the full lane.
Common standards designers cross-check
Cable sizing in practice is read against regional wiring rules and tabulated ampacity—for example IEC 60364, NEC ampacity tables, BS 7671, or AS/NZS 3008—plus manufacturer datasheets. This page does not cite table numbers or replace statutory design; it only provides a current-to-next-mm² screening ladder for early checks.
Installation method, grouping & fault current
Real ampacity depends on how the run is installed—for example in conduit, on cable tray, buried, or in free air—together with grouping/bundling and ambient temperature derating. This calculator does not change the mm² output by installation method; apply IEC/NEC correction factors on your project tables after you have a candidate size.
Fault current & protection: available fault current, device let-through, and adiabatic thermal limits can require a larger conductor than steady-state ampacity. Size OCPD and check short-circuit withstand after screening—see the breaker size calculator and motor starting & protection guide for coordination context.
More context: Voltage Drop Calculation Guide · kW to kVA Formula Explained · 3-Phase Power Calculator · When to Use kVA Instead of kW
Typical circuit examples (screening only)
Illustrative mm² from the same ladder as the calculator (not a catalog ampacity). Adjust current in the tool and verify derating for your installation.
| Application | Example design A | Screening mm² (Cu ladder) | Note |
|---|---|---|---|
| Small panel branch | 25 | 4 | Often first stop before derating |
| Production feeder | 90 | 25 | May step to 35 mm² after grouping/ambient |
| Distribution feeder | 180 | 70 | Confirm voltage drop on long runs |
Frequently Asked Questions
How do bundled cables affect ampacity?
Grouped conductors reduce heat dissipation, so tabulated ampacity must be multiplied by a grouping derating factor from IEC 60364 or NEC tables. This calculator does not apply those factors—upsize after you have a candidate mm² from the screening ladder.
What ambient temperature derating should I apply?
Use the correction factor from your wiring code for the conductor insulation rating (for example 70 °C or 90 °C PVC/XLPE) and the expected ambient. High ambient or enclosed trays often push you one mm² step—or more—above a current-only screening value.
When does fault current require a larger conductor?
Short-circuit thermal limits (adiabatic withstand) can force a larger cross-section than steady-state ampacity. Check available fault current, protection device let-through, and manufacturer k²S² limits—use the breaker size calculator and project fault studies after screening mm² here.
Can you provide a cable sizing example with numbers?
Given: 37 kW, PF 0.85, 400 V three-phase → kVA ≈ 43.5, line current I ≈ 63 A. Screening ladder → 16 mm² Cu. Then verify grouping derating and route voltage drop before locking the BOM.
How does mm² relate to AWG for NEC projects?
IEC 60228 uses metric mm²; NEC projects often specify AWG/kcmil. Approximate cross-checks: 6 mm² ≈ 10 AWG, 16 mm² ≈ 6 AWG, 25 mm² ≈ 4 AWG, 70 mm² ≈ 2/0 AWG—always confirm against the NEC ampacity table for your insulation and installation method.
How do harmonics affect cable sizing?
Non-linear loads add harmonic current that increases conductor heating and neutral loading. Derating or upsizing may be required beyond a fundamental-frequency ampacity check—this page assumes steady-state RMS design current only.
What is the difference between copper and aluminum cables?
Copper has higher conductivity, so it carries more current for the same cross-section and is more compact, but more expensive. Aluminum is lighter and cheaper but often requires a larger cross-section and special terminations.
How does voltage drop affect cable sizing?
Long runs at high current can cause significant voltage drop. In many standards you must keep voltage drop below a specified percentage, which may require a larger cable than indicated by current capacity alone—use the voltage drop calculator after screening.
What standards do engineers cross-check for cable sizing?
Designers cross-check IEC 60364, NEC ampacity chapters, BS 7671, or AS/NZS 3008 plus manufacturer data for insulation temperature rating. This tool does not replace those tables or statutory compliance.
Does the calculator change results by installation method (conduit, tray, buried)?
No. Results are keyed from design current only. Real ampacity depends on installation method (conduit, cable tray, buried, or free air), grouping, and ambient temperature; apply IEC/NEC correction factors separately on your project tables.
Is this calculator suitable for final design?
No. It is intended for quick checks and concept design. Final cable selection must follow detailed tables from standards and manufacturers, and should be validated by a qualified electrical engineer.