Cable Core Size Converter

The Cable Core Size Converter converts conductor sizes between AWG, SWG, and metric cross-sectional areas, suggesting equivalent cable core dimensions.

Cable Core Size Converter Explained

This tool proposes a conductor cross‑sectional area for power cables based on your design current, circuit length, and installation. It checks three main criteria at once: current‑carrying capacity, voltage drop, and short‑circuit thermal withstand. You can switch material between copper and aluminum and choose insulation types to reflect real installations.

Behind the scenes, the calculator applies common methods from international practice. It models resistance versus temperature, basic reactance, and allowable voltage drop. It then rounds up to the nearest standard size and provides quick AWG/mm² conversions. Use the results as a sizing guide and confirm against your local code and manufacturer data.

How the Cable Core Size Method Works

The method follows a simple sequence. It starts with design current, tests capacity, then checks performance under normal and fault conditions. The final size is the smallest cross‑section that passes all checks.

• Determine design current from load power, voltage, and power factor (or enter known current).
• Apply ampacity limits for the installation method, ambient temperature, and insulation class.
• Calculate expected voltage drop over the route and compare with your limit.
• Verify short‑circuit thermal withstand using the adiabatic equation for the fault level and clearing time.
• Round up to the next standard size and show alternatives with similar performance.

This approach mirrors common engineering practice. You get a balanced choice that handles heat in normal use, holds voltage within limits, and survives faults long enough for protection to operate.

Equations Used by the Cable Core Size Converter

The converter uses well‑known formulas for current, voltage drop, conductor resistance, and short‑circuit thermal stress. Where data are unknown, it applies conservative defaults and notes the assumption.

• Design current: I = P / (V × pf) for single‑phase; I = P / (√3 × V × pf × η) for three‑phase motors (η optional).
• Conductor resistance at temperature θ: Rθ = R20 × [1 + α × (θ − 20°C)], with R20 = ρ20 / A.
• Voltage drop (single‑phase): Vd = 2 × I × L × (R × cosφ + X × sinφ).
• Voltage drop (three‑phase): Vd = √3 × I × L × (R × cosφ + X × sinφ).
• Short‑circuit thermal check (adiabatic): S ≥ Isc × √t / k, where k depends on material and insulation.
• AWG ↔ mm²: A(mm²) ≈ 0.012668 × 92^((36 − AWG)/19.5); d(mm) ≈ 0.127 × 92^((36 − AWG)/39).

Typical constants: ρ20 for copper ≈ 1.724×10⁻⁸ Ω·m, for aluminum ≈ 2.826×10⁻⁸ Ω·m; α for copper ≈ 0.00393/°C, aluminum ≈ 0.00403/°C. Example k‑factors: copper PVC ≈ 115, copper XLPE ≈ 143, aluminum PVC ≈ 76, aluminum XLPE ≈ 94. The tool lists these options so you can match your cable type.

Inputs, Assumptions & Parameters

Provide the essentials and select the installation options that match your project. The converter will calculate and display the governing constraint with notes.

• Load current, or power with supply voltage and power factor.
• Phase type (single/three‑phase) and power factor angle if known.
• One‑way circuit length from source to load.
• Conductor material and insulation (copper/aluminum; PVC/XLPE).
• Installation method and ambient temperature (in air, in conduit, buried).
• Short‑circuit current at cable and fault clearing time.

Ranges and edge cases matter. Very long runs drive voltage drop, even at modest current. Very high fault levels or slow breakers can push size due to the adiabatic check. If your inputs fall outside common tables, the tool flags a note and recommends a conservative size.

Step-by-Step: Use the Cable Core Size Converter

Here’s a concise overview before we dive into the key points:

1. Select single‑phase or three‑phase power.
2. Enter load current, or enter power, voltage, and power factor.
3. Set one‑way circuit length and allowable voltage drop percent.
4. Choose conductor material, insulation, and installation method.
5. Enter ambient temperature, fault current at cable, and clearing time.
6. View the suggested size, review AWG/mm² options, and pick the final cable.

These points provide quick orientation—use them alongside the full explanations in this page.

Real-World Examples

A 7 kW single‑phase heat pump at 230 V with pf 0.95 is 25 m from the panel. Current I ≈ 7000 / (230 × 0.95) ≈ 32 A. Allow 3% voltage drop, so 6.9 V limit. For copper PVC in conduit at 30°C, 6 mm² has Rθ ≈ 3.0 mΩ/m. Vd ≈ 2 × 32 × 25 × 0.003 = 4.8 V, which passes. Ampacity for 6 mm² in this setup typically exceeds 32 A. With an end‑of‑circuit fault of 1.5 kA for 0.2 s and k = 115, Smin ≈ 1,500 × √0.2 / 115 ≈ 6 mm², which also passes. What this means: 6 mm² copper is adequate; 10 mm² offers extra margin if future load growth is likely.

A 45 kW, 400 V, three‑phase motor has pf 0.85 and efficiency 0.9, 120 m from the MCC. Current I ≈ 45,000 / (√3 × 400 × 0.85 × 0.9) ≈ 85 A. Set a 5% drop limit (20 V). Using aluminum XLPE in open tray at 40°C: trial S = 35 mm² with Rθ ≈ 0.87 mΩ/m and X ≈ 0.08 mΩ/m. Vd ≈ √3 × 85 × 120 × (0.00087 × 0.85 + 0.00008 × 0.526) ≈ 13.8 V, which passes. Ampacity is typically near 120–130 A, which passes. For Isc = 10 kA, t = 0.25 s, k = 94, Smin ≈ 10,000 × 0.5 / 94 ≈ 53 mm², so 35 mm² fails the adiabatic check; 50 mm² passes. What this means: Choose 50 mm² aluminum XLPE to satisfy fault thermal limits while meeting drop and ampacity.

Accuracy & Limitations

The converter provides a solid preliminary size with transparent notes. It simplifies complex installation conditions and uses generalized constants. For final design, confirm with local code tables and manufacturer data sheets.

• Ampacity depends on grouping, soil or duct thermal resistivity, and ventilation, which may vary widely.
• Voltage drop uses typical R and X; special cable constructions can shift reactance.
• Short‑circuit k‑factors assume adiabatic behavior; very long faults or small conductors may need non‑adiabatic checks.
• Protection device settings affect clearing time; always verify with actual time‑current curves.

Use the results as a guide to shortlist sizes and options. Then check the selected cable against the exact installation conditions and compliance requirements.

Units Reference

Cable sizing mixes electrical and geometric units. Consistent units prevent errors, especially in voltage drop and short‑circuit equations. Use this table as a quick reminder while you work through the steps.

Match the units in your inputs to these references. If you enter AWG, the tool converts it to mm² and back, so you can compare options quickly.

Common Issues & Fixes

Most sizing errors come from incomplete inputs or unrealistic assumptions. Use these quick fixes when results look off.

• Voltage drop too high: increase conductor size or reduce allowable length; consider copper if you used aluminum.
• Ampacity failure: choose a cooler installation option or a higher‑temperature insulation class.
• Short‑circuit failure: reduce protection clearing time or step up to the next cross‑section.
• Mismatch with code tables: adjust ambient, grouping, or soil parameters to mirror the table conditions.

When in doubt, add notes on your assumptions and try a sensitivity check. Small changes in length, temperature, or clearing time can move the governing constraint.

FAQ about Cable Core Size Converter

Does the converter pick the number of cores or just the core size?

It recommends the conductor cross‑sectional area. You still choose the number of cores based on the circuit (e.g., 2C, 3C, 4C, or single‑core sets).

How do I account for parallel conductors?

Size one conductor for the share of current, apply the same checks, then confirm grouping and spacing factors. Protection and fault sharing must be verified.

Can I use this for DC systems?

Yes. Use the single‑phase voltage drop formula with cosφ = 1 and X = 0. Enter the one‑way length and double it within the formula for the return path.

What if I only know AWG?

Enter the AWG and use the built‑in conversion to mm². The tool compares both and displays capacity and drop using the converted area.

Cable Core Size Terms & Definitions

Ampacity

The maximum continuous current a conductor can carry without exceeding its temperature rating under stated conditions.

Voltage Drop

The reduction in voltage from source to load due to conductor impedance at the operating current and power factor.

Adiabatic Equation

A formula that checks whether a conductor can thermally withstand a short‑circuit for the fault clearing time.

k‑Factor

A material and insulation constant used in the adiabatic equation to relate current, time, and allowable temperature rise.

Power Factor

The ratio of real power to apparent power in an AC circuit, affecting current and voltage drop.

Resistivity

A material property that describes electrical resistance per unit length and area, varying with temperature.

Reactance

The AC opposition due to inductance in a conductor or cable, contributing to voltage drop at non‑unity power factor.

Thermal Resistivity of Soil

A measure of how soil resists heat flow, affecting ampacity for buried cables.

References

Here’s a concise overview before we dive into the key points:

• American Wire Gauge (AWG) basics and formulas
• IET Wiring Matters: Understanding the adiabatic equation
• Engineering Toolbox: Voltage drop in electrical circuits
• Prysmian: Short‑circuit ratings of cables
• NFPA 70 (NEC) overview page
• IET: BS 7671 Wiring Regulations information

These points provide quick orientation—use them alongside the full explanations in this page.