Cable Size Calculator

1. Find the load current (Ib). 1-phase: I = (kW × 1000) ÷ (V × PF) 3-phase: I = (kW × 1000) ÷ (√3 × V × PF) For motors: kW = (HP × 0.746) ÷ efficiency.
2. Add a planning margin (optional). This calculator can add +20% for spare capacity. It is not a code demand or derating factor.
3. Choose a base size from current-carrying capacity. Pick the smallest mm² size whose capacity, multiplied by the total derating factor, is at least the design current — separately for copper or aluminium.
4. Apply derating. Correct the tabulated capacity for ambient temperature, grouping and installation method — see derating. The calculator applies the single total factor you enter; the final design must use the factors from your governing table.
5. Check voltage drop. 1-phase: Vd = 2 × I × L × (Rₜ cosφ + X sinφ) ÷ 1000 3-phase: Vd = √3 × I × L × (Rₜ cosφ + X sinφ) ÷ 1000 Rₜ is the conductor resistance at operating temperature and X the cable reactance (Ω/km). If Vd% exceeds the chosen limit, step up one size and re-check.
6. Take the larger of the two. The final screening size is whichever is larger: the size set by current-carrying capacity or the size set by voltage drop. Short, heavily loaded runs are usually capacity-governed; long runs are usually voltage-drop-governed.
7. Select protection, neutral and earth. Protective device: Ib ≤ In ≤ cable capacity. Neutral = phase size by default. Earth/CPC by rule of thumb (verify with an adiabatic check).

Worked Example: 20 kW, 3-Phase, 415 V, 60 m, Copper

1. Load current = (20 × 1000) ÷ (1.732 × 415 × 0.85) ≈ 32.7 A.
2. With +20% planning margin → sizing current ≈ 39.3 A.
3. Smallest size with screening capacity ≥ 39.3 A → 10 mm² (40 A).
4. Resistance at 70°C = 1.83 × [1 + 0.00393 × (70 − 20)] ≈ 2.19 Ω/km; with cosφ 0.85 and X 0.08 Ω/km, R cosφ + X sinφ ≈ 1.86 + 0.04 = 1.90 Ω/km.
5. Voltage drop at 60 m = √3 × 39.3 × 60 × 1.90 ÷ 1000 ≈ 7.8 V ≈ 1.9% → passes 3%. The size is capacity-governed here.
6. Result: 10 mm² copper, indicative protective device 40 A (39.3 ≤ 40 ≤ 40).

Screening result only — confirm with the applicable current-rating table for your installation method and derating.

Why Cable Size Matters

• Heat: current through conductor resistance creates I²R heat. An undersized cable overheats, ages its insulation and can cause a fire.
• Voltage drop: long runs lose voltage in the cable. Motors and equipment can overheat, underperform or trip on undervoltage.
• Cost: oversizing adds material and installation cost — but a larger size may be chosen deliberately for voltage drop, derating or future load.

• Formula: Vd% = Vd ÷ V × 100, with Vd = k × I × L × (Rₜ cosφ + X sinφ) ÷ 1000, where k = 2 for single-phase and √3 for three-phase.
• L = one-way length (m), R = conductor resistance (Ω/km), I = design current (A).
• Screening target: selectable, 3% by default. Codes set their own limits — e.g. IEC 60364-5-52 and BS 7671 guidance cite 3% for lighting and 5% for other uses on a public LV supply (6% and 8% from a private supply), and NEC informational notes suggest about 3% branch / 5% total.
• Temperature and reactance: resistance is corrected from 20°C to the selected conductor temperature (α = 0.00393/°C copper, 0.00403/°C aluminium), and a typical multicore reactance of 0.08 Ω/km is added. Reactance matters most on large sizes at low power factor; single-core cables laid apart have higher reactance.
• Motor starting: the check is steady-state. A motor’s starting current can briefly cause a much larger drop — check it separately for large DOL motors.
• For an R·cosφ + X·sinφ calculation, use the Voltage Drop Calculator.

Field Experience: When Cable Layout Caused the Unbalance (Industrial UPS Connection)

Parallel Single-Core Cables

• Used when one conductor per phase would be too large to pull or terminate.
• Cable arrangement changes mutual inductance — a poor layout makes cables share current unequally.
• Two families: trefoil (three phases grouped per circuit) and flat (evenly spaced).
• Follow the layout diagram in the applicable standard (e.g. AS/NZS 3008.1, IEC 60364-5-52) — typically defined for up to four parallel circuits per phase.

Trefoil vs Flat Layout for 3 Parallel Cables per Phase (Three-Phase)

With three parallel single-core cables per phase there are nine cables in total, laid as three groups. Each group carries one cable of every phase, so each group is a complete three-phase circuit. The two common ways to lay these groups are trefoil and flat.

1. Trefoil arrangement

Layout: three trefoil groups side by side. Each group holds one A, one B and one C cable, touching and held together with trefoil cleats.

Advantages

• Low external magnetic field, so less induced heating in nearby steel trays, supports and gland plates.
• Compact — needs the least tray width.
• Each group is a complete circuit, so the three paths stay alike when groups are spaced and cut to length equally.
• Widely used and covered in manufacturer guidance and standards (IEC, IEEE).

Limitations

• Touching cables lose heat less freely, so the rating can be slightly lower than for spaced flat layouts in some installations.
• Needs proper trefoil cleats at regular intervals to keep the shape, especially under fault forces.
• Current sharing between groups is not automatic — it still depends on equal spacing and equal lengths.

2. Flat arrangement (phase order rotated)

Layout: three flat groups of three cables in a row, with the phase order rotated between groups — A-B-C, C-B-A, A-B-C — and the same order kept at both ends.

Advantages

• Better heat dissipation when cables are spaced, because more of the surface is exposed to air.
• Rotating the phase order between groups helps even out inductive imbalance, so the parallel paths tend to share current more evenly.
• Easier to inspect, to clamp-meter each cable and to terminate.
• Practical on long ladder or tray runs with enough width, where trefoil is awkward.

Limitations

• Needs more tray width and more mechanical support.
• Higher external magnetic field than trefoil, so more induced heating in nearby steelwork unless spaced or non-magnetic.
• Imbalance returns if the rotation is not kept along the whole run and at both ends.
• Needs correct spacing and cleating as per the manufacturer.

Practical field comparison

Field observations, not code values. Results depend on spacing, phase order, bonding and run length.

Key practical points (both arrangements)

1. Use the same length, material, cross-section, insulation and rating for every parallel cable in a phase.
2. Keep the A-B-C phase sequence and the group order the same at both ends of the run.
3. Use cleats and spacing as per the manufacturer and the applicable standard (cable cleats: IEC 61914).
4. Check induced heating on metal supports and gland plates — pass all cables of one circuit through the same opening, or use a non-magnetic plate.
5. Verify current sharing after installation with a clamp meter or load test.
6. Provide sheath bonding and earthing as per the design.
7. For a calculated current-sharing check, IEC 60287-1-3 covers any number of parallel single-core cables in any layout.

3. Three-phase with neutral and earth — layout and procedure

This covers a three-phase, four-wire circuit with a separate neutral and separate earth, using three parallel single-core cables per phase. The neutral and earth are laid with the same groups, so each parallel path stays complete.

Neutral options

• Full-size neutral (default): same size as the phase cable, one neutral per group. Every group then carries the same share of neutral current. Prefer this with non-linear loads (triplen harmonics).
• Reduced neutral: only where the load is balanced, low in harmonics and the standard allows it. Use fewer neutral cables of equal size, place them symmetrically and check the sharing by measurement.

Earth options

• Separate earth conductor(s) (green-yellow) on the same route, close to the phase cables — one per group or a common conductor. Size for fault current and disconnection time with the Earthing Conductor Size Calculator.
• Armour, sheath or tray as the protective conductor only where the design and standard permit it, after checking continuity and impedance.

Procedure

1. Fix the number of parallel paths and the layout first (quad or flat) — the neutral and earth follow the same layout.
2. Select the neutral size. Default is the same size as the phase; reduce it only where the conditions above are met.
3. Lay one neutral with each group, with the same length, material and size as the phase cables. In a trefoil layout this makes a quad bundle; in a flat layout keep the neutral in the same position in every group.
4. Size the earth conductor for the fault current and disconnection time, then lay it on the same route, close to the groups.
5. Cleat each group with the correct trefoil or quad cleats as per the manufacturer, with the earth conductor alongside.
6. Terminate phase, neutral and earth in the same order at both ends. Land the neutrals on the neutral bar and the earth on the earth bar.
7. Provide sheath or armour bonding as per the design. In large single-core cables, bonding both ends can cause circulating current, so check it at design stage.
8. Before energising: insulation resistance, phase-sequence and neutral/earth continuity checks.
9. After energising: clamp-meter each phase and neutral cable in every group, and check the glands, plates and supports for heating.

Neutral Conductor Sizing

• Default: neutral = phase conductor size (this is what the calculator shows).
• Balanced, low-harmonic 3-phase loads: a reduced neutral may be allowed — it must still carry the out-of-balance current.
• Many nonlinear single-phase loads (LED drivers, SMPS, IT equipment): triplen harmonics (3rd, 9th…) add in the neutral — size it equal to or larger than the phase.

Earth / CPC Conductor Sizing

• Rule of thumb used here: same as phase up to 16 mm², 16 mm² for 16–35 mm², half the phase size above 35 mm².
• Final design needs the adiabatic check (fault current and disconnection time) — use the Earthing Conductor Size Calculator.

Common Cable Laying Methods

Laying Method Diagrams: Tray, Direct Buried and Ducts

The same cable carries a different current depending on how it is laid, because the laying method decides how easily heat leaves the cable. The three cross-sections below show the methods in the table above that most often decide the derating factor.

1. Perforated tray / ladder

• Touching: bunched cables heat each other, so a grouping factor applies.
• Spaced: a gap of at least one cable diameter (De) lets air move between cables and gets closest to the free-air rating.
• In the IEC / BS reference methods, multicore cable on perforated tray is Method E, single-core touching is Method F and single-core spaced is Method G. Check the exact conditions in your standard's table.
• Keep cables in a single layer where possible; extra layers add more derating.

2. Direct buried

• The rating depends on burial depth, ground temperature and soil thermal resistivity. Published tables commonly assume about 0.7 m depth, 20 °C ground and 2.5 K·m/W soil, so dry or sandy soil needs a derating factor.
• Sand bedding protects the sheath and carries heat away; the cover tile and warning tape protect against later excavation.
• Other cables or heat sources nearby also reduce the rating; groups of buried circuits need a grouping factor.

3. Ducts in ground

• The air inside a duct resists heat flow more than soil does, so the same cable has a lower rating in a duct than direct buried.
• Ducts let you pull or replace cables without digging; spare ducts and draw pits make later additions easy.
• For single-core cables, keep all phases of a circuit together in the same non-magnetic duct. Putting one phase in each steel duct causes heating and unequal current.

Illustrative cross-sections, not to scale. Depths, spacing and bedding are typical values only; follow your standard, the cable manufacturer and the utility specification. For the derated current, use the Current Carrying Capacity Calculator.

Cable Laying & Installation Checklist

The calculator gives a preliminary size only. Before and during installation, confirm these points against the project specification and the applicable standard.

A general engineering checklist — requirements differ between installations and are not identical for every cable.

Common Cable Sizing Mistakes

• Ignoring length: a cable rated for the current can still fail the voltage-drop check.
• Assuming PF = 1 for motors: underestimates current and undersizes cable and breaker.
• Skipping derating: hot rooms and grouped cables reduce capacity.
• Ignoring cores / loaded conductors: 2-, 3- and 4-core cables have different ratings (not modelled here).
• Swapping Cu ↔ Al without re-checking the size table.

Conceptual background only. For final design use the applicable standard (AS/NZS 3008.1/3000, IEC 60364-5-52, IS installation practice or BS 7671) and verify with a qualified electrical engineer.