400 V · 11 kV · 33 kV

Cable sizing calculator

Start with route duty in kW, then choose the cable family and compare conductor size, parallel runs and installation conditions.

Calculation basisUK / EN preliminary designSteady-state, 3-phase, 50 Hz

Assessment viewFast constraint-led selection

Duty

Define the supply and load at the receiving end.

Load

Power factorRoute length

Calculated design current (Ib)428 A8,152.2 kVA from P ÷ PF · Ib = S ÷ (√3 × U)

Required tabulated current (It), per run214 AIt = Ib ÷ (runs × Ctotal) — the reference-condition rating the selected cable must clear before derating factors are applied

Constraint search

What can be installed?

Set the largest cable and maximum parallel conductors you will permit.

10 of 10 methods available

Maximum CSA per conductorMaximum conductors per phase

Results use the current cable family, protection and environmental assumptions. Select an available method to apply its recommended conductor count. Switch to Full assessment to edit the detailed assumptions.

Cable family

Constrain construction, conductor and geometry.

Cable specificationCopper-wire screened XLPE/EPRConductorInsulationCable formFormation

Selected constructionBS 7870-4.10 — utility single-core1 conductor · Semiconductor screens and XLPE/EPR insulation · copper-wire screen · MDPE or low-smoke oversheath

Published product dataGeneric standard (no single manufacturer)

Basis: BS 7870-4.10 / HD 620 · 6.35/11 kV
Conductor: Class 2 copper or aluminium
Sheath: MDPE or low-smoke oversheath
Published range: See project or manufacturer cable schedule

Open published source ↗

Selected conductors per phase

Search boundaryUp to 4 × 630 mm² per phaseSet in Quick view

Installation

Refine the selected route's thermally limiting conditions.

Installation methodologySingle-way duct in soil

Ambient air

°C

Ground temperature

°C

Soil resistivity

K·m/W

Burial depth

Grouped circuitsGroup spacingTriple-n harmonics

Thermal insulationProject factor

Direct solar exposureApplies an additional preliminary 0.90 factorMaximum available bend radius

Protection constraints

Set fault duty and the project voltage-drop limit.

Prospective fault

Clearing time

Maximum voltage drop

Screen prospective earth fault

Screen cross-sectional area

mm²

Screen k-factor

Recommended arrangementAll checks pass

2 × 95 mm²per phase

Al / XLPE / single

1 conductor · Semiconductor screens and XLPE/EPR insulation · copper-wire screen · MDPE or low-smoke oversheath

Installed rating450 A+22 A headroom (4.9%)

Voltage drop0.63%limit 3.0%

Route losses50.4 kWconductor + dielectric

Operating temperature83.4 °C6.6 °C headroom to 90 °C

Low headroom — 4.9% of installed rating (below the 15% guide)

Consider stepping up to 2 × 120 mm² for 16.3% headroom.

Protection

Short-circuit withstand (adiabatic)

Conductor95 mm² ≥ 85.1 mm² required

ScreenEnter screen CSA, k-factor and earth fault current to check

Audit trail

Applied rating factors

C_total 1.000

Installation basisUnderground duct

0.780

Ambient air30 °C / 90 °C conductor

1.000

Ground temperature20 °C

1.000

Soil thermal resistivity2.5 K·m/W

1.000

Depth of laying0.7 m

1.000

Grouping & spacing1 circuit, touching

1.000

Formationtrefoil

1.000

Harmonicsnot applied to MV

1.000

Thermal insulationnone

1.000

Solar exposurenot exposed

1.000

Factors shown are transparent engineering seed models, not reproduced standards tables. Replace with licensed values or a validated IEC 60287 dataset before issue.

Verification

Constraint checks

Ampacity450 A ≥ 428 A

Short-circuit withstandS ≥ 85.1 mm² / conductor

Screen short-circuit withstandEnter screen CSA, k-factor and earth fault current

Voltage drop0.63% ≤ 3%

Parallel conductor equalitySame material, CSA, length & termination required

Option study

Passing cable sizes

9 options

2 × 95 mm²Best fit450 A0.63%50.4 kW

2 × 120 mm²511 A0.51%39.9 kW

2 × 150 mm²578 A0.42%32.0 kW

2 × 185 mm²649 A0.35%26.0 kW

2 × 240 mm²749 A0.29%20.2 kW

Calculation appendix

Full preliminary calculation set

Every equation, substitution, cable-data source and applied factor used by this comparison.

Unsaved live design · generated 27 August 2026

Preliminary engineering calculationManufacturer records describe product construction and published scope. Current rating remains the transparent seed model shown below until licensed BS 7671 or validated IEC 60287 data is loaded.

2 × 95 mm²per phase

Al / XLPE / single — 1 conductor · Semiconductor screens and XLPE/EPR insulation · copper-wire screen · MDPE or low-smoke oversheath

Installed rating450 A

Voltage drop0.63%

Route losses50.4 kW

Operating temperature83.4 °C

Duty7,500.0 kW at 0.92 PF11,000 V · 480 m

Selected cable familyBS 7870-4.10 — utility single-coreAluminium · single · 2 per phase

Recommended arrangement2 × 95 mm² / phaseUnderground duct · Ctotal 1.000

Overall resultPreliminary checks passAmpacity · voltage drop · adiabatic short circuit

Manufacturer / construction record

BS 7870-4.10 — utility single-core

Published source ↗

ProviderGeneric standard (no single manufacturer)

Published basisBS 7870-4.10 / HD 620 · 6.35/11 kV

ConductorClass 2 copper or aluminium

InsulationSemiconductor screens and XLPE/EPR insulation

Screen / armourcopper-wire · Unarmoured

Outer layerMDPE or low-smoke oversheath

Verified diagram sequence: 1 conductor · Semiconductor screens and XLPE/EPR insulation · copper-wire screen · MDPE or low-smoke oversheath

Worked calculations

Equations and substitutions

14 calculation steps

StepCheckEquation / substitutionResult

01Apparent power

S = P ÷ PF7,500.0 kW ÷ 0.92

8,152.2 kVA

02Three-phase design current

Ib = S × 1000 ÷ (√3 × U)8,152.2 × 1000 ÷ (√3 × 11,000 V)

427.9 A

03Current per parallel conductor

Irun = Ib ÷ n427.9 A ÷ 2

213.9 A

04Seed reference rating

It = 30.2 × S^0.55 × kmaterial × kform × kvoltage × kinstallationS = 95 mm²; installation basis 0.780

224.9 A / run

05Composite correction factor

Ctotal = Ca × Cg × Cs × Cd × Cn × Ch × Ci × Csolar × Cformation × Cproject1.000 × 1.000 × 1.000 × 1.000 × 1.000 × 1.000 × 1.000 × 1.000 × 1.000 × 1.000

1.000

06Installed current rating

Iz = It × Ctotal × n224.9 × 1.000 × 2

449.8 A

07Conductor AC resistance at operating temperature

Rθ = [(ρ20 × 1000 ÷ S) × (1 + α(θ − 20))] × (1 + ys + yp)S = 95 mm²; θ = 90 °C; ys 0.0006; yp 0.0010 (IEC 60287-1-1, estimated geometry)

0.3820 Ω/km

08Three-phase voltage drop

ΔV = √3 × Ib × (R cosφ + X sinφ) × L ÷ nR 0.3820 Ω/km; X 0.095 Ω/km; L 0.480 km

69.14 V · 0.63%

09Three-phase conductor route loss

Pc = 3 × Irun² × Rroute × n ÷ 1000Irun 213.9 A; Rroute 0.1834 Ω

50.36 kW

10Dielectric loss

Wd = 3 × ω × C × U0² × tanδ × L × n ÷ 1000C 0.289 µF/km (estimated geometry); U0 6,351 V; tanδ published typical (IEC 60287-1-1 Table 3)

0.011 kW

11Charging current (informational)

Ic = ω × C × U0 × LReactive current from estimated cable capacitance; not deducted from rated current below

0.28 A / run

12Adiabatic short-circuit minimum

Smin = (Ik ÷ n) × √t ÷ k16.00 kA × 1000 ÷ 2 × √1.00 ÷ 94

85.1 mm² / conductor

13Screen adiabatic short-circuit minimum

Sscreen,min = (Ik,earth ÷ n) × √t ÷ kscreenEnter a screen k-factor and prospective earth fault current (Protection constraints panel) to check

—

14Estimated conductor temperature at design load

θc = θamb + (θmax − θamb) × (Ib ÷ Iz)²θamb 20.0 °C; θmax 90 °C; Ib 427.9 A; Iz 449.8 A

83.4 °C (6.6 °C headroom to 90 °C rating)

Correction factors

Applied factor schedule

Product = 1.000

Installation basis0.780Underground duct

Ambient air1.00030 °C

Ground temperature1.00020 °C

Soil thermal resistivity1.0002.5 K·m/W

Depth of laying1.0000.7 m

Grouping1.0001 circuit(s), touching

Harmonics1.000Not applied

Thermal insulation1.000none

Solar exposure1.000Not exposed

Formation1.000trefoil

Project factor1.000User-entered

Calculation basisThree-phase, 50 Hz, steady-state comparison

Review against current licensed standards, the exact manufacturer datasheet, protection settings, bonding, earthing and project conditions before issue for construction.