HV Cable Sizing Calculator — help

Overview

Sizes three-phase high voltage copper XLPE cables for systems rated 3.3–33 kV per AS/NZS 3008.1.1 and IEC 60502. Cable data is sourced from Nexans Olex published tables for copper conductors with XLPE insulation rated at 90°C.

The calculator performs four checks simultaneously:

Current capacity — selects the smallest standard conductor whose derated ampacity meets the design current, then automatically upsizes if voltage drop or short circuit withstand checks require a larger conductor.
Voltage drop — verifies the three-phase voltage drop stays within the user-specified limit.
Short circuit withstand — verifies the conductor cross-section exceeds the adiabatic minimum for the prospective fault current and clearing time.
Power loss — calculates I²R losses in the cable run.

Input Parameters

Load Parameters

Parameter	Type	Default	Range	Description
`load_power`	Number	5000	min 10	Three-phase load power (kW or kVA)
`power_type`	Enum: kW / kVA	kVA	—	Whether load_power is active (kW) or apparent (kVA)
`power_factor`	Number	0.9	0.5 – 1.0	Lagging power factor (cos φ)
`system_voltage`	Enum: 3.3 / 6.6 / 11 / 22 / 33 kV	11	—	Nominal line-to-line voltage (kV)

Cable Type

Parameter	Type	Default	Range	Description
`cable_type`	Enum: Multi-core (3-core) / Single-core	Multi-core (3-core)	—	Cable construction
`cable_configuration`	Enum: Trefoil / Flat / N/A (Multi-core)	N/A (Multi-core)	—	Single-core laying arrangement (ignored for multi-core)

Installation Conditions

Parameter	Type	Default	Range	Description
`installation_method`	Enum	Direct Buried	—	Direct Buried / In Ducts (Underground) / In Cable Trays / In Air (Clipped) / Enclosed Trunking
`ambient_temperature`	Number	25	−20 – 80 °C	Ground temperature (buried) or air temperature (in air)
`thermal_resistivity`	Number	1.0	0.5 – 3.0 K·m/W	Ground thermal resistivity (buried cables only)
`depth_of_burial`	Number	0.8	0.5 – 2.0 m	Depth of burial (buried cables only)
`number_of_circuits`	Integer	1	1 – 6	Number of circuits sharing the same route
`circuit_spacing`	Enum: 0 / 0.15 / 0.3 / 0.6 m	0.3	—	Spacing between grouped circuits (0 = touching)

Route Parameters

Parameter	Type	Default	Range	Description
`cable_length_m`	Number	250	1 – 10,000 m	Total cable route length
`max_voltage_drop_percent`	Number	2.5	0.5 – 10%	Maximum allowable voltage drop percentage

Short Circuit Parameters

Parameter	Type	Default	Range	Description
`short_circuit_current_ka`	Number	25	1 – 100 kA	Prospective fault current (RMS symmetric)
`short_circuit_duration_s`	Enum: 0.1 / 0.5 / 1.0 / 3.0 s	1.0	—	Fault clearing time

Design Parameters

Parameter	Type	Default	Range	Description
`conductor_temp_rating`	Enum: 90 °C	90	—	Info only — R values always use 90°C from Nexans Olex tables

Output Results

Output	Unit	Description
`load_current_a`	A	Three-phase load current
`k1_temperature_factor`	—	Temperature correction factor
`k2_grouping_factor`	—	Grouping correction factor
`k3_thermal_resistivity_factor`	—	Soil thermal resistivity correction factor
`k4_depth_factor`	—	Depth of burial correction factor
`combined_derating_factor`	—	Product \( k_1 \times k_2 \times k_3 \times k_4 \)
`design_current_a`	A	Required cable rating = \( I_L / k_{total} \)
`selected_conductor_size_mm2`	mm²	Selected standard conductor cross-section
`cable_current_rating_a`	A	Tabulated ampacity at reference conditions
`cable_resistance_ohm_per_km`	Ω/km	AC resistance per phase at 90°C
`cable_reactance_ohm_per_km`	Ω/km	Reactance per phase at 50 Hz
`cable_impedance_ohm_per_km`	Ω/km	Cable impedance \( Z = \sqrt{R^2 + X^2} \)
`actual_voltage_drop_volts`	V	Three-phase voltage drop in volts
`actual_voltage_drop_percent`	%	Voltage drop as percentage of nominal voltage
`voltage_drop_compliant`	Boolean	Whether \( V_{d\%} \leq V_{d,max\%} \)
`min_size_short_circuit_mm2`	mm²	Minimum conductor area for SC withstand
`short_circuit_compliant`	Boolean	Whether selected size ≥ minimum SC size
`power_loss_w`	W	Total I²R loss in the cable run
`power_loss_percent`	%	Power loss as percentage of transmitted power
`current_utilization_percent`	%	Load current as percentage of cable rating
`overall_compliant`	Boolean	True when ampacity, voltage drop, and SC checks all pass
`compliance_status`	text	"COMPLIANT" or "NON-COMPLIANT"
`derated_current_rating_a`	A	Tabulated ampacity × combined derating factor
`governing_criterion`	text	"Ampacity", "Short Circuit Withstand", or "Voltage Drop"

Formulas

Load Current (Three-Phase)

For apparent power (kVA):

\[ I_L = \frac{S}{\sqrt{3} \times U} \]

For active power (kW):

\[ I_L = \frac{P}{\sqrt{3} \times U \times \cos\phi} \]

Where \(S\) is apparent power (kVA), \(P\) is active power (kW), \(U\) is line-to-line voltage (kV), and \(\cos\phi\) is the power factor.

Derating Factors

Four correction factors are applied to convert tabulated ampacity to site-specific conditions:

\(k_1\) — temperature correction (ambient vs reference)
\(k_2\) — grouping/proximity of adjacent circuits
\(k_3\) — soil thermal resistivity (buried cables only)
\(k_4\) — depth of burial (buried cables only)

\[ k_{total} = k_1 \times k_2 \times k_3 \times k_4 \]

Factors are interpolated from AS/NZS 3008 correction tables. For in-air installations, \(k_3 = k_4 = 1.0\).

Design Current

\[ I_z = \frac{I_L}{k_{total}} \]

The cable must have a tabulated rating \(I_{rated} \geq I_z\). The smallest standard size satisfying this condition is selected.

Cable Impedance

\[ Z_c = \sqrt{R_c^2 + X_c^2} \quad (\Omega/\text{km}) \]

Where \(R_c\) is the AC resistance at 90°C and \(X_c\) is the reactance at 50 Hz, both from Nexans Olex tables.

Three-Phase Voltage Drop

\[ \Delta V = \sqrt{3} \times I_L \times L \times (R_c \cos\phi + X_c \sin\phi) \]

Where \(I_L\) is the load current (A), \(L\) is the cable length (km), \(R_c\) is resistance (Ω/km), \(X_c\) is reactance (Ω/km), and \(\cos\phi\) is the power factor.

\[ V_{d\%} = \frac{\Delta V}{U} \times 100 \]

\(U\) is the nominal line-to-line voltage in volts.

Short Circuit Withstand (Adiabatic Equation)

\[ A_{min} = \frac{I_{sc} \times \sqrt{t}}{k} \]

Where \(I_{sc}\) is the prospective fault current (A, RMS symmetric), \(t\) is the fault clearing time (s), and \(k\) is the adiabatic constant. For copper XLPE conductors (90°C initial to 250°C final):

\[ k = 143 \;\text{A}\cdot\text{s}^{0.5}/\text{mm}^2 \]

The selected conductor must satisfy \(A_{selected} \geq A_{min}\).

Three-Phase Power Loss

\[ P_{loss} = 3 \times I_L^2 \times R_c \times L \]

Where \(R_c\) is resistance (Ω/km) and \(L\) is length (km). Factor of 3 accounts for all three phases.

Key Lookup Tables

11 kV Multi-core — Current Ratings & Impedance

Copper XLPE, direct buried, 0.8 m depth, 25°C ground, 1.0 K·m/W (Nexans Olex reference conditions)

CSA (mm²)	Rating (A)	R @ 90°C (Ω/km)	X (Ω/km)
16	120	1.540	0.110
25	155	0.969	0.105
35	185	0.668	0.102
50	220	0.524	0.100
70	270	0.363	0.095
95	325	0.261	0.090
120	375	0.207	0.087
150	425	0.168	0.084
185	485	0.134	0.082
240	565	0.102	0.079
300	645	0.0817	0.077
400	750	0.0638	0.074
500	850	0.0497	0.072
630	965	0.0384	0.070

Temperature Correction Factors (Buried, 90°C XLPE)

Ground Temp (°C)	k₁
15	1.11
20	1.05
25 (reference)	1.00
30	0.94
35	0.88
40	0.82

Grouping Factors — Multi-core Buried

Circuits	Touching	0.15 m	0.30 m	0.60 m
1	1.00	1.00	1.00	1.00
2	0.80	0.85	0.90	0.95
3	0.70	0.78	0.85	0.92
4	0.65	0.73	0.81	0.90
5	0.60	0.70	0.78	0.88
6	0.57	0.68	0.76	0.87

Worked Examples

The 4 examples below are the validation cases for this calculation. Each one was produced by the calculation engine from the inputs shown, every figure was checked against the independent reference quoted at the end of the example, and the same cases are run against every engine build (see validation scorecard, tool fe_calc_cable_size; worked examples for every calculation).

#	Case	Cable	I_L (A)	I_derated (A)	V_d (%)	Governing	Result
1	11 kV 5 MVA substation feeder, 500 m: short-circuit withstand governs	185 mm² Cu XLPE 3-core	262.4	485.0	0.32	short-circuit withstand	PASS
2	33 kV 20 MW sub-transmission, single-core trefoil in hot, dry soil	240 mm² Cu XLPE trefoil	411.7	440.7	0.55	current-carrying capacity	PASS
3	11 kV 3 MVA mine feeder, four touching circuits: ampacity governs	120 mm² Cu XLPE 3-core	157.5	175.9	0.83	current-carrying capacity	PASS
4	6.6 kV 1.5 MVA feeder, 3 km single-core trefoil: voltage drop governs	120 mm² Cu XLPE trefoil	131.2	424.0	2.29	voltage drop	PASS

Example 1 — 11 kV 5 MVA substation feeder, 500 m: short-circuit withstand governs

Reference buried conditions (no derating): 25 kA for 1 s needs 174.8 mm², so 185 mm² is selected although 150 mm² would carry the 262 A load.

Given: 5,000 kVA at 11 kV, PF 0.9; copper XLPE 3-core cable, direct buried, ground 25 °C (default), soil 1 K·m/W (default), depth 0.8 m (default), 1 circuit; route 500 m; fault 25 kA (default) for 1 s (default); voltage-drop limit 2.5 % (default).

Load current	I_L = 5,000 kVA × 1000 / (√3 × 11,000) = 262.43 A
Derating	k₁ = 1.00 (25 °C ground); k₂ = 1.00 (1 circuit); k₃ = 1.00 (1 K·m/W); k₄ = 1.00 (0.8 m); k_total = 1.00 × 1.00 × 1.00 × 1.00 = 1.00; I_z = 262.43 / 1.00 = 262.43 A
Short-circuit minimum	A_min = I_sc × √t / k = 25,000 × √1 / 143 = 174.83 mm²
Size selection	Multi-core (3-core) sizes tried smallest first for rating ≥ I_z, A ≥ A_min and ΔV within 2.5 %: selected 185 mm² — Nexans Olex 11 kV table: rating 485 A, R = 0.134 Ω/km at 90 °C, X = 0.0820 Ω/km (Z = 0.1571 Ω/km). Last check to reject a smaller size: short circuit withstand
Check — capacity	I_derated = 485 × 1.00 = 485.0 A vs I_L 262.43 A (utilisation 54.1 %) PASS
Check — voltage drop	ΔV = √3 × 262.43 × 0.500 × (0.134 × 0.9 + 0.0820 × 0.4359) = 35.53 V = 0.323 % of 11,000 V, limit 2.5 % PASS
Check — short circuit	A = 185 mm² ≥ A_min 174.83 mm² (margin 5.8 %) PASS
Power loss	P_loss = 3 × 262.43² × 0.134 × 0.500 = 13,843 W (0.28 % of the load)
Result	3/3 checks pass → PASS; 185 mm² Cu XLPE; power loss 13,843 W; governing: short circuit withstand
Reference	c0020 help-page Example 1 (calcs/test_c0020 test_example1_*): I = 5 MVA / (√3 × 11 kV) = 262.4 A; adiabatic minimum S = 25 000 × √1 / 143 = 174.83 mm² → 185 mm²; governed by short-circuit withstand. (validation case `c0020-5000kva-11kv-example1`)

Example 2 — 33 kV 20 MW sub-transmission, single-core trefoil in hot, dry soil

All four buried derating factors apply (0.88 × 0.93 × 0.79 × 0.96 = 0.62); 240 mm² carries the 412 A load and drops 0.55 % over 2 km.

Given: 20,000 kW at 33 kV, PF 0.85; copper XLPE single-core trefoil cable, direct buried, ground 35 °C, soil 2 K·m/W, depth 1.2 m, 2 circuits, 0.3 m apart; route 2,000 m; fault 31.5 kA for 1 s (default); voltage-drop limit 5 %.

Load current	I_L = 20,000 kW × 1000 / (√3 × 33,000 × 0.85) = 411.66 A
Derating	k₁ = 0.88 (35 °C ground); k₂ = 0.93 (2 circuits, 0.3 m apart); k₃ = 0.79 (2 K·m/W); k₄ = 0.96 (1.2 m); k_total = 0.88 × 0.93 × 0.79 × 0.96 = 0.621; I_z = 411.66 / 0.621 = 663.24 A
Short-circuit minimum	A_min = I_sc × √t / k = 31,500 × √1 / 143 = 220.28 mm²
Size selection	Single-core trefoil sizes tried smallest first for rating ≥ I_z, A ≥ A_min and ΔV within 5 %: selected 240 mm² — Nexans Olex 11 kV table: rating 710 A, R = 0.102 Ω/km at 90 °C, X = 0.0750 Ω/km (Z = 0.1266 Ω/km). Last check to reject a smaller size: ampacity
Check — capacity	I_derated = 710 × 0.621 = 440.7 A vs I_L 411.66 A (utilisation 93.4 %) PASS
Check — voltage drop	ΔV = √3 × 411.66 × 2.000 × (0.102 × 0.85 + 0.0750 × 0.5268) = 179.98 V = 0.545 % of 33,000 V, limit 5 % PASS
Check — short circuit	A = 240 mm² ≥ A_min 220.28 mm² (margin 9.0 %) PASS
Power loss	P_loss = 3 × 411.66² × 0.102 × 2.000 = 103,711 W (0.44 % of the load)
Result	3/3 checks pass → PASS; 240 mm² Cu XLPE; power loss 103,711 W; governing: current carrying capacity. Cable utilization is high (93.4%) Consider voltage drop carefully for long cable runs The HV engine uses its 11 kV cable data (ratings, R, X) for every system voltage; confirm with manufacturer data for this voltage.
Reference	c0020 help-page Example 2 (calcs/test_c0020 test_example2_*): I = 20 MW / (√3 × 33 kV × 0.85) = 411.7 A; derating 0.88 × 0.93 × 0.79 × 0.96 = 0.621; 240 mm² selected; ΔV ≈ 0.545 %. (validation case `c0020-20mw-33kv-trefoil-example2`)

Example 3 — 11 kV 3 MVA mine feeder, four touching circuits: ampacity governs

Heavy grouping (0.65) with 40 °C soil and 1.5 K·m/W soil takes the derating to 0.47, so 120 mm² is set by ampacity rather than the 98.9 mm² short-circuit minimum.

Given: 3,000 kVA at 11 kV, PF 0.9; copper XLPE 3-core cable, direct buried, ground 40 °C, soil 1.5 K·m/W, depth 0.8 m (default), 4 circuits, 0 m apart; route 1,500 m; fault 20 kA for 0.5 s; voltage-drop limit 3 %.

Load current	I_L = 3,000 kVA × 1000 / (√3 × 11,000) = 157.46 A
Derating	k₁ = 0.82 (40 °C ground); k₂ = 0.65 (4 circuits, 0 m apart); k₃ = 0.88 (1.5 K·m/W); k₄ = 1.00 (0.8 m); k_total = 0.82 × 0.65 × 0.88 × 1.00 = 0.469; I_z = 157.46 / 0.469 = 335.71 A
Short-circuit minimum	A_min = I_sc × √t / k = 20,000 × √0.5 / 143 = 98.90 mm²
Size selection	Multi-core (3-core) sizes tried smallest first for rating ≥ I_z, A ≥ A_min and ΔV within 3 %: selected 120 mm² — Nexans Olex 11 kV table: rating 375 A, R = 0.207 Ω/km at 90 °C, X = 0.0870 Ω/km (Z = 0.2245 Ω/km). Last check to reject a smaller size: ampacity
Check — capacity	I_derated = 375 × 0.469 = 175.9 A vs I_L 157.46 A (utilisation 89.5 %) PASS
Check — voltage drop	ΔV = √3 × 157.46 × 1.500 × (0.207 × 0.9 + 0.0870 × 0.4359) = 91.73 V = 0.834 % of 11,000 V, limit 3 % PASS
Check — short circuit	A = 120 mm² ≥ A_min 98.90 mm² (margin 21.3 %) PASS
Power loss	P_loss = 3 × 157.46² × 0.207 × 1.500 = 23,095 W (0.77 % of the load)
Result	3/3 checks pass → PASS; 120 mm² Cu XLPE; power loss 23,095 W; governing: current carrying capacity. Consider voltage drop carefully for long cable runs Review grouping factors and thermal considerations for multiple circuits
Reference	c0020 help-page Example 3 (calcs/test_c0020 test_example3_*): I = 3 MVA / (√3 × 11 kV) = 157.5 A; derating 0.82 × 0.65 × 0.88 × 1.00; 120 mm² selected, governed by ampacity; S_min = 20 000 × √0.5 / 143 = 98.9 mm²; ΔV ≈ 0.834 %. (validation case `c0020-3000kva-11kv-grouped-example3`)

Example 4 — 6.6 kV 1.5 MVA feeder, 3 km single-core trefoil: voltage drop governs

Two trefoil circuits 0.3 m apart in 30 °C soil: 95 mm² meets ampacity and the 16 kA / 0.5 s withstand but exceeds 2.5 % over 3 km, so 120 mm² is selected (the engine's 11 kV data is applied at 6.6 kV).

Given: 1,500 kVA at 6.6 kV, PF 0.9 (default); copper XLPE single-core trefoil cable, direct buried, ground 30 °C, soil 1 K·m/W (default), depth 0.8 m (default), 2 circuits, 0.3 m apart; route 3,000 m; fault 16 kA for 0.5 s; voltage-drop limit 2.5 % (default).

Load current	I_L = 1,500 kVA × 1000 / (√3 × 6,600) = 131.22 A
Derating	k₁ = 0.94 (30 °C ground); k₂ = 0.93 (2 circuits, 0.3 m apart); k₃ = 1.00 (1 K·m/W); k₄ = 1.00 (0.8 m); k_total = 0.94 × 0.93 × 1.00 × 1.00 = 0.874; I_z = 131.22 / 0.874 = 150.10 A
Short-circuit minimum	A_min = I_sc × √t / k = 16,000 × √0.5 / 143 = 79.12 mm²
Size selection	Single-core trefoil sizes tried smallest first for rating ≥ I_z, A ≥ A_min and ΔV within 2.5 %: selected 120 mm² — Nexans Olex 11 kV table: rating 485 A, R = 0.207 Ω/km at 90 °C, X = 0.0810 Ω/km (Z = 0.2223 Ω/km). Last check to reject a smaller size: voltage drop
Check — capacity	I_derated = 485 × 0.874 = 424.0 A vs I_L 131.22 A (utilisation 30.9 %) PASS
Check — voltage drop	ΔV = √3 × 131.22 × 3.000 × (0.207 × 0.9 + 0.0810 × 0.4359) = 151.10 V = 2.289 % of 6,600 V, limit 2.5 % PASS
Check — short circuit	A = 120 mm² ≥ A_min 79.12 mm² (margin 51.7 %) PASS
Power loss	P_loss = 3 × 131.22² × 0.207 × 3.000 = 32,076 W (2.14 % of the load)
Result	3/3 checks pass → PASS; 120 mm² Cu XLPE; power loss 32,076 W; governing: voltage drop. Voltage drop margin is tight (0.21%) Consider voltage drop carefully for long cable runs The HV engine uses its 11 kV cable data (ratings, R, X) for every system voltage; confirm with manufacturer data for this voltage.
Reference	Hand calculation with the engine's embedded Nexans Olex 11 kV single-core trefoil table (help page Key Lookup Tables): I = 1500 kVA / (√3 × 6.6 kV) = 131.22 A; k1 = 0.94 (30 °C ground), k2 = 0.93 (2 trefoil circuits, 0.3 m), k3 = k4 = 1.00 → 0.874, Iz = 150.1 A; Amin = 16 000 × √0.5 / 143 = 79.12 mm²; 95 mm² (R 0.261, X 0.083 Ω/km) gives ΔV = √3 × 131.22 × 3 × (0.261 × 0.9 + 0.083 × 0.4359) = 184.8 V = 2.80 % > 2.5 %; 120 mm² (R 0.207, X 0.081, rated 485 A) gives 151.1 V = 2.29 %; derated rating 485 × 0.874 = 424.0 A. (validation case `c0020-1500kva-6.6kv-trefoil-3km-vd`)

Standards & References

AS/NZS 3008.1.1:2017 — Electrical installations — Selection of cables — Cables for alternating voltages up to and including 0.6/1 kV (derating factor methodology also applied to HV cables)
IEC 60502-2 — Power cables with extruded insulation and their accessories for rated voltages from 1 kV up to 30 kV
IEC 60909 — Short-circuit currents in three-phase a.c. systems — used for adiabatic withstand calculation
IEC 60287 — Electric cables — Calculation of the current rating (thermal resistivity, grouping, depth corrections)
Nexans Olex — High Voltage Cable Systems product catalogue (ampacity tables, R and X data for copper XLPE cables)
Short circuit constant \(k = 143\) for copper/XLPE (90°C → 250°C) per AS/NZS 3008 Table 52

Tips & Limitations

Tips

Short circuit withstand often governs HV cable sizing rather than current capacity or voltage drop. Always check SC first to avoid upsizing late in the design.
Reducing fault clearing time from 1.0 s to 0.5 s reduces the minimum SC cross-section by approximately 29%.
Single-core trefoil cables typically have 15–25% higher current ratings than multi-core cables of the same CSA.
For long cable runs (>1 km), voltage drop may become the governing criterion even at HV. Check the VD margin carefully.
Increasing circuit spacing from touching to 0.6 m can recover 20–30% of ampacity lost to grouping derating.
The calculator reports power loss — for cables operating near full load, losses above 3% suggest an economic case for a larger conductor.

Limitations

Copper XLPE only — aluminium conductors and other insulation types (EPR, paper/lead) are not currently supported.
Balanced three-phase only — single-phase and unbalanced loading are not modelled.
Cable data referenced to 11 kV — the Nexans Olex tables are 11 kV reference data. For other voltage ratings (3.3, 6.6, 22, 33 kV) the same R and X values are used; actual insulation thickness differences may cause minor variations in ampacity.
Adiabatic SC only — the short circuit withstand uses the simplified adiabatic method. For fault durations >3 s or where heat dissipation is significant, a non-adiabatic analysis should be performed.
No motor starting analysis — inrush and starting current (typically 5–8× FLC) voltage drop is not calculated.
No earth/screen conductor sizing — earth fault withstand of the cable screen is not assessed; a separate earth fault calculation should be performed.
Derating factor tables use linear interpolation between tabulated points. For temperatures outside the range 15–50°C the nearest boundary factor is used.

Developer Guide

Architecture

File layout:

components/c0020_hv_cable_sizing/component.py — Component registration, parameter schema, calculator class (HVCableSizingCalculator), SVG diagram generator, summary cards builder
components/c0020_hv_cable_sizing/calculations.py — Pure math functions, Nexans Olex cable data tables, derating factor tables (no Django dependencies)
components/c0020_hv_cable_sizing/equations.py — LaTeX/HTML equation builder and table data builder for rendered output
components/c0020_hv_cable_sizing/excel_generator.py — Excel report generation

The calculator extends ElectricalCalculator (from calcs.core.base_calculator). The calculate() method delegates to the pure function calculate_hv_cable_sizing(data) in calculations.py.

Cable data tables are defined as Python dicts in calculations.py:

CABLE_DATA_11KV_MULTICORE
CABLE_DATA_11KV_SINGLECORE_TREFOIL
CABLE_DATA_11KV_SINGLECORE_FLAT

Derating factor tables:

TEMP_CORRECTION_BURIED_90C — reference 25°C
TEMP_CORRECTION_AIR_90C — reference 30°C
THERMAL_RESISTIVITY_FACTORS — reference 1.0 K·m/W
DEPTH_OF_BURIAL_FACTORS — reference 0.8 m
GROUPING_MULTICORE_BURIED
GROUPING_SINGLECORE_TREFOIL
GROUPING_SINGLECORE_FLAT

SVG generators registered via register_component():

cable_sizing_diagram — generate_cable_sizing_diagram_svg()
qr_code_display — generate_qr_code_svg()

Code Paths

Key functions in calculations.py:

Function	Purpose
`calculate_load_current(power, power_type, voltage_kv, pf)`	Three-phase load current (kW or kVA input)
`get_temperature_correction_factor(temp, method)`	k₁ from buried or in-air tables
`get_grouping_factor(circuits, spacing, type, config)`	k₂ from grouping tables
`get_thermal_resistivity_factor(resistivity)`	k₃ from soil resistivity table
`get_depth_of_burial_factor(depth)`	k₄ from burial depth table
`interpolate_factor(value, table)`	Linear interpolation helper for all factor tables
`get_cable_data(size, type, config, voltage)`	Lookup rating, R, X from Nexans Olex tables
`find_next_standard_size(min_size, cable_type)`	Next standard CSA ≥ minimum required
`calculate_voltage_drop_3phase(I, L, R, X, pf)`	Three-phase voltage drop in volts
`calculate_short_circuit_withstand(sc_ka, duration)`	Adiabatic minimum CSA (k = 143)
`calculate_hv_cable_sizing(data)`	Main orchestrator — runs all 11 steps

Calculation steps in calculate_hv_cable_sizing():

Load current
Derating factors (k₁–k₄)
Design current (load / combined derating)
Cable selection from ampacity tables
Voltage drop check
Short circuit withstand check
Power loss calculation
Current utilization & margins
Overall compliance
Recommendations generation
Standards reference attachment

Key functions in equations.py:

Function	Purpose
`build_equations(result)`	Builds LaTeX-formatted HTML for all calculation steps
`build_table_data(result)`	Builds input/output table data for rendering

Testing

Running tests:

pytest calcs/components/c0020_hv_cable_sizing/ -v
pytest calcs/test_c0020* -v

Worked examples: the examples above are generated from the MCP validation corpus (mcp_public/validation/cases/fe_calc_cable_size/) by python manage.py generate_help_examples --tool fe_calc_cable_size, which writes help/_hv_cable_sizing_examples.html; calcs/test_help_examples.py fails if the committed fragment is stale.

Key constants for test validation:

SC constant k = 143 A·s^0.5/mm² (copper XLPE, 90°C → 250°C)
Standard multi-core sizes: 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400, 500, 630 mm²
Standard single-core sizes: 50, 70, 95, 120, 150, 185, 240, 300, 400, 500, 630, 800, 1000, 1200 mm²

How to extend:

Add voltage-specific cable data tables (e.g., CABLE_DATA_33KV_MULTICORE) in calculations.py and update get_cable_data() to select by voltage.
Add aluminium conductor tables with corresponding k-factor (k = 92 for Al/XLPE) and update calculate_short_circuit_withstand().
Extend the parameter schema in component.py to expose new conductor material or insulation options.
Add derating tables for in-air installation methods (currently uses buried tables as reference for all methods).
Write tests covering new lookup values, edge cases (largest cable insufficient, boundary derating conditions), and cross-checks against published examples.