Installer tools · DC cable

DC cable calculator

Two minima, calculated independently — current-carrying capacity after correction, and the CSA needed to hold your volt-drop criterion — then the larger standard size, and which constraint set it.

The circuit

Design current (Ib)

Work it out from module Isc

Module Isc at STC

Parallel strings on this cable

Isc MAX = strings × Isc(STC) × 1.25, the 1.25 accounting for irradiance above STC (IET, April 2026).

Circuit operating voltage

What the volt drop is expressed as a percentage of.

Route length (one way)

Both legs are counted — 2 conductors in the path.

Installation arrangement

The string pair: positive and negative run together against a surface. This is a rated arrangement, so the pair's mutual heating is already in the figure — adding a grouping factor for those same two cables would derate them twice.

Volt-drop criterion

Or set your own

Common DC design practice for PV string circuits. Not a regulatory limit, and this tool does not present it as one.

Conditions, grouping and cable type

Cable type

Ambient temperature

°C

Rating basis is 60 °C. Values below it get a factor of 1.00 — the table offers no uplift.

Grouping factor (Cg)

The selected arrangement already rates 2 loaded cables together. Apply a factor only for circuits beyond that pair.

Where the factor came from

Grouping factors for larger groups come from tables this tool is not licensed to reproduce. Enter the factor you are designing to and name its table — it is recorded in the calculation trail alongside everything else.

Further correction factor (Cx)

Anything else you are derating for. 1.00 = none.

Where that factor came from

Conductor temperature for volt drop

°C

Defaults to the cable's maximum continuous conductor temperature (90 °C) — conservative, and comparable with tabulated mV/A/m figures. Lower it if you have a realistic operating temperature.

Conductors in the circuit path

2 for a two-wire DC circuit — positive and negative.

Recommended cable

6mm²

Voltage drop governs

Voltage drop sets the size. Current-carrying capacity alone would allow 1.5 mm². Cable range alone would allow 1.5 mm².

Tabulated capacity: 57 A
After correction: 57 A
Volt drop: 3.63 V
as % of circuit: 0.91 %

Minimum CSA · Current-carrying capacity

1.5 mm²

How this was reached

Design current (Ib)14.00 A
The current the circuit is designed to carry continuously.
Installation arrangementTwo loaded cables touching, on a surface
The string pair: positive and negative run together against a surface. This is a rated arrangement, so the pair's mutual heating is already in the figure — adding a grouping factor for those same two cables would derate them twice.
Source: Keystone Cable, Polycab, HES Kablo
Ambient correction (Ca)1.00
60 °C against a 60 °C rating ambient — taken from the 60 °C row of the conversion table, not interpolated.
Source: Keystone Cable, Polycab
Grouping (Cg)1.00
None applied. The "Two loaded cables touching, on a surface" rating already accounts for 2 loaded cables, so the string pair needs no further factor.
Combined correction factor1.000
Ca 1.00 × Cg 1.00
Required tabulated capacity (It)14.00 A
Ib ÷ combined factor = 14.00 ÷ 1.000. The cable must be tabulated at this or above BEFORE correction.
Smallest size that carries it1.5 mm²
Tabulated 24 A, 24.0 A after correction — against a design current of 14.00 A.
Source: Keystone Cable, Polycab, HES Kablo

Minimum CSA · Voltage drop

Governing

6 mm²

How this was reached

Volt-drop criterion1.00 %
4.00 V at a circuit voltage of 400 V. This is the designer's chosen criterion, not a figure this tool imposes.
Conductor path60.0 m
30.0 m route × 2 conductors — volt drop is over the complete circuit path, not one leg of it.
Conductor temperature assumed90 °C
Resistance corrected from 20 °C by a factor of 1.2751, using α₂₀ = 0.00393 /K for copper.
Source: Material constant
Smallest size within the criterion6 mm²
3.3900 Ω/km at 20 °C → 0.2594 Ω over the whole path at 90 °C.
Source: Keystone Cable, Polycab, HES Kablo, Prysmian, IEC
Volt drop at that size3.63 V (0.91 %)
Ib × path resistance = 14.00 A × 0.2594 Ω, against a 4.00 V criterion.

Minimum CSA · Cable range

1.5 mm²

How this was reached

Smallest size in the range1.5 mm²
BS EN 50618 PV cable (H1Z2Z2-K) is not manufactured below this. A manufacturing fact, not a regulatory minimum.
Source: Keystone Cable, IET
Standards-imposed minimum CSAnone recorded
This pack records no standards-imposed minimum CSA. That is an absence of data, not a statement that none applies — check your licensed copy of BS 7671 and any scheme requirements.

Every size in this range, against this circuit

CSA	Capacity after correction	Volt drop	Loss at Ib	Meets
1.5 mm²	24 A	3.67 %	205.4 W	current only
2.5 mm²	33 A	2.20 %	123.1 W	current only
4 mm²	44 A	1.36 %	76.3 W	current only
6 mm²recommended	57 A	0.91 %	50.8 W	both
10 mm²	79 A	0.52 %	29.2 W	both
16 mm²	107 A	0.33 %	18.6 W	both
25 mm²	142 A	0.21 %	11.9 W	both
35 mm²	176 A	0.15 %	8.5 W	both
50 mm²	221 A	0.10 %	5.9 W	both
70 mm²	278 A	0.07 %	4.2 W	both
95 mm²	333 A	0.06 %	3.1 W	both
120 mm²	390 A	0.04 %	2.5 W	both
150 mm²	453 A	0.04 %	2.0 W	both
185 mm²	515 A	0.03 %	1.6 W	both
240 mm²	620 A	0.02 %	1.2 W	both

Loss is I²R over the whole conductor path at the design current. Going one size up reduces it — comparing that saving against the extra cable cost is a design optimisation this tool does not yet do.

Assumptions, factors and data sources

Assumptions

Cable and rating basis: BS EN 50618 PV cable (H1Z2Z2-K): Capacities tabulated at 120 °C conductor and 60 °C ambient, on the BS EN 50618 / IEC 62930 rating basis.
Conductor: copper, resistance at 20 °C from the pack: Maximum DC resistance values, corrected to 90 °C with α₂₀ = 0.00393 /K.
Circuit path: 2 conductors × 30.0 m: A two-wire DC circuit: volt drop is taken over both the positive and the negative leg.
Volt-drop criterion: 1.00 % of 400 V: Chosen by the designer. This tool does not impose a volt-drop limit.
Ambient temperature: 60 °C: Applied by stepping up to the next tabulated row. The conversion table is not interpolated.

Factors applied

Ambient temperature (Ca): 1: 60 °C is at or below the 60 °C rating ambient, so the table gives 1.00 — it offers no uplift below its rating ambient.
Grouping (Cg): 1: None applied. The "Two loaded cables touching, on a surface" rating already accounts for 2 loaded cables, so the string pair needs no further factor.

Data sources

Keystone Cable — Solar Cables 1.0/1.0kV Single-Core, H1Z2Z2-K / 62930 IEC 131 — Table 4 (Current-Carrying Capacity of PV Cables) and Table 5 (Current Rating Conversion Factors) (2024.1), read 2026-08-11Tabulated at 120 °C maximum conductor temperature (EN 50618) and 60 °C ambient (IEC 62930).
Polycab — POLYCAB SOLAR H1Z2Z2-K BS EN 50618 — Photovoltaic Power Cable, product specification (Document No. 00519, Rev 00, 11-01-2024), read 2026-08-11Independent second transcription of the same EN 50618 rating basis. Agrees with Keystone on every capacity, factor and resistance.
HES Kablo — H1Z2Z2-K solar cable, EN 50618 / TS EN 50618 — mechanical and electrical properties, read 2026-08-11Independent third transcription. Agrees on the free-in-air and on-a-surface capacities and on every conductor resistance.
Prysmian — PRYSUN H1Z2Z2-K 1/1kV — rubber cables for photovoltaic plants (GB00_PRYSUN_H1Z2Z2_K_1KV), read 2026-08-11Corroborates the 4 mm² (5.09 Ω/km) and 6 mm² (3.39 Ω/km) conductor resistances, and states the class 5 tinned copper conductor to IEC 60228.
IEC — IEC 60228, Conductors of insulated cables — class 5 flexible tinned copper, maximum DC resistance at 20 °CNamed, not reproduced: the resistance values used here were read from the manufacturer datasheets above, which each state IEC 60228 class 5 as their conductor standard. Verify against a licensed copy if the numbers matter contractually.
Material constant — Temperature coefficient of resistance of copper at 20 °C, α₂₀ = 0.00393 /KThe value conventionally used for annealed copper conductors (as in IEC 60287-1-1). A physical property of the metal rather than a tabulated design value. Note that BS 7671 Appendix 4 uses 0.004 for its own corrections — if you are cross-checking against Appendix 4, expect a difference of well under 1 % in the resulting volt drop.
IET — Exploring wiring systems for unearthed DC solar PV systems, Part 1 — Wiring Matters issue 104 (March 2025), read 2026-08-11States that cables to BS EN 50618, "without metallic armouring or screens", are commonly used to meet the DC-side insulation requirements, and cites BS 7671 Regulation 712.521.101.

Data pack en50618-h1z2z2k v1.0.0 · engine v1

Calculated on the BS EN 50618 / IEC 62930 rating basis published in the cable datasheets cited above, cross-checked across three manufacturers. This is not a statement of BS 7671 compliance: BS 7671's own tabulated capacities and its Table 4B1 and 4C1 correction factors are not used here and are not reproduced. Verify the result against your licensed copy of BS 7671 and against the datasheet for the cable you are actually installing.