Voltage drop & cable size calculator

Current ratings and correction factors calculated by East Coast Electrical Supplies with the IEC 60287 method from public physics; not AS/NZS 3008 table values.

Ratings basis

Estimates calculated with the IEC 60287 steady-state method (not AS/NZS 3008 table values)

Air temperature (reference)

40 °C, still air, shaded (no sun)

Ground temperature (reference)

25 °C

Soil thermal resistivity

2.5 K·m/W (dry soil, allowing for drying out around the cable)

Burial depth (reference)

0.5 m to the cable (or conduit) centre

Maximum conductor temperature

90 °C XLPE (X-90), 75 °C PVC (V-75)

Conductor

Plain copper, ρ = 1.7241e-8 Ω·m at 20 °C, α = 0.00393/K. Stranded (class 2) × 1.13 for lay and area tolerance; flexible (class 5) a further × 1.05, and its rating × 0.97

Frequency, losses

50 Hz; skin and proximity effect included; no dielectric, sheath or armour losses (LV, unarmoured)

Thermal resistivity

XLPE 3.5, PVC insulation 5, PVC fillers, sheath and conduit 6 K·m/W

Cable dimensions

Typical outside diameters from makers' datasheets (the Job Kit Builder data) where held, otherwise typical insulation and sheath thicknesses

Three-phase circuits

3 loaded conductors (balanced, neutral not loaded); multicore = 4-core cable, single-core = 3 cables in trefoil

Single-phase circuits

2 loaded conductors; multicore = 2C+E cable; single-core cables use the three-phase rating (safe side)

Clipped / on tray (touching)

The lower of clipped to a wall (75% of the surface sheds heat) and on a tray (85%)

Small cables in conduit or insulation

Ratings up to 16 mm² × 0.9 (conservative margin)

Partly surrounded by thermal insulation

Half the cable surface against insulation (treated as no heat flow)

Spaced / on ladder

One circuit: single layer, one cable diameter apart, free air. Two or more circuits: treated as touching or stacked (the touching grouping factor)

Conduit

Smallest PVC conduit with ≤ 40% fill, one circuit per conduit; in air, fixed to a wall

Grouped circuits

Identical, equally loaded circuits; in air bunched together (touching methods) or in a single layer (spaced); in the ground side by side, touching. The lowest factor across all sizes is used.

Air and ground temperature factors

√((θmax − θ) / (θmax − θref))

Voltage drop

Vd = k × I × L × (R cos φ + X sin φ), k = √3 three phase, 2 single phase; R at the maximum conductor temperature (safe side); reactance from cable geometry

DC mode (solar PV and battery cables)

Ratings basis

Estimates calculated with the IEC 60287 steady-state method (not AS/NZS 3008 or AS/NZS 5033 values)

DC circuit

2 loaded single-core cables (positive and negative), each carrying the full current; touching, or one diameter apart on the spaced method

Resistance

d.c. only (no skin or proximity effect, no reactance). Fine stranded (class 5) copper: ρ/A × 1.1865; tinned (solar cable) a further × 1.03; at the maximum conductor temperature (safe side)

Voltage drop

Vd = 2 × I × L × R ÷ 1000 (R in Ω/km per conductor, L one way); % of the battery nominal voltage or the PV string's Vmp

Solar PV current

Cable rating checked at strings × Isc × the Isc factor (1.25 to start: sunlight above the 1000 W/m² test level); voltage drop worked at strings × Isc (above the working current Imp: safe side)

Maximum conductor temperature

90 °C solar cable and XLPE / rubber, 75 °C PVC. Solar cable is often marked for 120 °C, but that is a limited-life rating, so 90 °C is used

Solar cable

Insulation and sheath 5 K·m/W (safe side for cross-linked compounds); wall 1.2 + 0.2 × √A mm (typical double insulation); rated 1.5 kV DC (general rating of the cable family)

Battery cable dimensions

Typical single-core flexible outside diameters from makers' datasheets (Job Kit Builder data) where held, otherwise typical insulation and sheath thicknesses

Battery inverter cables

At 60 V or less, the cable rating is checked at the DC current × 1.25 to allow for inverter ripple current (can be turned off). The voltage drop is worked at the DC current. Fuses, isolators, shunts and lug joints are not included in the drop

Air temperature

Reference 40 °C, shaded. Solar starts at 70 °C (under modules or on a roof). No allowance for direct sun on the cable: enter a higher temperature for cable in the sun

Ground, soil, depth

25 °C ground, soil 2.5 K·m/W, 0.5 m deep

Grouped circuits, conduit, small cables

As for AC: the lowest factor across all sizes; one DC circuit per conduit; sizes up to 16 mm² in conduit or insulation × 0.9

Not checked

Fault current from batteries or PV, reverse current, string fusing, DC isolation, polarity, cable DC voltage rating (battery leads), connector ratings