Cable Sizing

Quick pick (TL;DR)

Find the full-load current.
Apply the design factor (×1.25 for motors).
Divide by derating factors (ambient × grouping) to get the required tabulated capacity Iz.
Pick a size whose rated capacity ≥ Iz.
Check voltage drop ≤ 5% (3% for lighting).
Check short-circuit withstand.
Use XLPE/Cu as the default; Al only for large feeders to save cost.

Step 1 — The three checks

Check	Criterion	Why
Current capacity	Iz ≥ I_design / derating	Insulation temperature
Voltage drop	≤ 5% power / 3% lighting	Equipment performance
Short-circuit	k²S² ≥ I²t	Conductor survives fault

Step 2 — Worked example

90 kW motor, 400 V, 80 m

Step 1 — Full-load current I = P / (√3 · V · PF · η) I = 90000 / (1.732 · 400 · 0.85 · 0.92) I = 90000 / 541.7 = 166 A Step 2 — Design current motor factor ×1.25 I_d = 166 × 1.25 = 208 A Step 3 — Apply derating ambient 40°C (XLPE) = 0.91 , grouping (2 ckt) = 0.80 combined = 0.91 × 0.80 = 0.728 Check 1 — Capacity installed = base × 0.728 95 mm² : 290 × 0.728 = 211 A ≥ 208 → 95 mm² Check 2 — Voltage drop VD = 0.42 × 166 × 80 / 1000 = 5.6 V %VD = 5.6 / 400 = 1.4% ≤ 5% → OK Check 3 — Short-circuit Smin = √((31,000)² × 0.2) / 143 Smin = 13,864 / 143 = 97 mm² → need 120 mm² Answer → 120 mm² XLPE/Cu — short-circuit governs

Three checks — the largest wins. Here short-circuit (31 kA, 0.2 s → 97 mm²) governs, pushing to 120 mm².

Cable sizing calculator

Cable sizing (3-phase, XLPE/Cu)

LV ≤ 1 kV (1.5–630 mm², Cu/Al, XLPE/PVC). MV cables use separate maker tables.

Power (kW)

Voltage (V)

Phases

Power factor

Efficiency

Load type

Conductor

Insulation

Installation

Ambient temp (°C)

No. of circuits

Run length (m)

VD limit (%)

Fault current (kA)

Fault clear time (s)

Parallel runs / phase

Screening tool only — final design uses project tables & a short-circuit study.

Reference — indicative ampacity (XLPE/Cu)

Size (mm²)	Ampacity (A)	Volt drop (mV/A/m)
1.5	19	26
2.5	26	16
4	35	9.5
6	45	6.4
10	62	3.8
16	85	2.4
25	112	1.5
35	138	1.1
50	168	0.81
70	213	0.57
95	290	0.42
120	340	0.33
150	390	0.27
185	446	0.22
240	528	0.18
300	608	0.15
400	726	0.115
500	827	0.10
630	941	0.087

Indicative IEC 60364 free-air values — apply derating & confirm with the project schedule.

Decision flow

1. Full-load current → ×1.25 (motor)

2. ÷ derating → required Iz

Pick sizeSmallest size with ampacity ≥ Iz

Check VD ≤ 5%If fail → upsize

ConfirmShort-circuit withstand

Result: the largest size demanded by capacity, voltage drop or fault current wins.

Step 3 — Conductor & insulation

Cu default; Al for large feeders.
XLPE default; PVC legacy.
Method & derating strongly change capacity.

Common mistakes

No ×1.25 / no derating.
Ignoring voltage drop on long runs.
Ignoring grouping on shared trays.

Requisition & datasheet

Load data, conductor, insulation, method.
Size, capacity, Iz, %VD, fault, standard.

Frequently asked questions

What should I know about Cable Sizing?

1) Find full-load current. 2) Apply design factor (×1.25 for motors). 3) Divide by derating factors (ambient × grouping) to get the required tabulated capacity Iz. 4) Pick a size whose rated capacity ≥ Iz. 5) Check voltage drop ≤ 5% (3% for lighting). 6) Check short-circuit withstand. Use XLPE/Cu as the default; Al only for large feeders to save cost.

Which standards or codes apply?

IEC 60364-5-52 (current-carrying capacity & derating), IEC 60502 (power cables), IEC 60909 (fault current), BS 7671, NEC Article 310.

Standards — IEC 60364-5-52; IEC 60502; IEC 60909; BS 7671; NEC 310.