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Cables

Cable Size for a 100 A Three-Phase Submain (XLPE, Clipped Direct)

A submain from the main switchboard to a floor or building distribution board is a typical case where both checks matter: the cable must carry the design current at the site's ambient temperature, and the route is often long enough that voltage drop decides the size.

The calculator below is preset for a three-phase 400 V copper XLPE cable, three loaded conductors, installed by method C (clipped direct to a wall) as a single circuit, 100 A design current, 40 °C ambient, 80 m route, power factor 0.9 and a 3 % voltage drop limit. It reports the size needed for current-carrying capacity, the size needed for voltage drop and the larger of the two.

Last reviewed

Heat from the conductor must escape through the insulation into the surroundings. A hotter ambient or grouped neighbours slow that, so the tabulated capacity is multiplied by correction factors: Iz = I₀ · ka · kg.

For estimation and education. Not a substitute for a licensed engineer, the applicable codes (IEC 60364, NEC/NFPA 70, BS 7671, local regulations) or manufacturer data. Read the disclaimer.

Inputs

Capacity source

Cable

Insulation
Loaded conductors

Copper only. Source: IEC 60364-5-52 Table B.52.5.

Load and protection

A
A

When entered, the cable is sized so that Iz ≥ In

Correction factors

°C

Fills Ca from Ca = √((θmax − θa)/(θmax − 30))

IEC 60364-5-52 Table B.52.14, typical values: 25 °C 1.04 · 30 °C 1.00 · 35 °C 0.96 · 40 °C 0.91 · 45 °C 0.87 · 50 °C 0.82

Grouping arrangement

Circuits or multi-core cables grouped together (1 to 9)

IEC 60364-5-52 Table B.52.17 (circuits: factor): 1: 1.00 · 2: 0.85 · 3: 0.79 · 4: 0.75 · 5: 0.73 · 6: 0.72 · 7: 0.72 · 8: 0.71 · 9: 0.70

Voltage-drop check

System
V
m
Length unit
mΩ/m

Default 0.08 mΩ/m

%

Conductor taken at 90 °C for the voltage drop

Result

Cable size by current-carrying capacity — Engineerly. Estimate for education only; verify against applicable codes and manufacturer data.

Recommended size
25mm²
Iz of the recommended size
115.6A

25 mm²: current-carrying capacity and voltage drop give the same size.

Required reference capacity It
109.89A
Size by current-carrying capacity
25 mm² (It 127 A, Iz 115.6 A)
Size by voltage drop (3 %)
25 mm²
Voltage drop at the recommended size
11.45 V (2.86 %)
Source table
IEC 60364-5-52 Table B.52.5
Reference capacities used: IEC 60364-5-52 Table B.52.5, method C

Copper conductors, ambient air 30 °C. Iz = It × Ca × Cg with your factors.

mm²It (A)Iz (A)
1.52320.93
2.53128.21
44238.22
65449.14
107568.25
1610091
25127115.6
35158143.8
50192174.7
70246223.9
95298271.2
120346314.9
150399363.1
185456415
240538489.6

Working, with your numbers

  1. 1. Required reference capacity
    It ≥ Ib / (Ca × Cg)
    It ≥ 100 A / (0.91 × 1)
    = 109.89 A
  2. 2. Smallest tabulated size meeting It
    It(S) ≥ It,required
    It(25 mm²) = 127 A ≥ 109.89 A
    = 25 mm²
  3. 3. Current-carrying capacity after correction
    Iz = It × Ca × Cg
    Iz = 127 A × 0.91 × 1
    = 115.57 A
  4. 4. Resistance at conductor temperature
    R_T = ρ20 (1 + α (T − 20)) / A
    R_T = 0.017241 × (1 + 0.00393 × (90 − 20)) / 25 mm²
    = 0.87936 mΩ/m
  5. 5. Voltage drop (three-phase, line-to-line)
    ΔV = √3 × I × L × (R cosφ + X sinφ) / n
    ΔV = 1.73205 × 100 A × 80 m × (0.00087936 × 0.9 + 8.00000e-5 × 0.4359) Ω/m / 1
    = 11.45 V
  6. 6. Percentage of nominal voltage
    ΔV% = ΔV / V × 100
    ΔV% = 11.4495 V / 400 V × 100
    = 2.86 %

Assumptions

  • Copper conductors; reference capacities from IEC 60364-5-52 Table B.52.5, method C, 3 loaded conductors, ambient air 30 °C.
  • Ca = 0.91 and Cg = 1 as entered; Ca × Cg = 0.91.
  • Voltage drop with one conductor per phase at 90 °C, resistance from resistivity (IEC sizes up to 630 mm²).
  • Recommended size = the larger of the current-carrying-capacity size and the voltage-drop size.
Mode: Built-in IEC table (copper)
Insulation: XLPE / EPR (90 °C)
Installation method: C, 3 loaded conductors
Design current Ib: 100 A
Ambient temperature factor Ca: 0.91
Grouping factor Cg: 1
System: Three-phase
Voltage: 400 V
Route length (one way): 80 m
Power factor: 0.9
Allowed drop: 3 %

Check before you rely on this

  • Estimate only. The cable must also meet fault protection (automatic disconnection, IEC 60364-4-41) and short-circuit withstand k²S² ≥ I²t (IEC 60364-4-43).
  • Harmonic currents (for example from LED drivers, IT equipment and drives) can load the neutral; IEC 60364-5-52 Annex E then reduces the capacity or sizes on the neutral current.
  • Buried cables, ducts in the ground, trays, ladders and the other installation methods are not covered here; take their values and factors from IEC 60364-5-52 or the cable maker.
  • Estimation and education only. Not a substitute for a licensed engineer, the applicable code (IEC 60364, NEC/NFPA 70, BS 7671 or local regulations) or manufacturer data.

Notes for this case

  • Reference capacities are copper values from IEC 60364-5-52 Table B.52.5 (XLPE, three loaded conductors), method C.
  • The protective device rating must satisfy Ib ≤ In ≤ Iz; check it with the breaker sizing calculator.
  • Cables in trays, ducts or ground use other installation methods and factors that are not built in.

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How these formulas were checked: methodology and testing.