Cable Sizing and Voltage Drop

Current rating and derating, voltage drop, short-circuit withstand and a worked three-phase feeder.

On this page

  1. The three checks
  2. Current rating and derating
  3. Worked example: the feeder to the press motor
  4. Voltage drop
  5. Short-circuit withstand
  6. Copper, aluminum and sizes
  7. Cable losses and economic size
  8. FAQ

The three checks

A cable must pass three tests, and the size is the largest that any of them demands:

  1. Current-carrying capacity (ampacity): the conductor must not overheat at the load current, allowing for the installation method, the ambient temperature and grouping with other cables.
  2. Voltage drop: the loss along the cable must leave enough voltage at the load (usually 3 to 5 percent in total, and at the motor’s start too).
  3. Fault protection: the cable must survive the short-circuit current until the protective device clears it, and the device must trip on an earth fault (see Protection, Earthing and RCDs).

Current rating and derating

Tables in IEC 60364-5-52, BS 7671 (Appendix 4), NEC Table 310.16 and equivalents give the current a cable can carry for a given insulation (PVC at 70 °C, XLPE at 90 °C), material and method of installation (clipped to a surface, in conduit, buried, in a tray). Typical values for copper, PVC, three loaded conductors on a surface (reference method C) are:

Cross-sectionTabulated ampacity (typical)After derating (40 °C, 3 circuits grouped)
1.0 mm²18 A12.4 A
2.0 mm²24 A16.5 A
4.0 mm²32 A22.0 A
6.0 mm²41 A28.2 A
10.0 mm²57 A39.2 A
16.0 mm²76 A52.2 A
25.0 mm²101 A69.4 A
35.0 mm²125 A85.9 A

Derating factors multiply the tabulated value: ambient temperature above 30 °C (0.87 at 40 °C for PVC, 0.71 at 50 °C), grouping of several loaded circuits together (0.79 for three, 0.70 for five, 0.65 for six), burying in thermally poor soil, thermal insulation around the cable (halving the rating if completely surrounded), and harmonic currents in the neutral. The cable is sized so that Ib ≤ In ≤ Iz (design current ≤ protective device rating ≤ cable rating after derating). These tables are indicative; use the current edition of the standard for design.

Worked example: the feeder to the press motor

Worked example: 30 m of copper PVC cable to the 11 kW motor

  • Design current Ib = 20.5 A (Three-Phase Systems). Protective device: a type C breaker, In = 25 A (the next size above Ib; motor starting is handled by the magnetic trip at 5 to 10 times).
  • Derating: ambient 40 °C (0.87) × three grouped circuits (0.79) = 0.687. Required tabulated rating = 25 ÷ 0.687 = 36.4 A.
  • The smallest size at or above that is 6 mm² (41 A tabulated; 28.2 A after derating, above In). A 4 mm² cable (32 A tabulated, 22.0 A derated) is too small.
  • Voltage drop: ΔV = √3 I L (R cos φ + X sin φ) with copper at 70 °C: 3.2 V = 0.80 percent of 400 V at full load: well within the 3 to 5 percent limit (1.20 percent for 4 mm²).
  • At starting (7 × 20.5 = 144 A at a power factor of 0.35) the drop is 9.7 V = 2.4 percent in the cable alone; with the transformer’s drop the dip at the motor is a few percent more, still under the 10 to 15 percent that a contactor and the motor can tolerate.
  • The same cable length with a 2 kW single-phase heater on 2.5 mm²: current 8.7 A, out and back 2 × 20 m: ΔV = 2 I R L = 3.0 V (1.3 percent of 230 V).
Voltage drop against cable length0255075100125150051015202530cable length (m)voltage drop (V, line to line)4 mm²6 mm²10 mm²16 mm²5 percent of 400 V = 20 V6 mm², 30 m: 3.2 VΔV = √3 I L (R cos φ + X sin φ) with copper at 70 °C (resistivity 0.0213 Ω·mm²/m) and a reactance of 0.08mΩ/m. Doubling the length doubles the drop.
Voltage drop against cable length for four copper sizes at the motor current.

Voltage drop

Three-phase: ΔV = √3 I L (R cos φ + X sin φ)    single-phase: ΔV = 2 I L (R cos φ + X sin φ)

Limits are about 3 percent for lighting and 5 percent for other loads (IEC 60364-5-52 suggests a total of 4 to 5 percent from the origin), but motors want less: the torque falls with V², so a 5 percent drop costs 10 percent of the torque. Long runs usually size on voltage drop, short ones on ampacity: at 30 m the ampacity decides, at 125 m (see the figure) the 4 mm² reaches the 5 percent limit even though its ampacity is fine.

Short-circuit withstand

During a fault the conductor heats adiabatically (no time for the heat to escape), and the permitted energy is given by the standard equation I² t ≤ k² S², with S in mm² and k = 115 for PVC-insulated copper, 143 for XLPE, 76 for PVC aluminum. The protective device must therefore clear the fault in less than t = k² S² ÷ I².

Worked example: fault at the motor end

  • The 100 kVA transformer (Transformers and Distribution) has a phase impedance of 0.064 Ω; the 6 mm² cable adds 0.088 Ω (copper at 20 °C, which gives the highest fault current: the case for the breaker’s rating and the cable’s withstand).
  • Three-phase fault current at the motor terminals: Isc = (400 ÷ √3) ÷ (0.064 + 0.088) = 1 524 A, compared with 3 608 A at the transformer terminals.
  • Thermal withstand time = (115 × 6)² ÷ 1 524² = 0.20 s. The 25 A breaker trips magnetically in under 0.01 s, so the cable is safe by a wide margin.

Copper, aluminum and sizes

Aluminum has 1.61 times the resistivity of copper, so it needs about 1.6 times the cross-section for the same resistance (and is 1.5 to 2 times larger in diameter than copper of the same rating), yet weighs only about 49 percent as much and costs less per amp, so it is used in large feeders (above 16 to 25 mm²) and overhead lines. Terminals must be rated for aluminum (oxide layer, creep) and the joint made with the right compound. North American cables use AWG and kcmil: 10 AWG is 5.26 mm² (close to 6 mm²), 8 AWG 8.37 mm², 6 AWG 13.3 mm², 4 AWG 21.2 mm², 2 AWG 33.6 mm² and 1/0 AWG 53.5 mm². The AWG table is in the downloads library and the Wire Ampacity and Voltage Drop Calculator uses it.

Cable losses and economic size

A cable wastes I² R as heat. In the motor feeder the 6 mm² cable loses 3 I² R = 135 W at full load, 538 kWh a year over 4 000 hours (about 65); a 4 mm² cable would lose 202 W and a 16 mm² cable 50 W. For continuously loaded circuits a cable one or two sizes up than the minimum often pays back through the energy it saves (IEC 60287-3-2 gives an economic method), and it also reduces voltage drop and heating.

Related

FAQ

Do I size the cable from the load or the breaker?

Both: the cable must carry the load current and match the protective device, so that Ib ≤ In ≤ Iz. If the breaker is bigger than the load needs, the cable must be sized to the breaker.

Why does grouping reduce the rating?

Cables heat each other. A bundle of loaded cables has less surface per cable to lose heat.

Is 5 percent voltage drop too much?

For lights and heaters it is acceptable. For motors, a long cable and a starting dip can stall the motor; keep the total near 3 percent and check the start.

What is the difference between mm² and AWG?

mm² is the metric cross-section; AWG is a wire-gauge number where a smaller number is a thicker wire. Convert with the table in the AWG download.

References

  • IEC 60364-5-52 (selection and erection of wiring systems) and IEC 60364-4-43/-4-41 (protection); BS 7671 (IET Wiring Regulations) and NFPA 70 (NEC) Article 310.
  • IEC 60287 (calculation of the current rating of cables) and IEC 60949 (short-circuit thermal effects).
  • Kiessling, F., et al., Overhead Power Lines; Cadick, J., Electrical Safety Handbook; manufacturers’ cable catalogs.

Last reviewed: October 2026. Ampacities are typical illustrations; use the tables of the standard that applies.