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  • August 20, 2026

Cable Sizing Motor Feeders: 5 Essential Steps with Worked Example

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Electrical Design & Calculations
Cable Sizing for Motor Feeders: 5 Essential Steps with Worked Example

Selecting a cable for a motor feeder is not simply picking a size that matches the motor nameplate current.

Two derating factors (ambient temperature and cable grouping) can reduce current carrying capacity by 30 to 40%, forcing a larger cable size than nameplate current alone suggests.

This guide covers the five step IEC method with a full worked example for a 22 kW motor feeder and an interactive calculator.

IEC 60364 Method Derating Factors Voltage Drop Check Ampacity Tables

A cable that is correctly sized for current carrying capacity can still fail the voltage drop check on a long run. Both criteria must be satisfied independently -- the larger of the two results is the final cable size.

Why Motor Feeders Need Special Attention

A motor draws more current than its full load rating under certain conditions. At startup, inrush can reach 6 to 7 times the full load value, though this lasts only seconds.

The cable does not carry startup current continuously, but the overcurrent protection device must tolerate it.

Cable Sizing

IEC 60364-4-43 requires the design current to be at least 1.25 times the motor full load current (FLC).

This 25% margin covers service factor and thermal reserve. See the MCB sizing guide for overcurrent device selection.

×1.25
IEC 60364 design current multiplier for motor circuits
0.70
Typical grouping derating for 3 cables touching in a conduit
0.87
Ambient temperature derating at 45°C for PVC cables
5%
IEC 60364 maximum voltage drop for motor and power circuits
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The 5-Step Cable Sizing Procedure

1
Calculate full load current (FLC)

Use the three phase motor current formula: FLC = P / (√3 × V × pf × η). P is motor output power in watts, V is line voltage, pf is power factor, and η (eta) is efficiency. All values are from the motor nameplate.

2
Apply the 1.25 motor design current multiplier

Design current I_design = 1.25 × FLC. This is the minimum current the cable must carry continuously. The cable's derated ampacity must exceed this value.

3
Determine derating factors for site conditions

Find the ambient temperature correction factor Ca from IEC 60364-5-52 Table B.52.14, and the grouping factor Cg from Table B.52.17. Combined derating = Ca × Cg. If no other derating applies, use 1.0 for that factor.

4
Select cable from ampacity table

Required minimum ampacity: Iz_min = I_design / (Ca × Cg). Pick the smallest standard cable from the IEC 60364 table whose rated ampacity Iz_table equals or exceeds Iz_min. Verify: Iz_table × Ca × Cg ≥ I_design.

5
Check voltage drop on the selected cable

Apply the three phase voltage drop formula using the FLC (not design current): V_drop = 1.732 × FLC × L × R / 1000.

Percentage drop must be within 5%. See the voltage drop calculation guide for the full procedure.

Derating Factor Reference

These are the most commonly used derating factors from IEC 60364-5-52 for PVC insulated copper cables. XLPE cables have different correction factors -- always check the manufacturer's datasheet for the specific insulation type.

Ambient Temperature Correction Factor Ca (PVC Insulation, rated at 70°C)
25°C (cooler than standard)
Ca = 1.03
30°C (standard reference temperature)
Ca = 1.00
35°C
Ca = 0.94
40°C
Ca = 0.87
45°C (common in Indian plants)
Ca = 0.79
50°C
Ca = 0.71
55°C
Ca = 0.61
Grouping Factor Cg -- Cables Touching in Conduit or Trunking
1 circuit (no grouping)
Cg = 1.00
2 circuits
Cg = 0.80
3 circuits
Cg = 0.70
4 circuits
Cg = 0.65
5 circuits
Cg = 0.60
6 circuits
Cg = 0.57
In Indian process plants, an ambient temperature of 45°C is the standard design assumption for outdoor and non air-conditioned indoor installations. Combined with grouping, the effective derating can reach 0.55 or lower, often forcing the cable size up by two steps from what the nameplate current alone would suggest.

IEC 60364 Ampacity Table: Copper PVC in Conduit (30°C, Single Circuit)

Cable Size (mm²)Ampacity Iz at 30°C (A)Resistance at 70°C (Ω/km)Typical Motor Rating at 415V
1.51515.00Up to 0.55 kW
2.5209.18Up to 0.75 kW
4255.72Up to 1.5 kW
6323.82Up to 2.2 kW
10442.27Up to 4 kW
16571.43Up to 7.5 kW
25730.780Up to 15 kW
35900.554Up to 22 kW
501080.420Up to 30 kW
701360.332Up to 45 kW
951640.247Up to 55 kW
1201880.196Up to 75 kW
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Worked Example: 22 kW Motor Feeder, 120 m Run

A 22 kW, 415V, three phase induction motor has a power factor of 0.85 and efficiency of 0.92. The feeder runs 120 m from the MCC. Site ambient temperature is 45°C. Three motor feeder cables are routed together in the same conduit.

Motor Full Load Current Formula
FLC = P / (√3 × V × pf × η)
P = 22,000 W  |  V = 415 V  |  pf = 0.85  |  η = 0.92
FLC = 22,000 / (1.732 × 415 × 0.85 × 0.92) = 39.1 A
Worked Example
22 kW Motor, 415V, 120 m Run, 45°C Ambient, 3 Cables in Conduit
1
Full load current: FLC = 22,000 / (1.732 × 415 × 0.85 × 0.92) = 39.1 A
2
Design current: I_design = 1.25 × 39.1 = 48.9 A
3
Derating factors: Ca = 0.79 (45°C, PVC), Cg = 0.70 (3 circuits), Combined = 0.79 × 0.70 = 0.553
4
Minimum ampacity required: Iz_min = 48.9 / 0.553 = 88.4 A
5
Try 25 mm² (Iz = 73 A): 73 × 0.553 = 40.4 A < 48.9 A
Fail -- 25 mm² derated capacity (40.4 A) is below design current (48.9 A). Upsize to 35 mm².
6
Try 35 mm² (Iz = 90 A): 90 × 0.553 = 49.8 A > 48.9 A
Pass -- 35 mm² derated capacity (49.8 A) exceeds design current (48.9 A). Proceed to voltage drop check.
7
Voltage drop (use FLC, not design current): V_drop = 1.732 × 39.1 × 120 × 0.554 / 1000 = 4.51 V = 1.09%
Pass -- 1.09% is well within the 5% IEC limit. Final selection: 35 mm² copper, 3-core PVC.
Note the difference between the table value (Ca = 0.79 at 45°C used above) and the value from the derating table shown earlier (Ca = 0.87 at 40°C). Always select the factor corresponding to the actual design ambient temperature for your site, not the nearest round number.

Motor Cable Sizing Calculator

Motor Feeder Cable Sizing Calculator
IEC 60364 method -- five step result with voltage drop check
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Cable Sizing for Motor Feeders -- Common Questions

Why multiply by 1.25 for motor circuits?
IEC 60364-4-43 requires it to account for service factor, thermal reserve, and the fact that motors rarely run at exactly their nameplate load.
Do I use FLC or design current for voltage drop?
Use FLC (full load current) for voltage drop. The 1.25 multiplier is for ampacity sizing only. Using design current in the voltage drop formula overstates the drop.
What if both voltage drop and ampacity give different cable sizes?
Always select the larger of the two results. Both criteria must be satisfied independently -- a cable that passes ampacity but fails voltage drop must be upsized.
Does the earth conductor need to be sized separately?
Yes. The earth conductor is sized using the IEC 60364 adiabatic method based on fault current and disconnection time, not on load current. See the earth conductor sizing guide. The energy consumption calculation guide covers related load analysis methods.
Does a VFD drive change the cable sizing?
Yes. VFDs generate harmonic currents that add heat to the cable. Many engineers apply an additional 1.1 to 1.25 derating factor on VFD output cables and use shielded cable to reduce EMI on instrument circuits nearby.
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External References

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What We Learn Today

  • Design current for motor circuits = 1.25 × FLC (IEC 60364-4-43 requirement)
  • Combined derating = Ca × Cg -- can reduce effective capacity to below 55% in hot, grouped installations
  • Required ampacity = I_design / (Ca × Cg) -- select the next standard cable above this value
  • Voltage drop uses FLC, not design current -- V_drop = 1.732 × FLC × L × R / 1000
  • Both ampacity and voltage drop must pass -- take the larger cable if they give different answers
  • VFD output cables need an extra derating allowance for harmonic heating
“A cable sized only for nameplate current will overheat in a hot plant. A cable sized only for ampacity may starve the motor of voltage on a long run. Check both -- always.”
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