Cable Sizing Calculator

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Size AC power cables to IEC 60364: design current, derating for ambient temperature and grouping, ampacity selection and a voltage-drop cross-check.

Inputs

Load (kW)

System voltage (V)

Supply

Power factor

Route length (one way) (m)

Conductor material

Ambient temperature derating (Ca)

0.87 for 40 °C on PVC

Grouping derating (Cg)

Permitted voltage drop (%)

How to use this calculator: Cable Sizing Calculator

Size AC power cables to IEC 60364: design current, derating for ambient temperature and grouping, ampacity selection and a voltage-drop cross-check. The example below is calculated by this page's real engine from the displayed inputs.

1Confirm that the Cable Sizing Calculator matches the quantity or design check you need.
2Enter Load, System voltage, and Power factor using the units printed beside each field.
3Select the applicable Supply and Conductor material options; these choices change the calculation method or factors.
4Calculate, then follow the substituted equations in the worked example and compare the result with any stated limit.
5Read the assumptions, warnings and cited references before using the result for a financial, medical or engineering decision.

Input guide and example values

Use values from the same measurement basis and time period. Conditional fields appear only when the related option is selected.

Input	Example value	Why it matters
Load	30 kW	Measured or known load used by the calculation engine.
System voltage	415 V	Measured or known system voltage used by the calculation engine.
Supply	Three phase (3/4-wire)	Select the option that matches the real installation or scenario.
Power factor	0.85	Measured or known power factor used by the calculation engine.
Route length (one way)	60 m	Measured or known route length (one way) used by the calculation engine.
Conductor material	Copper	Select the option that matches the real installation or scenario.
Ambient temperature derating (Ca)	0.87	0.87 for 40 °C on PVC
Grouping derating (Cg)	0.8	Measured or known grouping derating (cg) used by the calculation engine.
Permitted voltage drop	3 %	Measured or known permitted voltage drop used by the calculation engine.

Formula inputs & variables for Cable Sizing Calculator

These are the named quantities used by this calculator. When the source formula does not define a mathematical symbol, OneCalcApp keeps the real input label instead of inventing one.

Variable / input	Unit	Meaning in this calculation
Load	kW	Measured or known load used by the calculation engine.
System voltage	V	Measured or known system voltage used by the calculation engine.
Supply	—	Select the option that matches the real installation or scenario.
Power factor	—	Measured or known power factor used by the calculation engine.
Route length (one way)	m	Measured or known route length (one way) used by the calculation engine.
Conductor material	—	Select the option that matches the real installation or scenario.
Ambient temperature derating (Ca)	—	0.87 for 40 °C on PVC
Grouping derating (Cg)	—	Measured or known grouping derating (cg) used by the calculation engine.

Formula, derivation and worked example

IEC 60364-5-52 requires a cable to satisfy three independent checks: thermal capacity after derating, voltage drop, and short-circuit withstand. This calculator performs the first two. Derating factors reduce the tabulated ampacity for hot environments and for cables bunched together, so the cable actually installed must be sized on the corrected value, not the table value.

Single phase: I_b = P / (V × cos φ); Three phase: I_b = P / (√3 × V × cos φ)

Required ampacity I_z = I_b / (Ca × Cg)

ΔV = k × ρ × L × I / A, k = 2 (1-ph) or √3 (3-ph)

Selected size satisfies I_z(cable) ≥ I_b / (Ca·Cg) and ΔV% ≤ limit

Substitution steps

1. Design current
P / (√3 × V × cos φ)
= 49.101 A
2. Total derating
Ca × Cg
= 0.696
3. Required ampacity
I_b / (Ca × Cg)
= 70.55 A
4. Ampacity selection
first size with I_z ≥ required
= 16 mm²
5. Voltage drop check
√3 × ρ × L × I / A
= 5.485 V

Computed example results

Recommended cable: 16 mm² Copper; governed by current capacity
Design current I_b: 49.1 A
Required capacity after derating: 70.55 A
Cable rated capacity: 72 A; at 30 °C in conduit
Voltage drop: 5.49 V; 1.32% of 415 V
Size on ampacity alone: 16 mm²
Size on voltage drop alone: 10 mm²
Suggested protective device: 63 A

Understanding the result

Read the main result together with supporting checks, assumptions, limits and intermediate values.

For a manual check, repeat the first equation, confirm the units and change one input at a time.

Common mistakes when using Cable Sizing Calculator

Do not mix units for Load (kW), System voltage (V), Route length (one way) (m). A unit mismatch changes the input magnitude even when the typed number looks reasonable.
Do not leave Supply on the default choice unless that choice matches the real scenario; the selected option can change the calculation path or factor.
Do not replace the displayed Single phase: I_b = P / (V × cos φ); Three phase: I_b = P / (√3 × V × cos φ) relationship with a different convention without also changing the underlying assumptions; compare like-for-like methods when checking the result.
Do not treat Recommended cable = 16 mm² Copper from the worked example as a universal answer. It belongs to the displayed example inputs and must be recalculated for the actual case.

When the Cable Sizing Calculator is useful

Cable Sizing Calculator is designed for cases where Load, System voltage, Supply, Power factor are known and you need Recommended cable, Design current I_b, Required capacity after derating. The page keeps the live calculator, calculation method and worked example together so the result can be checked instead of treated as a black-box number.

Use the calculator for the scope described by its inputs and notes. The displayed method is Single phase: I_b = P / (V × cos φ); Three phase: I_b = P / (√3 × V × cos φ). If the real project or decision needs factors that are not represented here, treat the result as an estimate and add the missing checks separately.

Load and System voltage: what changes the answer

The worked example uses Load = 30 kW, System voltage = 415 V, Supply = Three phase (3/4-wire), Power factor = 0.85. With those values, Recommended cable is 16 mm² Copper. Changing an input should be interpreted according to that field's unit, range, option and hint rather than by the number alone.

For this calculator, the main input roles are: Load (kW): Measured or known load used by the calculation engine. System voltage (V): Measured or known system voltage used by the calculation engine. Supply: Select the option that matches the real installation or scenario. Available choices include Single phase (2-wire), Three phase (3/4-wire). Power factor: Measured or known power factor used by the calculation engine.

How to sanity-check a Cable Sizing Calculator result

Start by confirming the entered values and units, then compare the substituted working with the displayed formula or calculation steps. Pay particular attention to Recommended cable, because it is the first worked-example output shown by the live engine.

Finally, compare the result with the assumptions, warnings and related calculators on this page. A nearby calculator can be useful as a cross-check when it measures the same workflow from a different input or output direction.

Next logical calculator

Continue with Voltage Drop Calculator

Useful next check because both tools use System voltage and System voltage, while Voltage Drop Calculator answers a different part of the same workflow.

Open Voltage Drop Calculator

Standards, source trail and limitations

References show the method used. Check the current local edition, amendments and project specification before a regulated decision.

Formula

Single phase: I_b = P / (V × cos φ); Three phase: I_b = P / (√3 × V × cos φ)
Required ampacity I_z = I_b / (Ca × Cg)
ΔV = k × ρ × L × I / A, k = 2 (1-ph) or √3 (3-ph)
Selected size satisfies I_z(cable) ≥ I_b / (Ca·Cg) and ΔV% ≤ limit

Worked example

130 kW, 415 V three phase, pf 0.85, 60 m run in copper, Ca 0.87, Cg 0.8.
2I_b = 30 000 / (1.732 × 415 × 0.85) = 49.1 A → I_z required = 49.1 / 0.696 = 70.5 A.
316 mm² copper (72 A) meets ampacity; voltage drop = 1.732 × 0.0172 × 60 × 49.1 / 16 = 5.5 V = 1.32% — acceptable.

Assumptions

Ampacity table is for PVC-insulated copper/aluminium in conduit at 30 °C ambient.
Balanced load and steady-state operation; motor starting currents are not included.

Tips

For long runs voltage drop, not heating, usually governs the size — check both as this tool does.
Aluminium needs roughly one to two sizes larger than copper for the same duty.

Warnings

Always verify the short-circuit withstand A ≥ I√t / k separately before finalising a size.
Protective device rating must satisfy I_b ≤ I_n ≤ I_z, and I₂ ≤ 1.45 I_z.

Standards & references

IEC 60364-5-52
IS 732:2019
IEC 60287

Frequently asked questions

What cable size do I need for a 30 kW three-phase load?

At 415 V and 0.85 power factor the design current is about 49 A. With typical derating you need 16 mm² copper, or 25 mm² aluminium, subject to a voltage-drop check on the route length.

What is a derating factor?

A multiplier below 1.0 that reduces a cable's tabulated current capacity for high ambient temperature, grouping with other cables, thermal insulation or burial conditions.

How much voltage drop is allowed?

IEC 60364 recommends 3% for lighting and 5% for power circuits, measured from the origin of the installation to the point of use.

What inputs does the Cable Sizing Calculator use?

It uses Load, System voltage, Supply, Power factor, Route length (one way), Conductor material, Ambient temperature derating (Ca), Grouping derating (Cg), and Permitted voltage drop. Follow the unit printed for each field and choose any selectable option to match the real scenario.

What does the Cable Sizing Calculator calculate?

It calculates Recommended cable, Design current I_b, Required capacity after derating, Cable rated capacity, Voltage drop, Size on ampacity alone, Size on voltage drop alone, and Suggested protective device. With the displayed default inputs, Recommended cable is 16 mm² Copper.

Which formula does the Cable Sizing Calculator use?

The primary relationship is Single phase: I_b = P / (V × cos φ); Three phase: I_b = P / (√3 × V × cos φ). The page also shows substituted values and calculation steps so the result can be checked independently.

How can I verify a Cable Sizing Calculator result?

First verify the units for Load, System voltage, and Supply. Then compare the substituted formula steps with Recommended cable and its displayed precision.

What are the limitations of the Cable Sizing Calculator?

Cable Sizing Calculator: Use this result for preliminary design and cross-checking. Confirm the applicable code edition, manufacturer data, site conditions and qualified-engineer approval before final design or installation.

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