Cable Sizing vs Ampacity: What Are You Actually Checking?

Ampacity is an input to cable sizing, not a synonym for cable sizing. Ampacity answers a thermal steady-state question: how much current can this conductor carry under defined installation conditions without exceeding its permitted temperature? Cable sizing adds several other constraints, including protective-device coordination, voltage drop, short-circuit thermal withstand, minimum cross-section rules and project-specific conditions.

1. Cable sizing and ampacity are different checks

For an IEC-style low-voltage workflow, start with the circuit design current Ib. Select a protective device with rated current In, then verify that the cable's corrected current-carrying capacity Iz is adequate for the actual installation. The familiar screening relationship is:

That relationship is essential, but it does not prove that the cable is fully sized. A conductor can pass the ampacity/protection check and still fail because the run is too long for the voltage-drop target, because the available fault energy exceeds its short-circuit thermal withstand, or because a minimum cross-section or special installation rule governs.

2. Core equations behind the workflow

A reference current-carrying capacity is corrected for real installation conditions using the factors applicable to the adopted standard or manufacturer data:

For a preliminary AC voltage-drop check:

For the adiabatic short-circuit thermal check:

The final conductor is therefore the smallest standard size that passes every active criterion, not merely the one whose table ampacity exceeds the load current.

3. Worked example: why “Iz > Ib” is not enough

Assume a feeder has:

• design current Ib = 42 A;
• protective-device rating In = 50 A;
• verified reference ampacity Iz,ref = 63 A;
• temperature factor = 0.91;
• grouping factor = 0.80;
• no additional correction factor.

The cable can carry the 42 A design current under these assumptions, but the coordination check fails because:

If the 50 A protective device is to remain, the minimum required reference ampacity under the same two factors is:

The designer must therefore move to a candidate whose verified reference ampacity is at least 68.68 A under the relevant table/manufacturer basis, then still check voltage drop and short-circuit withstand. This example also shows why applying derating after selecting a cable can invalidate what initially looked like a comfortable choice.

4. Practical cable-selection workflow

1. Determine Ib from the actual load model, including duty and applicable demand assumptions.
2. Select the protective-device concept and candidate In.
3. Classify the real installation/reference method.
4. Obtain Iz,ref from the adopted standard or verified manufacturer data.
5. Apply only the correction factors that belong to that rating basis.
6. Check Ib ≤ In ≤ Iz and any additional overload requirements.
7. Check voltage drop at normal operating conditions.
8. Check short-circuit thermal withstand against prospective current/clearing time or verified total-clearing I²t.
9. Check minimum cross-section, neutral/harmonics, mechanical and project-specific requirements.
10. Select the largest minimum section required by all active criteria.

Run the IEC Cable Sizing Calculator

5. Assumptions and limits

• The correction-factor model must match the same cable construction and reference-rating basis used for Iz,ref.
• Do not multiply factors blindly where the adopted standard or manufacturer provides a combined/special case.
• Harmonic-rich circuits, loaded neutrals, buried circuits, thermal insulation and unusual cable groupings can require additional treatment.
• Voltage-drop limits are jurisdiction/project dependent; do not treat a calculator default as a universal limit.
• The short-circuit check depends on the actual protective-device clearing behavior and conductor/insulation temperature basis.

Adjacent checks

Use the Cable Ampacity Calculator when you already have a verified base rating and want to inspect derating explicitly. Use the Voltage Drop Calculator for a deeper R/X-based drop check, and continue with the cable short-circuit thermal-withstand guide for the I²t constraint.

Engineering references

Current wiring-system reference: IEC 60364-5-52:2009+AMD1:2024 consolidated version. Practical method reference: Electrical Installation Guide — conductor sizing methodology.