Cable Size Calculator: NEC and IEC Methods Explained

Three checks, not one

A conductor size is acceptable only when it passes all of these:

1. Ampacity after correction. Tabulated current-carrying capacity adjusted for ambient temperature, grouping and installation method must be at least the design current (and, in NEC practice, at least the rating of the protecting device).
2. Voltage drop. Usually 3% branch and 5% total under NEC guidance, 3%/5% under IEC 60364-5-52 Annex G. The formulas are in the voltage drop guide.
3. Short-circuit withstand. The conductor must survive the let-through energy until the device clears, checked adiabatically (IEC) or by the equipment grounding rules and device listing (NEC).

Add a fourth practical constraint: termination temperature rating. A 90 °C conductor landed on a 75 °C breaker is a 75 °C conductor.

NEC method, step by step

1. Determine the load current, and apply 125% to continuous loads (NEC 210.19(A), 215.2(A)(1)).
2. Select a base ampacity from Table 310.16 for the conductor and termination temperature.
3. Apply ambient correction (Table 310.15(B)(1)) and conductor adjustment for more than three current-carrying conductors (Table 310.15(C)(1)).
4. Confirm the result protects at the device rating (NEC 240.4), respecting the small-conductor limits of 240.4(D).
5. Check voltage drop; increase size if needed.
6. Size the equipment grounding conductor per Table 250.122, and upsize it proportionally if the phase conductors were increased for voltage drop (250.122(B)).

The arithmetic is:

text required base ampacity = load × 1.25 / (k_ambient × k_bundle)

Worked NEC example: 125 A continuous at 480Y/277 V

Conditions: 125 A continuous load, four current-carrying conductors in one raceway, 40 °C ambient, 75 °C terminations, THWN copper.

• Load with the continuous factor: 125 × 1.25 = 156.25 A
• Ambient correction at 40 °C, 75 °C column: 0.88
• Adjustment for 4 current-carrying conductors: 0.80
• Combined: 0.88 × 0.80 = 0.704
• Required base ampacity: 156.25 / 0.704 = 221.9 A

From Table 310.16, 75 °C copper: 3/0 = 200 A (too small), 4/0 = 230 A. So 4/0.

Cross-check using the 90 °C column for adjustment, as permitted when terminations are rated 75 °C: 4/0 at 90 °C is 260 A; 260 × 0.91 (40 °C, 90 °C column) × 0.80 = 189.3 A. That exceeds the 156.25 A load, and the final ampacity is capped at the 75 °C value of 230 A, which also exceeds it. 4/0 stands.

If the ambient were 30 °C and the raceway held only three conductors, the same load would need 156.25 A of base ampacity and 2 AWG (115 A) would still be short — 1/0 at 150 A is also short, so 2/0 at 175 A. Derating is what pushed this from 2/0 to 4/0, and it is the step most often skipped.

IEC method, step by step

IEC 60364-5-52 states the relationship directly:

text Ib ≤ In ≤ Iz Iz = It × k_temperature × k_grouping × k_soil × ...

where Ib is the design current, In the protective device rating, It the tabulated capacity for the installation method (the reference methods A1, A2, B1, B2, C, D1, D2, E, F), and the k factors correct for actual conditions.

1. Compute Ib from the load, including diversity.
2. Choose the device rating In ≥ Ib.
3. Pick the installation method and insulation (PVC 70 °C or XLPE 90 °C).
4. Correct It by the k factors; the result Iz must be ≥ In.
5. Check voltage drop, then disconnection time and earth-fault loop impedance.
6. Size the protective conductor per IEC 60364-5-54.

Worked IEC example: 80 A feeder, 35 mm²

Conditions: Ib = 80 A, three-phase 400 V, 45 m, copper XLPE, installation method C (clipped direct), 35 °C ambient, two circuits grouped, In = 100 A.

• Ambient correction at 35 °C for XLPE: 0.96
• Grouping, two circuits: 0.80
• Combined: 0.96 × 0.80 = 0.768
• Required tabulated capacity: 100 / 0.768 = 130.2 A
• 25 mm² at method C, XLPE, is about 119 A: 119 × 0.768 = 91.4 A — below In, rejected.
• 35 mm² is about 147 A: 147 × 0.768 = 112.9 A ≥ 100 A. Accepted.

Voltage drop check at 1.10 mV/A/m: 1.10 × 80 × 45 / 1000 = 3.96 V, or 0.99% of 400 V. Well inside the 5% limit, so ampacity governs here — the opposite of the long-run case in the voltage drop article.

Both methods run in the demo calculator: change one factor and the selected size updates.

NEC ampacities, copper, Table 310.16

Correction factors that actually bite

Two factors multiply. Four conductors at 40 °C is 0.88 × 0.80 = 0.704, a 30% loss of capacity — enough to move a feeder up two sizes.

Short-circuit withstand (adiabatic check)

The conductor must survive the fault until the device clears:

text S = √(I² × t) / k

with S in mm², I in amperes, t in seconds, and k = 143 for copper XLPE, 115 for copper PVC, 94 for aluminium XLPE (IEC 60364-5-54, Table 43.1).

A 25 kA fault cleared in 0.1 s: S = √(25,000² × 0.1) / 143 = √62,500,000 / 143 = 7,906 / 143 = 55.3 mm².

With a current-limiting device clearing in 0.02 s: S = √(625,000,000 × 0.02) / 143 = √12,500,000 / 143 = 3,536 / 143 = 24.7 mm².

Same fault level, less than half the copper — because the device limited the let-through energy. This is why "the breaker will protect it" is not an answer without the device's I²t data. On the NEC side, the equivalent obligations live in 110.10 and 250.122; the arithmetic is the same physics.

Five mistakes that survive review

• Sizing on connected load instead of design current. The 125% continuous factor is applied before the table lookup.
• Using the 90 °C ampacity with 75 °C terminations. The conductor may be 90 °C rated; the breaker lug is not.
• Ignoring ambient in the ceiling void. A 50 °C void turns an 8 AWG into a 6 AWG.
• Forgetting EGC upsizing. When phase conductors grow for voltage drop, 250.122(B) requires the equipment grounding conductor to grow proportionally.

Sizing is only half the packet: the results have to land in the panel schedule and the single-line diagram, or the crew builds from a number nobody can trace.

Next step

If you want ampacity, derating, voltage drop and the panel schedule to stay in step, open the demo project and change one conductor size — the checks rerun across the circuit. Export formats are documented in the docs, and plan limits are on the pricing page.

Sources

• NFPA 70 (NEC) — code development and current edition: https://www.nfpa.org/codes-and-standards/nfpa-70-standard-development/70
• NFPA free access to codes and standards: https://www.nfpa.org/codes-and-standards/free-access
• IEC 60364-5-52:2009 — wiring systems and current-carrying capacity: https://webstore.iec.ch/en/publication/1878
• IEC 60364-5-53:2001 — selection and erection of switchgear: https://webstore.iec.ch/en/publication/1880
• Southwire voltage drop calculator (cross-check for the voltage drop step): https://www.southwire.com/calculator-vdrop

Ampacities and factors quoted are representative of the editions current at the time of writing; tables and clause numbers change between editions. Verify against the current edition of the applicable code and any local amendments. This is technical guidance, not a compliance opinion.