What is ampacity?
Ampacity is the maximum continuous current a conductor can carry without overheating past its insulation’s temperature rating. It depends on the conductor material, cross-section, insulation type, ambient temperature, and how the conductor is installed. A #12 AWG copper wire hanging in free air can handle far more current than the same wire crammed into a hot conduit with a dozen other conductors.
How to use this calculator
Pick your standard first — NEC 310.16 for North America (AWG sizes) or IEC 60364-5-52 for Europe and most of the rest of the world (mm² sizes). Then:
- Select conductor material (copper or aluminium)
- Select wire size
- Set insulation temperature rating (NEC) or reference installation method (IEC)
- Enter ambient temperature — defaults to 30°C, which is the base for both codes
- Enter number of current-carrying conductors (NEC) or grouped circuits (IEC)
The calculator returns the base table ampacity, applies the temperature and grouping corrections, and shows the effective allowable current — the number you actually design around.
The formula
Both codes use the same conceptual formula:
I_effective = I_base × k_temperature × k_grouping
The differences are in the base tables (NEC 310.16 vs IEC 60364 Method A/C/E), the correction factor tables, and additional rules like NEC 110.14(C) (terminal temperature limits) and 240.4(D) (small conductor OCPD limits).
NEC quirks: the 60/75/90°C columns
NEC 310.16 lists ampacity in three columns:
- 60°C — for older insulation types like TW and UF
- 75°C — for THW, THWN, USE — the standard “wet-location wire”
- 90°C — for THHN, XHHW-2, RHW-2 — modern thermoplastic and cross-linked insulations
Modern wire is almost always 90°C-rated. But here’s the catch: NEC 110.14(C) caps the final ampacity at the 75°C column for most terminations, because most breakers and lugs are only rated for 75°C. So THHN in a raceway derates from the 90°C column, but the final answer can’t exceed the 75°C column. This calculator applies that cap automatically and shows when it’s the binding limit.
NEC 240.4(D) small-conductor rule
Regardless of what the ampacity table says, NEC 240.4(D) caps overcurrent protection for the smallest wire sizes: 15 A for #14 Cu, 20 A for #12 Cu, 30 A for #10 Cu. This is why residential 12 AWG copper is always on a 20 A breaker even though its 75°C ampacity is 25 A — the small-conductor rule limits the breaker size, and the breaker limits the effective ampacity.
IEC installation reference methods
IEC 60364-5-52 defines many “reference methods” describing how a cable is installed. This calculator supports the three most common:
- Method A — insulated wall / conduit within thermally insulated wall. Poor heat dissipation, lowest ampacity.
- Method C — clipped directly to a surface or in conduit on a wall. Middle-ground cooling.
- Method E — free air, cable tray, or ladder. Best cooling, highest ampacity.
For the same conductor size, Method E can allow 30–50% more current than Method A. Getting this right is what separates a professional calculation from a table-guess.
Temperature correction
Every ampacity table is based on a reference ambient temperature — 30°C for both NEC and IEC. In practice, cables in machine rooms, motor control centers, or attics can see 40–50°C. The correction factor drops rapidly:
- NEC 90°C wire at 40°C ambient: factor 0.91
- NEC 90°C wire at 50°C ambient: factor 0.82
- IEC PVC insulation at 40°C: factor 0.87
- IEC PVC insulation at 50°C: factor 0.71
For high-ambient installations, XLPE-insulated cable (rated 90°C conductor) is often used instead of PVC to preserve ampacity.
Grouping / bundling derating
Multiple current-carrying conductors in the same raceway or bundle each generate heat, and each one loses cooling because the others surround it. NEC 310.15(C)(1) and IEC 60364 Table B.52.17 both apply grouping factors — and they can be aggressive:
- 4–6 conductors (NEC): 0.80
- 7–9 conductors (NEC): 0.70
- 10–20 conductors (NEC): 0.50
- 5 circuits grouped (IEC): 0.60
- 10 circuits grouped (IEC): 0.48
This is why control panels with many conductors often use oversized wire — the ampacity per wire is derated so heavily by the grouping that individual conductors can’t carry their nominal ampacity.
Important limitations
This calculator is based on published table data and standard correction factors. It does not apply:
- Direct-buried cable ampacity (soil thermal resistivity is a separate topic)
- Cable in cable ducts underground (NEC Table 310.20, IEC Method D)
- Solar / PV-specific derating (NEC 690.31)
- National code variations (BS 7671 UK, VDE Germany, LST EN Lithuania, etc.)
- Ampacity of multi-core cables with individual insulated conductors (uses different tables)
For final installations, always cross-check with the applicable code and have installations verified by a licensed electrician. This calculator gets you 90% of the way to the right answer for typical installations — but the last 10% is exactly where code violations happen.