Drawing No. EH–EE–003 // Electrical Engineering
Cable Ampacity & Derating Calculator
Reviewed August 2026
Look up a conductor's base ampacity from NEC Table 310.16, then apply ambient temperature correction and conductor-count bundling adjustment together to find its actual allowable ampacity for real installation conditions.
What problem does this solve?
NEC Table 310.16 gives a conductor's base ampacity under standard conditions — but real installations are rarely standard: a hot attic, a bundle of many current-carrying conductors sharing one conduit, or both together, all reduce how much current a conductor can safely carry before its insulation overheats. This tool looks up the base value and applies both corrections together, so you can see the actual allowable ampacity for your real installation condition, not just the table's reference value.
Inputs
Results
Background
Lists allowable ampacity by conductor size, material and insulation temperature rating, under standard reference conditions: 30°C (86°F) ambient, no more than 3 current-carrying conductors in the raceway or cable. Every other condition (hotter ambient, more conductors bundled together) requires applying a correction factor to this base value — the base table alone is a reference point, not the final answer for most real installations.
Table 310.15(B)(1) gives a multiplier for ambient temperatures other than the standard 30°C, separately for each insulation temperature rating. Higher ambient temperature reduces allowable ampacity (the conductor has less thermal margin before reaching its insulation's maximum temperature); lower ambient temperature increases it. Note the correction factor depends on which temperature-rating column you're using for the base value, not just the ambient temperature itself.
Table 310.15(C)(1) reduces allowable ampacity when more than 3 current-carrying conductors share a raceway or cable, since each conductor's heat has less opportunity to dissipate when several are generating heat together in a confined space. The reduction is a flat percentage based on the total conductor count: 80% for 4–6 conductors, dropping to 35% for 41 or more — a substantial reduction for heavily loaded conduits.
Allowable ampacity = Base ampacity × Ambient correction factor × Bundling adjustment factor. Both corrections apply simultaneously whenever both conditions exist — a hot attic run with several bundled circuits needs both factors multiplied together, not just the larger of the two, since they represent two independent physical effects (surrounding heat, and mutual heating between conductors) both reducing the same thermal margin.
Equipment termination temperature can cap the usable ampacity even when a higher-temperature conductor insulation is used for derating. This calculator now applies the selected termination-temperature column as a screening cap, but overcurrent-device rules, equipment-specific listings, conductor-use restrictions and other adopted-Code provisions still require a separate final check.
Frequently asked questions
Practical questions about inputs, assumptions and interpretation.
The three columns (60°C, 75°C, 90°C) correspond to different insulation types rated for different maximum operating temperatures — a 90°C-rated insulation (like THHN) can tolerate more heat, and therefore more current, than a 60°C-rated insulation (like TW) in the exact same conductor size. Which column applies depends on the actual insulation type installed, and separately, on what temperature the equipment terminals in the circuit are rated for (per NEC 110.14(C)).
Only if every termination point in the circuit — every breaker, lug, and device the conductor connects to — is also rated for 90°C, which is uncommon in standard residential and commercial equipment. In practice, the 90°C column is most often used only as the starting point for derating calculations (ambient and bundling corrections), with the final result still capped at the lower of the 75°C or 60°C column per the actual terminal ratings.
Each current-carrying conductor generates heat proportional to its current squared, and when several share a confined raceway, that heat has to escape through the same limited surface area and surrounding air — so adding more conductors doesn't just add their individual heat outputs, it also reduces each conductor's own ability to shed heat to its surroundings, compounding the effect. This is why the adjustment factor drops so much for larger bundles (down to 35% for 41+ conductors) rather than declining gradually.
Equipment grounding/bonding conductors are never counted. A neutral conductor is only counted if it carries significant current under normal conditions — a neutral serving only the unbalanced current of a balanced multi-wire circuit is not counted, but a neutral in a circuit with substantial non-linear (harmonic-generating) loads, or the common conductor of certain three-phase configurations, generally is counted. This calculator assumes you've already applied this rule when entering the conductor count.