Ampacity Calculator
The base NEC ampacity table only tells part of the story. This applies the two adjustments most often missed — ambient temperature and conductor bundling — to give the real, usable ampacity.
| Base ampacity (NEC 75°C table) | |
|---|---|
| Temperature correction factor | |
| Conductor-count adjustment factor | |
| Adjusted (usable) ampacity |
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Quick reference: how much the two adjustments cost you
| Condition | Correction factor | Effective loss |
|---|---|---|
| 30°C ambient (baseline) | 1.00 | None |
| 40°C ambient (hot attic) | 0.88 | -12% |
| 50°C ambient | 0.75 | -25% |
| 4-6 bundled conductors | 0.80 | -20% |
| 7-9 bundled conductors | 0.70 | -30% |
These stack multiplicatively, not by adding percentages — a hot attic run bundled with five other conductors combines both rows above, not just the worse of the two. See the full breakdown in understanding NEC derating.
How to use this calculator
Pick the wire gauge and material, then enter the actual ambient temperature the run passes through and the number of other current-carrying conductors bundled with it in the same raceway or cable. The result is the real, usable ampacity after both NEC-required adjustments — which is very often lower than the base table number most people stop at.
Why the base table isn't the whole answer
The NEC ampacity table assumes a 30°C (86°F) ambient and no more than three current-carrying conductors bundled together. Real installations routinely violate both assumptions — an attic run in summer easily exceeds 40°C, and a raceway carrying six or more circuits is common in a subpanel feed. Both conditions reduce how much current a wire can safely carry without the insulation overheating, which is exactly the failure mode ampacity ratings exist to prevent. This is the single most common wire-sizing mistake covered in common wire sizing mistakes — checking the base table and stopping there. See understanding NEC derating for the full correction tables and why the two factors multiply rather than add.
Worked example
10 AWG copper is rated 35A on the base table. Run it through a 40°C attic (correction factor 0.88) bundled with five other current-carrying conductors — six total (adjustment factor 0.80) — and the real, usable ampacity drops to 24.64A: 35 × 0.88 × 0.80. That's a 30% reduction from the number on the base table, and it's the exact scenario that turns a "correctly sized" circuit into one that runs hot in practice.
Frequently asked questions
Does ambient temperature really matter that much?
Yes — the correction factor for 75°C-rated conductors drops from 1.00 at 30°C down to 0.88 at 40°C and 0.75 at 50°C. An uninsulated attic run in a hot climate can realistically see 40-50°C, which is a 12-25% ampacity reduction before bundling is even considered.
What counts as a "current-carrying conductor" for bundling?
Hot and neutral conductors under continuous, non-cancelling load count. A properly balanced 3-wire circuit's neutral often doesn't count under NEC rules, and grounding conductors never count — but working out which conductors in a specific raceway actually count is exactly the kind of judgment call worth confirming with a licensed electrician rather than guessing.
Do I need to apply both adjustments, or just one?
Both, when both conditions apply — they're independent, multiplicative factors, not alternatives. A run that's both hot and bundled needs both corrections applied together, as in the worked example above, not just whichever one seems more significant.
Based on NEC Table 310.16 (base ampacity) and Table 310.15(B)(1)/(C)(1) (temperature and conductor-count adjustments). These figures can shift between NEC editions. Reference only — verify against the current NEC and local code, and consult a licensed electrician before installing.