Ampacity Calculator — Derating for Heat & Bundling
The number in Table 310.16 assumes a 30 °C ambient and no more than three current-carrying conductors. Real installations violate both assumptions constantly — hot attics, rooftop EMT, and raceways full of circuits. This calculator applies the ambient correction and bundling adjustment, then shows the two ceilings people forget: the 110.14(C) termination-temperature cap and the 240.4(D) small-conductor breaker limit.
Derate your conductor
For rooftop raceway in direct sun, add the 310.15(B)(3) adder before entering the value.
Exclude grounds (310.15(E)(3)) and the neutral of a balanced circuit (310.15(E)(1)).
Usable ampacity
Base 40 A × 0.91 ambient × 0.8 bundling.
Table 310.16 base
40 A
Max breaker
30 A
See the breakdown
Planning estimate. Rooftop sun exposure, underground duct banks, and conductors in ambient above 40 °C carry extra requirements. A licensed electrician and your AHJ have final say.
The formula, explained in plain English
Derating is multiplication, then two ceilings. The multiplication reduces what the conductor can carry; the ceilings cap what you are allowed to use regardless of the math.
Why start at 90 °C?
Because derating is allowed to begin from the insulation's own rating. THHN starting at 40 A and derating to 29 A beats starting at the 35 A termination value and derating to 25 A. You just can't finish above the termination cap.
The factors multiply
Heat and bundling are independent problems, so their factors compound. 0.91 × 0.80 = 0.728 — a conductor keeping less than three-quarters of its table value.
Ambient is where the wire is
Not the outdoor air temperature and not the room the panel is in — the temperature surrounding the conductor along its run. An attic in July and a walk-in freezer are both derating cases.
The 24-inch exception
Bundling adjustment applies only where conductors run together for more than 24 inches. A short nipple between two panels does not trigger it.
Worked examples
The default case, the "why can't I put 10 AWG on a 40 amp breaker" case, and a large feeder losing more than most people expect.
Hot attic, shared raceway — 10 AWG THHN
Copper · 90 °C insulation · 104 °F (40 °C) ambient · 6 current-carrying conductors · 75 °C terminations. The defaults above.
× 0.91 (36–40 °C ambient) × 0.80 (4–6 conductors) = 29.1 A
110.14(C) cap at 75 °C = 35 A → not binding
240.4(B) rounds up to 30 A; 240.4(D) caps 10 AWG at 30 A
→ 29.1 A usable · 30 A max breaker
Result: a conductor most people would call "good for 30 amps" is carrying 29.1 A of real capacity here. It still lands on a 30 A breaker via the 240.4(B) round-up, but the margin is gone — a seventh conductor in that raceway drops it to 25.5 A, and at ten conductors the 0.50 factor takes it to 18.2 A, which forces a 20 A device.
Ideal conditions — 10 AWG THHN, nothing derated
Copper · 90 °C insulation · 86 °F (30 °C) ambient · 3 conductors · 75 °C terminations.
110.14(C) cap at 75 °C = 35 A ← governs
240.4(D) caps 10 AWG copper at 30 A
→ 35 A usable · 30 A max breaker
Result: this is the answer to "why can't I put 10 AWG on a 40 amp breaker?" Even with zero derating, two separate rules pull it down — the termination cap to 35 A and the small-conductor rule to a 30 A device.
Crowded industrial raceway — 4/0 THHN
Copper · 90 °C insulation · 113 °F (45 °C) ambient · 9 current-carrying conductors · 75 °C terminations.
× 0.87 (41–45 °C) × 0.70 (7–9 conductors) = 158.3 A
110.14(C) cap at 75 °C = 230 A → not binding
→ 158.3 A usable · 175 A max breaker
Result: 4/0 copper that "everyone knows" is good for 230 A is down to 158 A here — a 31% loss. This is the single most common cause of undersized conductors on commercial jobs, because the derating gets skipped and the table value gets used directly.
The two derating tables
Ambient temperature correction from Table 310.15(B)(1), and conductor bundling adjustment from Table 310.15(C)(1). Multiply both.
Ambient correction — Table 310.15(B)(1)
| Ambient | 60 °C | 75 °C | 90 °C |
|---|---|---|---|
| ≤ 10 °C (50 °F) | 1.29 | 1.20 | 1.15 |
| 11–15 °C (52–59 °F) | 1.22 | 1.15 | 1.12 |
| 16–20 °C (61–68 °F) | 1.15 | 1.11 | 1.08 |
| 21–25 °C (70–77 °F) | 1.08 | 1.05 | 1.04 |
| 26–30 °C (79–86 °F) | 1.00 | 1.00 | 1.00 |
| 31–35 °C (88–95 °F) | 0.91 | 0.94 | 0.96 |
| 36–40 °C (97–104 °F) | 0.82 | 0.88 | 0.91 |
| 41–45 °C (106–113 °F) | 0.71 | 0.82 | 0.87 |
| 46–50 °C (115–122 °F) | 0.58 | 0.75 | 0.82 |
| 51–55 °C (124–131 °F) | 0.41 | 0.67 | 0.76 |
| 56–60 °C (133–140 °F) | — | 0.58 | 0.71 |
Bundling adjustment — Table 310.15(C)(1)
| Current-carrying conductors | Factor |
|---|---|
| 1–3 conductors | 1.00 |
| 4–6 conductors | 0.80 |
| 7–9 conductors | 0.70 |
| 10–20 conductors | 0.50 |
| 21–30 conductors | 0.45 |
| 31–40 conductors | 0.40 |
| 41 or more conductors | 0.35 |
Applies where conductors are bundled for more than 24 inches. Grounds never count; the neutral counts only per 310.15(E).
Sources & standards: NEC (NFPA 70) 2023 — Table 310.16 ampacities, Table 310.15(B)(1) ambient correction, 310.15(B)(3) rooftop sunlight exposure, Table 310.15(C)(1) bundling adjustment, 310.15(E) neutral and grounding conductor counting, 110.14(C) termination temperature limitations, 240.4(B) and 240.4(D) overcurrent protection. Local amendments override the model code.
Frequently asked questions
Common questions about ampacity, derating, and termination temperature limits.
When do I have to derate a conductor?
Two triggers, and they stack. Ambient temperature correction from Table 310.15(B)(1) applies whenever the ambient is anything other than 26–30 °C (79–86 °F) — attics, rooftops, boiler rooms, and cold storage all qualify. Conductor bundling adjustment from Table 310.15(C)(1) applies when more than three current-carrying conductors share a raceway or cable for more than 24 inches. If both apply, multiply the base ampacity by both factors.
Do neutrals count as current-carrying conductors?
Usually yes, with important exceptions in NEC 310.15(E). The neutral of a balanced 3-wire single-phase circuit, or of a balanced 4-wire three-phase circuit, does not count. But the neutral of a 4-wire three-phase circuit supplying substantially nonlinear loads (LED drivers, computers, VFDs) does count, because harmonic currents add rather than cancel there.
Does the equipment grounding conductor count?
No. NEC 310.15(E)(3) excludes grounding and bonding conductors from the conductor count — they carry no current under normal operation. Only the ungrounded conductors, and any neutral that counts under 310.15(E), go into the bundling number.
Why can 90 °C wire never actually be used at its 90 °C ampacity?
Because of NEC 110.14(C). Almost all breakers, panelboards, and equipment terminals are listed for 75 °C (some small equipment only 60 °C), and a conductor may not be used above its ampacity at the temperature rating of the terminations. So THHN's 90 °C column is real, but you may only use it as the starting point for derating math — the final usable ampacity is capped at the 75 °C value. This calculator shows both numbers and which one governs.
How many conductors before bundling derating kicks in?
More than three current-carrying conductors triggers Table 310.15(C)(1): 4–6 conductors × 0.80, 7–9 × 0.70, 10–20 × 0.50, 21–30 × 0.45, 31–40 × 0.40, 41 or more × 0.35. There is a 24-inch exception — conductors bundled for 24 inches or less do not require adjustment.
Does conduit in direct sunlight need extra correction?
Yes. NEC 310.15(B)(3) requires an adder for raceways and cables exposed to direct sunlight on or above rooftops. Table 310.15(B)(3)(c) historically added up to 33 °C for raceways very close to the roof deck. Enter the sun-adjusted ambient temperature above rather than the shade air temperature — a 95 °F day can produce a 120 °F+ effective ambient inside rooftop EMT.
Can I round up to the next breaker size?
Often, yes. NEC 240.4(B) permits the next higher standard overcurrent rating when the conductor ampacity does not correspond to a standard rating, for circuits rated 800 A or less that do not supply receptacles for cord-and-plug-connected portable loads. That allowance is still bounded by 240.4(D) for 14, 12, and 10 AWG. The Breaker Size Calculator works the problem from the load side.
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