Want this on your own domain?
We build this calculator as an embeddable tool — engineered, SEO- and AI-ready, delivered in 5 business days for a flat $2,800.
Get yours in 5 daysHow it's calculated
Ampacity is the current a conductor can carry continuously without exceeding its insulation temperature rating. NEC Table 310.16 lists it by size, material, and insulation column (60/75/90 °C). Correction factors reduce it for high ambient temperature and for more than three current-carrying conductors bundled together; terminations then cap the usable column (commonly 75 °C).
- Continuous load 40 A → 40 × 1.25 = 50 A design.
- THHN Cu, 75 °C terminations → 75 °C column.
- #8 Cu = 50 A at 75 °C ✓ (no derating needed).
Reference tables
Copper ampacity — NEC Table 310.16 (excerpt)
| Size | 60 °C | 75 °C | 90 °C |
|---|---|---|---|
| #14 | 15 | 15 | 15 |
| #12 | 20 | 20 | 25 |
| #10 | 30 | 35 | 40 |
| #8 | 40 | 50 | 55 |
| #6 | 55 | 65 | 75 |
| #4 | 70 | 85 | 95 |
| 1/0 | 125 | 150 | 170 |
| 4/0 | 195 | 230 | 260 |
Ambient temperature correction (90 °C column)
| Ambient | Factor |
|---|---|
| 21–25 °C | 1.04 |
| 26–30 °C | 1.00 |
| 31–35 °C | 0.96 |
| 36–40 °C | 0.91 |
| 41–45 °C | 0.87 |
A number the code tables do not give directly: for 4–6 current-carrying conductors in one raceway, apply an 80% bundle factor before choosing the size.
See a live tool in action
The interactive version of this calculator is on our build queue. Meanwhile, these live example simulators show exactly how yours would look and behave on your own domain:
FAQ
Which temperature column do I use?
Use the column that matches the lowest-rated part in the circuit — usually the equipment terminations. Most breakers and lugs are rated 75 °C, so even 90 °C wire is limited to the 75 °C ampacity for sizing.
Why multiply the load by 1.25?
NEC requires conductors and overcurrent devices for a continuous load (3 hours or more) to be sized at 125% of the load, so the equipment runs cool.
When does bundling derating apply?
When more than three current-carrying conductors share a raceway or cable, ampacity is reduced (e.g., 80% for 4–6, 70% for 7–9). Neutrals that carry current count.
Copper vs aluminum sizing?
Aluminum carries less per size, so you typically go up one or two sizes versus copper for the same load. Always confirm terminations are rated for aluminum.
Does this replace an engineered design?
No — it sizes conductors to Table 310.16 for orientation. Verify against the current NEC and your AHJ.
Build it for your catalog
Put this calculator on your domain so the engineers sizing it land on your site — not a competitor's. See how it works or start the conversation.