Cable Ampacity Calculator (NEC)
Screen a common insulated conductor against a Table 310.16 ampacity column, ambient-temperature correction, the number of current-carrying conductors, equipment-terminal temperature limits, and a continuous or noncontinuous load. The result exposes every reduction instead of returning a mysterious wire size.
Local adoption controls. NFPA publishes the National Electrical Code, but states and local authorities choose the edition, amendments, permits, and inspection requirements that apply. Confirm the adopted code with the authority having jurisdiction before design or installation.
Ampacity gate result
The entered conductor passes this limited ampacity screen by 4.60 A. Complete all installation-specific and protection checks before use.
Why the answer passes through three heat gates
Start with an ampacity column
The table value depends on conductor material, size, and the temperature rating permitted for the insulation and application. A 90°C insulation column can provide a starting value for adjustment calculations when allowed, but it does not automatically make every connected terminal a 90°C termination.
Account for trapped heat
Higher ambient temperature reduces the conductor’s ability to reject heat. Multiple current-carrying conductors grouped in a raceway or cable can heat one another. The calculator multiplies the selected table ampacity by both applicable screening factors and shows them separately for review.
Respect the terminal ceiling
Equipment termination provisions can restrict the ampacity associated with the conductor size. The result uses the lower of the adjusted insulation-column value and the selected terminal-column value. Listing, labeling, conductor type, and equipment instructions still decide which temperature rating is permitted.
The tool deliberately reports the arithmetic before suggesting a passing size. A conductor is not selected merely because its unadjusted table value exceeds the load. If the heat factors reduce that value below the required load ampacity, the scenario fails. Likewise, a large adjusted value does not override a lower termination limitation. Seeing each gate makes it easier to find whether changing routing, reducing conductor grouping, reducing ambient exposure, or selecting a larger conductor is the meaningful design response.
Worked example: 6 AWG copper in a hotter, crowded raceway
The default scenario uses 6 AWG copper, a 90°C insulation ampacity column, 75°C terminations, 40°C ambient temperature, six current-carrying conductors, and a 40 A continuous load. The starting 90°C table value is 75 A. At 40°C, the selected temperature-correction factor is 0.91. Six current-carrying conductors produce an 80 percent adjustment factor in this simplified lookup.
Multiplying 75 A by 0.91 and 0.80 gives 54.60 A. The 75°C table value for 6 AWG copper is 65 A, so the terminal ceiling does not reduce the already-adjusted value. The final screening ampacity remains 54.60 A. The continuous-load comparison uses 40 A × 125 percent, producing a required ampacity of 50 A. The screen therefore shows 4.60 A of margin and identifies 6 AWG copper as the smallest size in its included table that passes the same modeled factors.
This is not a complete branch-circuit, feeder, service, motor, photovoltaic, HVAC, or electric-vehicle-supply-equipment design. Some loads have specific sizing articles, demand factors, conductor minimums, overcurrent-device rules, neutral treatment, rooftop temperature considerations, or equipment instructions. A calculated passing margin only says that the entered scenario survives the arithmetic implemented in this calculator.
Selecting the insulation and terminal columns
Insulation column
Choose a column only when the conductor insulation type and governing installation rule allow it. Common conductors may carry multiple type markings, but a marking alone does not settle every application. Wet-location status, cable assembly rules, equipment listing, and the conductor’s actual temperature rating matter.
Using the 90°C starting column for correction and adjustment does not mean the final termination may be loaded to the 90°C table value. This calculator therefore keeps the insulation starting point and the equipment-terminal ceiling as two different inputs.
Terminal column
The terminal input is a screening ceiling, not a field determination. Equipment rated 100 A or less, conductor sizes, separately identified equipment, listed assemblies, and equipment instructions affect permitted termination temperature provisions. When uncertain, stop and confirm the marked terminal rating and applicable NEC section.
The calculator offers 60°C and 75°C terminal columns because they are common planning cases. It does not infer a terminal rating from the load or conductor size, and it does not model splices, connectors, lugs, parallel conductors, or separately rated components.
What this calculator includes—and what it leaves out
| Design question | Included here | Still requires project review |
|---|---|---|
| Table starting ampacity | Common copper sizes 14 AWG through 4/0 and common aluminum sizes 12 AWG through 4/0 in the 60°C, 75°C, and 90°C columns. | Other sizes, special conductor constructions, flexible cords, fixture wires, cable-specific tables, and installations governed by other ampacity provisions. |
| Ambient correction | Discrete correction factors for an ambient entry through 85°C using the selected insulation temperature column. | Rooftop adders where applicable, multiple ambient zones, thermal insulation, sunlight, soil conditions, duct banks, and engineered ampacity calculations. |
| Conductor grouping | A general factor based on the entered number of current-carrying conductors. | Which neutrals count, raceway length exceptions, nipples, bundled cables, diversity provisions, cable tray rules, and conductor arrangements covered by specific exceptions. |
| Load comparison | A 125 percent continuous-load scenario or a 100 percent noncontinuous-load scenario. | Mixed continuous and noncontinuous loads, demand factors, motor rules, HVAC nameplates, listed 100-percent-rated assemblies, harmonics, nonlinear loads, and article-specific requirements. |
| Protection | No overcurrent-device selection is claimed. | Standard ratings, small-conductor rules, next-size-up allowances, equipment protection, interrupting rating, selective coordination, and available fault current. |
OSHA’s construction wiring rule states that conductors must have sufficient ampacity to carry the load and that conductors and equipment must be protected from overcurrent according to their ability to conduct current safely. That broad safety requirement is consistent with treating ampacity and overcurrent protection as coordinated decisions, not interchangeable numbers.
Neither linked result replaces the NEC calculation or an engineered study.
How to use the screening result responsibly
- Identify the governing circuit. Classify the load and locate any article-specific sizing rule before using a general ampacity table.
- Verify conductor markings and installation. Material, insulation type, location rating, cable or raceway method, and listing determine which table and column may apply.
- Count current-carrying conductors correctly. Do not assume every grounded conductor is excluded or included. The circuit and harmonic content affect the determination.
- Use the worst credible ambient condition. Ambient temperature is not necessarily the room thermostat setting. Consider the actual route and heat sources.
- Confirm termination ratings. Check equipment markings, conductor size range, connector listing, and manufacturer instructions at both ends and at intermediate devices.
- Coordinate protection and voltage drop. A conductor can pass ampacity yet have unacceptable voltage drop, mechanical limitations, or insufficient protection coordination.
- Check local adoption and inspection requirements. The 2026 NEC is available, but a jurisdiction may enforce an earlier edition with amendments. The locally adopted rule is the relevant legal baseline.
Record the inputs with the project file. A future reviewer should be able to see why the material, temperature columns, ambient condition, conductor count, and continuous-load treatment were chosen. If any input is a placeholder, label it and rerun the calculation once field conditions and equipment submittals are known.
Frequently asked questions
Is this a complete NEC wire-size calculator?
No. It is a transparent Table 310.16-style screening calculator for a limited group of common conductors and factors. It does not choose the governing article, verify an insulation type, count neutrals, size equipment grounding conductors, calculate voltage drop, select protection, or apply local amendments. A qualified designer must complete those tasks.
Why does the suggested size sometimes change when ambient temperature changes?
Higher ambient temperature can reduce the correction factor because a conductor has less thermal margin to dissipate internally generated heat. The adjusted ampacity may fall below the required load ampacity, causing the next included conductor size to become the smallest passing screening option.
Can I always use the 90°C column for THHN or THWN-2?
No. A conductor marking may permit a 90°C insulation rating, but terminations, equipment listings, cable construction, wet or dry location requirements, and other application rules still govern. A 90°C starting point may be permitted for adjustment and correction while the final ampacity remains capped by a lower terminal rating.
Does the calculator apply the 125 percent rule to every load?
It applies 125 percent only when you select the continuous-load scenario. Real circuits can combine continuous and noncontinuous portions or follow equipment-specific rules. Determine the load under the applicable NEC article before entering one current value.
Why is 14 AWG aluminum rejected?
The limited aluminum dataset begins at 12 AWG. The calculator does not invent an ampacity where its included reference set has no entry. Select a supported size or use the exact table and product rules for the conductor under consideration.
Can a passing ampacity result be paired with any breaker?
No. Breaker selection is a separate code and equipment-coordination task. Standard ratings, conductor protection rules, article-specific loads, terminal ratings, equipment maximum overcurrent protection, interrupting rating, and available fault current can all control. Passing this screen does not approve an overcurrent device.
References
- National Fire Protection Association, NFPA LiNK available publications: 2026 NFPA 70, National Electrical Code
- National Fire Protection Association, Errata No. 70-26-1 for the 2026 edition of NFPA 70
- Occupational Safety and Health Administration, 29 CFR 1926.404, Wiring Design and Protection
Educational screening only. Use the complete locally adopted NEC edition, applicable amendments, listed equipment instructions, permits, and qualified electrical professionals. Do not install or modify energized equipment based in this calculator.