Wire & Cable calculator
Cable Ampacity Calculator
This cable-ampacity screen evaluates conductor size, material, installation method, insulation, ambient temperature, current-carrying conductor count, and optional load or voltage-drop inputs. It does not identify a listed NM-B, MC, or AC product by itself; use the result with the cable marking, adopted code edition, equipment terminations, and installation conditions.
Updated October 29, 2026
Example Calculations
How to Use
NEC Multi-Conductor Cable Ampacity Standards
Cable vs Conductor: Multi-conductor cables contain 2+ insulated conductors in a single jacket/armor. Ampacity differs from individual conductors due to thermal characteristics and NEC application rules.
Calculator input boundary: Select the installation method (raceway/conduit, cable assembly, direct burial, free air, or cable tray), conductor gauge and material, ambient temperature, current-carrying conductor count, and insulation type. The wire-sizing, voltage-drop, and complete-analysis modes also use the load current, cable length, and system voltage fields. The tool does not have a NM-B/MC/AC product selector or a terminal-rating input, so those product and termination checks remain part of the installation review.
Cable Type Comparison: NM-B vs MC vs AC (Critical Selection Guide)
| Feature | Type NM-B (Romex) | Type MC (Metal-Clad) | Type AC (BX) |
|---|---|---|---|
| Temperature Rating | Use the 60°C basis unless the adopted NEC edition, product marking, and installation method allow otherwise | 90°C (THHN/XHHW) | 60°C typical |
| Max Ampacity | Common residential branch-circuit range; verify Article 334 limits for the adopted edition | Use the applicable ampacity table, cable marking, and terminal temperature rating | Typically ≤30A branch circuits |
| Use Scope to Verify | Dry-location residential use when Article 334, the building type, and local amendments allow it | Review Article 330, product marking, support, exposure, and location limits | Review Article 320, existing-work conditions, support, and connector requirements |
| Commercial Review | Verify Article 334 scope before using NM-B in nonresidential work | Verify Article 330 scope, listing, and project conditions | Verify Article 320 scope; MC is often reviewed for new commercial runs |
| Mechanical Protection | PVC jacket only | Aluminum/steel armor | Galvanized steel armor |
| Best For | Residential branch circuits | Commercial, high-ampacity feeders | Residential retrofit (old work) |
Selection screen: Start with NM-B only when the project is a dry-location residential branch-circuit use that Article 334 and local amendments allow. Review Type MC when the project needs commercial scope, mechanical protection, feeders, exposed runs, or a 90°C derating starting point. Treat Type AC as a separate Article 320 review rather than an automatic substitute for MC.
Type NM-B Cable Ampacity (Romex) - NEC 334.80
| Cable Size | Conductors | Ampacity (60°C) | Max Breaker | Typical Use |
|---|---|---|---|---|
| 14/2 NM-B | 14 AWG Cu | 15A | 15A | Lighting, receptacles |
| 12/2 NM-B | 12 AWG Cu | 20A | 20A | General receptacles, kitchen |
| 10/2 NM-B | 10 AWG Cu | 30A | 30A | Dryer, water heater |
| 10/3 NM-B | 10 AWG Cu | 30A | 30A | Electric range, MWBC |
| 8/3 NM-B | 8 AWG Cu | 40A | 40A | Electric range (50A receptacle) |
| 6/3 NM-B | 6 AWG Cu | 55A | 55A | Subpanel feeders |
NM-B restrictions to verify: Review Article 334 in the NEC edition adopted for the project, the cable marking, the building type, exposed or damp/wet-location limits, and local amendments before using an NM-B result as the final cable selection.
Type MC Cable Ampacity (Metal-Clad) - NEC 330.80
| Cable Size | Insulation | Ampacity | Applications |
|---|---|---|---|
| 12/2 MC-THHN | 90°C (THHN) | 30A (use 25A @ 75°C term) | Commercial, exposed areas |
| 10/3 MC-THHN | 90°C (THHN) | 40A (use 35A @ 75°C term) | Feeders, industrial |
| 8/4 MC-THHN | 90°C (THHN) | 55A (use 50A @ 75°C term) | Subpanel feeders |
| 6/4 MC-THHN | 90°C (THHN) | 75A (use 65A @ 75°C term) | Large feeders, 60A circuits |
MC cable review points: 90°C insulation can provide a higher derating starting point, armor adds mechanical protection, and Article 330 may fit commercial or industrial routing when the listed cable, supports, fittings, location, and equipment terminations are suitable. Still apply terminal limits, overcurrent protection, load rules, and the adopted NEC edition before final selection.
Type AC Cable Ampacity (BX/Armored) - NEC 320.80
| Cable Size | Ampacity (60°C) | Max Use |
|---|---|---|
| 14/2 AC | 15A | Lighting circuits |
| 12/2 AC | 20A | General circuits |
| 10/2 AC | 30A | Limited to branch circuits |
AC Cable Limitations: Type AC (BX) typically 60°C rated, not suitable for high-ampacity feeders. Consider Type MC for commercial/industrial installations requiring >30A capacity.
For conductor ampacity without cable assembly restrictions, use the NEC Ampacity Calculator. For voltage drop analysis, use the Voltage Drop Calculator. For final conductor sizing, use the Wire Size Calculator. Move to the Breaker Sizing Calculator only after the load current and final conductor basis are known. Keep conduit-fill questions on the Conduit Fill Calculator; this page is for cable assemblies, not raceway occupancy.
After confirming ampacity, check that the conductor maintains voltage within acceptable limits—see the Voltage Drop Formulas Guide for NEC-based analysis.
Common Applications
Frequently Asked Questions
Does this calculator choose NM-B, MC, or AC by name?
No. The calculator screens conductor and installation inputs, including a cable-assembly installation method, but it does not select a listed product or verify its marking, fittings, location limits, or terminal rating. Match the screen to the actual cable documentation and the adopted code path before final selection.
Why does NM-B cable usually use the 60°C ampacity basis?
Use the 60°C ampacity basis for Type NM-B unless the adopted NEC edition, cable marking, equipment terminations, and installation details support a different review path. The calculator separates conductor table values from the cable-assembly and terminal limits so a larger feeder is not selected from the 75°C or 90°C column alone. For higher-current feeders, compare NM-B with a listed MC cable or individual conductors in raceway after checking Article 334 limits, local amendments, and the equipment termination ratings.
When using 90°C rated MC cable, why should terminal temperature limits be checked?
NEC 110.14(C) makes the equipment termination rating part of the final ampacity review. A 90°C insulation rating can be useful as a starting point for temperature or bundling corrections, but the final result still needs to be compared with the marked terminal rating, the overcurrent device, the load type, and the adopted NEC edition. For example, a 90°C starting value may remain useful after ambient correction while the final branch-circuit review is still limited by 60°C or 75°C equipment terminations.
What cable should I use for Multi-Wire Branch Circuits (MWBC) and why does cable type matter?
An MWBC review normally starts with a 3-wire cable or raceway grouping that keeps the ungrounded conductors and shared neutral together. Check NEC 210.4, simultaneous disconnect requirements, neutral sizing, conductor grouping, and the equipment layout before selecting 12/3 NM-B, 10/3 NM-B, MC, or individual conductors in raceway. Separate cable runs need a documented design and inspection basis because conductor grouping and neutral current can become safety issues.
How do I apply temperature derating to NM-B cable in hot attics and why is it more severe than MC cable?
Hot locations make the insulation temperature column important. A 12/2 NM-B screen at 50°C starts from the 60°C basis and may fall below the intended circuit load after correction, while a listed MC cable with 90°C insulated conductors may have more derating headroom before the final terminal check. Use the calculator result as a prompt to compare conductor size, cable type, ambient assumptions, terminal rating, and the adopted NEC correction table before deciding whether to upsize or change wiring method.
Related Calculators
Ampacity Calculator
Individual conductor ampacity and derating per NEC 310.16 with ambient-temperature correction, current-carrying conductor adjustment, and terminal-rating limits.
Wire Size Calculator
Handle conductor gauge selection from load current, standard OCPD, and voltage drop after ampacity and raceway constraints are known.
Voltage Drop Calculator
Calculate voltage drop in conductors
Conduit Fill Calculator
Check conduit fill, conductor count, and raceway loading against NEC-style limits.
Wire Gauge Converter
Convert between AWG, mm², and circular mils for conductor sizing and documentation.
Circuit Breaker Sizing Calculator
NEC-oriented standard breaker review-point screen with simplified conductor protection checks.