Wire & Cable calculator

Cable Ampacity Calculator

Q: What cable should I use for Multi-Wire Branch Circuits (MWBC) and why does cable type matter?

MWBC requires 3-wire cable (2 hots + 1 shared neutral) per NEC 210.4. Use 12/3 NM-B for residential 20A MWBC or 10/3 NM-B for 30A MWBC. Cable type is critical because: (1) All conductors must be in same cable/raceway per NEC 210.4(D) to maintain magnetic balance, (2) Shared neutral must be sized for unbalanced load - NM-B cable ensures proper conductor grouping, (3) Handle-tie requirement for breakers (simultaneous disconnect) ensures proper MWBC protection. Never use separate 12/2 cables for MWBC - neutral overloading risk. Type MC-THHN 12/3 or 10/3 acceptable for commercial MWBC installations.

Q: How do I apply temperature derating to NM-B cable in hot attics and why is it more severe than MC cable?

NM-B starts at 60°C rating vs MC at 90°C, making temperature derating more severe. Example: 12/2 NM-B in 50°C (122°F) attic: Base 20A × 0.58 (temp factor for 60°C insulation @ 50°C per NEC Table 310.15(B)(1)) = 11.6A final ampacity - insufficient for standard 15A circuit. Solution: Upsize to 10/2 NM-B (30A × 0.58 = 17.4A, meets 15A requirement). Same scenario with 12/2 MC-THHN: 30A × 0.82 (90°C insulation @ 50°C) = 24.6A, but limited to 25A @ 75°C termination - still adequate for 20A circuit. MC cable better for high-temperature installations due to 90°C rating advantage.

Multi-conductor cable ampacity calculator implementing NEC 2023 standards for assembled cables with multiple insulated conductors in a single jacket. Essential for Type NM-B (Romex), Type MC (Metal-Clad), Type AC (Armored Cable/BX) installations per code requirements.

Updated June 8, 2026

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Example Calculations

Residential Kitchen: 12/2 NM-B Cable

12/2 NM-B (Romex) for 20A kitchen receptacle circuit. NEC 334.80 limits NM-B to 60°C rating (20A for 12 AWG). 12 AWG copper at 60°C = 20A max per NEC Table 310.16.

Inputs

Cable Type: 12/2 NM B
Conductor Size: 12 AWG Copper
Temperature: 30°C (86°F)
Installation: Through Wall Cavities

Commercial Building: 10/3 MC-THHN

10/3 Type MC cable with THHN conductors for commercial feeder. 90°C rating allows 40A starting point, but use 75°C termination limit (35A) per NEC 110.14(C).

Inputs

Cable Type: 10/3 MC THHN
Conductor Size: 10 AWG Copper
Temperature: 30°C Ambient
Installation: Exposed In Cable Tray

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.

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	60°C only (NEC 334.80)	90°C (THHN/XHHW)	60°C typical
Max Ampacity	60A limit (NEC 334.80)	No limit (use Table 310.16)	Typically ≤30A branch circuits
Permitted Locations	Residential, ≤3 story buildings	All occupancies, exposed OK	Residential, concealed locations
Commercial Use	Prohibited >3 floors (334.10)	Permitted (NEC 330.10)	Limited - prefer MC
Mechanical Protection	PVC jacket only	Aluminum/steel armor	Galvanized steel armor
Typical Cost (12/2)	$0.40-0.60/ft	$1.20-1.80/ft	$0.90-1.20/ft
Best For	Residential branch circuits	Commercial, high-ampacity feeders	Residential retrofit (old work)

Selection Rule: Use NM-B for residential ≤60A. Use Type MC for: (1) Commercial buildings, (2) Ampacity >60A, (3) Exposed locations, (4) Where 90°C derating advantage needed. Type AC is legacy - prefer MC for new commercial work.

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: (1) Max 60°C rating per NEC 334.80. (2) Cannot exceed 60A per NEC 334.80. (3) Prohibited in commercial buildings >3 stories per NEC 334.10(A)(3). (4) Not permitted in exposed locations or wet areas.

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 Advantages: (1) 90°C insulation allows higher derating starting point. (2) Permitted in commercial/industrial per NEC 330.10. (3) Can exceed 60A (NM-B cannot). (4) Armor provides mechanical protection. (5) Use 75°C column for terminations per NEC 110.14(C).

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 NEC Ampacity Calculator. For voltage drop analysis, use Voltage Drop Calculator. For final conductor sizing, use Wire Size Calculator. 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

Residential NM-B Cable Selection - 12/2 for 20A circuits, 10/2 for 30A dryer/water heater, 6/3 for subpanels

Commercial MC Cable Sizing - Type MC-THHN for exposed commercial installations, 90°C rating advantage

Multi-Wire Branch Circuits (MWBC) - 10/3 or 12/3 cable for split-phase 240V circuits per NEC 210.4

Kitchen/Bath Circuits - 12/2 NM-B for 20A GFCI-protected receptacles, 10/2 for dedicated appliances

Subpanel Feeders - Calculate cable size for 60A-100A subpanels using NM-B (residential) or MC (commercial)

Temperature Derating - Apply corrections for attic (hot), conduit on roof (+30°C per NEC), buried installations

Cable Bundling Analysis - Multiple cables in single raceway require ampacity adjustment per NEC 310.15(B)(3)(a)

Code Compliance Verification - Ensure cable type matches occupancy: NM-B residential only, MC for commercial

Frequently Asked Questions

Why is NM-B cable limited to 60A when the conductors can handle more current?

NEC 334.80 explicitly limits Type NM-B cable to circuits rated 60A or less, regardless of conductor size. This restriction exists because: (1) NM-B jacket/insulation rated only 60°C vs 75-90°C for individual THHN conductors, (2) Heat dissipation limitations in multi-conductor assembly, (3) Historical safety margins for residential wiring. For circuits >60A, use Type MC cable or individual conductors in conduit. Example: 100A subpanel feeder requires 1 AWG conductors (110A @ 75°C) but cannot use NM-B cable - must use Type MC or conduit.

When using 90°C rated MC cable (THHN), why must I use 75°C ampacity for circuit sizing?

Per NEC 110.14(C), equipment terminations are rated for 75°C maximum (unless specifically marked for 90°C, rare for <100A equipment). While 12/2 MC-THHN conductors have 30A ampacity at 90°C, you must use 25A (75°C column) for final circuit sizing because breakers and receptacles are 75°C rated. The 90°C rating is valuable for derating calculations: Start with 30A, apply temperature/bundling corrections, then verify final derated ampacity meets load requirements. Example: 40°C ambient reduces 30A to 26.4A (30A × 0.88), still exceeds 25A 75°C requirement for 20A circuit.

What cable should I use for Multi-Wire Branch Circuits (MWBC) and why does cable type matter?

MWBC requires 3-wire cable (2 hots + 1 shared neutral) per NEC 210.4. Use 12/3 NM-B for residential 20A MWBC or 10/3 NM-B for 30A MWBC. Cable type is critical because: (1) All conductors must be in same cable/raceway per NEC 210.4(D) to maintain magnetic balance, (2) Shared neutral must be sized for unbalanced load - NM-B cable ensures proper conductor grouping, (3) Handle-tie requirement for breakers (simultaneous disconnect) ensures proper MWBC protection. Never use separate 12/2 cables for MWBC - neutral overloading risk. Type MC-THHN 12/3 or 10/3 acceptable for commercial MWBC installations.

How do I apply temperature derating to NM-B cable in hot attics and why is it more severe than MC cable?

NM-B starts at 60°C rating vs MC at 90°C, making temperature derating more severe. Example: 12/2 NM-B in 50°C (122°F) attic: Base 20A × 0.58 (temp factor for 60°C insulation @ 50°C per NEC Table 310.15(B)(1)) = 11.6A final ampacity - insufficient for standard 15A circuit. Solution: Upsize to 10/2 NM-B (30A × 0.58 = 17.4A, meets 15A requirement). Same scenario with 12/2 MC-THHN: 30A × 0.82 (90°C insulation @ 50°C) = 24.6A, but limited to 25A @ 75°C termination - still adequate for 20A circuit. MC cable better for high-temperature installations due to 90°C rating advantage.

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