NM Cable Ampacity Lookup
NEC 334.80 is the residential cable-assembly checkpoint that keeps NM-B ampacity separate from individual THHN or THWN-2 conductors in raceway. Use it after the conductor ampacity and derating basis are identified, but before treating the result as usable for the load.
| Field Item | NEC Reference | Field Meaning |
|---|---|---|
| NM-B ampacity cap | NEC 334.80 | Final NM-B ampacity cannot exceed the applicable 60°C ampacity basis, even when higher insulation ratings are used for permitted adjustment math. |
| 90°C insulation use | NEC 334.80, NEC 310.15 | The 90°C insulation basis may be used for ampacity adjustment and temperature correction where permitted, but it does not become the final NM-B ampacity. |
| Bundled NM cables | NEC 334.80, NEC 310.15(C)(1) | Multiple NM cables routed together, through framing openings, or in heat-retaining conditions can require conductor-count adjustment review. |
| Current-carrying conductors | NEC 310.15 context | Count current-carrying conductors based on the circuit condition. Equipment grounding conductors are not normally counted for ampacity adjustment. |
| Thermal insulation contact | NEC 334.80 context | Cable surrounded by insulation, foam, caulk, or other heat-retaining material may need derating review before the result is trusted. |
| Terminal temperature limit | NEC 110.14(C) | Terminal ratings still control the final usable ampacity. NM-B residential circuits commonly remain limited by 60°C assumptions. |
| Small conductor OCPD limits | NEC 240.4(D) | Small copper conductors still have overcurrent limits. A derating calculation does not remove those branch-circuit limits. |
| Raceway handoff | NEC Chapter 9 context | NM-B is a cable assembly and is not treated like separate THHN conductors for raceway fill or ampacity workflows. |
Field Example
NM-B Derating and the 60°C Final Cap
NM-B ampacity review separates permitted 90°C derating math from the final 60°C ampacity cap.
NM-B review = permitted 90°C adjustment/correction math, then final 60°C ampacity limit
Field example: bundled NM-B cables may require adjustment using the permitted higher-temperature basis, but the final usable ampacity still cannot exceed the applicable 60°C value. Check NEC 310.15(C)(1) Ampacity Adjustment, NEC 310.15(B)(1) Temperature Correction, and NEC 110.14(C) Terminal Temperature Limits before trusting the result. Use the Ampacity Calculator for the derating screen.
Cable Assembly Boundary
Cable Assembly Boundary
NM-B is a cable assembly. The conductors inside may have a higher insulation rating, but the cable assembly is still governed by Article 334 rules and the final 60°C ampacity limit.
Individual THHN or THWN-2 conductors in raceway follow a different installation path. Do not pull THHN-style final ampacity assumptions into NM-B sizing just because a 90°C insulation basis appears in the adjustment calculation.
Use 90°C only as the permitted adjustment starting point. Pull the final NM-B result back to the 60°C cap.
Ampacity Sequence
90°C Adjustment and 60°C Cap
The safe field sequence is to keep the adjustment basis and final ampacity cap separate. This avoids the common mistake of treating NM-B like it can use the 75°C or 90°C column as the final answer.
Start with the permitted basis. Identify conductor material, insulation rating, and the table column allowed for adjustment and correction.
Apply derating checks. Correct for ambient temperature and adjust for conductor count, bundling, and installation conditions.
Compare to the 60°C cap. Do not allow final NM-B ampacity to exceed the applicable 60°C ampacity limit.
Confirm OCPD and equipment limits. Recheck terminal ratings, overcurrent limits, equipment markings, and the actual field installation before trusting the result.
Field Conditions
Bundling and Insulation Checks
NM-B ampacity review becomes more important when cable assemblies are grouped, sealed, insulated, or routed through heat-retaining areas. These conditions can reduce available ampacity before the final 60°C cap is applied.
Bored holes and framing paths. Multiple NM cables through the same bored hole or framing path can create conductor-count, conductor protection, and heat-dissipation concerns.
Insulation and sealing materials. Thermal insulation, foam, caulk, or firestopping around cable paths can change the heat condition that the ampacity result depends on.
Current-carrying conductor count. Count only conductors that carry current under the applicable circuit condition; equipment grounding conductors are not normally counted for adjustment.
Neutral-count conditions. Multiwire branch circuits, shared neutrals, and nonlinear load conditions can change whether a neutral is counted.
Calculator Use
TradeHub Calculator Application
TradeHub calculators treat NEC 334.80 as an NM cable boundary where permitted 90°C adjustment math still must land under the final 60°C ampacity limit.
Related TradeHub Calculators
Field Checks
Common Field Misses
Treating NM-B as if the 75°C or 90°C column can be used for final ampacity.
Forgetting that bundled NM cables or insulation-covered cable paths can require derating review.
Counting equipment grounding conductors as current-carrying conductors for ampacity adjustment.
Missing neutral-count rules on multiwire branch circuits, shared neutrals, or nonlinear load conditions.
Upsizing for voltage drop without rechecking ampacity, OCPD, cable assembly limits, and terminal temperature boundaries.
Related References
Related NEC Field References
Ambient correction, conductor-count adjustment, neutral counting, and derating review.
NEC 310.16 Ampacity TableBase conductor ampacity context before NM-B cable assembly limits are applied.
NEC 110.14(C) Terminal Temperature LimitsTerminal cap review before final conductor ampacity is trusted.
NEC 240.4(B) Next-Size-Up RuleConditional OCPD rule that should not override NM-B or small-conductor limits.
NEC 310.12 Dwelling Service and Feeder ConductorsDwelling service and feeder sizing context before installation derating checks.
NEC Chapter 9 Conduit Fill TablesRaceway fill context when wiring method changes from cable assembly to conductors in raceway.
Source Scope
Source Alignment and Use Scope
This page is a field reference based on NEC 334.80, NEC 310.15, NEC 310.16, NEC 110.14(C), NEC 240.4(D), and related TradeHub source alignment records. It explains NM-B cable ampacity screening logic for planning and field review only. It does not reproduce proprietary NEC tables, approve an installation, replace adopted-code or local-amendment review, or override equipment markings, product listings, manufacturer instructions, engineered documents, or AHJ direction. Review the TradeHub Code Citation & Source Log for source alignment records and the TradeHub Methodology page for how field references are scoped.
FAQ
NM Cable Ampacity FAQ
Can NM-B use the 90°C column?
The 90°C insulation basis may be used for permitted adjustment and correction checks, but final NM-B ampacity cannot exceed the applicable 60°C ampacity limit and must still respect terminal, overcurrent, and equipment requirements.
Does bundling NM-B cable require derating?
It can. Multiple current-carrying conductors in bundled cable assemblies, bored framing paths, or insulation-covered locations can require ampacity adjustment review under the applicable NEC rules.
Is NM-B the same as THHN in conduit?
No. NM-B is a cable assembly covered by Article 334 rules. Individual THHN or THWN-2 conductors in raceway follow a different installation path and should not be substituted for NM-B ampacity logic without checking the wiring method.