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• NEC Table 310.16 is the master ampacity chart for insulated conductors (0-2000 V) installed in raceway, cable, or earth and limited to no more than three current-carrying conductors in a single raceway/cable and an ambient of 30 °C (86 °F).
• It lists allowable continuous current (ampacity) for each conductor size (AWG/kcmil), material (Cu or Al/Cu-clad Al) and insulation temperature rating (60 °C, 75 °C, 90 °C).
• The values in the table are the starting point; you must apply further NEC adjustment and correction factors, then respect equipment-terminal temperature limits before final conductor sizing.
Key points
– Covers 14 AWG through 2000 kcmil.
– Copper has higher ampacity than the same size aluminum.
– Table number changed: formerly 310.15(B)(16) (pre-2020 NEC).
– Final conductor ampacity must satisfy 110.14(C) terminal ratings (generally 60 °C ≤100 A or #1 AWG and smaller; 75 °C otherwise, unless 90 °C equipment is listed).
Table scope and title
Official (2023 NEC) title:
“Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts, 60 °C Through 90 °C (140 °F Through 194 °F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30 °C (86 °F).”
Table anatomy
• Rows: Conductor size (AWG for 14 AWG→4/0 AWG, then kcmil).
• Columns: Three temperature ratings for Copper followed by three for Aluminum/Cu-Clad Al.
• Example excerpt (2023 NEC, Cu):
| Size | 60 °C | 75 °C | 90 °C (A) |
|---|---|---|---|
| 14 AWG | 20* | 20* | 25 |
| 12 AWG | 25* | 25* | 30 |
| 10 AWG | 30* | 35* | 40 |
| 8 AWG | 40 | 50 | 55 |
| 6 AWG | 55 | 65 | 75 |
| 3 AWG | 85 | 100 | 115 |
| 1/0 | 125 | 150 | 170 |
| 500 kcmil | 310 | 380 | 430 |
(*Limited by 240.4(D) over-current protection rules.)
Mandatory adjustment/correction workflow
Step-by-step conductor sizing (NEC compliant):
a) Determine design current (I_load).
b) Multiply by 125 % if load is “continuous” (≥3 h).
c) Select insulation type (THHN, XHHW-2, UF, etc.) → choose matching 60/75/90 °C column for starting ampacity.
d) Apply ambient temperature correction from Table 310.15(B)(1). Example: THHN at 45 °C → 0.91 factor.
e) Apply conductor grouping adjustment from Table 310.15(C)(1). Ex: 4-6 current-carrying conductors → 80 %.
f) Compare resulting ampacity with equipment terminal rating per 110.14(C); limit to the lower value (usually 60 °C or 75 °C).
g) Check voltage-drop separately (recommended ≤3 % branch, ≤5 % feeder+branch).
h) Select next larger standard conductor size if necessary.
Practical example
• 75 A continuous load, THHN Cu in 1-½″ EMT with 4 current-carrying conductors, attic ambient 45 °C.
– Design current = 75 A ×1.25 = 93.8 A.
– Base ampacity (90 °C column, 3 AWG) = 115 A.
– Ambient factor (45 °C) = 0.91 → 115 A×0.91 = 105 A.
– Adjustment for 4 conductors = 0.80 → 105 A×0.80 = 84 A.
– Equipment terminals 75 °C → 75 °C column for 3 AWG = 100 A (limit).
– Final allowable ampacity = 84 A (<100 A), so 3 AWG Cu is acceptable.
Voltage-drop check (if run is long) may still dictate upsizing even when thermal capacity is adequate.
• Table renumbering: 2020 NEC reorganised Article 310; what was 310.15(B)(16) in 2017 and earlier again became 310.16 in 2020-2023 editions.
• Growing use of 90 °C rated conductors (THHN, XHHW-2, PV-wire) to gain derating “head-room,” especially in solar PV, data centers, and rooftop conduit subjected to high temperatures (see 310.15(B)(3)(c)).
• Cost and weight pressures are increasing use of compact aluminum alloy feeders; designers rely heavily on 75 °C aluminum columns plus derating.
• Software (BIM, Revit, SKM PTW, ETAP) now embeds Table 310.16 with automatic derating and voltage-drop modules.
• Why only three conductors? Heat generated by mutual impedance rises exponentially when more than three current paths share the same space, hence the conservative base condition.
• Circular mil (CM) vs kcmil: 1 kcmil = 1000 circular mils = area of a circle with 1 mil (0.001 in.) diameter multiplied by 1000.
• THHN vs THWN-2: Both are dual-rated; THHN dry 90 °C / THWN-2 wet 90 °C. Using THHN/THWN-2, designer may start in 90 °C column even when the final termination is 75 °C, because the higher starting value often means a smaller conductor after derating.
• The NEC is a minimum safety standard; local jurisdictions (AHJs) may amend stricter rules.
• Only licensed electricians/engineers should stamp electrical drawings; improper ampacity sizing can lead to overheating, fire, or equipment failure.
• Follow NFPA 70E for arc-flash and shock hazards when working on or near energized conductors.
• Always keep an updated copy of the NEC edition enforced by your AHJ.
• For field work, carry laminated quick-reference strips or mobile apps with Table 310.16 plus correction/adjustment tables.
• Document all derating calculations on the design drawings to facilitate plan-check approval.
• If using conductors in ambient >50 °C (rooftops, boiler rooms) or in bundles >30 highlighted in 310.15(C)(1), expect substantial derating—often a larger size is cheaper than adding additional raceways.
• Values reproduced here are condensed; consult the official NEC for the complete table.
• Over-current protection limits for small conductors (240.4(D)) can override Table 310.16 (e.g., 14 AWG Cu limited to 15 A OCP).
• Ampacities do not ensure acceptable voltage regulation or energy efficiency.
• High-temperature polymer and silicone-based insulations (125 °C-150 °C) for electric vehicle charging infrastructure.
• Impact of harmonics (non-linear loads) on conductor heating; consider IEEE 519 and derating neutrals.
• Integration of NEC ampacity tables into AI-assisted electrical design platforms.
• Comparison with IEC 60364 current-carrying capacity methods (Adiabatic equation).
NEC Table 310.16 is the designer’s starting point for sizing conductors by ampacity. Select the row (size), column (temperature rating & material), then apply ambient-temperature corrections and conductor-count adjustments from Article 310, finally cap the result at equipment-terminal limits in 110.14(C). The table’s numbering reverted to 310.16 in the 2020-2023 NEC, and its correct application—together with voltage-drop, over-current protection and local amendments—ensures both code compliance and electrical safety.
