NEC 310.16 · Wire ampacity explained

How to read the NEC 310.16 wire ampacity table

Which column to use, what the footnotes change, how derating works, and the field mistakes that fail inspections — with worked examples and a link to the free official NEC viewer.

Need the actual numbers?

This page teaches you how to use NEC Table 310.16 — it deliberately does not reproduce the table. Look up Table 310.16 in the free NFPA 70 (NEC) online viewer (free NFPA account, read-only), or open your printed code book to Article 310.

Section numbers and values vary by NEC edition, and your jurisdiction may adopt an older edition or local amendments. The edition your AHJ enforces governs — confirm before you order wire.

How to read Table 310.16, step by step

Step	What you do
1. Confirm the table applies	Table 310.16 covers conductors up to 2000 V in raceway, cable, or earth, with no more than three current-carrying conductors, at 30°C ambient. Single conductors in free air, higher voltages, and dwelling service/feeder shortcuts use other sections (see the decision table below).
2. Pick the material side	The table is split into a copper half and an aluminum / copper-clad aluminum half. Make sure you are reading the half that matches the wire on the reel.
3. Find the insulation column	Each half has three temperature columns (60°C, 75°C, 90°C). The column headers list the insulation types that belong to each — find your insulation (THWN-2, XHHW-2, NM-B, etc.) to know its highest column.
4. Read the row for the conductor size	Rows run from small AWG sizes up through kcmil sizes. Where the row meets your column is the base (uncorrected) ampacity.
5. Apply correction and adjustment	Multiply the base value by the ambient-temperature correction factor and by the adjustment factor for more than three current-carrying conductors. For 90°C wire, start these calculations from the 90°C column.
6. Check the termination column	The final ampacity cannot exceed the value in the column matching the termination rating (110.14(C)) — usually 60°C or 75°C. Use the lower of the derated value and the termination-column value.
7. Check the table footnotes and 240.4	The table notes point to the small-conductor overcurrent limits in 240.4(D). Then apply 240.4 for the breaker: the conductor must be protected at its ampacity, with the next-size-up allowance only up to 800 A.

Worked examples

Example	Inputs	Steps	Result
A. 200 A feeder, no derating	Copper THWN-2 in conduit, 75°C lugs at both ends, 3 current-carrying conductors, 30°C ambient.	Terminations are 75°C, so read the copper 75°C column. A 3/0 Cu conductor in that column is 200 A per Table 310.16 — it matches the 200 A breaker without the next-size-up rule.	3/0 Cu
B. Six #12 in one conduit	Six current-carrying 12 AWG copper THHN in one EMT, 30°C ambient, 20 A breakers, 75°C terminations.	THHN is 90°C wire, so start at the 90°C column: 12 AWG Cu is 30 A. Six conductors take an 80% adjustment (310.15(C)(1)): 30 × 0.80 = 24 A. The termination-column check also exceeds 20 A, and 240.4(D) limits 12 AWG copper to a 20 A device.	20 A circuits OK

Values are quoted only to illustrate the calculation. Look up your own conductor in the official table for your adopted edition.

Common field mistakes

Mistake	Why it happens	Fix
Using the 90°C column as the final answer	THHN is 90°C wire, but most breakers and lugs are rated 75°C (or 60°C on small equipment).	Use the 90°C column only as the starting point for derating; cap the result at the termination-column value.
Skipping ambient correction	Rooftops, attics, boiler rooms, and hot process areas routinely exceed the 30°C table basis.	Apply the 310.15(B) correction factor for the actual ambient — and check the rooftop sunlight rules for raceways on roofs.
Miscounting current-carrying conductors	Adjustment factors kick in at four or more. Neutrals sometimes count (e.g., shared neutrals on wye systems with nonlinear load); equipment grounds never do.	Count per 310.15(E) and (F), then apply 310.15(C)(1).
Ignoring 240.4(D) on small wire	The table shows small conductors carrying more than their breaker is allowed to be.	Size the breaker from 240.4(D) for 14–10 AWG copper (and the aluminum equivalents), not from the table cell.
Reading the copper half for aluminum	Aluminum needs a larger size for the same ampacity.	Read the aluminum half, and verify the lugs are rated for aluminum (AL/CU).
Using 310.16 for cords, free air, or medium voltage	Other conditions have their own tables.	Flexible cords use Article 400; single conductors in free air use 310.17; over 2000 V use the medium-voltage article.
Applying the dwelling rule to commercial feeders	The 83% dwelling shortcut in 310.12 only applies to 120/240 V dwelling services and the main feeder carrying the whole dwelling load.	For everything else, size straight from 310.16 with corrections.

Which section applies?

Situation	Where to look
Building wire in conduit, cable, or earth, ≤ 3 current-carrying conductors	Table 310.16
Single insulated conductors in free air (e.g., on insulators, in cable tray per tray rules)	Table 310.17 (and the cable-tray article, 392.80)
Single-family / dwelling-unit 120/240 V service or main feeder	310.12 — size at 83% of the service rating, then use 310.16
Ambient above 30°C	310.15(B) correction factors (2020/2023 numbering)
Four or more current-carrying conductors together	310.15(C)(1) adjustment factors
Conductors over 2000 V	The medium-voltage conductor article (311 in 2020; 315 in 2023)
Flexible cords and cables	Article 400 ampacity tables

What changed between editions

Edition	What to know
2017 NEC	The ampacity table was numbered Table 310.15(B)(16); correction and adjustment tables were 310.15(B)(2)(a) and 310.15(B)(3)(a); the dwelling table was 310.15(B)(7).
2020 NEC	Article 310 was reorganized: the table became Table 310.16, ambient correction moved to 310.15(B), adjustment to 310.15(C)(1), and the dwelling table was replaced by the 83% method in 310.12. Medium-voltage conductors moved to a new Article 311.
2023 NEC	Kept the 2020 numbering for 310.16 and its companion sections; medium-voltage conductors moved to Article 315. Check the edition's change markers for any revised footnotes.
2026 NEC and later	Being adopted state by state. Always confirm the section numbers and footnotes in the edition your AHJ enforces.

Summary of numbering changes for orientation only — confirm in the edition your jurisdiction has adopted.

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When you need Table 310.16

Any time you size a branch circuit, feeder, or service conductor, NEC Table 310.16 is where the ampacity comes from. It answers one question — how much current a given conductor can carry continuously under standard conditions — and everything else (derating, terminations, breaker size) is applied on top of that base value.

The most common error is reading one cell and stopping. The table value is only the start: correct for ambient temperature, adjust for bundled conductors, cap at the termination rating, and then pick the breaker under 240.4. Work it in that order and the conductor will pass inspection and run cool.

Matching wire to a specific breaker? See how to pick wire size for a breaker, the correction & adjustment factor guide, and the equipment grounding conductor guide. Check long runs with the voltage drop calculator. Field PM keeps panel schedules, QA/QC, and field documentation together in the job book.

FAQ

What is NEC Table 310.16?+

It is the NEC table of allowable ampacities for insulated conductors rated up to 2000 volts, installed in raceway, cable, or directly buried. Its values assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors together. It is the primary table for sizing building wire. You can read it for free in the NFPA online viewer at nfpa.org/freeaccess.

Which temperature column do I use?+

The one matching the lowest-rated part of the circuit — usually the terminations. NEC 110.14(C) generally limits circuits of 100 A or less (or 14 AWG through 1 AWG) to the 60°C column unless the equipment is listed and marked for 75°C, and larger circuits to the 75°C column. The 90°C column is a starting point for derating 90°C-insulated wire, not a final termination ampacity.

Why are 14, 12, and 10 AWG copper limited below the table value?+

NEC 240.4(D) caps the overcurrent device on small conductors regardless of what Table 310.16 shows. For example, 12 AWG copper is limited to a 20 A device even though its 90°C value is 30 A. Check 240.4(D) in your adopted edition for the full list.

Do I need to derate ampacity?+

Yes, whenever conditions differ from the table basis. Ambient temperatures above 30°C require a correction factor (310.15(B) in the 2020/2023 NEC), and more than three current-carrying conductors in a raceway or cable require an adjustment factor (310.15(C)(1)). Example: six current-carrying conductors take an 80% adjustment.

Where can I see the full 310.16 table for free?+

NFPA publishes NFPA 70 (the NEC) in a free, read-only online viewer at nfpa.org/freeaccess — create a free account, open NFPA 70 for your adopted edition, and go to Article 310. Your local code book or AHJ-adopted edition is the authority.

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