TradeCalcs

Wire Size Calculator — Ampacity with NEC 240.4(D) Caps

Who this is for: For electricians laying out branch circuits who want the Table 310.16 answer with the 240.4(D) small-conductor limits baked in — no flipping between tables mid-job.

Enter load current and terminal temperature rating — get the minimum AWG by ampacity, with the 240.4(D) small-conductor caps already applied.

Quick answer: Wire size by ampacity: pick the smallest Table 310.16 conductor that meets your load at the terminal temperature rating, then apply the 240.4(D) caps — 14 AWG max 15 A, 12 AWG max 20 A, 10 AWG max 30 A. A 50 A load at 75°C lands on 8 AWG copper (table value exactly 50 A).

Amps the circuit carries at steady state.

Temperature rating
Minimum size
10 AWG
Table 310.16 @ 75°C
Effective ampacity
30 A
table value
Standard breaker pair
30 A
next standard size at or below
SizeTable @ 75°C240.4(D) capEffective
14 AWG20 A15 A15 A
12 AWG25 A20 A20 A
10 AWG35 A30 A30 A
The 240.4(D) caps are why “12 AWG = 25 A at 75 °C” still can’t feed a 25 A breaker: everyday branch circuits cap 14 AWG at 15 A, 12 AWG at 20 A, 10 AWG at 30 A. Exceptions (motor circuits per Article 430) exist — this tool covers the everyday rule.
Ampacity pick only — Table 310.16 with 240.4(D) small-conductor caps. Long runs also need the 3% voltage-drop check; ambient/bundle derating stacks separately.
Core facts
Ampacity tableNEC Table 310.16 (60/75/90°C)
240.4(D) caps14 AWG → 15 A · 12 AWG → 20 A · 10 AWG → 30 A
Working column75°C for most terminations
Example50 A @ 75°C → 8 AWG copper
Not includedVoltage drop, ambient/bundle derating — check separately
StatusFigures follow NEC 2017+ reference tables (310.16 ampacity, Chapter 9 conduit fill, 314.16 box fill, 240.4(D) small-conductor caps) and standard equal-friction duct sizing, compiled September 2026. Estimates only — final designs belong to a licensed electrician / HVAC designer per local code.

How does this wire size calculator pick a size?

It reads NEC Table 310.16 at the insulation temperature you choose — 60°C (older NM terms), 75°C (most modern terminations) or 90°C (heat-rated cable, but terminations usually still govern at 75°C) — and finds the smallest conductor whose allowable ampacity meets your load. Then it applies 240.4(D), the small-conductor rule that caps 14 AWG at 15 A, 12 AWG at 20 A and 10 AWG at 30 A regardless of the table figure. The result shows both the table value and the effective cap, so you can see when 240.4(D) is the binding constraint. Note this is the ampacity pick only — long runs also need the voltage-drop check above, and derating for ambient temperature or bundled conductors stacks on top.

Common uses

  • Picking branch-circuit conductors for a named breaker size
  • Sanity-checking a plan marked with generous but oversized copper
  • Explaining to an apprentice why 12 AWG can't feed a 25 A breaker even though the table says 25 A
  • Comparing 60°C vs 75°C terminals on replacement equipment

Frequently Asked Questions

What is NEC 240.4(D)?
The small-conductor rule: after all other corrections, 14 AWG copper may not protect above 15 A, 12 AWG above 20 A, 10 AWG above 30 A. It overrides the higher ampacity figures in Table 310.16 (which show 14 AWG at 20 A in the 75°C column) for everyday branch circuits.
Which temperature column should I use?
Use the lowest of: conductor insulation rating, termination rating, or the device rating. Most residential breakers and receptacles are 75°C or less, so the 75°C column is the default working answer. The 90°C column is mainly for derating starting points.
Does this account for voltage drop?
No — ampacity and voltage drop are separate checks. A 15 A circuit 200 ft long passes ampacity on 14 AWG but badly fails the 3% drop test. Run the voltage drop calculator for distance-sensitive circuits and take the larger of the two sizes.
Where's the aluminum table?
Aluminum carries less current per size — a rough equivalence is two trade sizes up from copper (8 AWG Cu ≈ 6 AWG Al). Use the voltage drop calculator's material selector for the aluminum numbers until the dedicated aluminum table lands.
What about derating for attic heat or conduit fill?
Multiply the table ampacity by the ambient-temperature and bundling correction factors before comparing to your load. This calculator gives the clean-table answer; derated designs need those factors applied first.

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