What is wire ampacity and why does it matter?

Ampacity is the maximum continuous current a conductor can carry without exceeding its temperature rating. Undersized wire overheats, degrades insulation, and creates a fire hazard. NEC Table 310.16 sets the minimum ampacity for every AWG conductor size.

What does NEC Table 310.16 say about wire sizing?

NEC Table 310.16 lists allowable ampacity for copper and aluminum conductors at 60°C, 75°C, and 90°C temperature ratings, based on 3 or fewer current-carrying conductors in a raceway at 30°C ambient. More conductors require derating per NEC 310.15(B)(3).

How do I use this wire size calculator?

Enter your load current in amps, choose copper or aluminum, select the conductor temperature rating, and enter the number of current-carrying conductors in the raceway. The calculator finds the minimum AWG from NEC Table 310.16, applies any required derating, and shows the NEC code citation.

What happens to wire ampacity when you bundle conductors in conduit?

When more than 3 current-carrying conductors share a raceway, ampacity must be derated per NEC 310.15(B)(3): 4–6 conductors = 80%, 7–9 = 70%, 10–20 = 50%, 21–30 = 45%, 31–40 = 40%, 41+ = 35%. The calculator applies this automatically.

Should I use copper or aluminum wire?

Copper is standard for branch circuits and has higher ampacity per AWG. Aluminum is common for large feeders and service conductors (100A+) because it is lighter and cheaper, but requires AL-rated terminations and anti-oxidant compound.

How is NEC wire ampacity calculated?

NEC Table 310.16 values assume 30°C ambient and no more than 3 current-carrying conductors. For higher ambients, apply correction factors from NEC Table 310.15(B)(1). For more than 3 conductors, apply adjustment factors from NEC 310.15(B)(3).

Frequently asked questions about wire size and ampacity

Common sizing: 20A circuit = 12 AWG copper (75°C). 30A circuit = 10 AWG copper (75°C). 50A circuit = 8 AWG copper (75°C). 100A feeder = 3 AWG copper or 1 AWG aluminum (75°C). 200A service = 3/0 AWG copper or 4/0 AWG aluminum (75°C). Always verify with NEC Table 310.16 and local amendments.

⚡ Electrical Calculator

Wire Size / Ampacity Calculator

NEC Table 310.16 — Copper & Aluminum · 60/75/90°C · Conduit derating per NEC 310.15(B)(3)

⚙️ Circuit Parameters

Load Current (Amps)

Conductor Material

Aluminum min: 12 AWG — use AL-rated terminations

Conductor Temperature Rating

Most equipment terminals are 75°C rated — use 75°C column unless terminations are verified 90°C per NEC 110.14(C)

Current-Carrying Conductors in Raceway (for derating)

NEC 310.15(B)(3) — Neutral conductors carrying only unbalanced current and EGCs do not count

Please enter a valid load current greater than zero.

Minimum Conductor Size

—

Table Ampacity

—

Derated Ampacity

—

Max OCPD

✓ Verify an Existing Wire Size

Wire Size to Check

Load Current (Amps)

Conductor Material

Temperature Rating

Current-Carrying Conductors in Raceway

Please select a wire size and enter a valid load current.

PASS

—

Load Current

—

Table Ampacity

—

Derated Ampacity

Size	Cu 60°C	Cu 75°C	Cu 90°C	Al 75°C	Max OCPD (Cu 75°C)
14 AWG	15 A	20 A	25 A	N/A	15 A *
12 AWG	20 A	25 A	30 A	20 A	20 A *
10 AWG	30 A	35 A	40 A	30 A	30 A *
8 AWG	40 A	50 A	55 A	40 A	50 A
6 AWG	55 A	65 A	75 A	50 A	60 A
4 AWG	70 A	85 A	95 A	65 A	80 A
3 AWG	85 A	100 A	115 A	75 A	100 A
2 AWG	95 A	115 A	130 A	90 A	110 A
1 AWG	110 A	130 A	145 A	100 A	125 A
1/0	125 A	150 A	170 A	120 A	150 A
2/0	145 A	175 A	195 A	135 A	175 A
3/0	165 A	200 A	225 A	155 A	200 A
4/0	195 A	230 A	260 A	180 A	225 A
250 kcmil	215 A	255 A	290 A	205 A	250 A
300 kcmil	240 A	285 A	320 A	230 A	275 A
350 kcmil	260 A	310 A	350 A	250 A	300 A
400 kcmil	280 A	335 A	380 A	270 A	325 A
500 kcmil	320 A	380 A	430 A	310 A	375 A

* NEC 240.4(D) hard limits: 14 AWG Cu = 15 A max, 12 AWG Cu = 20 A max, 10 AWG Cu = 30 A max. Max OCPD shown is the largest standard breaker size at or below the 75°C ampacity. Assumes ambient 30°C, 3 or fewer CCC.

💡

Need NEC code lookup on the job site?

VoltageLab's AI Code Lookup has every NEC article explained in plain English — searchable from your phone in seconds. Used by journeymen, masters, and exam candidates across the USA.

Download on iOS Download on Android

Based on NEC Table 310.16 (2023 NEC) — allowable ampacities for insulated conductors rated 0–2000V, in a raceway or cable, ambient 30°C (86°F). Derating per NEC 310.15(B)(3). Max OCPD per NEC 240.4 and 240.4(D). Results are indicative — always verify against the locally adopted NEC edition and consult a licensed electrician before installation.

Voltage Lab

Wire Size / Ampacity Calculator

ABOUT US

Tools

FOLLOW US