Wire Gauge Charts Editorial · October 19, 2026
Wire gauge chart amps: what 15, 20 and 30 amperes really ask of your wire
Every wire size question eventually becomes an amps question. How many amperes can this cable carry? Which gauge do I need for a 20-amp breaker? Why does one website say 12 AWG is good for 20 amps and another says 25? The answers all come from the same two tables — and the wire gauge chart amps page keeps them side by side. This post walks through how to read them and where beginners go wrong.
Two tables, one wire
The first table is ampacity: how much continuous current a conductor can carry before its insulation cooks. The second is overcurrent protection: the breaker or fuse size that code allows on that conductor. They are related but not identical, which is the root of most confusion.
Ampacity comes from NEC Table 310.16, printed for three insulation temperatures. A copper 12 AWG conductor in a raceway carries 20 A in the 60 °C column, 25 A at 75 °C, and 30 A at 90 °C. Same copper, three numbers. The insulation — THWN-2, XHHW-2, old NM-B — decides which column applies to your run.
Overcurrent protection in small residential sizes obeys NEC 240.4(D), the famous “small conductor rule”. It caps protection below the theoretical ampacity: 15 A for 14 AWG, 20 A for 12 AWG, 30 A for 10 AWG. That is why household answer is “20-amp breaker, 12 gauge wire” even though the copper itself holds 25 A. The cap exists because terminations, devices and everyday handling errors eat into the margin.
The lookups that cover 90% of questions
Working from the electrical wire gauge chart sizes, these pairings answer most home questions:
| Load | Breaker | Copper |
|---|---|---|
| Lighting circuit | 15 A | 14 AWG |
| Kitchen/bath receptacles | 20 A | 12 AWG |
| Bathroom fan + heat, small workshop | 20 A | 12 AWG |
| Water heater, dryer | 30 A | 10 AWG |
| Range (sub-branch), small oven | 40 A | 8 AWG |
| Range, cooktop | 50 A | 8 AWG (6 AWG for margin) |
| Sub-panel feeder | 60 A | 6 AWG |
| Whole service | 100 A | 4 AWG copper / 2 AWG aluminium |
| Whole service | 200 A | 4/0 copper / 250 kcmil aluminium |
Memorize three anchors — 15/14, 20/12, 30/10 — and interpolate the rest with the full gauge chart, which doubles the copper area roughly every three gauges.
Aluminium answers shift two sizes
Aluminium conducts about 61–64% as well as copper by area, so every aluminium lookup steps up several gauges. In practice aluminium has left the small branch sizes entirely — today’s market is effectively copper-only below feeder sizes, partly because standard aluminium ampacity entries begin at feeder territory, and partly because of the termination reputation built in the 1960s–70s. Modern aluminium lives where big cross-section pays: service entrances and feeders. A 100 A service runs 4 AWG copper or 2 AWG aluminium; 200 A steps to 4/0 copper or 250 kcmil aluminium. The rule that keeps aluminium safe is terminations, not size: AL/CU-rated lugs, anti-oxidant paste, and a torque wrench at every landing.
The trap: ampacity is not the only limit
A wire sized purely by amps passes the heat test and can still fail the voltage test. Ampacity protects the conductor; voltage drop protects the load, and it grows with every foot of run. A 12 AWG, 20 A circuit is electrically fine at 40 ft and lossy at 100 ft — 4 V lost in round-trip resistance, which compressors, motors and dimmers all feel. Professional sizing therefore runs two checks: ampacity from the table, drop from length × Ω/ft × amps (the calculator does both in one pass and tells you which limit decided the size).
Three seconds of estimation help: copper resistance divides by two every ~3 gauges thicker, so stepping one gauge buys ~26% more allowable length — three gauges doubles it. If your run loses more than 3% at the table’s size, step up and re-check.
Where the ampacity numbers come from
The 310.16 values assume “raceway” conditions: continuous conductors in conduit, 30 °C ambient, three current-carrying wires. Change any assumption and correction factors stack — hot attics derate every conductor’s rating, bundled or heavily packed runs add more, and continuous loads must be sized at 125%. Automotive, marine and solar worlds run a different table because open wire in free air cools differently from wire in a bundle; chassis free-air limits (on the 12V chart) are much higher for short runs but carry no code authority.
Bottom line: use the amps chart to confirm the size the breaker needs, then confirm your run length still fits the voltage budget. When both agree, the wire is sized — and when they disagree, the calculator shows you exactly where to spend one more gauge.