The baseline
10 AWG copper: the 30 amp standard
NEC Table 310.16 rates 10 AWG copper at 30 amps for 60°C insulation (NM-B) and 35 amps at 75°C
(THHN/THWN). For a 30 amp breaker, 10 AWG meets the minimum ampacity under standard conditions: no
more than three current-carrying conductors, 30°C ambient temperature, and typical installation.
For aluminum conductors, you need 8 AWG to reach 30 amps at 60°C. Aluminum is
less common for 30 amp branch circuits but is sometimes used for longer feeder runs where cost
savings justify the larger conduit.
Common applications
Where 30 amp circuits show up
30 amp circuits serve several standard residential and commercial loads:
-
Electric dryers — the most common 30 amp load in residential work. Typically a
240V circuit with 10/3 NM-B cable. Most dryer runs are short (under 30 ft from panel) so voltage
drop is rarely an issue.
-
Window and wall air conditioners — larger units (18,000+ BTU) often require a
dedicated 30 amp, 240V circuit.
-
Water heaters — standard residential electric water heaters (4500W element)
draw about 18.75 amps at 240V, well within a 30 amp circuit. The breaker is sized at 125% for
continuous loads, requiring a 25 or 30 amp breaker.
-
Small sub-panels — a 30 amp feeder to a small outbuilding or workshop. These
runs can be long enough to make voltage drop the primary sizing concern.
-
RV outlets (TT-30) — the TT-30 receptacle is a 30 amp, 120V outlet standard for
RV hookups. Note that at 120V, voltage drop limits are reached much sooner than at 240V.
12 AWG warning
Why 12 AWG is not acceptable for 30 amps
12 AWG copper is rated for 20 amps at 60°C. Placing it on a 30 amp breaker exceeds the wire's
ampacity by 50%, creating a serious risk of overheating, insulation damage, and fire.
NEC 240.4(D) explicitly limits overcurrent protection for 12 AWG copper to 20 amps. This is a hard
code requirement, not a recommendation. Even if the actual load is below 20 amps, the breaker size
determines the minimum wire gauge.
Voltage drop
When to upsize beyond 10 AWG
At full 30 amp load with 10 AWG copper, voltage drop reaches the 3% recommendation at these
approximate one-way distances:
- 120V source: about 35 feet one-way.
- 240V source: about 70 feet one-way.
Beyond these distances, step up to 8 AWG copper or
6 AWG aluminum. For very long runs (100+ ft at 240V), you may need 6 AWG copper.
The RV outlet scenario (30 amp at 120V) is particularly tight — 35 feet is not much distance. If
the outlet is at the far end of a property, 8 AWG or even 6 AWG may be necessary.
Check your specific run length