The Correct Wire Gauge for 30 Amp Breakers at 120V and 240V, and the Mistakes That Cause Overheating
What size wire for a 30 amp circuit breaker? For copper conductors, the standard answer is 10 AWG. That single fact covers most residential situations, but the right 30 amp wire size still depends on whether you’re wiring a 120V or 240V circuit, how far the run is, and whether the conductor is copper or aluminum.
This 30 amp breaker wire size question comes up constantly for RV shore power hookups, well pump and submersible pump circuits, electric water heaters, window air conditioners, and subpanel feeds — and getting the wire gauge for 30 amp circuits wrong is one of the most common (and most dangerous) DIY electrical mistakes.
This guide breaks down the correct wire size for a 30 amp breaker at both 120V and 240V, explains the difference between 10/2 and 10/3 wire, and covers the code reasoning behind why 10 gauge wire is the standard for 30 amp circuits.
What Size Wire Do You Need for a 30 Amp Breaker?
The core rule behind every wire size for 30 amp breaker question is ampacity — how much current a conductor can safely carry without overheating. Copper 10 AWG wire is rated for 30 amps under standard NEC tables, which is why it’s the go-to answer for what gauge wire for 30 amp circuits almost everywhere.
30 Amp Wire Size Chart
| Wire Gauge | Material | Typical Ampacity |
|---|---|---|
| 12 AWG | Copper | 20 Amps — undersized for a 30A breaker |
| 10 AWG | Copper | 30 Amps — standard for a 30A circuit |
| 8 AWG | Copper | 40–50 Amps — oversized but acceptable |
| 8 AWG | Aluminum | 30 Amps — aluminum equivalent to 10 AWG copper |
30 Amp Wire Size for 120V vs. 240V Circuits
The wire gauge itself — 10 AWG copper — doesn’t change between 120V and 240V, but the cable configuration does. Ampacity is about current, not voltage, so the same 30 amp wire size applies to both; what changes is how many conductors the cable needs.
10/2 vs. 10/3 Wire — Which Do You Need?
10/2 wire has two current-carrying conductors plus ground and is used for straight 240V circuits with no neutral required — think well pump or submersible pump circuits and some water heaters. 10/3 wire adds a neutral conductor and is used when a circuit needs both 120V and 240V, such as an electric dryer or a 30 amp RV service pedestal that also needs 120V for lighter loads.
Common 30 Amp Circuit Applications
RV Shore Power (30 Amp Service)
Standard 30 amp RV wire size is 10 AWG, typically as 10/3 cable to supply both 120V and the shared neutral RV pedestals need.
Well Pump and Submersible Pump Circuits
Most residential well pump wire runs on a dedicated 30 amp, 240V circuit using 10/2 cable, since pumps don’t need a neutral conductor.
Electric Water Heaters
Many electric water heaters run on a 30 amp, 240V circuit with 10/2 wire, though always confirm against the unit’s nameplate rating.
Subpanel Feeds
A 30 amp subpanel feed typically uses 10/3 wire so the subpanel can distribute both 120V and 240V circuits downstream.
Copper vs. Aluminum Wire for 30 Amp Circuits
Aluminum wire needs a larger gauge than copper to carry the same current safely, since it has higher resistance. For a 30 amp circuit, that generally means 8 AWG aluminum instead of 10 AWG copper. If you’re working with an older aluminum-wired MCB (miniature circuit breaker) panel, confirm the breaker itself is also rated for aluminum conductors, since not all are.
How to Confirm You Have the Right Wire Size
Tools You’ll Need
- Wire Gauge Tool or Caliper – Measures the conductor if the jacket isn’t labeled
- Flashlight – For reading the gauge and type printed on the cable jacket
- Non-Contact Voltage Tester – Confirms the circuit is de-energized before inspecting it
- Manufacturer’s Nameplate Data – Confirms the actual amperage the appliance requires
Step 1: Check the cable jacket for its printed gauge and conductor count (for example, “10/2” or “10/3”). Step 2: Confirm the wire is rated 10 AWG copper or 8 AWG aluminum before connecting it to a 30 amp breaker. Step 3: For long runs — beyond roughly 100 feet — consider upsizing the wire to offset voltage drop. Step 4: Match the wire’s insulation type (THHN, NM-B/Romex, etc.) to the installation environment, since ampacity ratings vary by insulation and conduit fill. If the breaker trips as soon as you restore power, see our guide on whether circuit breakers can reset themselves before assuming the wire size is the problem.
Common Mistakes to Avoid
- Installing a 30 amp breaker on 12 or 14 gauge wire that’s only rated for 15–20 amps
- Using 10/2 wire on a circuit that actually needs a neutral, like a dryer or mixed-voltage RV pedestal
- Assuming aluminum and copper wire of the same gauge carry the same current — see our guide on breaker load capacity for how ampacity math works
- Ignoring voltage drop on long wire runs to a well pump, subpanel, or detached structure
- Mixing wire insulation types without checking their ampacity rating for the installation method
When to Call a Licensed Electrician
You should contact a professional if:
- You’re adding a brand-new 30 amp circuit rather than replacing existing wire like-for-like
- The run is long enough that voltage drop could be a concern
- You’re not sure whether existing wiring is aluminum or copper
- You’re feeding a subpanel or a detached structure, which involves additional code requirements
- A breaker trips immediately after installing new wiring — see our guide on how to change a circuit breaker safely for troubleshooting steps
Frequently Asked Questions
What Size Wire Do I Need for a 30 Amp Breaker?
10 AWG copper wire, or 8 AWG aluminum wire, is the standard size for a 30 amp circuit under typical NEC ampacity ratings.
Can I Use 12 Gauge Wire on a 30 Amp Breaker?
No. 12 AWG copper is rated for 20 amps, which is undersized for a 30 amp breaker and creates a fire risk from overheating before the breaker trips.
What Size Wire Do I Need for a 30 Amp 240V Circuit?
Still 10 AWG copper. Whether you need 10/2 or 10/3 wire depends on whether the circuit also requires a neutral conductor for 120V loads alongside the 240V.
Does 10 Gauge Wire Need to Be Upsized for Long Runs?
For runs beyond roughly 100 feet, voltage drop becomes more significant, and upsizing to 8 AWG is often recommended even though 10 AWG technically meets the ampacity requirement. See our guide on installing a new circuit breaker and outlet safely for related wiring considerations.
Need Reliable Breakers for Your 30 Amp Circuit?
Dvolt manufactures MCBs, MCCBs, and RCCBs engineered for consistent performance across residential and commercial panels alike.
Contact Our Team Browse Circuit BreakersFinal Thoughts
The wire size for a 30 amp breaker comes down to one core number: 10 AWG copper, or 8 AWG aluminum. What changes between installations is the cable configuration — 10/2 for straight 240V loads like a well pump, 10/3 when a neutral is needed for mixed-voltage equipment like a dryer or RV pedestal.
Always size the wire to the breaker, never the other way around, and confirm the conductor material and run length before finalizing an installation. If anything about the existing wiring or the intended load is unclear, a licensed electrician can confirm the safest setup for your specific circuit.
