What Size Wire Do You Need for a 30-Amp Breaker?
ZacharyWilliamMost of the time, a 30A breaker pairs with #10 AWG copper (or #10 AWG aluminum where permitted), but the “right” wire size depends on cable type, terminals, derating, and voltage drop. This guide breaks it down in plain English with tables you can actually use.
Quick answer: For typical residential branch circuits, use #10 AWG copper on a 30A breaker. Aluminum (or copper-clad aluminum) often requires larger conductors for equivalent performance, and local rules/terminal ratings matter.
Safety note: Electrical work can be hazardous. Follow your local code and manufacturer instructions, and hire a licensed electrician if you’re not qualified to work inside panels.
1) The standard wire size for a 30A breaker
For most household branch circuits, a 30-amp breaker is paired with #10 AWG copper. This is the common pairing you’ll see for things like a dedicated RV receptacle (TT-30), some air conditioners, or small water heaters.
| Conductor material | Typical minimum for a 30A breaker | What you’ll commonly buy | Notes |
|---|---|---|---|
| Copper | #10 AWG | 10/2 w/ground (120V circuits) or 10/3 w/ground (240V circuits needing neutral) | “10 gauge for 30 amps” is the standard rule-of-thumb in U.S. residential wiring. Upsize for long runs or derating conditions. |
| Aluminum / Copper-Clad Aluminum | #10 AWG is shown at 30A in common ampacity charts | Less common for short branch circuits; more common in larger feeders | Aluminum requires correct terminals, antioxidant (where required), and proper torque. In practice, many electricians prefer copper for 30A branch circuits. |
Don’t mix this up: breaker size does not automatically mean the wire is safe in all conditions. Ampacity can be reduced by heat, bundling, or conduit fill—so “#10 for 30A” is the starting point, not the finish line.
2) Why #10 copper is the “default” (NEC context)
There are two big reasons #10 copper is the standard for a 30A breaker:
- Small-conductor protection limits: The NEC restricts overcurrent protection for small conductors (commonly referenced as NEC 240.4(D)), which is why you don’t normally see a breaker larger than 30A protecting #10 conductors. See the NEC notes summarized by Schneider Electric (source).
- Terminal temperature ratings matter: Many devices and breakers are installed under 60°C/75°C terminal rules. Even if a conductor insulation rating looks high, the usable ampacity can be limited by how the terminals are rated/installed.
In plain terms: even though some charts show #10 copper can carry more than 30A under certain insulation/conditions, a 30A breaker is the standard match for #10 in most typical branch-circuit installations.
Continuous-load reminder: If a load runs for long periods (often defined as 3+ hours in code discussions), circuit and breaker sizing typically uses a 125% factor for the continuous portion. Schneider summarizes this under NEC 210.20(A) (source). Practically, many people treat a 30A circuit as “about 24A continuous” unless equipment is listed otherwise.
3) Wire size by wire/cable type (NM-B vs THHN vs UF-B)
A lot of confusion comes from mixing “wire size” (AWG) with “wire type” (NM-B, UF-B, THHN/THWN, etc.). The gauge might be the same (#10), but the installation method changes what you buy and how it’s rated.
| Wire gauge | 60°C column (often used for NM-B / UF-B in many residential contexts) | 75°C column (THW/THWN/USE/SE, etc.) | 90°C column (THHN/THWN-2, etc.) | What this means for a 30A breaker |
|---|---|---|---|---|
| #10 copper | 30A | 35A | 40A | #10 copper remains the standard choice for a 30A breaker |
| #8 copper | 40A | 50A | 55A | Often chosen when you need margin (long runs, heat, bundling) |
What to buy, in real terms:
-
NM-B (Romex) in walls: typically
10/2 w/ground(120V) or10/3 w/ground(240V with neutral). - THHN/THWN in conduit: you’ll buy individual conductors (often #10 copper THHN/THWN) plus a ground, sized and colored appropriately.
-
UF-B outdoors / underground: often
10/2 UF-Bor10/3 UF-Bdepending on circuit needs, with burial depth/protection rules applied.
4) 120V vs 240V: same gauge, different cable
Whether a 30A circuit is 120V or 240V, the wire gauge requirement is driven by amperage—so it’s still commonly #10 copper. What changes is how many conductors you need.
| Use case | Voltage | Breaker | Typical cable | Common receptacle | Notes |
|---|---|---|---|---|---|
| RV 30A shore power outlet | 120V | 30A single-pole | 10/2 w/ground | TT-30R | Hot + neutral + ground (no second hot). Don’t confuse with a dryer outlet. |
| Water heater (typical) | 240V | 30A double-pole | 10/2 w/ground | Hardwired | No neutral needed for many heaters (verify nameplate). |
| Dryer (modern 4-wire) | 240V | 30A double-pole | 10/3 w/ground | NEMA 14-30R | Two hots + neutral + ground. |
| Small compressor / A/C circuit | 120V or 240V | 30A (varies) | #10 copper (type varies) | Often hardwired | Motor starting current can push you to upsize for performance. |
5) How far can you run #10 at 30 amps? (voltage drop table)
Even if #10 copper is code-acceptable for a 30A breaker, long runs can cause voltage drop. Many electricians aim for about 3% drop on a branch circuit (good practice), especially for sensitive electronics and motor loads.
The table below assumes a full 30A load and uses commonly referenced resistance values: #10 copper resistance at 75°C is often cited from NEC conductor property tables (example shown by Mike Holt) and larger-gauge resistance values are available from Southwire engineering data for THHN/THWN conductors. See: Mike Holt (NEC Ch. 9 Table examples) and Southwire THHN/THWN-2 copper data.
| Copper conductor | Approx. max one-way length @ 120V, 30A | Approx. max one-way length @ 240V, 30A | Good takeaway |
|---|---|---|---|
| #10 AWG | ~48 ft | ~97 ft | Great for short runs; long runs can feel “weak” under heavy loads. |
| #8 AWG | ~76 ft | ~153 ft | A common upgrade when the panel is far away. |
| #6 AWG | ~121 ft | ~242 ft | Helps if you expect sustained near-maximum draw. |
| #4 AWG | ~194 ft | ~387 ft | Usually overkill for most 30A branch circuits, but useful for very long runs. |
Rule of thumb: If your 30A receptacle is far from the panel and you expect heavy loads, upsizing from #10 to #8 (or even #6) can reduce nuisance dimming, warm cords, and equipment stress.
6) When you should upsize (continuous loads, derating, heat)
You may need to go bigger than #10 even on a 30A breaker if the installation reduces ampacity or increases losses. Here are the most common reasons:
- Continuous loads: If the load is expected to run for hours at a time, design margin matters (often treated as ~24A continuous on a 30A circuit per 125% sizing concepts).
- More than 3 current-carrying conductors: Bundling several circuits in one conduit can require adjustment factors.
- High ambient temperatures: Hot attics, rooftop conduit, or mechanical rooms can reduce allowable ampacity.
- Motor/compressor starts: Voltage drop hits hardest during startup, so upsizing can improve reliability.
- Aluminum conductors: Must be terminated correctly; upsizing is sometimes chosen for performance and durability.
| # of current-carrying conductors | Percent of table ampacity (typical reference) | Why it matters |
|---|---|---|
| 4–6 | 80% | Less ability to shed heat |
| 7–9 | 70% | Heat buildup becomes significant |
| 10–20 | 50% | Often forces a conductor size increase |
7) Fast installation checklist (30A circuits)
- Confirm the load: nameplate amps, voltage (120 vs 240), and whether it needs a neutral.
- Pick the correct cable: 10/2 w/ground (common) vs 10/3 w/ground (if neutral is required).
- Match the receptacle to the breaker: don’t put 15A/20A receptacles on a 30A breaker.
- Mind distance: if the run is long, consider #8 or #6 for voltage drop performance.
- Use proper terminations: torque to spec; use AL-rated devices if using aluminum.
- Follow local requirements: GFCI/AFCI rules vary by location and application.
If you’re unsure: the safest move is to have a licensed electrician verify conductor type, insulation rating, terminations, and local code requirements before you buy materials.
8) Where UDPOWER fits (portable power & 30A circuits)
Many people research “30A breaker wire size” because they’re installing an RV inlet, a shop receptacle, or a generator-style connection. Even if you plan to use portable power (instead of utility power), the wiring rules for the circuit itself don’t change—you still size the wiring for the breaker and installation method.
For example, the UDPOWER S1200 is rated at 1200W AC output (2400W surge) with 1024Wh capacity, and supports up to 400W solar input (12V–75V, 12A) according to the product specs on UDPOWER’s site: UDPOWER S1200 product page. That’s plenty for many portable loads—but it’s not the same as a full 30A/120V shore service (which can be up to 3600W).
Practical tip: If you’re using any 30A-rated cords/adapters (like RV-style cords), choose cables built with 10 AWG conductors and keep them as short as practical to reduce voltage drop and heat. (Cord selection is separate from permanent building wiring—follow the cord and device manufacturer instructions.)
Product mention is for planning context only—always match your system to your real load requirements.
9) FAQ
Is 10/2 wire good for a 30-amp breaker?
Often yes—10/2 w/ground is commonly used for a 30A circuit when you don’t need a neutral (many 240V loads like water heaters), or for a 120V 30A TT-30 RV outlet (hot + neutral + ground).
Do I ever need 8 gauge wire on a 30A breaker?
You might choose #8 copper to reduce voltage drop on long runs, or when derating conditions apply (heat, bundling in conduit). It’s also a common “margin” upgrade for better real-world performance.
Can I put a 30A receptacle on a 20A breaker (or vice versa)?
Receptacles and breakers should be matched appropriately for the circuit design and code rules. In practice: don’t put 15A/20A outlets on a 30A breaker, and don’t expect a 20A breaker to support a true 30A load.
Does wire size change if it’s 240V instead of 120V?
The gauge is driven mainly by amps, not volts—so 30A is still commonly #10 copper. But 240V often changes the number of conductors (two hots, sometimes a neutral, plus ground).
What’s the biggest mistake people make with RV 30A wiring?
Confusing TT-30 (120V) with dryer-style 30A (240V) receptacles. They are not wired the same, and mixing them can damage equipment. A TT-30 product listing example is shown by Legrand (source).
Sources
- Cerrowire ampacity chart (copper & aluminum): view
- Helukabel allowable ampacity tables (NEC table references + adjustment factors): PDF
- Schneider Electric NEC information of note (includes NEC 210.20(A), 240.4(D) discussion): view
- TT-30 receptacle listing example (wire size range, 125V/30A TT-30): Legrand
- Conductor resistance references used for voltage drop planning: Mike Holt (NEC Ch. 9 examples), Southwire THHN/THWN-2 data
- UDPOWER S1200 product specifications: view
Last updated: December 22, 2025. Always verify requirements against your local code authority and the latest NEC edition used in your area.
