Introduction
A breaker size is only as good as the hardware that implements it — the right breaker type for the location, the right wire for the breaker, and a panel the two are both listed for. Pick the wrong combination and even a perfect calculation fails inspection or, worse, fails silently in the wall. The Breaker Size Calculator tells you the rating and minimum wire gauge; this guide covers what to actually buy: breaker types, wire types, panel compatibility, and the small parts that make the difference between a clean install and a callback.
Prices below are typical US planning ranges and vary by region and brand. Compatibility notes reflect general NEC principles — your panel’s door label is the final authority on what it’s listed for.
Breaker Types
Standard thermal-magnetic breakers
The ordinary breaker: a thermal element trips on sustained overload, a magnetic element trips instantly on short circuit. Available single-pole (120 V) and double-pole (240 V). Typical cost: $5–$25. This is the default for locations where code doesn’t require additional protection — but in modern homes, that’s fewer locations than most people expect.
GFCI breakers
Ground-fault circuit interrupters trip on tiny current imbalances (around 5 mA) that indicate current leaking to ground — the shock-protection device. Required by NEC 210.8 in kitchens, bathrooms, garages, outdoors, laundry areas, basements, crawl spaces, and other specified locations. Typical cost: $40–$80. They need a neutral connection (including on 240 V circuits) and are physically larger than standard breakers.
AFCI breakers
Arc-fault circuit interrupters detect the arcing signatures of damaged wiring — the fire-protection device. Required by NEC 210.12 across most dwelling-unit living areas (bedrooms, living rooms, hallways, and more, depending on code cycle). Typical cost: $40–$120. Like GFCI breakers, they need proper neutral connections and don’t tolerate shared neutrals the way standard breakers do.
Dual-function (GFCI + AFCI) breakers
Combine both protections in one device — increasingly the default choice for new dwelling-unit circuits, since many locations now require both. Typical cost: $50–$150. When in doubt about which protection a location requires, the dual-function breaker is the safe specification; confirm with the code compliance guide or your inspector.
| Breaker type | Protects against | Typical locations | Typical cost |
|---|---|---|---|
| Standard | Overload, short circuit | Where no GFCI/AFCI required | $5–$25 |
| GFCI | Ground faults (shock) | Kitchen, bath, garage, outdoor, laundry, basement | $40–$80 |
| AFCI | Arc faults (fire) | Bedrooms, living areas, hallways | $40–$120 |
| Dual-function | Both | Locations requiring both | $50–$150 |
Panel Compatibility: The Non-Negotiable Rule
Breakers are not interchangeable between brands. NEC 110.3(B) requires equipment to be installed per its listing and labeling, and every panel’s door label lists the breaker types it’s rated for. A breaker that physically clips in but isn’t listed for the panel is a code violation and can void the panel’s UL listing — with real insurance consequences after a fire.
- Check the door label first. It lists approved breaker series (e.g., the manufacturer’s own line). Buy from that list.
- Watch for “classified” breakers. Some third-party breakers are UL-classified for specific competitor panels. These are legitimate only for the exact panels on their classification listing — not a general license to mix brands.
- Match the series, not just the brand. Manufacturers make multiple incompatible lines; the label specifies which.
- Older panels need extra care. Breakers for discontinued panels (Pushmatic, Zinsco, Federal Pacific) are expensive, hard to find, and in some cases the panel itself should be replaced rather than fed new breakers.
Wire Types and Sizes
Copper conductor sizing
For residential branch circuits, the NEC 240.4(D) small-conductor limits are the governing numbers for copper:
| Copper AWG | Max breaker (NEC 240.4(D)) | Common cable | Typical uses |
|---|---|---|---|
| 14 AWG | 15 A | 14/2 NM-B | Lighting, general receptacles |
| 12 AWG | 20 A | 12/2 NM-B | Kitchen/bath receptacles, heaters |
| 10 AWG | 30 A | 10/2 or 10/3 NM-B | Dryer, small water heater |
| 8 AWG | 40 A | 8/3 NM-B or THHN in conduit | Range, cooktop |
| 6 AWG | 50–60 A | 6/3 NM-B or THHN in conduit | Range, EV charger, subpanel |
Two caveats shrink these limits: high ambient temperature (hot attics) and conduit fill (many conductors in one conduit) both require derating the conductor’s ampacity. The table values never increase — conditions only reduce them. For long runs, also check voltage drop with the Voltage Drop Calculator, which can force a heavier gauge than the breaker minimum.
Cable and insulation types
- NM-B (“Romex”): the standard indoor residential cable — sheathed, easy to pull, dry locations only. The “/2” or “/3” counts current-carrying conductors plus ground (12/2 = two 12 AWG conductors + ground).
- THHN/THWN-2 in conduit: individual conductors pulled through conduit — required in some jurisdictions (notably Chicago and surrounding areas) and for wet, outdoor, or exposed locations.
- UF-B: underground feeder cable for direct burial to outbuildings, landscape lighting, and similar.
- Aluminum conductors: require larger gauges than copper for the same ampacity and listed connectors at every termination. If your existing wiring is aluminum, involve a licensed electrician — termination practices are unforgiving.
The Small Parts That Matter
- Boxes: every splice and device needs an accessible, properly sized box — box-fill calculations (NEC 314.16) limit how many conductors each box can hold. Plastic boxes are fine for NM-B; metal boxes need grounding.
- Connectors and clamps: cable entering a box or panel needs a listed connector to protect the sheathing from the knockout edge.
- Wire connectors: use listed wire nuts or lever connectors sized for the wire combination; never tape-only splices.
- Torque tools: NEC 110.14(D) requires terminations tightened to the manufacturer’s specified torque — a torque screwdriver is a real tool, not a luxury.
- Labels: NEC 408.4 requires circuit identification. A label maker and an accurate panel directory are the cheapest inspection insurance available.
For what the full job typically costs with these materials, see the cost guide. For the habits that keep an installation clean, the pro tips guide covers planning practices the parts list can’t.
Related Guides
- Pro Tips — planning habits that make the materials perform: torquing, labeling, honest load classification.
- Breaker Installation Cost Guide — typical price ranges for circuits, swaps, and panel upgrades.
- Code Compliance Guide — where GFCI and AFCI are required and what inspectors check.
- Complete Breaker Sizing Guide — the sizing method that produces the numbers on your shopping list.
- Breaker Sizing Checklist — verify breaker, wire, and panel compatibility before buying.
Frequently Asked Questions
Can I mix breaker brands in my panel?
Only breakers listed for your specific panel (per its door label) or UL-classified for that exact panel model. A breaker that merely fits is a code violation.
Is 12/2 Romex enough for a 20-amp circuit?
Yes — 12 AWG copper is the minimum for a 20 A breaker per NEC 240.4(D), and 12/2 NM-B is the standard indoor cable for the job. Longer runs may need heavier wire for voltage drop.
Do I need GFCI, AFCI, or both?
It depends on the location: GFCI where shock risk dominates (wet areas, outdoors, garage), AFCI across most living areas, both where the requirements overlap. Dual-function breakers cover both. Check the code compliance guide for the location list.
Why are AFCI breakers so much more expensive?
They contain arc-detection electronics that a standard thermal-magnetic breaker doesn’t. The cost reflects the protection: detecting damaged-wiring arcs that standard breakers can’t see.
Get Your Number
Once you know the breaker size and minimum wire gauge from the Breaker Size Calculator, this guide turns those numbers into a shopping list: the right protection type for the location, the right cable for the environment, a breaker listed for your panel, and the small parts that make the install clean. Buy once, to the label — the most expensive breaker is the one you have to replace after the inspection.
Disclaimer: Product types and prices are general planning information and vary by region, brand, and code cycle. Compatibility is governed by your panel’s listing label and local amendments. Verify your materials list with a licensed electrician and your local authority having jurisdiction (AHJ) before purchasing.
