How do I size a circuit breaker?

Divide the load watts by voltage to get amps. For continuous loads (on 3+ hours), multiply by 1.25 per NEC 210.20. Round up to the next standard breaker size: 15, 20, 30, 40, 50, 60A, etc.

What size breaker do I need for a 240V appliance?

Divide watts by 240 to get amps. Multiply by 1.25 for continuous loads. For example, a 5,000W dryer: 5000/240 = 20.8A × 1.25 = 26A, so a 30A breaker is required.

When is a GFCI breaker required?

NEC requires GFCI protection in bathrooms, kitchens (within 6 ft of sink), garages, unfinished basements, outdoors, crawl spaces, and near pools/spas. GFCI breakers or GFCI outlets provide this protection.

What is an AFCI breaker?

AFCI (Arc Fault Circuit Interrupter) breakers detect dangerous arcing in wiring — the type of arc that causes electrical fires. NEC 2020 requires AFCI protection in bedrooms, living rooms, dining rooms, family rooms, and most other habitable areas.

Circuit Breaker Size Calculator — NEC Breaker Sizing

Circuit / Appliance	Breaker	Wire (Cu)	Voltage	Protection	Notes
General Lighting	15A	#14 AWG	120V	AFCI	Most rooms NEC 2020+
General Outlets	15–20A	#14–12 AWG	120V	AFCI	Habitable rooms
Kitchen Small Appliance (×2)	20A each	#12 AWG	120V	GFCI	NEC requires 2 minimum
Refrigerator (dedicated)	20A	#12 AWG	120V	GFCI	Dedicated circuit
Dishwasher (dedicated)	20A	#12 AWG	120V	GFCI	Dedicated circuit
Clothes Washer	20A	#12 AWG	120V	None	Dedicated circuit
Microwave (dedicated)	20A	#12 AWG	120V	None	Dedicated if 1,000W+
Bathroom	20A	#12 AWG	120V	GFCI	All bathroom outlets
Garage	20A	#12 AWG	120V	GFCI	All outlets
Outdoor Outlets	20A	#12 AWG	120V	GFCI	All outdoor locations
Electric Range	50A	#6 AWG	240V	None	Double-pole breaker
Electric Dryer	30A	#10 AWG	240V	None	Double-pole breaker
Electric Water Heater	30A	#10 AWG	240V	None	Dedicated, double-pole
Central AC (3-ton)	30–50A	#10–6 AWG	240V	None	Per nameplate MCA/MOCP
EV Charger (Level 2)	50–60A	#6–4 AWG	240V	GFCI	Dedicated circuit, 40A min
Hot Tub / Spa	50–60A	#6–4 AWG	240V	GFCI	GFCI required, dedicated
Subpanel Feed (100A)	100A	#4 AWG Cu	240V	None	Disconnect required at subpanel

Circuit / Appliance

Breaker

Wire (Cu)

Voltage

Protection

Notes

General Lighting

15A

#14 AWG

120V

AFCI

Most rooms NEC 2020+

General Outlets

15–20A

#14–12 AWG

120V

AFCI

Habitable rooms

Kitchen Small Appliance (×2)

20A each

#12 AWG

120V

GFCI

NEC requires 2 minimum

Refrigerator (dedicated)

20A

#12 AWG

120V

GFCI

Dedicated circuit

Dishwasher (dedicated)

20A

#12 AWG

120V

GFCI

Dedicated circuit

Clothes Washer

20A

#12 AWG

120V

None

Dedicated circuit

Microwave (dedicated)

20A

#12 AWG

120V

None

Dedicated if 1,000W+

Bathroom

20A

#12 AWG

120V

GFCI

All bathroom outlets

Garage

20A

#12 AWG

120V

GFCI

All outlets

Outdoor Outlets

20A

#12 AWG

120V

GFCI

All outdoor locations

Electric Range

50A

#6 AWG

240V

None

Double-pole breaker

Electric Dryer

30A

#10 AWG

240V

None

Double-pole breaker

Electric Water Heater

30A

#10 AWG

240V

None

Dedicated, double-pole

Central AC (3-ton)

30–50A

#10–6 AWG

240V

None

Per nameplate MCA/MOCP

EV Charger (Level 2)

50–60A

#6–4 AWG

240V

GFCI

Dedicated circuit, 40A min

Hot Tub / Spa

50–60A

#6–4 AWG

240V

GFCI

GFCI required, dedicated

Subpanel Feed (100A)

100A

#4 AWG Cu

240V

None

Disconnect required at subpanel

Breaker Size	Min. Wire (Cu)	Min. Wire (Al)	Common Use
15A	#14 AWG	N/A (Cu only)	Lighting, general outlets
20A	#12 AWG	#10 AWG	Kitchen, bathroom, appliances
30A	#10 AWG	#8 AWG	Dryer, water heater, AC
40A	#8 AWG	#6 AWG	Range, large AC
50A	#6 AWG	#4 AWG	Range, EV charger, spa
60A	#6 AWG	#4 AWG	Subpanel, large AC
100A	#4 AWG	#2 AWG	Subpanel, garage feed
150A	#1/0 AWG	#2/0 AWG	Service entrance
200A	#3/0 AWG	#4/0 AWG	Main service panel

Breaker Size

Min. Wire (Cu)

Min. Wire (Al)

Common Use

15A

#14 AWG

N/A (Cu only)

Lighting, general outlets

20A

#12 AWG

#10 AWG

Kitchen, bathroom, appliances

30A

#10 AWG

#8 AWG

Dryer, water heater, AC

40A

#8 AWG

#6 AWG

Range, large AC

50A

#6 AWG

#4 AWG

Range, EV charger, spa

60A

#6 AWG

#4 AWG

Subpanel, large AC

100A

#4 AWG

#2 AWG

Subpanel, garage feed

150A

#1/0 AWG

#2/0 AWG

Service entrance

200A

#3/0 AWG

#4/0 AWG

Main service panel

How to Size a Circuit Breaker in 3 Steps

Calculate Load Amps

Divide the load's wattage by the voltage: Amps = Watts ÷ Volts. A 1,800W baseboard heater on 120V draws 15 amps.

Apply Continuous Load Factor

If the load runs for 3+ hours continuously, multiply amps by 1.25 per NEC 210.20. This ensures the breaker operates at no more than 80% capacity.

Round Up to Standard Size

Choose the next standard breaker size above your design amps: 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 100A. Match wire gauge to breaker size per NEC 310.15.

Frequently Asked Questions

What size breaker do I need for a 1500-watt heater?

1,500W ÷ 120V = 12.5A. This is a continuous load (runs 3+ hours), so multiply by 1.25 = 15.6A. Round up: you need a 20-amp breaker with #12 AWG wire. Most baseboard heaters are on dedicated 20A circuits.

Can I use a 20-amp breaker with #14 wire?

No — this is a serious code violation and fire hazard. #14 AWG copper wire is rated for only 15 amps. A 20-amp breaker on #14 wire will allow the wire to overheat before the breaker trips. Always use #12 AWG minimum on 20-amp circuits.

How do I know if I need a single or double-pole breaker?

120V circuits use single-pole breakers (one hot wire). 240V circuits use double-pole breakers (two hot wires), which occupy two slots in your panel. Appliances like dryers, ranges, water heaters, and central AC require double-pole breakers.

What is the difference between a standard and AFCI breaker?

Standard breakers trip on overcurrent (overload or short circuit). AFCI breakers additionally detect series and parallel arc faults — dangerous arcing in damaged wiring that can start electrical fires. AFCI breakers cost more ($25–$50 each vs $5–$15) but are required in most living areas by NEC 2014+.

Do I need a dedicated circuit for my refrigerator?

The NEC recommends (but does not always require) a dedicated 20A circuit for the refrigerator. Many local codes do require it. A dedicated circuit prevents tripped breakers from interrupting the refrigerator, and GFCI protection is required for kitchen counter receptacles.

What size breaker do I need for a dryer?

Electric dryers typically require a 30-amp, 240V double-pole breaker with #10 AWG wire. Always verify the dryer's nameplate, as some larger dryers require 40A. Gas dryers only need a 120V/15-20A circuit for the motor and controls.

How many circuits can I run on a 200-amp panel?

A 200-amp panel typically has 40–42 circuit slots. Each single-pole breaker uses one slot; double-pole uses two. The 200A service limit is the total amperage draw, not the number of circuits. In practice, most circuits aren't running at full load simultaneously, so 40+ circuits is adequate for most homes.

Can I replace a 15-amp breaker with a 20-amp breaker?

Only if the wiring is #12 AWG or larger. If the circuit uses #14 AWG wire (the standard for 15A circuits), you must not upgrade to 20A. If nuisance tripping is the issue, investigate the actual load rather than simply upsizing the breaker.

What is an interlock kit vs. a transfer switch?

Both prevent backfeed from a generator to utility lines. An interlock kit is a mechanical device that prevents the main breaker and generator input breaker from both being on simultaneously — lower cost ($50–$100) but requires manual operation. A transfer switch ($200–$600+) is a dedicated switching device, sometimes with automatic operation (ATS).

When does a circuit require both GFCI and AFCI?

In NEC 2017+, kitchen counter receptacles require both AFCI and GFCI protection. Combination AFCI/GFCI breakers (available from most major brands) provide both in one device. Some areas also require dual protection in bathrooms and finished basements.

What is the maximum number of outlets on a 15-amp circuit?

NEC doesn't specify a maximum number of outlets per circuit, but a common rule of thumb is 8–10 outlets per 15A circuit (based on 1.5A per outlet × 80% = 12A for 15A circuit / 1.5 = 8 outlets). For rooms with heavy loads, fewer outlets per circuit is safer.

Does an air conditioner need its own dedicated circuit?

Yes — the NEC and virtually all AC manufacturers require a dedicated circuit for central AC and most window AC units over 7.5 amps. The breaker size must match the nameplate MOCP (Maximum Overcurrent Protection) rating, not simply the running amperage.

What is a tandem breaker and when can I use one?

Tandem (twin) breakers fit two 120V circuits in one panel slot. They're allowed only in panels rated for them (check the load center label for "CTL" rating). Tandem breakers help add circuits to a full panel, but they cannot be used everywhere — panels limit how many tandems are permitted.

How do I size a breaker for a motor?

NEC Article 430 governs motor circuits. The branch circuit breaker for a motor is sized at 250% of the motor's full-load current (for standard motors) to allow starting surges. This is different from continuous loads, which use the 125% rule. Always check the motor nameplate FLA (Full Load Amps).

Can a circuit breaker be too large for my wire?

Yes — oversized breakers are dangerous. The breaker's job is to protect the wire. A 30A breaker on #14 AWG wire (15A rated) means the wire will overheat to 200°F+ before the breaker trips. Always size the breaker at or below the wire's ampacity rating.

Key Terms

Ampacity — The maximum current a wire can carry continuously without exceeding its temperature rating. Determined by wire size, insulation type, and installation method.

Overcurrent Protection — A breaker or fuse that disconnects a circuit when current exceeds the safe limit, protecting wiring from overheating and fire.

Trip Curve — The relationship between overcurrent magnitude and the time it takes a breaker to trip. Motor circuits use breakers with slower trip curves to tolerate starting surges.

AFCI — Arc Fault Circuit Interrupter. Detects dangerous electrical arcing in damaged wiring. Required by NEC 2014+ in bedrooms and most living spaces.

GFCI — Ground Fault Circuit Interrupter. Trips within milliseconds when current leaks to ground (as little as 5 mA). Required near water sources to prevent electrocution.

Short Circuit Current (SCC) — The very high current that flows instantly during a fault between hot and neutral or hot and ground. Breakers must have an interrupting rating (AIC) equal to or greater than the available SCC.

Key Terms

GFCI

Ground Fault Circuit Interrupter — trips within 5ms when current leaks to ground (as low as 5mA). Required near water and outdoors. Prevents electrocution.

AFCI

Arc Fault Circuit Interrupter — detects dangerous electrical arcing that can cause fires. Required in bedrooms and most habitable rooms in NEC 2014+ jurisdictions.

Continuous Load

A load expected to be on for 3 or more hours at a time. NEC 210.20 requires these circuits to be sized at 125% (×1.25) of the continuous load current.

Ampacity

The maximum current (amps) a wire can carry continuously without exceeding its temperature rating. Larger-diameter wires have higher ampacity.

Double Pole

A 240V circuit uses a 2-pole (double-pole) breaker that occupies two slots in the panel and connects to both hot bus bars, providing 240V between poles.

Demand Factor

In NEC Article 220, the demand factor reduces calculated load to reflect that not all circuits run at full capacity simultaneously. Used for service sizing.

Real-World Examples

Example 1

EV Charger Circuit

Load: Level 2 EVSE, 7,200W (32A continuous) at 240V

Calculation: 7,200 ÷ 240 = 30A. Continuous × 1.25 = 37.5A. Next standard: 50A breaker.

Wire: #6 AWG copper. NEC 625.40 requires a dedicated circuit with GFCI protection.

Example 2

Bathroom Circuit

Load: Hair dryer (1,875W) + outlets at 120V

Calculation: 1,875 ÷ 120 = 15.6A. Rounded up: 20A breaker.

Wire: #12 AWG copper. GFCI protection required for all bathroom outlets per NEC 210.8(A)(1).

Example 3

Central AC Unit

Load: 3-ton AC, 3,500W running at 240V. Nameplate shows MOCP (max overcurrent protection) of 35A.

Result: Use nameplate MOCP — 35A breaker. For new installs, always use nameplate MCA and MOCP, not calculated watts.

Circuit Breaker Sizing: What Every Homeowner Should Know

The 80% Rule Explained

Circuit breakers are designed to handle 100% of their rated current for short periods, but continuous loads (those running 3+ hours) must be limited to 80% of breaker capacity. This is why a 20-amp breaker should not supply more than 16 amps of continuous load. The calculator automatically applies the 1.25× multiplier when you select "Continuous Load."

Why Wire Gauge Must Match Breaker Size

The wire gauge and breaker size must be coordinated. The breaker protects the wire — if you use a 20A breaker with #14 AWG wire (rated for 15A), the wire can overheat and burn before the breaker trips. This is one of the most common and dangerous code violations. Never upsize a breaker to solve a nuisance tripping problem without also upsizing the wire.

GFCI vs. AFCI: Different Hazards

GFCI and AFCI protect against different types of electrical hazards. GFCI protects against ground faults (shock from current flowing through a person to ground) and is required near water. AFCI protects against arc faults (high-temperature arcing in faulty wiring that ignites surrounding materials) and is required in living areas. Combination AFCI/GFCI breakers provide both protections and are increasingly the default for new installations.

When to Upgrade Your Electrical Panel

If your panel is consistently at 80%+ of capacity, if you're adding large loads (EV charger, hot tub, room addition), or if you have a Federal Pacific or Zinsco panel (known safety issues), it may be time to upgrade. Modern 200-amp panels provide plenty of capacity for most homes; larger homes with multiple EV chargers and all-electric appliances may benefit from a 400-amp service.

Circuit Breaker Size Calculator

How to Size a Circuit Breaker in 3 Steps

Frequently Asked Questions

Key Formulas

Key Terms

Key Formulas

Key Terms

Real-World Examples

EV Charger Circuit

Bathroom Circuit

Central AC Unit

Circuit Breaker Sizing: What Every Homeowner Should Know

The 80% Rule Explained

Why Wire Gauge Must Match Breaker Size

GFCI vs. AFCI: Different Hazards

When to Upgrade Your Electrical Panel

Circuit Breaker Size Calculator

How to Size a Circuit Breaker in 3 Steps

Frequently Asked Questions

Key Formulas

Key Terms

Key Formulas

Key Terms

Real-World Examples

EV Charger Circuit

Bathroom Circuit

Central AC Unit

Circuit Breaker Sizing: What Every Homeowner Should Know

The 80% Rule Explained

Why Wire Gauge Must Match Breaker Size

GFCI vs. AFCI: Different Hazards

When to Upgrade Your Electrical Panel

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