Circuit Breaker Size Calculator: 125% Rule, Standard Breaker Sizes and Worked Examples

For study estimates only, not engineering design

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Circuit Breaker Size Calculator: turn “how many amps do I need?” into clear numbers

Quick answer: Estimate load current from the appropriate voltage, power and power-factor inputs. For applicable continuous loads, the common sizing calculation uses 125%, subject to code exceptions and equipment ratings. Breaker selection also depends on conductor ampacity and circuit requirements.

Studying for the Texas Journeyman exam? Sizing overcurrent protection by hand is part of the calculations portion of the Texas Journeyman exam. See what the calculations part tests.

Sizing one breaker is one piece of the bigger picture. If you want the rest of the toolkit — voltage drop, three-phase power, wire sizing — see the Electrical Calculators & Career Tools hub. Or if you’d rather check your grasp of the underlying fundamentals first, try the free Electricity Fundamentals Mock Exam — a self-practice quiz, not an official certification exam.

If you’re looking for a simple, code-aware circuit breaker size calculator for 120/240 V home and light commercial branch circuits in the US, this page walks you through both the math and real-world examples.

Any time you add a new air conditioner, run more outlets in the kitchen, or get ready for an EV charger, you’ll run into the same question:

“What size breaker do I actually need for this branch circuit – 15A, 20A, 40A, or something else?”

If you only go by a rough idea of the total watts, it’s very easy to swing too far in either direction:

• Oversize everything and blow up the material cost, or
• Undersize the circuit, and the breaker starts tripping every summer or under continuous load.

This circuit breaker size calculator for branch circuits is here to fix that. It pulls together your watts, voltage, power factor and whether the load is continuous, then runs a simple breaker sizing calculation to give you:

• Estimated load current (A)
• Minimum recommended breaker amp rating (continuous vs non-continuous)
• Suggested standard breaker size, plus a common wire size pairing for reference

The results are for education and preliminary design. Before anyone installs or changes wiring, you still need to go back to the NEC, local electrical code, and ampacity tables, and have a qualified electrical professional review the design.

On this page, you can:

• Use the calculator to quickly go from “load in watts” → “recommended breaker amp size”
• Understand the 125% rule for continuous loads and the 100% rule for non-continuous loads
• Walk through real-world examples (AC, EV charger, kitchen small appliances) and see what “safe amp rating” looks like in practice
• Jump over to voltage drop, power and energy calculators to build a more complete wiring workflow

On the Texas Journeyman exam

Where this calculation shows up

On the PSI Texas Journeyman exam this is part of the Calculations part: Branch Circuit Calculations and Branch Circuit Overcurrent Protection. That part has 26 questions in 110 minutes, and you need 70% on it on its own. The NEC Knowledge part asks the code rule behind the number.

Try one free question from our Texas Journeyman set: A 20-ampere, 120-volt branch circuit supplies several receptacle outlets in an office. What is the maximum rating of any single piece of cord-and-plug-connected utilization equipment, not fastened in place, that this circuit may supply?

1. A. 10 A
2. B. 12 A
3. C. 16 A
4. D. 20 A

Show the answer

C. 16 A. On a 15- or 20-ampere branch circuit supplying two or more outlets, one cord-and-plug-connected piece of equipment not fastened in place may not exceed 80% of the branch-circuit rating: 0.80 x 20 A = 16 A. Reference: NEC 210.23(B)(1) (2026 NEC).

Try 70 free Journeyman questions How the exam tests calculations

Exam format from the PSI candidate bulletin, read October 3, 2026.

1. Why you need a branch circuit load / circuit breaker size calculator

Real-world wiring design is almost never as simple as “one wire, one load.” In a typical home you’ll see things like:

• One receptacle branch circuit feeding the TV, soundbar, robot vacuum, phone chargers
• A whole row of kitchen outlets taking turns with the rice cooker, microwave, coffee machine and air fryer
• An AC unit, dehumidifier and air purifier running at the same time

If you just “go by feel,” two extreme outcomes are common:

• Breaker is too small. It seems fine at first, but in hot weather or long run times the breaker gets hot and starts tripping.
• Breaker is too large. Material costs go up, the panel fills up faster, but you don’t actually gain meaningful safety.

On top of that, a lot of people have heard one more rule—but haven’t really used it in calculations:

“Continuous loads (3 hours or more) must be sized at 125%.”

In other words, for the same 16 A load:

• If it’s non-continuous, a 20 A breaker might be fine.
• If it’s continuous (sign stays on all night, office lighting, EV charging), you treat the breaker as if only 80% of its rating is available for that load.

This calculator’s whole job is to take:

• Load in watts (W)
• Circuit voltage (V)
• Power factor (PF)
• Whether it’s a continuous load

…and convert that into a “design current” and “recommended breaker size”, so you have a solid starting point before you ever pull a wire.

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2. How to use this circuit breaker size calculator

Here’s the basic flow for using this circuit breaker size calculator step by step.

1. Choose system type and voltage
   Select single-phase or three-phase, then enter the circuit voltage, such as 120/240 V or 208/240/277/480 V.
2. Choose your input: watts or amps
   • If you know the equipment’s rated power, choose “Total load (W)”.
   • If you already have the design current from the spec sheet (for example, “max 18 A”), choose “Design current (A)”.
3. Enter the load
   • For watts: add up all the loads that reasonably might run at the same time on that branch circuit.
   • If you’re factoring in diversity / demand, apply that on your side first, then plug the effective watts into the calculator.
4. Set a power factor (PF)
   • For general lighting and receptacle loads, starting with PF ≈ 0.95 is usually fine.
   • Pure resistive heating (toasters, baseboard heaters, some ovens) can be treated as PF ≈ 1.0.
   • For three-phase motors, EV chargers and other power electronics, use the PF given in the manufacturer’s data whenever possible.
5. Tell the calculator if it’s a continuous load
   • If any part of the branch is expected to run 3 hours or more at a similar load level (signs, office lighting, many EV charging setups), check “continuous load”.
   • The calculator will automatically apply the 125% factor to reach a minimum breaker size for that circuit.
6. Choose the standard breaker size steps
   • For US residential and light commercial, that usually means standard values like 15/20/30/40/50/60 A, etc.
   • In other regions you might see different series—the calculator lets you pick the series that matches your local panel hardware.
7. Hit “Calculate”
   You’ll get a summary card with:
   • Estimated load current
   • Minimum recommended breaker rating
   • A suggested standard breaker size (rounded up to the next standard value)
   • Load percentage on that breaker
   • A “common wire size pairing” as a rough mental reference

If the load is already very close to the breaker’s limit, the card will shift to a yellow or red tone, reminding you to pause and re-check wire size, voltage drop and the actual use scenario.

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3. The 125% rule for continuous loads, in plain English

The first time people hear “continuous loads must be sized at 125%,” the reaction is usually:

“Why do I have to add 25%? Isn’t that just wasting copper and money?”

In plain English: breakers are not meant to sit at 100% of their rating all day long.
With long run times, ambient temperature changes and accumulated thermal stress, the device has a harder life—so the code wants a bit of headroom.

Roughly speaking:

• Non-continuous loads
  Things that cycle on and off, or only run at full load for shorter periods—hair dryers, microwaves, hand tools, etc.
• Continuous loads
  Things that stay on for 3 hours or more at a relatively steady level—sign lighting, office lighting, many HVAC systems, EV charging, some industrial process loads.

For continuous loads, the common design logic is:

• Design current = load current × 125%
• Breaker rating ≥ design current

If you’d like a more code-focused explanation of how the 125% rule shows up in real designs, the International Association of Electrical Inspectors has a helpful article on how NEC Article 215 handles continuous loads and the 125% sizing rule.

When you check “continuous load” in the calculator, that’s what it does behind the scenes. Then it picks the next higher standard breaker size, so you can instantly see:

• Roughly what percentage of the breaker this branch circuit will use
• Whether it looks comfortably loaded, borderline, or clearly over-stressed

The real design still has to go back to the code rules and ampacity tables used in your country or state. The calculator’s job is to turn “gut feeling” into actual numbers you can talk through with a customer, inspector or engineer.

Worked example 1: continuous plus noncontinuous load at 240 V

• Continuous load: 5,000 W ÷ 240 V = 20.83 A, × 125% = 26.04 A
• Noncontinuous load: 6,000 W ÷ 240 V = 25.00 A, counted at 100%
• Minimum breaker capacity: 26.04 A + 25.00 A = 51.04 A
• Next standard size above 51.04 A: 60 A

Worked example 2: three-phase feeder with 75 A continuous and 75 A noncontinuous

• Continuous part: 75 A × 125% = 93.75 A
• Add the noncontinuous part: 93.75 A + 75 A = 168.75 A
• Next standard size above 168.75 A: 175 A

Both follow the classic exam pattern: size the continuous part at 125% and the noncontinuous part at 100%, add them, and round up to the next standard rating. The conductors still have to carry the load and be protected at or below their own limit.

Standard breaker sizes (NEC 240.6(A))

Typical use	Standard ratings (A)
Home branch circuits	15, 20, 25, 30, 35, 40, 45, 50, 60
Larger circuits and feeders	70, 80, 90, 100, 110, 125, 150, 175, 200
Services and large feeders	225, 250, 300, 350, 400, 450, 500, 600

Copper wire	Maximum breaker (NEC 240.4(D))
14 AWG	15 A
12 AWG	20 A
10 AWG	30 A

Larger standard ratings continue above 600 A. The small-conductor limits apply unless an exception in 240.4(E) through (G) covers the equipment, such as motor or air-conditioning circuits sized from the nameplate. Check the code edition your jurisdiction has adopted.

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4. Example 1 – 15A or 20A for a living room AC?

Let’s walk through a very common question:

“Should my living room AC be on its own 20A circuit?”

Say you have a split-system AC, and the nameplate says:

• Rated voltage: 240 V
• Rated power: 3,000 W
• Type: cooling / heating, runs most of the night during the summer

Here’s how you’d run it through the calculator:

1. System: Single-phase, voltage 240 V
2. Input mode: Total load (W), enter 3000 W
3. PF: start with 0.95 (unless the manufacturer gives a more precise value)
4. Check “continuous load”, because summer nights can easily run 6–8 hours
5. Standard breaker series: choose a set that includes 15 A, 20 A, 30 A, 40 A…

Hit Calculate, and you’ll typically see something like:

• Estimated load current in the mid-teens (A), depending on PF
• Minimum continuous-load breaker capacity a bit higher than that
• Suggested standard breaker size: 20 A, with a common pairing of around 12 AWG copper (or the local equivalent mm² size)

From here, you can go back to your voltage drop calculator and ampacity tables to check:

• Conductor size vs. ampacity
• Circuit length and voltage drop
• Ambient temperature, bundling with other circuits, etc.
• Whether it belongs on a dedicated circuit, and how much future expansion you want

The calculator doesn’t replace that deeper design work—it just gets you to a reasonable starting point in seconds.

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5. Example 2 – EV charger, 32 A at 240 V

Now let’s look at something that’s becoming very common in US garages: an EV charger.

Suppose the manufacturer’s specs say:

• Rated voltage: 240 V single-phase
• Maximum input current: 32 A
• Recommended breaker: 40 A
• Typical use: charging 4–6 hours per night

For EV charging, US codes generally treat this as a continuous load, which is why you often see a 32 A charger on a 40 A breaker.

Here’s how you’d use the “design current” mode:

1. System: Single-phase, voltage 240 V
2. Input mode: Design current (A), enter 32 A
3. PF: use 1.0 as a simple starting point, or whatever the manufacturer provides
4. Check “continuous load”
5. Standard breaker series: use your region’s residential / light commercial series

Click Calculate, and you should see:

• Estimated load current: 32 A (exactly what you entered)
• Minimum continuous-load breaker capacity: 32 A × 1.25 ≈ 40 A
• Suggested standard breaker size: 40 A
• A reminder to double-check wire size and voltage drop, not just breaker rating

The good part is: you’re no longer just trusting “because the spec sheet says so.” You can see the math under that 40 A recommendation, and adjust if you change the circuit length, ambient temperature or panel configuration.

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6. Example 3 – Kitchen outlets & small appliances

The kitchen is where loads can “explode” without you noticing: rice cooker, induction cooktop, coffee machine, toaster oven, air fryer… all wanting power in the same window around meal times.

Let’s say you’re planning one small appliance branch circuit expected to feed:

• Rice cooker: 1,000 W
• Microwave: 1,200 W
• Coffee maker: 800 W

They won’t all be at full power all the time, but it’s still useful to check a worst-case scenario.

In the calculator you could:

1. Add the loads: 1,000 + 1,200 + 800 = 3,000 W
2. Assume 120 V, PF ≈ 0.95
3. At first, leave “continuous load” unchecked and look at the result
4. See what percentage of a 20 A breaker that would consume
5. Optionally, toggle on “continuous” and compare how the recommended breaker bumps up

If you discover that:

• A 20 A breaker would be pushed close to or beyond 100% in your worst-case estimate, and
• There’s no other branch circuit available to share the load,

…that’s a good hint this branch might need a different plan:

• Split into two circuits,
• Separate the high-wattage receptacles, or
• Reserve a dedicated circuit for the biggest appliance.

With the calculator, you can very quickly test different “what-if” layouts and hunt for a balance between material cost, panel space and day-to-day usability.

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7. Design & safety notes: what this calculator does not do for you

One more reminder up front: this circuit breaker size calculator for branch circuits is a quick design-check tool, not a full-blown design engine.

Here are the big things it does not handle for you:

• It does not look up wire ampacity tables.
  Conductor ampacity depends on insulation rating, installation method, ambient temperature, bundling with other cables and more.
• It does not calculate voltage drop.
  On long runs, you can be “ampacity-legal” and still fail on voltage drop. Use a voltage drop calculator alongside this tool for long circuits.
• It does not handle protection coordination.
  The relationship between the main breaker, downstream breakers and the protected equipment curves still requires detailed design and, often, specialized software.
• It never overrides the code.
  The National Electrical Code (NEC), any applicable IEC-based standards, local amendments and utility requirements always come first. Online tools are only helpers.

So think of this calculator as a good sketchpad for:

• Roughing out a design,
• Estimating material needs, and
• Explaining a concept to a homeowner or stakeholder,

…but still always bringing the final design back to stamped drawings, code references and ampacity tables before anyone starts pulling wire.

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8. Related electrical calculators & tools

If you’ve just run a few circuits through this circuit breaker size calculator, chances are you’ll also want these tools within reach:

• Voltage Drop Calculator
  Use circuit length, current, wire size and conductor material to estimate voltage drop and end voltage—so you don’t end up with “legal amps but unacceptable voltage.”
• Single-Phase / Three-Phase Power Calculator
  Quickly convert between kW, kVA, kVAR and line current (A), which is especially handy when you’ve got a mix of loads with different rating styles.
• Energy Cost / kWh Estimator
  Turn circuit power into estimated monthly energy cost, so you can have better conversations about whether an equipment or lighting upgrade is worth it.
• (Coming soon) Wire Ampacity Quick Reference Sheet
  A compact cheat sheet of common wire sizes, typical amp ratings and everyday use cases, so you don’t have to flip through a thick code book every time.

Bookmark the set of tools that fits your work. Over time it becomes your own mini design toolbox.

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9. Next step: turn this one-time check into a standard checklist

If this circuit breaker size calculator helped you get a handful of branch circuits under control, the next step is to make this “get it right the first time” approach your new normal.

For every new branch circuit you touch:

• Make a simple load list, run it through the calculator and record the results on your drawings or project notes.
• When you move into a new home—or help a friend—use it to spot whether high-load equipment has sensible circuits and breakers behind it.
• Gradually build your own “typical case library”: what you usually use for AC units, water heaters, kitchen circuits, EV chargers and more.

If you’d like a printable checklist you can physically walk through the house with, start here:

Disclaimer: This calculator is for educational and preliminary design use only. Values are for planning and discussion. Actual installation must follow applicable codes and standards, and be reviewed and performed by qualified electricians and, where required, qualified design professionals or inspectors.

Breaker sizing with the 125% rule is the kind of math you practice for an electrician license exam. If you are working toward that license, a free Tsai Electro account keeps your practice scores, your hours ledger and an exam date plan in one place.

Circuit breaker size calculator FAQ

One more thing, before you trust the number this calculator gave you

I’m Tsai, creator of Tsai Electro. Quick check on the math this calculator runs behind the scenes: if a circuit runs at 120V and draws 15A, how many watts is that? (Answer: 1,800W — P = V × I. If that felt instant, good — that’s exactly the kind of fundamentals this calculator assumes you already have.)

If it didn’t feel instant, or you just want to check where your gaps actually are before you rely on a tool like this one, I put together a free Electricity Fundamentals Mock Exam. It’s a self-practice quiz — Ohm’s law, basic circuit math, that kind of thing — not an official certification or licensing exam, and it doesn’t replace your local code or a real electrician’s judgment. It just tells you, honestly, which basics are already solid and which ones are worth reviewing before you size your next circuit.

Looking for other calculators, or the career-path tools mentioned above? See the full Electrical Calculators & Career Tools hub.

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