Breaker Size Calculator

Size a circuit breaker from load amps or watts using the NEC 80%/125% continuous-load rule, then round up to a standard breaker rating.

Topic · Breaker Size Calculator

Breaker Size Calculator – how to size breakers correctly and safely

Learn how to size circuit breakers from load current or power, apply the NEC 80%/125% continuous-load rule, and use the Breaker Size Calculator above to check your work with clear, step-by-step examples.

Read time Breaker sizing basics NEC 80% / 125% rule Single & three-phase examples

How to use the Breaker Size Calculator step by step

The Breaker Size Calculator above is built to mirror how an engineer or electrician actually thinks through breaker sizing: identify the load, decide whether it is continuous, apply the 125% rule when needed, and finally round up to the next standard breaker rating. This section walks you through the workflow so that the numbers you enter match what the calculator expects.

The calculator supports two main tasks: recommending a breaker size from a given load, or finding the maximum load allowed on a breaker you already know. It also lets you work from either load current (amps) or load power (watts / kW / VA / kVA) and supports both single-phase and three-phase systems by applying the correct current formulas in the background.

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    Choose Solve For: pick “Recommended Breaker Size (A)” if you know the load and want a breaker, or “Maximum Allowable Load (A)” if you already have a breaker rating and want to know how much current it can carry under continuous or non-continuous assumptions.

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    Select the Input Type: use “Known Load Current” when you already have \( I_{load} \) from nameplates or previous calculations, or “Known Load Power” when you know power and voltage. For the power mode, the calculator automatically converts power and voltage (plus power factor and phase) into current using the appropriate single-phase or three-phase formula.

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    Set Electrical Phase and Load Type: choose Single-Phase or Three-Phase, then specify whether the load is Continuous (expected to run for 3+ hours) or Non-Continuous. The calculator applies a 125% factor to continuous loads to reflect common NEC sizing practice for 80%-rated breakers, and a 100% factor for non-continuous or when you simply want a straight 1:1 relationship.

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    Enter your load data and read the results: type in load current or power, voltage, and power factor if applicable. The calculator computes the effective load current, applies the chosen sizing factor, and then rounds up to the nearest standard breaker rating. The Quick Stats table summarizes the calculated load current, the sizing factor used, and the final recommended breaker size so you can quickly verify that the assumptions match your design intent.

Single-line schematic showing a load fed through a correctly sized circuit breaker using the Breaker Size Calculator concept
Conceptual view of a load protected by a correctly sized breaker. The calculator helps convert load amps or watts into a practical breaker choice that respects continuous-load rules.

What breaker sizing means and why it matters

Breaker sizing is the process of selecting a circuit breaker rating that will protect conductors and equipment from overcurrent without nuisance tripping during normal operation. In practice, that means choosing a breaker that is high enough to allow your load to run continuously at its design current, but low enough that faults or overloads trip the breaker before conductors overheat or equipment is damaged.

Most common molded-case circuit breakers are 80% rated, which means they are intended to carry no more than 80% of their nameplate rating for continuous loads. To accommodate this, design practice (and many NEC rules) treat continuous loads by multiplying the load current by 125% to determine the required breaker rating. Non-continuous loads can typically be sized at 100% of the load current.

If you undersize the breaker, it may trip during normal operation—especially on motor starting or when a continuous load is near the rating. If you oversize the breaker relative to conductor ampacity or equipment ratings, you may fail to protect the wiring during overloads. The Breaker Size Calculator helps you stay in a realistic zone by always sizing from actual load current, applying an appropriate factor, and then rounding up to a standard breaker size that you can actually buy.

Remember that breaker sizing is only one part of a safe design. Conductor ampacity, installation conditions, ambient temperature, and equipment ratings all interact. Use this calculator as a planning and learning tool, then verify your final choices against the NEC, manufacturer data, and local codes before construction. For load planning at the panel or feeder level, you can pair this page with the Electrical Load Calculator to understand how individual breaker choices roll up into overall system demand.

Key breaker sizing formulas and NEC-style rules

Under the hood, the Breaker Size Calculator uses a small set of core formulas that you would also use by hand. Understanding them makes it easier to sanity-check the results, and to know whether your input assumptions make sense for your project.

If you already know the load current \( I_{load} \), the basic continuous-load rule for 80%-rated breakers is:

\[ I_{\text{breaker, req}} = \begin{cases} 1.25 \, I_{\text{load}} & \text{for continuous loads} \\[4pt] 1.00 \, I_{\text{load}} & \text{for non\text{-}continuous loads} \end{cases} \]

In plain language: for a continuous load, multiply the actual load current by 125% to find the minimum breaker rating; for non-continuous loads, 100% is usually acceptable. The calculator evaluates this automatically based on the Load Type you choose.

When you start from power instead of current, the calculator first converts power to current using standard power equations. For single-phase systems:

\[ I_{\text{load}} = \frac{P}{V \cdot PF} \]

and for three-phase systems:

\[ I_{\text{load}} = \frac{P}{\sqrt{3} \, V \cdot PF} \]

where:

  • \( P \) is real power in watts or kilowatts.
  • \( V \) is line-to-line voltage for three-phase, or line voltage for single-phase.
  • \( PF \) is power factor (dimensionless, typically 0.7–1.0).

After computing \( I_{load} \) from these formulas, the calculator applies the appropriate sizing factor (125% or 100%) to get the required breaker current, then rounds up to the nearest standard breaker size. If you choose Solve For → Maximum Allowable Load (A), the math runs in the opposite direction: the calculator takes the breaker rating and divides by the sizing factor to find the maximum continuous or non-continuous load current that would be acceptable on that breaker.

When you see the dynamic equation banner update above the calculator, it is essentially displaying a version of these equations tailored to your current settings—single vs three-phase, current vs power input, and continuous vs non-continuous load type.

Standard breaker ratings and how rounding up works

Real hardware is available only in discrete sizes, so any practical breaker sizing tool must take your calculated requirement and round up to a standard rating. The Breaker Size Calculator uses a typical list of common molded-case breaker sizes—values like 15 A, 20 A, 25 A, 30 A, 40 A, 50 A, 60 A, 70 A, 80 A, 100 A, and upward. Once it has computed a required current, it chooses the smallest standard rating that is greater than or equal to that requirement.

For example, if a continuous load calculation produces a required breaker current of 23.5 A, it is not valid to choose a 20 A breaker; the logical choice is the next size up. The calculator will typically recommend a 25 A breaker in that case. This is consistent with real-world design practice: you size conductors for the load and its factors, then select the smallest breaker rating that still satisfies those requirements.

Rounding up does not override code requirements. If the rounded breaker rating exceeds the ampacity of your chosen conductors or the ratings of connected equipment, you must either increase conductor size, reduce the load, or adjust your design so everything remains coordinated. Use the recommended breaker as a starting point and cross-check it against the Cable Sizing Calculator and equipment datasheets to keep breaker and conductor choices aligned.

In some specialized cases—such as 100%-rated breakers installed exactly as the listing requires—continuous loads may be allowed at 100% of breaker rating. The Breaker Size Calculator assumes the more common 80%-rated scenario and the corresponding 125% continuous-load factor, so always verify whether your specific breaker and installation conditions differ from that assumption.

Worked breaker sizing examples using the calculator

To make the formulas concrete, this section walks through realistic scenarios that match common search patterns: sizing a breaker from a known load current, sizing from power and voltage, and checking the maximum load on an existing breaker. You can either follow along by hand or click the example button to push the values straight into the Breaker Size Calculator above.

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Example 1 – Continuous 20 A load on a 120 V circuit

Suppose you have a single-phase 120 V circuit feeding a resistive load that draws a steady 20 A for more than three hours. You want to know what breaker size is appropriate for an 80%-rated breaker.

For a continuous load, the required breaker current is:

\[ I_{\text{breaker, req}} = 1.25 \, I_{\text{load}} = 1.25 \times 20 \,\text{A} = 25 \,\text{A} \]

You then round up to the next standard breaker rating. 25 A is itself a standard size, so a 25 A breaker can be appropriate, provided the conductor ampacity and all other design requirements are satisfied.

To reproduce this example in the calculator, set: Solve For = Recommended Breaker Size, Input Type = Known Load Current, Phase = Single-Phase, Load Type = Continuous, then enter 20 A for the load current. The calculator will show a calculated required current of 25 A and recommend a 25 A standard breaker.

Result: Continuous 20 A load → required breaker current 25 A → recommended standard breaker size 25 A.

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Example 2 – Three-phase 15 kW motor at 480 V, PF = 0.85

Next, consider a three-phase 480 V motor with a rated real power of 15 kW and a power factor of 0.85. Assume it is a continuous industrial load and you want to find a suitable breaker size.

First, convert power to current for a three-phase system:

\[ I_{\text{load}} = \frac{P}{\sqrt{3} \, V \cdot PF} = \frac{15\,000}{\sqrt{3} \times 480 \times 0.85} \approx 21.3 \,\text{A} \]

Because this is a continuous load with an 80%-rated breaker, apply the 125% factor:

\[ I_{\text{breaker, req}} = 1.25 \times 21.3 \,\text{A} \approx 26.6 \,\text{A} \]

Rounding up to the nearest standard breaker rating, you would typically select a 30 A breaker, then verify that conductors and terminations are sized appropriately for this current.

To mirror this example in the calculator, choose Solve For = Recommended Breaker Size, Input Type = Known Load Power, Phase = Three-Phase, Load Type = Continuous, enter 15 kW at 480 V with PF = 0.85, and review the Quick Stats and final recommended breaker rating.

Result: 15 kW three-phase motor at 480 V, PF 0.85 → 21.3 A load → ~26.6 A required → recommended standard breaker 30 A.

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Example 3 – Maximum continuous load on a 40 A breaker

Finally, imagine you already have a 40 A breaker installed and want to know the maximum continuous load current you should plan for if the breaker is 80% rated. This scenario matches the “Maximum Allowable Load (A)” mode in the calculator.

For a continuous load:

\[ I_{\text{load, max}} = \frac{I_{\text{breaker}}}{1.25} = \frac{40 \,\text{A}}{1.25} = 32 \,\text{A} \]

So a 40 A, 80%-rated breaker should not be planned to carry more than about 32 A of continuous load. The Breaker Size Calculator computes this for you when you select Solve For = Maximum Allowable Load, choose Load Type = Continuous, and enter a 40 A breaker rating.

Result: 40 A breaker → maximum planned continuous load ≈ 32 A.

Example chart comparing breaker ratings and allowable continuous load currents for different breaker sizes
Visual guide: as breaker size increases, the maximum continuous load rises in proportion, but continuous loads are limited to 80% of the breaker rating for typical 80%-rated devices.

Design tips, edge cases, and limitations of breaker sizing calculators

While a tool like the Breaker Size Calculator is extremely helpful, it is not a replacement for detailed design and code review. It focuses on the relationship between load current and breaker rating; real-world installations introduce additional layers that you should keep in mind as you interpret the results.

Coordinate breaker size with conductor ampacity. Breakers are there primarily to protect the conductors. Once the calculator recommends a breaker, cross-check that your wire size (including insulation type, ambient temperature, and bundling / derating factors) has sufficient ampacity for both the continuous load and the breaker rating.
Watch motor starting and inrush. Motors and transformers can draw several times their running current during starting or energization. In those cases, breaker trip curves, time-delay characteristics, and dedicated NEC motor rules are just as important as steady-state current and may push you toward larger breakers or specific trip units.
Do not ignore manufacturer instructions or local code. Different breaker families, panelboards, and equipment listings may allow or require specific approaches to continuous loading, 100%-rated operation, or conductor terminations. Use the calculator as a starting point, then confirm all final decisions against the NEC, product documentation, and local authority-having-jurisdiction (AHJ) requirements.

A few other practical guidelines that align well with how the calculator is structured:

  • Keep power factor realistic; values between 0.7 and 0.95 are typical for many inductive loads.
  • Use single-phase mode only for actual single-phase systems; do not approximate three-phase loads as single-phase.
  • Never size a breaker below the calculated required current; always round up to the next standard size.
  • Recalculate if the load mix changes—multiple loads on a feeder may create different continuous/non-continuous combinations.
  • Document your assumptions (continuous vs non-continuous, PF, voltage) so future engineers can understand why a particular breaker was chosen.
  • Consider voltage drop, conduit fill, and installation details; pairing this page with tools like the Voltage Drop Calculator can help ensure that your chosen breaker, conductor, and layout all work together in practice.

If your project becomes complex—multiple motors on one feeder, long runs with voltage drop constraints, or selective coordination requirements—you can still use the Breaker Size Calculator to sanity-check individual load segments while handling the more advanced system design separately.

Breaker Size Calculator – frequently asked questions

How do I choose the right breaker size from load current?

Start with the actual load current \( I_{load} \). If the load is continuous (expected to run for 3+ hours), multiply by 125% to get a required breaker current; otherwise, use 100%. Then pick the next standard breaker size above that value. The Breaker Size Calculator automates this: you select Known Load Current, choose continuous or non-continuous, and it returns a recommended standard breaker rating plus a summary of the math used.

What is the 80% rule for breakers and how does this calculator handle it?

Many molded-case breakers are 80% rated, meaning they are intended to carry no more than 80% of their nameplate rating for continuous loads. To satisfy that, designers typically size the breaker at 125% of the continuous load current. The Breaker Size Calculator bakes this into the Continuous Load (125% Rule) option: it multiplies the load current by 1.25 before picking a standard breaker size. Non-continuous loads are sized at 100% of their current.

Can I use the Breaker Size Calculator for three-phase motors?

Yes—set Electrical Phase to Three-Phase and use the Known Load Power mode. Enter motor kW (or kVA), line-to-line voltage, and a realistic power factor. The calculator uses \( I = P / (\sqrt{3} V PF) \) to find the running current, applies the selected continuous/non-continuous factor, and then recommends a standard breaker rating. You should still confirm motor sizing against NEC motor tables and manufacturer recommendations, especially for starting and inrush conditions.

How do I find the maximum load on an existing breaker?

Use the Maximum Allowable Load (A) mode. Enter your breaker rating (for example, 40 A) and choose whether the load will be continuous or non-continuous. For a continuous load with an 80%-rated breaker, the calculator divides the breaker rating by 1.25 to find the recommended maximum continuous load current. For a non-continuous load, it assumes a 1.0 factor and returns a maximum load equal to the breaker rating (subject to conductor ampacity and other code limits).

Does the Breaker Size Calculator replace code checks or an engineer’s stamp?

No. The calculator is an educational and preliminary design tool that helps you apply common breaker-sizing formulas quickly and consistently. It does not account for every NEC article, selective coordination, special trip curves, ambient temperature effects, or manufacturer-specific requirements. Always verify final designs against the NEC, equipment listings, manufacturer instructions, and local code, and involve a qualified engineer or electrician for critical or permitted projects.

What if the calculator’s recommended breaker doesn’t match my conductor size?

If the recommended breaker rating exceeds the ampacity of the conductors you planned to use, you have a few options: increase conductor size so that ampacity matches or exceeds the breaker rating, reduce the load or split it into multiple circuits, or reconsider assumptions such as continuous duty. The calculator intentionally rounds up to a safe standard size relative to the load; it is your responsibility to adjust conductor sizing and equipment choices so everything remains coordinated and code-compliant.

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