How to Select the Right Breaker & Fuse Size: Standard Ratings & Rounding Rules

This guide clarifies selecting breaker and fuse sizes for electrical circuits using recognized standards worldwide.

Target audience includes electrical designers, installers, inspectors, and engineers responsible for protection coordination and compliance.

Breaker / Fuse Sizing: Select Correct Rating and Rounding Rule Based on Load Current

Design load current Ib (A)

Load duty type / design factor (-) General-purpose non-continuous (100 % of load current) Continuous load ≥ 3 h (125 % design factor) Motor / high inrush load (≈150 % design factor)

Protection device family Molded-case / miniature circuit breaker (MCB/MCCB) Thermal-magnetic circuit breaker Full-range gG / gL fuse Motor protection aM fuse

Standard rating series IEC miniature breaker ratings 0.5–63 A IEC 60947 common ratings 6–630 A UL489 / NEC common ratings 15–600 A

Rounding rule for standard rating selection Round up to next higher standard rating (default) Round to nearest standard rating (ensure ≥ load current Ib) Round down if still ≥ load current Ib, otherwise round up

Advanced options

Nominal system voltage (V)

Effective conductor ampacity Iz (A)

Max device utilisation vs cable ampacity (%)

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Enter the basic data to compute the recommended breaker / fuse rating.

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Formulas and sizing logic used:

• Design load current: Ib [A] = steady-state load current at design conditions (user input).
• Load duty factor: kload [-] = factor accounting for continuous duty or inrush, selected by load duty type.
• Required protective device current (before standardization): Ireq [A] = Ib × kload.
• Standard rating series: a predefined list of manufacturer / standard ratings In,std [A] (depends on selected IEC or UL/NEC series).
• Rounding rule:
  • Round up: select the smallest In,std ≥ Ireq.
  • Nearest: select In,std with minimum |In,std − Ireq|, but always In,std ≥ Ib.
  • Down if within: prefer the largest In,std ≤ Ireq if In,std ≥ Ib; otherwise use the next higher rating.
• Effective conductor ampacity (optional check): Iz [A] = continuous current-carrying capacity of the cable after derating.
• Maximum allowed device rating from cable: In,max,cable [A] = Iz × (futil / 100), where futil [%] is the maximum utilisation vs cable ampacity.
• Validation condition (if Iz is provided): selected protective device rating In,std must satisfy In,std ≤ In,max,cable, otherwise the cable is undersized for the chosen rating.

Standard rating series	Typical standard ratings (A)
IEC miniature breaker ratings (MCB, 0.5–63 A)	0.5, 1, 2, 3, 4, 6, 10, 13, 16, 20, 25, 32, 40, 50, 63
IEC 60947 common breaker / fuse ratings (6–630 A)	6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630
UL489 / NEC common breaker / fuse ratings (15–600 A)	15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600

When should I use the 125% factor for continuous loads?

Use a 125% design factor when the load is expected to operate at or near full current for 3 hours or more, as by many codes for continuous loads. This increases the breaker or fuse rating so that it does not operate too close to its thermal limit in steady-state conditions.

What is the difference between rounding up and rounding to the nearest rating?

Rounding up always selects the next higher standard rating above the calculated requirement, which is the safest rule for most protection design. Rounding to the nearest may result in a device slightly below the calculated design level if the lower standard rating is closer, so the calculator enforces at least the actual load current Ib to avoid undersizing.

Why does the calculator warn when the device rating exceeds cable ampacity?

If the selected breaker or fuse rating exceeds the effective cable ampacity (including derating), the cable may be thermally overloaded without the protective device tripping in time. In this case, either the cable size or the protective device rating must be adjusted.

Can I use this tool for motor short-circuit and ground-fault protection?

You can use the tool as a first sizing step by applying a higher design factor for motor or high inrush loads, but detailed motor protection must also consider starting current, time-current curves, and specific code rules for maximum protective device multipliers.

Scope and applicability

This article explains principles, formulas, and practical rounding rules for selecting breaker and fuse sizes for low-voltage installations (typically up to 1000 V AC). It covers conductor ampacity interactions, continuous and non-continuous loads, motor protection considerations, and how to round calculated currents to standard device ratings under common international standards (IEC, NEC/NFPA 70, UL guidance). This is a technical reference for engineering design and compliance; always verify with local code and equipment manufacturer data.

Fundamental sizing principles

Protection device selection must satisfy multiple, simultaneous constraints:

• The protective device rating (ampere rating) must be greater than the expected continuous or operating current using applicable multiplication factors.
• The protective device's interrupting capacity (breaking capacity, short-circuit rating) must be equal to or exceed prospective fault current at the point of installation.
• Conductors must have ampacity greater than or equal to the expected current after all correction and grouping factors; conductor insulation temperature rating must match terminal ratings.
• Coordination (selectivity) and inrush or starting currents (motors, transformers) must be considered so that nuisance trips and damage are avoided while still providing protection.

Key definitions

• Continuous load: a load expected to run for three hours or more (NEC definition commonly used internationally).
• Design current (I_design): the actual steady-state current drawn by the load or group of loads, before rounding.
• Overcurrent protective device (OCPD): breaker or fuse selected to protect conductors and equipment.
• Prospective short-circuit current (PSCC): calculated or measured available fault current at the point of installation.

Relevant standards and normative references

Key normative documents that inform sizing and rounding rules include:

• NFPA 70: National Electrical Code (NEC) — general rules for conductor ampacity, continuous loads, and motor protection. https://www.nfpa.org/NEC
• IEC 60364: Electrical installations of buildings — international installation rules. https://www.iso.org/standard/53783.html (via IEC/ISO portals)
• IEC 60947-2: Low-voltage switchgear and controlgear — circuit breakers. https://www.iec.ch/
• IEC 60269: Low-voltage fuses — selection and characteristics. https://www.iec.ch/
• UL 489: Molded-case circuit breakers and circuit-breaker enclosures (for North America). https://standardscatalog.ul.com/standards/en/standard_489
• IEEE Std 242 (Buff Book) and IEEE Std 399 (Brown Book) for coordination and grounding guidance. https://standards.ieee.org/

Note: links point to authoritative standards organizations; access may require subscription.

Standard ratings and commonly available device sizes

Selecting a breaker or fuse requires mapping a calculated design current to the next higher commercially available standard rating. Below are typical standard ratings used internationally; verify current local market offerings. Notes: values in the conductor table are typical approximations for copper conductors in standard installation conditions (single-core, 30 °C ambient, short installation length). Always consult the manufacturer's cable data sheets and local code ampacity tables (e.g., NEC 310.15(B)(16)).

Basic calculation formulas and variable explanations

All formulas are expressed in plain HTML for clarity. Explanations of variables and typical values follow each formula.

Single-phase; steady power load

Variable explanations:

• I = current in amperes (A).
• P = real power in watts (W) or kilowatts (kW × 1000).
• V = line-to-neutral voltage in volts (V) for single-phase (e.g., 230 V).
• PF = power factor (unitless, between 0 and 1). Typical PF for resistive heater = 1.0; for motors often 0.8–0.95.

Typical values: For a 9 kW heater at 230 V: I = 9000 / (230 × 1.0) = 39.13 A.

Three-phase balanced load (line-to-line voltage)

Variable explanations:

• I = line current in amperes (A).
• P = total real power in watts (W) or kW.
• V = line-to-line voltage (V), e.g., 400 V, 480 V.
• PF = power factor (unitless).
• √3 = 1.732 (square root of three).

Typical values: For a 30 kW three-phase load at 400 V, PF 0.9: I = 30000 / (1.732 × 400 × 0.9) ≈ 48.1 A.

Continuous load rule (common international practice)

For loads considered continuous: apply a multiplication factor (generally 125%) to the design current when selecting conductor ampacity and often OCPD: Variable explanations:

• I_cont = design value used for conductor and protective device selection.
• I_design = steady-state operating current from previous formulas.

Note: NEC 210.19(A)(1) and 210.20(A) define continuous load rules; IEC national applications may vary but the 125% multiplier is widely used. Always confirm local code.

Rounding rules and selection procedure (step-by-step)

Engineers must follow a consistent procedure to avoid under- or over-protection: 1. Calculate the expected steady-state current for each circuit using the formulas above. 2. Identify whether the load is continuous (apply 1.25 factor), motor starting, or non-continuous. 3. For conductor sizing, apply ampacity correction factors: ambient temperature, grouping (multiple conductors in conduit), and insulation temperature rating. Compute required conductor ampacity: 4. Choose a conductor with ampacity ≥ I_req_conductor. 5. For OCPD selection:

1. Compute required OCPD current for conductors and for equipment (motors require specific rules).
2. Round up to the next standard device rating from the standard rating table. This is the core rounding rule: select the next readily available standard rating greater than or equal to the required current after factors.
3. Verify the selected OCPD does not exceed maximum protective limits for the equipment (e.g., motor branch-circuit protection limits in NEC 430 or manufacturer recommendations).

6. Check selective coordination and manufacturer trip curve curves or fuse characteristic (time-current curves) to confirm discrimination with upstream devices. 7. Confirm the OCPD interrupting capacity meets or exceeds the calculated prospective short-circuit current at the point of installation. 8. Document all assumptions, code references, and manufacturer data.

Practical rounding examples of the rule

• If calculated requirement is 39.1 A and continuous, I_cont = 48.9 A → select next standard breaker rating = 50 A.
• If calculated requirement is 41.0 A non-continuous → select next standard rating = 50 A (or 63 A if preferred for coordination, but verify conductor sizing).
• If calculated requirement is 48.1 A → select 50 A standard rating.

Interrupting capacity (breaking capacity) and short-circuit considerations

Always verify:

• OCPD Icu (ultimate breaking capacity) and Ics (service breaking capacity) ratings meet or exceed the available prospective fault current at the installation point. Typical values in low-voltage switchgear are 10 kA, 25 kA, 50 kA, 100 kA depending on equipment and system.
• Fuses have distinct I2t and link characteristics; select fuse type (gG, aM, gR, etc.) appropriate for the protected equipment. IEC 60269 classification guidance must be followed for application-specific selection.

For arc-flash and safety calculations, accurate PSCC calculation at the point of installation is mandatory.

Ampacity correction factors and derating considerations

Several modifiers reduce permissible current for a conductor:

• Ambient temperature factor (e.g., NEC Table 310.15(B)(2)(a) or manufacturer tables).
• Number of current-carrying conductors in the same conduit or duct (grouping factor).
• Insulation type and terminal temperature rating—use the lowest applicable temperature rating.
• Altitude corrections for high-elevation installations in some standards.

Always apply these factors to the conductor ampacity before selecting conductor size.

Example 1 — Continuous heater circuit (single-phase) — full development

Problem statement: A building requires a 9 kW electric resistance heater connected to a single-phase 230 V supply. The load is continuous (operates more than three hours). Select conductor size and protective device following typical 125% continuous rule and rounding to standard device ratings. Step 1 — Compute design current: Assume PF = 1.0 for pure resistive load. I_design = 9000 / (230 × 1.0) = 39.130 A. Step 2 — Apply continuous load multiplier:

I_cont = I_design × 1.25 = 39.130 × 1.25 = 48.913 A.

Step 3 — Select OCPD rating by rounding up to next standard rating: From the standard device table, the next standard breaker/fuse rating ≥ 48.913 A is 50 A. Therefore OCPD selected = 50 A. Step 4 — Conductor selection (typical approach): Use the conductor table (representative values). For 50 A, choose conductor whose continuous rating is ≥ 48.913 A. From the table, 6 mm² copper = 41 A (insufficient). 10 mm² copper = 57 A (sufficient). Therefore select 10 mm² copper conductor (or nearest AWG equivalent: AWG 8 is approx 55–50 A depending on insulation; consult NEC table for exact AWG sizing). Step 5 — Verify terminal ratings and ambient conditions: If ambient temperature or grouping reduces conductor ampacity, adjust accordingly. Example: if ambient requires derating factor 0.91, then corrected ampacity of 10 mm² = 57 × 0.91 = 51.9 A (still > 48.913 A); acceptable. Document calculations and manufacturer tables. Step 6 — Verify interrupting capacity and coordination: Ensure the chosen 50 A breaker has an adequate short-circuit rating and is coordinated with upstream devices. Result summary: - Calculated continuous current: 48.913 A. - Selected OCPD: 50 A standard breaker or 50 A fuse. - Selected conductor: 10 mm² copper (typical) subject to final verification with local code and temperature correction.

Example 2 — Three-phase motor feeder (industrial) — full development

Problem statement: Select branch-circuit breaker for a three-phase induction motor rated 30 kW, 400 V, nameplate PF = 0.85, efficiency = 92%, supply is 50 Hz, motor full-load current not directly provided. Use international standard rounding and motor circuit rules. The motor is not a continuous connected load; it will have frequent starts but runs less than three hours continuously. Step 1 — Compute full-load electrical input power and current: Motor output P_out = 30 kW. Motor electrical input power P_in = P_out / efficiency. P_in = 30000 / 0.92 = 32608.7 W. Compute current using three-phase formula: Calculate denominator: 1.732 × 400 × 0.85 = 589.88. I_design = 32608.7 / 589.88 ≈ 55.28 A. Step 2 — Motor branch-circuit protection rules (general approach): Motor branch-circuit short-circuit and overload device sizing often follows manufacturer data and national code. Common practice derived from NEC 430 (industry standard) for circuit breakers:

• Motor branch-circuit short-circuit and ground-fault protection may be set up to 250% of motor full-load current for inverse-time breakers (varies by type and standard).
• Overload (thermal) protection must be set to protect motor windings and is often 115%–125% of FLC depending on device type.

For this example we aim to provide a branch-circuit breaker sized for normal operation, while relying on separate overload relay for motor overload protection. A conservative engineering approach for a circuit-breaker OCPD that protects conductors while allowing motor starting uses 125% of FLC for conductor and then selects a CB rating rounded to the next standard value while ensuring breaker trip curve allows starting current. Step 3 — Determine conductor sizing (125% rule for motor feeder conductors if considered continuous): Assume we will apply a conductor design factor of 1.25 (if motor loads are considered continuous per local rules) for thermal capacity. I_cont_for_conductor = I_design × 1.25 = 55.28 × 1.25 = 69.10 A. From conductor table, choose conductor with ampacity ≥ 69.10 A: 16 mm² copper = 76 A meets requirement. So conductor = 16 mm² copper (verify with temperature and grouping factors). Step 4 — Select CB rating for branch-circuit protection: For the breaker rating itself, conservative selection often rounds the actual operating current up without additional 1.25 factor if the breaker is intended to protect the motor with separate overload device. However when the breaker must protect conductor from continuous duty, one may choose 80 A breaker because:

• I_design = 55.28 A. If we apply rounding to standard device ratings for circuit breaker that must not exceed motor allowed maximum protective device, select next standard rating after either I_design or conductor requirement depending on final protection strategy.
• From standard ratings: 63 A is next above 55.28 A. But conductor selection required 76 A rating equivalent (16 mm²). If choosing 63 A breaker, conductor is oversized but acceptable. If motor starting currents are high, 63 A breaker may nuisance-trip. Selecting 80 A breaker avoids nuisance trips but ensure motor overload protection remains as a separate thermal relay set to motor FLC.

Decision path (example final): - Protect conductors: conductor sized to 16 mm² (76 A ampacity). - Select breaker: choose 80 A standard molded-case breaker to allow starting current margin. - Set separate thermal overload relay in motor starter to nameplate FLC (or per manufacturer's recommendations) to protect motor windings. Step 5 — Verify breaking capacity and coordination: Confirm 80 A CB has sufficient short-circuit breaking capacity at point of installation. Confirm upstream device coordination and that motor inrush is allowed by breaker time-current characteristics or by using a motor starter with appropriate inrush-limiting device. Result summary: - Calculated motor operating current: 55.3 A. - Selected conductor: 16 mm² copper (typical 76 A rating). - Selected protective device: 80 A CB with separate overload protector set to motor full-load current (or as required by local code).

Selective coordination and time-current curves

Effective protection requires time-current coordination:

• Use manufacturer time-current curves for breakers and fuses (log-log plots of current vs time) to assess selectivity between upstream and downstream devices.
• For fuses, consider I2t let-through energy and the motor’s ability to withstand short-duration currents.
• For breakers, consider fixed trip or adjustable long-time pickup settings; adjust settings within code limits and manufacturer ranges.

Selective coordination is a detailed design task; typical targets include ensuring downstream device clears faults without tripping upstream devices except for faults beyond upstream protection reach.

Common practical rounding rules checklist

Use this checklist when finalizing selections:

1. Always round the calculated design current up to the next available standard OCPD rating.
2. When conductor ampacity is limiting, ensure conductor ampacity is equal or greater than the final OCPD operating current adjusted by code multipliers.
3. For continuous loads, apply 125% factor before rounding to standard OCPD ratings, unless the standard specifically provides alternative allowances.
4. For motors, follow the motor protection rules of the applicable code: some codes allow higher OCPD ratings for starting currents but require separate overload protection set to motor FLC.
5. Always check short-circuit breaking capacity (Icu/Ics for CBs; rated breaking capacity for fuses).
6. Verify terminal temperature ratings of equipment and use the lower of conductor insulation rating and terminal rating when using ampacity tables.

Manufacturer resources and tools

Use manufacturer catalogs and coordination software to determine:

• Exact trip curves and adjustable ranges for specific breaker models.
• Fuse time-current characteristics and melting/fuse link behavior.
• Prospective fault current calculators provided by switchgear vendors.

Examples of reputable manufacturer resources:

• Schneider Electric technical catalogs and coordination tools. https://www.se.com/
• Siemens product data and time-current curves. https://new.siemens.com/
• Eaton product documentation for breakers and fuses. https://www.eaton.com/

Documentation and compliance

Document the following elements in the project design package:

• Load calculations and assumptions (power, PF, efficiency).
• Derating factors applied and source tables (ambient, grouping, altitude).
• Standard device tables used for rounding and the final selected device sizes and types.
• Manufacturer data sheets that justify the interrupting capacity and coordination.
• References to applicable code clauses (NEC, IEC, or national standards).

Closing technical reminders and best practices

• Always treat motor starting current and inrush as separate design considerations—do not rely on steady-state current alone.
• Prefer selecting devices that provide a reasonable margin for future load growth while maintaining coordination and safety.
• Never allow the selected OCPD rating to exceed equipment maximum protection ratings on nameplates unless specifically permitted by code and manufacturer guidance.
• When in doubt, consult the equipment manufacturer or an accredited testing laboratory for confirmation of protective device compatibility and breaking ratings.

References and further reading:

• NFPA 70, National Electrical Code (NEC). https://www.nfpa.org/NEC
• IEC 60364 series (electrical installations of buildings). https://www.iec.ch/
• IEC 60947-2, Circuit-breakers. https://www.iec.ch/
• IEC 60269, Low-voltage fuses. https://www.iec.ch/
• UL 489, Molded-case circuit breakers and circuit-breaker enclosures. https://standardscatalog.ul.com/
• IEEE recommended practice for electric power distribution for industrial plants (IEEE 141, The Red Book). https://standards.ieee.org/

By following the formulas and stepwise procedure above, applying the 125% rule for continuous loads where required, rounding up calculated currents to the next standard equipment rating, and validating conductor ampacity and interrupting capacity, designers will reliably select appropriate breaker and fuse sizes that satisfy safety, continuity, and code requirements.