A Circuit Breaker Sizing Calculator is a web tool that computes the electrical current a breaker must safely interrupt based on load (kW, kVA, or amps), phase, voltage, power factor, continuous-load conditions, derating, and inrush/startup multipliers, then recommends the nearest standard breaker rating and visualizes the comparison.

How to Use the Circuit Breaker Sizing Calculator

This article explains how the calculator works, how to interpret results, and how to embed and use it correctly on a WordPress site. It’s written for electricians, designers, engineers, contractors and site owners who need fast, practical guidance when choosing breaker sizes for typical installations.

Why this calculator matters

Sizing a circuit breaker correctly ensures protection of conductors, safe operation of equipment, and compliance with practical installation guidelines. An undersized breaker will trip unnecessarily or risk damage during overloads; an oversized breaker may not protect conductors and equipment adequately. This calculator gives a practical starting point by converting load values into current, applying common continuous-load and derating rules, and matching the result to standard, commercially available breaker sizes.

What the tool asks for (inputs explained)

The calculator accepts the following inputs:

Phase

Select Single-phase or Three-phase. This matters because three-phase current is calculated using a √3 factor.

Voltage (V)

Supply voltage of the circuit (e.g., 230 V, 400 V). The tool uses this to convert kW/kVA to amps.

Load (kW / kVA / A)

You can enter the load as kW (real power), kVA (apparent power), or directly as amps. If you enter kW, a power factor field is used to convert to apparent power (amps).

Power Factor (PF)

Used only when you input kW. Typical motor/industrial loads range from 0.8–0.95; resistive loads (heaters) are near 1.0.

Continuous Load

If marked as continuous, the calculator applies a 125% multiplier to ensure the breaker-selected rating covers a continuous duty load (a common conservative design approach used in many practices).

Derating Factor (%)

Enter a percentage for derating (for ambient temperature, conductor grouping, or cable derating). The tool uses this to increase the required capacity (entered as, e.g., 90–100% meaning how much of rated capacity is available).

Inrush / Starting Multiplier

Motors and large inductive loads can draw brief high currents. Enter a starting/inrush multiplier (e.g., 1.5× or 2×) if you want the selection current to account for startup.

What the calculator computes

1. Base current (A) — calculated from your inputs:
   • Single-phase: I = P / (V × PF) for kW; I = kVA×1000 / V for kVA.
   • Three-phase: I = P / (√3 × V × PF) for kW; I = kVA×1000 / (√3 × V) for kVA.
2. Adjusted current (A) — base current × continuous multiplier (if selected) ÷ derating fraction.
3. Selection current (A) — adjusted current × inrush multiplier (if provided). This is the value used to look up the nearest available standard breaker size.
4. Recommended breaker (A) — the next standard commercial breaker rating that is equal to or greater than the selection current. Common sizes include 10, 16, 20, 25, 30, 40, 50, 63, 80, 100 A, etc. The tool includes a broad set of sizes and picks the first that safely exceeds the required selection current.

Visual output and interpretation

The tool draws a clear bar chart (Plotly.js) showing base current, adjusted current, and selection current, plus a separate visual block representing the recommended breaker capacity. This helps you see margins between the calculated load and the protective device rating — useful for quick verification and decision-making.

Practical tips and examples

• For a 5 kW three-phase motor at 400 V, PF 0.85, continuous operation and 100% derate: the tool converts kW to amps, applies 125% for continuous, and recommends the next standard breaker (e.g., 16 A, 20 A, 25 A depending on calculation). Always check motor startup requirements — motors often need dedicated motor starters or inrush-rated breakers and protection.
• If you know cable ampacity constraints before picking a breaker, ensure the selected breaker does not exceed conductor ratings after derating. The calculator helps you choose the breaker; conductor sizing should be verified separately.

Limitations & best practice

This calculator provides guidance, not a final compliance decision. It uses conservative engineering multipliers (continuous load 125%) but does not replace local code requirements (NEC, IEC, local regs), manufacturer datasheets, or full coordination studies. Always confirm selections with a qualified electrical engineer or electrician for critical systems, motor protection, and life-safety circuits.

Disclaimer

This calculator is intended for general guidance only. It does not replace local electrical codes, the advice of a licensed electrician, or specific manufacturer recommendations. Use it as a design aid, not the final authority. Always verify breaker and conductor selections with local regulations and certified professionals.

FAQ

Q1: Can I use this for motor protection?
A1: The calculator estimates steady-state and inrush-adjusted currents, but motor protection often requires thermal overload relays, inrush/current-limited starters, and specific coordination. Use manufacturer guidance and motor-protection devices; consult an engineer for critical motors.

Q2: Should I always apply the 125% continuous load multiplier?
A2: The 125% factor is a common conservative guideline for continuous loads in many applications. Local regulations or installation specifics may require different treatments. Use it when loads are truly continuous; otherwise, it may be unnecessarily conservative.

Q3: Does the tool check cable ampacity?
A3: No. This tool recommends breaker sizes from standard ratings. After selecting a breaker, you must check conductor ampacity, terminations, and derating per local rules.

Q4: What if my recommended breaker is larger than conductor rating?
A4: Don’t install that combination. Reduce load, increase conductor size, or apply appropriate protective coordination. Consult an electrician.

Q5: Is this code SEO-safe to embed in WordPress?
A5: Yes — the tool is client-side HTML/JS. Place it in a Custom HTML block. For SEO, surround the block with clear textual content on the page (headings, explanations, and schema if desired), because search engines index surrounding content more readily than embedded JS-only widgets.