TryBuildCalc3-Phase Breaker Size Calculator (Three-Phase Circuit Sizing)
Calculate 3-phase breaker size.
🕒 Last updated: August 27, 2026
Inputs
ℹ️Common values: 120V, 208V, 240V, 277V, 480V (US); 230V, 400V (many other countries).
ℹ️Flags GFCI (NEC 210.8) and AFCI (NEC 210.12) protection requirements for dwelling units — always confirm against local code amendments.
Wire Cross-Check
Cost
Recommended Breaker: 50A (3-pole)
From 50.0A design current at 480V, three-phase
Circuit Summary
Design current basis: NEC 210.20(A) — 100% non-continuous + 125% continuous
Standard size: NEC 240.6(A)
Poles: 3-pole
Assumptions Used
NEC 210.20(A) continuous-load 125% factor, NEC 240.6(A) standard ampere ratings. GFCI/AFCI flags are a general dwelling-unit planning reference per NEC 210.8/210.12 — always confirm against local code amendments. This is a reference estimate — confirm with a licensed electrician before final installation.
Need to find the right wire size for this breaker from scratch? Wire Size Calculator →
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Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Circuit Breaker Size Calculator.
Three-phase breaker sizing
Three-phase circuits are typically protected by a 3-pole breaker — this page is pre-set to a common commercial three-phase scenario at 480V.
Edit the load, voltage, or continuous status above to check your actual three-phase circuit.
- Three-phase circuits use a 3-pole breaker, switching all three phase conductors together.
- The same 125% continuous-load factor and NEC 240.6(A) standard sizes apply regardless of phase.
- Switch Phase back to Single-phase for a standard residential branch circuit instead.
Circuit Breaker Formula: How Is Size Calculated?
The core calculation is the same whether there's one load or several sharing a circuit — the difference is how the design current is built up before rounding to a standard size.
Step 1 — Design Current per Load
Design Current = Load Current × 1.25 (if continuous — 3+ hours)
Design Current = Load Current (if not continuous)
Per NEC 210.20(A), a continuous load needs a 25% safety margin; a non-continuous load uses its actual current directly. This is applied per load before combining multiple loads onto one circuit.
Step 2 — Combined Design Current (Multiple Loads)
Total Design Current = Sum of every load's own Design Current
When several loads share one circuit, sum each load's own design current (not the raw currents first, then apply 125% once) — mathematically identical to NEC's own phrasing of noncontinuous-sum plus 125% of continuous-sum, since multiplication distributes over the sum.
Step 3 — Standard Breaker Size
Recommended Breaker = smallest NEC 240.6(A) standard rating ≥ Total Design Current
Standard ampere ratings (15, 20, 25, 30, 35, 40, 45, 50, 60A, and larger) come from NEC 240.6(A) — a breaker is always sized up to the next standard rating, never to a custom in-between value.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Load(s) | 40.0 A (continuous) | Sets the total design current |
| System | 480 V, three-phase | Sets number of poles |
| Location | Not a Dwelling-Unit Circuit (commercial/industrial) | Sets GFCI/AFCI requirement flags |
Step 1 — Design Current
| Calculation | Result |
|---|---|
| Total base current | 40.0 A |
| Total design current | 50.0 A |
Therefore, for 50.0A of design current at 480V, you need a 50A, 3-pole breaker.
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Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.