TryBuildCalcMultiple Loads Circuit Breaker Calculator (Combined Breaker for a Shared Circuit)
Combine multiple loads into one breaker size.
🕒 Last updated: August 27, 2026
Inputs
Load 1 — Load 1
Load 2 — Load 2
Each row is a separate load sharing this one circuit/breaker — up to 20 loads, combined into one total design current.
ℹ️Common values: 120V, 208V, 240V, 277V, 480V (US); 230V, 400V (many other countries).
ℹ️Determines pole count — voltage alone doesn't tell you this. 120V and 277V circuits are typically line-to-neutral (1-pole); 240V split-phase and 208V are typically line-to-line (2-pole).
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: 20A (1-pole)
From 20.0A design current at 120V, single-phase
Protection Required — General / Other (living area, hallway, closet)
• AFCI protection required (NEC 210.12(A))
Load Breakdown
| Load | Current | Design Current |
|---|---|---|
| Load 1 (continuous) | 8.0 A | 10.0 A |
| Load 2 | 10.0 A | 10.0 A |
| Total | 18.0 A | 20.0 A |
Circuit Summary
Design current basis: NEC 210.20(A) — 100% non-continuous + 125% continuous
Standard size: NEC 240.6(A)
Poles: 1-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 →
Checking a long run's voltage drop instead? Voltage Drop Calculator →
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Circuit Breaker Size Calculator.
Combined breaker sizing for multiple loads
This page is pre-set to Multiple Loads mode with two example loads — edit, add, or remove rows to match your actual shared circuit.
Each load's own continuous status is applied individually before summing, matching NEC's own phrasing rather than a simplified shortcut.
- Add up to 20 loads sharing one circuit.
- Each load's 125% continuous factor is applied per-load, then summed — not applied once to a pre-summed total.
- Result is one combined breaker size for the whole shared circuit.
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) | 2 loads combined | Sets the total design current |
| System | 120 V, single-phase | Sets number of poles |
| Location | General / Other (living area, hallway, closet) | Sets GFCI/AFCI requirement flags |
Step 1 — Design Current
| Calculation | Result |
|---|---|
| Total base current | 18.0 A |
| Total design current | 20.0 A |
Therefore, for 20.0A of design current at 120V, you need a 20A, 1-pole breaker, with AFCI protection required for general / other (living area, hallway, closet).
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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.