TryBuildCalc20 Amp Breaker Calculator (Load Check for a 20A Circuit)
Check your load against a 20A breaker.
🕒 Last updated: August 27, 2026
Inputs
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))
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.
20A breaker load check
20A is one of the most common residential breaker sizes, typically paired with 12 AWG copper wire — this page is pre-set to a 16A continuous-equivalent load, the maximum recommended continuous load for a 20A circuit.
Edit the load above to check whether your actual circuit fits 20A or needs a different standard size.
- A 20A circuit's maximum recommended continuous load is 16A (20 ÷ 1.25), per NEC 210.20(A).
- 12 AWG copper is the standard wire pairing for a 20A circuit.
- Increase the load above to see when 20A is no longer sufficient.
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) | 16.0 A (continuous) | 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 | 16.0 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).
Related Calculators
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.