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Circuit Breaker Size Calculator (NEC 210.20 / 240.6 — Single or Multiple Loads)

Calculate breaker size instantly.

Inputs

Multiple Loads on One Circuit?
Load Entered As
A
Continuous Load (3+ hours)?
V

ℹ️Common values: 120V, 208V, 240V, 277V, 480V (US); 230V, 400V (many other countries).

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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).

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ℹ️Flags GFCI (NEC 210.8) and AFCI (NEC 210.12) protection requirements for dwelling units — always confirm against local code amendments.

Wire Cross-Check

Cross-Check Against a Specific Wire Size?

Cost

Enable Cost Estimation?

Recommended Breaker: 20A (1-pole)

From 16.0A 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 →

Breaker Panel VisualizationPanel20A1-pole16.0 ADiagram simplified for clarity (not to scale)

What Is a Circuit Breaker Size Calculator?

A circuit breaker size calculator recommends the correct standard breaker (overcurrent protective device) rating for a circuit, based on its load current — applying NEC 210.20(A)'s continuous-load 125% safety factor, then rounding up to the nearest NEC 240.6(A) standard ampere rating (15, 20, 25, 30A, and so on).

Unlike the Wire Size Calculator (which starts from a load and finds the right wire, with the breaker as one part of that result), this tool goes straight from load to breaker — and can combine several loads sharing one circuit into a single combined breaker size, which the Wire Size Calculator doesn't do.

Why getting breaker size right matters:

  • An undersized breaker trips nuisance-fashion under normal load, or worse, doesn't protect the circuit as intended
  • An oversized breaker fails to trip before the wire is damaged — a genuine fire safety risk, not just an inconvenience
  • Combining multiple loads onto one circuit without properly summing their design currents is a common source of undersized breakers on shared circuits
  • GFCI/AFCI protection requirements vary by circuit location and are easy to overlook when focused only on the amperage question

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

InputValueWhy it is used
Load(s)16.0 ASets the total design current
System120 V, single-phaseSets number of poles
LocationGeneral / Other (living area, hallway, closet)Sets GFCI/AFCI requirement flags

Step 1 — Design Current

CalculationResult
Total base current16.0 A
Total design current16.0 A

Therefore, for 16.0A of design current at 120V, you need a 20A, 1-pole breaker, with AFCI protection required for general / other (living area, hallway, closet).

Essential Checklist+

Complete these critical checks before approving the work or proceeding to the next construction stage.

✓13 Inspection Points
✓4 Verification Categories
✓Load Classification+
  • Load current confirmed from equipment nameplate, not estimated
  • Continuous vs. non-continuous classification confirmed for every individual load, not applied as one blanket assumption
  • Loads entered in Multiple Loads mode genuinely share one physical circuit/breaker
  • No informal "diversity" or simultaneous-use reduction applied to a single branch circuit's load
✓Formula Application+
  • 125% continuous-load factor applied per load, then summed — not applied once to a pre-summed raw total
  • Final breaker size confirmed as a genuine NEC 240.6(A) standard rating, rounded up
  • Single-phase vs. three-phase confirmed against the actual service/panel
✓GFCI/AFCI Protection Requirements+
  • Circuit location selected matches the actual room/area the circuit will serve
  • Combination AFCI/GFCI (dual-function) breaker specified where a location requires both protections
  • Local code edition and amendments checked for GFCI/AFCI requirements, not assumed from a general reference
✓Wire Cross-Check & Code Compliance+
  • Recommended breaker checked against the actual wire size being installed, not assumed automatically compatible
  • Permit and inspection requirements confirmed for the scope of work
  • Work beyond a simple single-circuit breaker check reviewed or performed by a licensed electrician
Full QC Checklist+

Verification checklist for circuit breaker sizing — covering load classification, formula application, GFCI/AFCI protection requirements, and wire/code compliance. Use the Essential Checklist for critical checks before wiring; expand to Full QC Checklist for complete verification.

✓20 Inspection Points
✓4 Verification Categories
✓Load Classification+
  • Load current confirmed from equipment nameplate, not estimated
  • Continuous vs. non-continuous classification confirmed for every individual load, not applied as one blanket assumption
  • Loads entered in Multiple Loads mode genuinely share one physical circuit/breaker
  • Power factor entered correctly when sizing from a wattage nameplate rating on a motor or reactive load
  • No informal "diversity" or simultaneous-use reduction applied to a single branch circuit's load
✓Formula Application+
  • 125% continuous-load factor applied per load, then summed — not applied once to a pre-summed raw total
  • Final breaker size confirmed as a genuine NEC 240.6(A) standard rating, rounded up
  • Circuit configuration (line-to-neutral vs. line-to-line) confirmed, not assumed from voltage alone
  • Single-phase vs. three-phase confirmed against the actual service/panel
  • HACR-rated breaker specified for air conditioning, heat pump, or refrigeration compressor loads
✓GFCI/AFCI Protection Requirements+
  • Circuit location selected matches the actual room/area the circuit will serve
  • Method of providing required GFCI protection clarified — breaker-type GFCI vs. GFCI receptacle
  • Combination AFCI/GFCI (dual-function) breaker specified where a location requires both protections
  • Local code edition and amendments checked for GFCI/AFCI requirements, not assumed from a general reference
  • Weatherproof in-use cover confirmed for outdoor or wet-location receptacles, alongside GFCI protection
✓Wire Cross-Check & Code Compliance+
  • Recommended breaker checked against the actual wire size being installed, not assumed automatically compatible
  • Breaker's interrupting (AIC) rating confirmed adequate for the available fault current at its location
  • An existing installed breaker re-verified against actual current load, not assumed correctly sized because it's already installed
  • Permit and inspection requirements confirmed for the scope of work
  • Work beyond a simple single-circuit breaker check reviewed or performed by a licensed electrician

NEC 240.6(A) Standard Breaker Sizes

Standard ampere ratings for fuses and inverse time circuit breakers — a breaker rating always rounds up to one of these, never to a custom in-between value.

Standard Sizes (A)
15, 20, 25, 30, 35, 40, 45, 50, 60
70, 80, 90, 100, 110, 125, 150, 175, 200
225, 250, 300 (this calculator's upper limit)

NEC 240.6(A) lists further sizes above 300A (350, 400, 450A, and beyond) for larger service and feeder work — outside this calculator's scope.

When should you use this circuit breaker calculator?

  • Sizing a breaker for a new single-load branch circuit.
  • Combining several loads sharing one panel circuit into one correctly-sized breaker.
  • Checking whether an existing breaker is adequate — or oversized — for its actual load.
  • Confirming GFCI/AFCI protection requirements for a specific room or location before purchasing a breaker.
  • Cross-checking a recommended breaker against a specific wire size already chosen.

Quick Circuit Breaker Sizing Tips

  • Classify each load's continuous status carefully — most lighting and HVAC circuits run 3+ hours and need the 125% factor.
  • Use Multiple Loads mode for any shared circuit — summing raw currents without each load's own 125% factor understates the requirement.
  • Check GFCI/AFCI requirements by the room the circuit actually serves, not just its amperage — this is easy to overlook.
  • Always cross-check the recommended breaker against the actual wire size, especially for 14/12/10 AWG conductors capped by NEC 240.4(D).
  • Never round a breaker size down to save cost — always round up to the next NEC 240.6(A) standard size.

Common Mistakes

  • Forgetting the 125% continuous-load factor, or applying it to non-continuous loads that don't need it.
  • Summing multiple loads' raw currents first and applying 125% once to the total, instead of per-load — this understates the requirement whenever the mix of continuous/non-continuous loads differs.
  • Sizing the breaker to the load without cross-checking it against the actual wire — a breaker that exceeds the wire's rating defeats the wire's overcurrent protection.
  • Assuming GFCI/AFCI requirements only apply to bathrooms — kitchens, garages, outdoor circuits, and most habitable rooms have their own requirements too.
  • Rounding a breaker size down to the nearest standard rating instead of up — NEC 240.6(A) sizing always rounds up.

Limitations

  • GFCI/AFCI flags are a general dwelling-unit planning reference per NEC 210.8/210.12 — local amendments and edition differences can change specifics; always confirm against the code actually adopted locally.
  • The wire cross-check is informational only (small-conductor cap or base 75°C ampacity, no derating) — use the Wire Size Calculator for a full ampacity, derating, and conduit-fill check.
  • Covers standard breaker sizes up to 300A — larger service/feeder overcurrent devices are outside this calculator's scope.
  • NEC-based only (US) — other regions use different overcurrent protection standards and conventions and this calculator does not model them.

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.

FAQ

Start from the load's current (or watts and voltage), apply NEC 210.20(A)'s 125% factor if the load is continuous (running 3+ hours), then round up to the nearest NEC 240.6(A) standard breaker size (15, 20, 25, 30A, and so on). For example, a 16A continuous load needs 16 × 1.25 = 20A of design current, which rounds up to a standard 20A breaker. A 16A non-continuous load stays at 16A design current, which also rounds up to 20A — the same final answer here, but the underlying math differs, and it matters more once multiple loads with different continuous status are combined.
The Wire Size Calculator starts from your load and searches for the smallest wire that satisfies both ampacity and voltage drop, with the correct breaker as one part of that combined result — a single-load tool. This calculator goes straight from load to breaker, and can combine several loads sharing one circuit into a single combined breaker recommendation, which the Wire Size Calculator doesn't do. Use the Wire Size Calculator when you need to size the wire itself; use this one when you already have (or are deciding on) a wire and just need the correct breaker, or need to combine multiple loads onto one shared circuit.
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