Circuit Breaker Sizing Calculator

Size a standard branch-circuit breaker with the NEC 100%/125% rule, a motor circuit under NEC Article 430, an HVAC/AC compressor circuit under NEC 440.22, or a parallel-conductor feeder — all in one tool. Every mode rounds up to a standard NEC Table 240.6(A) breaker size, pairs it with a minimum conductor from NEC Table 310.16, applies ambient-temperature derating and the NEC 240.4(D) small-conductor cap, flags likely AFCI/GFCI requirements by circuit location, and offers a selectable IEC-style trip curve for installations using international equipment. Results include an ampacity-utilization meter, a breaker-headroom comparison chart, and a print view that captures the chart and results exactly as they appear on screen. Built for electricians, electrical engineers, facility managers, and students studying for licensing exams.

Pick a calculation mode, set the circuit parameters, and click Calculate. The breaker must clear its own sizing rule and the paired conductor must have adequate ampacity.

Phase

Conductor Temp

Ambient Temp

Trip Curve

Load Type

Circuit Location (drives AFCI/GFCI flag)

Quick Presets

Conductor Material

Supply Voltage (select or enter custom)

Load (value + unit)

Power Factor (AC — used for kW/kVA/W conversion)

Motor Horsepower (NEC Table 430.248/430.250 FLC lookup)

Protection Device Type (NEC Table 430.52)

Rated-Load Amps (RLA) (compressor nameplate)

Locked-Rotor Amps (LRA) (optional, informational)

Conductor Size (per parallel set, AWG/kcmil)

Number of Parallel Sets (NEC 310.10(H): 1/0 AWG minimum)

⚠️ Motor branch-circuit protection follows NEC Article 430, not the standard 125% rule. Starting current, service factor, and overload protection (separate from short-circuit/ground-fault protection) are not fully modeled here — verify with a licensed electrician and, for conductor sizing specifically, cross-check with the Motor Starting Current Estimator.

⚠️ HVAC/AC compressor protection per NEC 440.22 is sized from rated-load current (RLA), not locked-rotor amps (LRA). If a 175%-sized device trips on startup, NEC 440.22(A) Exception 1 permits sizing up to 225% of RLA — verify manufacturer nameplate maximum overcurrent protection (MOCP) if provided, which takes precedence.

⚠️ NEC 310.10(H) permits paralleling conductors 1/0 AWG and larger only, with all sets identical in length, material, size, insulation, and terminations. The size selected here is smaller than that — verify with a licensed electrician before paralleling.

Results

Recommended Breaker

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Paired Min. Wire

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Wire Ampacity

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Load Current

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Breaker Rating Used—

Formula: —

Standard Breaker Sizes — Utilization

Each bar is a standard breaker size, and the height is how much of that breaker's rating the adjusted load uses. The size this page recommends is marked in gold. The dashed line is 100 percent of the rating.

Neighboring Sizes Recommended Size 100% Capacity

📏 Need the full conductor sizing picture, including voltage drop over distance? Use the Wire Gauge Calculator to verify the paired conductor for this breaker.

🌀 Sizing a motor circuit's conductors, not just the breaker? Starting current, service factor, and NEC Article 430 conductor requirements are covered in more depth by the Motor Starting Current Estimator.

Results are engineering estimates based on published NEC standard breaker sizes (240.6(A)), NEC Table 310.16 general conductor ampacity, NEC Table 310.15(B)(1) ambient-temperature correction factors, NEC 240.4(D) small-conductor limits, and NEC Table 430.248/430.250 motor full-load current values (cross-checked against independent published reproductions). The breaker size still has to be checked against the installation and the adopted edition of the National Electrical Code (NFPA 70). Verify with a licensed electrician before any electrical work.

How to Use the Circuit Breaker Sizing Calculator

Choose the Calculation Mode

Pick Standard Load for a general branch circuit, Motor Circuit for NEC Article 430 motor protection, HVAC/AC Circuit for NEC 440.22 compressor sizing, or Parallel Conductors for multiple runs feeding one load.

Set the Circuit Parameters

Select phase, conductor material, temperature rating, ambient temperature, trip curve, and the circuit location (drives the AFCI/GFCI flag).

Enter the Load Details

Enter amps, watts, kW, or kVA for a standard circuit; horsepower for a motor; RLA for HVAC/AC; or wire size and parallel set count for parallel conductors. Or tap a Quick Preset.

Calculate, Review, and Print

Click Calculate to see the recommended standard breaker, paired minimum wire gauge, ampacity headroom, and any AFCI/GFCI requirement. Use Print to capture results and the chart exactly as they appear on screen.

Comprehensive Guide to Circuit Breaker Sizing

The 100% / 125% rule (standard loads)

NEC 210.20(A) and 240.6(A) require a breaker sized at 100% of a non-continuous load, or 125% of a continuous load (one expected to run at maximum current for three hours or more), rounded up to the next standard breaker size. The same 125% factor applies to the paired conductor's required ampacity.

Motor circuits are sized by percentage of full-load current, not the 125% rule

NEC Article 430, specifically Table 430.52, sizes motor branch-circuit short-circuit and ground-fault protection as a percentage of the motor's full-load current (FLC) from NEC Table 430.248 (single-phase) or 430.250 (three-phase) — not nameplate current. Typical maximums are 250% for an inverse-time breaker, 175% for a time-delay fuse, 300% for a non-time-delay fuse, and up to 800% for an instantaneous-trip breaker (motor circuit protector only). This headroom accommodates motor starting (inrush) current, which can be 6–8× running current for a few cycles. Motor overload protection — protecting the motor itself from sustained overload — is a separate NEC 430 Part III requirement not modeled by this calculator.

HVAC/AC compressor sizing (NEC 440.22)

Air-conditioning and refrigeration compressor branch-circuit protection is sized from the rated-load current (RLA) or branch-circuit selection current, not locked-rotor amps (LRA), at up to 175% under NEC 440.22(A). If this trips on startup, NEC 440.22(A) Exception 1 permits an increase up to 225% of that same current. Manufacturer nameplate "Maximum Overcurrent Protection" (MOCP), when provided, takes precedence over a calculated value.

Ambient temperature derating

NEC Table 310.15(B)(1) publishes correction factors for conductor ampacity when the ambient air temperature exceeds the 30°C (86°F) baseline used in the base ampacity tables. Higher ambient temperature reduces a conductor's usable ampacity — a wire that comfortably carries a load at 30°C may not be adequate in a hot attic or rooftop conduit run without upsizing.

AFCI and GFCI requirements

NEC 210.12 requires arc-fault circuit-interrupter (AFCI) protection on most 120V, 15A and 20A branch circuits supplying outlets in dwelling-unit living areas, bedrooms, and similar rooms. NEC 210.8 requires ground-fault circuit-interrupter (GFCI) protection in kitchens, bathrooms, garages, outdoor locations, unfinished basements, crawl spaces, and laundry areas. Some locations require both protections, available as a combination AFCI/GFCI breaker. Local amendments can expand these requirements beyond the base NEC — this calculator's flag is a starting point, not a final determination.

Parallel conductors and the breaker

NEC 310.10(H) permits paralleling conductors 1/0 AWG and larger, with every parallel set matching in length, material, size, insulation, and terminations. Paralleling changes the conductor arrangement, not the overcurrent-protection sizing math — the breaker is still sized from the total load using the standard rule. What must be separately verified is that the combined ampacity of all parallel sets meets or exceeds that breaker rating.

Trip curves (B/C/D)

Trip curve is an IEC 60898 classification, common on international/DIN-rail breakers, describing the multiple of rated current at which the instantaneous magnetic trip operates: roughly 3–5× for Curve B (resistive loads), 5–10× for Curve C (general/mixed loads, small motors), and 10–20× for Curve D (transformers, capacitor banks, high-inrush motors). Standard North American thermal-magnetic breakers don't typically publish a curve letter in the same way, so this selector is most relevant for installations using IEC-style equipment or side-by-side comparison.

Worked Examples

Four real-world scenarios, one per mode. Click "Load This Example" to run it in the calculator above.

1. Standard — Continuous 32A EV Charger, 240V

A Level 2 EV charger draws 32A continuously at 240V single-phase. Continuous loads are sized at 125% for both the breaker and the conductor.

32A × 1.25 = 40A required → next standard size = 40A breaker. Paired conductor: 8 AWG copper (50A @ 75°C) clears both the 40A requirement and the 240.4(D) cap (n/a above 10 AWG).

2. Motor — 10 HP, 3-Phase, 230V, Inverse-Time Breaker

A 10 HP three-phase motor at 230V has a tabulated FLC of 28A (NEC Table 430.250). Sized with an inverse-time breaker at 250% of FLC.

28A × 2.50 = 70A max → next standard size ≤ that value = 70A breaker. Conductor sized separately at 125% of FLC: 28A × 1.25 = 35A → 8 AWG copper (50A @ 75°C).

3. HVAC/AC — 22A RLA Compressor, 240V

An outdoor AC condensing unit has a nameplate RLA of 22A. NEC 440.22(A) permits sizing up to 175% of RLA.

22A × 1.75 = 38.5A max → next standard size ≤ that value = 35A breaker (or 40A if 35A causes nuisance tripping, still within the 225% exception ceiling of 49.5A).

4. Parallel Conductors — 350A Feeder, Three 2/0 AWG per Phase, 480V 3Ø

A 480V three-phase feeder carries 350A. Three parallel 2/0 AWG copper conductors are proposed per phase — is the combined ampacity sufficient, and what breaker fits?

Breaker: 350A (standard, non-continuous) → next standard size = 350A. Combined ampacity: 3 × 175A (75°C) = 525A, well above 350A — parallel set clears NEC 310.10(H)'s 1/0 AWG minimum.

Quick Reference Table

NEC 240.6(A) standard breaker/fuse sizes, with the minimum paired 75°C copper conductor (before ambient derating or the 240.4(D) small-conductor cap).

Standard Breaker	Min. Copper (75°C)

General ampacity per NEC Table 310.16, 30°C ambient, not more than 3 current-carrying conductors. Verify with a licensed electrician for your actual installation.

NEC 240.4(D) Small-Conductor Overcurrent Protection Caps — these conductors are capped at a maximum breaker/fuse rating regardless of their table ampacity, to protect terminations.

Size	Copper Cap	Aluminum Cap

Frequently Asked Questions

How do I size a circuit breaker for a standard load?

NEC 210.20(A) and 240.6(A) require a breaker rated at 100% of a non-continuous load or 125% of a continuous load (one running 3 hours or more), rounded up to the next standard breaker size. This calculator applies that rule automatically and pairs the result with a minimum conductor size.

How is a motor circuit breaker sized differently?

Motor branch-circuit short-circuit and ground-fault protection follows NEC Article 430 Table 430.52, not the standard 125% rule. The maximum protective device rating is a percentage of the motor's full-load current (FLC) from NEC Table 430.248 or 430.250 — commonly up to 250% for an inverse-time breaker, 175% for a time-delay fuse, or 300% for a non-time-delay fuse — because motors draw high inrush current on starting that ordinary branch-circuit protection would trip on.

Why can an HVAC/AC compressor breaker be larger than its running current?

NEC 440.22(A) permits air-conditioning and refrigeration compressor protection sized up to 175% of the rated-load current or branch-circuit selection current, to accommodate compressor starting current. If 175% still causes nuisance tripping, NEC 440.22(A) Exception 1 permits an increase up to 225% of that same current.

When is AFCI or GFCI protection required?

NEC 210.12 requires AFCI protection on most 120V, 15A and 20A branch circuits supplying outlets in dwelling-unit living areas, bedrooms, and similar rooms. NEC 210.8 requires GFCI protection in kitchens, bathrooms, garages, outdoor locations, unfinished basements, crawl spaces, and laundry areas. Some locations require both. This calculator flags likely requirements from the selected circuit location, but local amendments can expand these requirements.

Can I run parallel conductors instead of one large conductor, and how does that affect the breaker?

NEC 310.10(H) permits paralleling conductors 1/0 AWG and larger, with all sets identical in length, material, size, insulation, and terminations. The breaker is still sized from the total load using the standard 100%/125% rule — paralleling changes the conductor arrangement, not the overcurrent-protection math — but the combined ampacity of all parallel sets must still meet or exceed the breaker rating.

What is a breaker trip curve (B, C, D)?

Trip curve is an IEC 60898 classification describing how much inrush current a breaker tolerates before its instantaneous magnetic trip fires. Curve B trips at roughly 3–5× rated current, suited to resistive loads; Curve C trips at roughly 5–10×, suited to general mixed loads and small motors; Curve D trips at roughly 10–20×, suited to transformers, capacitor banks, and motors with high inrush. Standard North American thermal-magnetic breakers do not typically list a curve letter, but the concept is offered here for installations using IEC-style breakers or comparison against international equipment.