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
Quick Presets
✓
⚠️ 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
—
A
Paired Min. Wire
—
Wire Ampacity
—
A
Load Current
—
A
Breaker Rating Used—
Formula: —
Standard Breaker Sizes — Utilization

Percent of rated capacity used by your (continuous-adjusted) load across neighboring standard breaker sizes. Your recommended size is highlighted in gold; the dashed line marks 100% capacity.

Neighboring Sizes Recommended Size 100% Capacity
Breaker size comparison chart.
📏 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). Actual sizing varies with installation conditions and the adopted NEC edition. Verify with a licensed electrician before any electrical work.

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

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.

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.