Circuit Breaker Size Calculator — NEC 240 / 430 OCPD Sizing

TL;DR — What this calculator returns. Enter load amps or kW (resistive / general), or motor HP (NEC 430 branch circuit) — the tool returns the recommended overcurrent protective device (OCPD / breaker) sized per NEC 240.6(A) standard ratings, with NEC 240.4(B) next-larger OCPD logic when no standard OCPD matches the conductor ampacity, and the NEC 210.20(A) / 215.3 / 625 125 % continuous-load rule applied automatically when the load runs more than 3 hours. For motor branch circuits, the tool follows NEC 430.52 (inverse-time breaker = 250 % of FLA for NEMA Design A/B, 150 % for Design C/E). 100 % client-side, no signup, no upload.

What size breaker do I need?

A circuit breaker size calculator solves one specific problem: given a load, find the smallest NEC-compliant standard breaker that will protect the conductor without nuisance tripping. The answer depends on three pieces of information — running amps, whether the load is continuous (NEC Article 100: expected to run 3 hours or more), and which NEC Article applies to your circuit type.

For most residential and small-commercial branch circuits, the chain is NEC 240.4 → 240.6(A) → 210.20(A) — conductor ampacity is matched to one of the 27 standard OCPD sizes listed in NEC 240.6(A), and a 125 % multiplier is layered on when the load is continuous (EV chargers, commercial lighting, storage heaters). For motor branch circuits the chain is fundamentally different: NEC 430.52 → Table 430.250 (3Φ) / Table 430.6 (1Φ) → 250 % or 150 % multiplier depending on NEMA/IEC design letter. For service-entrance and feeder conductors the chain is NEC 215.3 → 220 (demand factors optional). And for hot tubs, spas, and pool equipment, NEC 680 layers GFCI protection onto the sizing chain.

The most common inspector question on a permit drawing is: “What NEC Article sized this breaker?” This calculator answers that question in line with the result card — every recommended OCPD is tagged with the Article (240.6(A), 240.4(B), 210.20(A), 430.52, etc.) that produced it. Once you have the breaker size, pair the breaker with the conductor using our Wire Size Calculator — that’s the second half of the NEC 110.14(C) + 240.4(B) pairing flow.

Why this circuit breaker calculator?

Most online breaker-sizing tools return “round to the nearest 5 A,” which is not NEC-compliant — inspectors reject non-standard OCPD sizes during plan review. This calculator is built around the actual NEC 2023 NFPA-70 logic, not around a smoothing function. Six things make it different:

Standard OCPD ladder (NEC 240.6(A))

NEC 240.6(A) lists exactly 27 standard ratings in amperes: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800. Anything else — including “55 A breaker” — is non-standard and will be rejected on a permit drawing. The calculator always returns a value from this list, with no rounding exceptions.

Round-up logic (NEC 240.4(B))

NEC 240.4(B) permits the next-larger standard OCPD only when no standard OCPD corresponds to the conductor ampacity (the next-larger OCPD is also capped at 800 A). For example, 6 AWG copper at 75 °C terminal rating has an ampacity of 65 A — the next standard OCPD larger than 55 A is 60 A, which is permitted. But 50 A conductor + 50 A OCPD is already a direct match — bumping that to 60 A is an over-size, not a 240.4(B) round-up. The calculator surfaces this distinction with a 🟢 / 🟡 / 🔴 compliance badge.

Continuous-load 125 % rule (NEC 210.20(A) / 215.3 / 625)

A continuous load (NEC 100 definition: expected to run ≥ 3 hours) requires OCPD ≥ 1.25 × load. EV chargers, solar PV inverters, battery storage PCS units, and most commercial lighting all trigger this rule. The calculator applies it by default for EV-charger presets and exposes a checkbox for manual entries. (Note: NEC 215.3 governs feeder OCPD sizing; the 125 % multiplier is identical on both sides of the service.)

Motor branch-circuit path (NEC 430.52)

NEC 430.52 sizes OCPD for motors using FLA × multiplier, where FLA comes from NEC Table 430.250 (3-phase) or Table 430.6 (single-phase) and the multiplier depends on NEMA design letter: Design A or B = 250 % of FLA, Design C or E = 150 % of FLA, Design D = 150 % of FLA (verify against the manufacturer’s OCPD list for Design D). Most breaker-sizing calculators skip this path entirely; the calculator handles it as a dedicated tab.

Wire-size pairing (NEC 110.14(C))

NEC 110.14(C) requires conductor terminations to be rated at not less than the OCPD rating — for OCPDs ≤ 100 A, the 75 °C column of NEC Table 310.16 is the inspector-favored reference. The calculator flags the appropriate ampacity column and cross-links to the Wire Size Calculator for the conductor side of the pair.

100 % client-side, no signup

All math runs in your browser. No form submission, no account, no email collection, no upload. Verify with DevTools → Network tab: zero outbound traffic beyond the initial page load.

How to use this circuit breaker calculator

The calculator has two modes (resistive/general load and motor branch circuit) and ten presets (30 A dryer, 40 A range, 50 A subpanel, 60 A EV charger, 100 A subpanel, 200 A service, 32/40/48 A EV charger variants, 10/25 HP motors). Pick a preset for a one-click answer, or enter load data manually.

Step 1 — Choose a preset or enter load manually

Click any preset chip (e.g., EV L2 48A (continuous)) to fill the inputs and surface a result instantly. To enter manually, switch to Resistive mode and type your load amps (or kW) into the corresponding field. For motor branch circuits, click the Motor Branch Circuit (NEC 430) tab and enter motor HP + voltage + phase + design letter.

Step 2 — Set voltage, phase, and power factor

In Resistive mode, choose system voltage (120 / 208 / 220 / 240 / 277 / 347 / 380 / 480 / 600 V), phase (1Φ or 3Φ), and power factor (default 0.95; pure resistive loads can use 1.0). Toggle the Continuous load checkbox for loads expected to run more than 3 hours — EV chargers, grid-tie PV inverters, and storage heaters all qualify. In Motor mode, set motor voltage, phase, and NEMA/IEC design letter (A, B, C, E, D); power factor is not used in NEC 430 sizing.

Step 3 — Read the OCPD result card

The result card shows three numbers: running amps (the actual load current), design amps (running × 1.25 if continuous, otherwise running amps directly), and the recommended OCPD — the smallest NEC 240.6(A) standard rating ≥ design amps. Below the OCPD number, a compliance badge indicates which rule fired:

• 🟢 NEC 240.6(A) Direct Match — running amps already align with a standard OCPD; no round-up required.
• 🟢 NEC 240.4(B) Round-Up Applied — next-larger OCPD permitted because no standard OCPD matches the conductor ampacity.
• 🟡 NEC 430.52 × 250 % Applied — motor branch circuit, Design A/B (verify motor nameplate design letter).
• 🟡 NEC 430.52 × 150 % Applied — motor branch circuit, Design C/E.
• 🔴 Exceeds 800 A — design current exceeds the 240.4(B) 800 A cap; use parallel conductors (NEC 310.10(H)) or service-entrance equipment.

Below the badge, the formula trace (e.g., 48 A × 1.25 = 60 A → 240.6(A) → 60 A OCPD) shows exactly how the result was derived, so you can cite the Article number on a permit drawing.

NEC breaker sizing basics

The full NEC 240 + 430 + 210.20(A) chain is the spine of US residential, commercial, and industrial electrical work. The four sub-sections below cover each link in that chain with the formulas, tables, and Inspector Reference Tables an electrician actually needs on the job site.

NEC 240.6(A) standard sizes

NEC 240.6(A) lists the standard ampere ratings for fuses and inverse-time breakers. Only these sizes are permitted on a permit drawing:

NEC 240.4(B) next-larger rule

NEC 240.4(B) is the most-misunderstood article in residential wiring. The verbatim text says: “The next higher standard rating of overcurrent device … shall be permitted for protection of conductors … where the ampacity of the conductors does not correspond to a standard ampere rating …” — capped at 800 A. Translated to the field:

• Conductor sized at 50 A (e.g., 6 AWG copper at 60 °C = 55 A or 75 °C = 65 A, designed against 50 A OCPD) → 50 A OCPD is a direct match; no round-up. Bumping to 60 A is over-sizing and a code violation.
• Conductor sized at 65 A (6 AWG copper at 75 °C terminal rating) → no 65 A OCPD exists; round up to 70 A OCPD (the next standard).
• Conductor sized at 110 A (e.g., 1 AWG copper at 75 °C = 130 A, designed for a 100 A subpanel but corrected to 110 A) → no 110 A OCPD exists; round up to 125 A OCPD.

The rule does not apply if a standard OCPD already matches the conductor ampacity. The calculator surfaces this distinction with a 🟢 direct-match badge vs. a 🟡 round-up badge.

NEC 210.20(A) / 215.3 continuous-load rule

NEC 210.20(A) governs branch-circuit OCPD sizing; NEC 215.3 governs feeder OCPD sizing. Both apply the same 125 % multiplier when the load is continuous (expected to run ≥ 3 hours, per NEC Article 100):

OCPD ≥ 1.25 × non-continuous load + continuous load

For a single-load continuous circuit, this simplifies to OCPD ≥ 1.25 × full load. Worked examples:

• 32 A continuous (Level-2 EV charger, 7.7 kW) → 32 × 1.25 = 40 → 40 A OCPD (direct match).
• 40 A continuous (Level-2 EV charger, 9.6 kW) → 40 × 1.25 = 50 → 50 A OCPD (direct match).
• 48 A continuous (Level-2 EV charger, 11.5 kW) → 48 × 1.25 = 60 → 60 A OCPD (direct match).

EV charging (NEC 625) is always treated as continuous by modern AHJ interpretation — that interpretation is the standard across the US and Canada.

NEC 430.52 motor branch-circuit sizing

NEC 430.52 sizes OCPD for motors using FLA × multiplier:

• Design A or B (most common, NEMA standard): 250 % of FLA on an inverse-time breaker.
• Design C or E (high starting torque, IEC high-efficiency): 150 % of FLA.
• Design D (very high locked-rotor current): 150 % of FLA, then verify against the manufacturer’s published OCPD list.

Worked example — a 460 V 3-phase 25 HP Design B motor: FLA from NEC Table 430.250 = 34 A; 34 × 2.50 = 85 A; next standard OCPD = 90 A. Per NEC 430.52(C)(1) Exception 1, the next-larger standard OCPD is permitted when the calculated value doesn’t match a standard. For motor feeder overcurrent protection, see NEC 430.62 (a different calculation); this calculator focuses on branch-circuit OCPD only.

NEC 220 demand-factor pairing (advanced)

For services and subpanel feeders in residential and small-commercial work, NEC 220 demand factors can reduce the design load below the nameplate sum. NEC Table 220.55 (range demand) and 220.82 (optional method, single-family residence) are common. The calculator exposes a demand-factor toggle in advanced mode — apply a percentage (e.g., 0.65 for 12 kW range demand) to scale design amps before the 240.6(A) match. This is the only calculator in the cluster that surfaces demand-factor math explicitly; the other tools expect nameplate inputs.

Breaker size for common services (presets)

The presets below pre-fill the calculator inputs with typical residential / small-commercial values. Click any preset for a one-click answer; the result card carries the NEC Article number for permit review.

30 A dryer circuit

A 240 V 24 A dryer (nameplate FLA, NEC 220.54 demand = 5,000 VA or 24 A minimum) → non-continuous → 24 A design amps → next standard OCPD = 30 A. Pair with 10 AWG copper (75 °C ampacity 35 A) on a 30 A 2-pole breaker. Verify with the dryer nameplate — modern dryers with steam cycles may draw 30 A continuously and require a 40 A OCPD under NEC 210.20(A).

40 A electric range circuit

A 240 V 33 A electric range (NEC Table 220.55 demand applied to the nameplate) → non-continuous in most jurisdictions → 33 A design amps → next standard OCPD = 40 A. Pair with 8 AWG copper (75 °C ampacity 50 A) on a 40 A 2-pole breaker. Range hoods and warming drawers on the same circuit may push the load into continuous-load territory — check with the AHJ.

50 A RV / subpanel feeder

A 240 V 50 A RV outlet (NEC 551) or small subpanel feeder → non-continuous (typical intermittent RV use) → 50 A direct match → 50 A OCPD. Pair with 6 AWG copper (75 °C ampacity 65 A) on a 50 A 2-pole breaker. For a continuous-load subpanel (one that feeds an EV charger downstream, for example), bump to 60 A OCPD under NEC 210.20(A).

60 A EV charger (Level 2, 11 kW)

Level-2 EV charging is always continuous (NEC 625 + 210.20(A)). A 48 A continuous load (a typical 11 kW Level-2 unit: Tesla, ChargePoint, JuiceBox, Emporia) → 48 × 1.25 = 60 → 60 A OCPD (direct match). Pair with 4 AWG copper (75 °C ampacity 85 A) on a 60 A 2-pole breaker. Always check the charger nameplate for actual FLA — some Level-2 units are 40 A continuous (requires 50 A OCPD) or 32 A continuous (requires 40 A OCPD). Verify the charger’s actual demand with our EV Charging Power Calculator.

100 A subpanel feeder

A 240 V 100 A subpanel feeder serving a workshop or detached garage → typically non-continuous at the feeder level (NEC 215.3) → 100 A direct match → 100 A OCPD. Pair with 3 AWG copper or 1 AWG aluminum on a 100 A 2-pole breaker. For subpanels that feed continuous loads downstream (EV chargers, PV inverters), apply the 125 % multiplier at the load level, not at the feeder; feeder amps remain 100 A.

200 A service entrance

A 240 V 200 A residential service entrance → typically non-continuous at the service level when calculated per NEC 220.82 optional method → 200 A direct match → 200 A OCPD. Pair with 2/0 AWG copper or 4/0 AWG aluminum on a 200 A 2-pole main breaker. For all-electric residences with continuous loads (PV inverter + EV + heat-pump compressor running simultaneously), check with the AHJ — some jurisdictions require 320 A or 400 A service to keep feeder OCPDs under the 80 % continuous-load rule.

10 kW / 25 kW motor branch circuits

NEC 430.52 motor branch-circuit OCPD sizing — fundamentally different from resistive loads.

• 460 V 3-phase 10 HP Design B motor → FLA = 14 A (NEC Table 430.250) → 14 × 2.50 = 35 → next standard OCPD = 35 A. Pair with 8 AWG copper (75 °C ampacity 50 A, NEC 110.14(C) ≥ 35 A OCPD).
• 460 V 3-phase 25 HP Design B motor → FLA = 34 A → 34 × 2.50 = 85 → next standard OCPD = 90 A. Pair with 3 AWG copper (75 °C ampacity 100 A).
• 230 V 3-phase 25 HP Design C motor → FLA = 68 A → 68 × 1.50 = 102 → next standard OCPD = 110 A. Pair with 1 AWG copper (75 °C ampacity 130 A).

For the kW → FLA pre-step, use our Three-Phase Power Calculator.

Wire size ↔ breaker size pairing (NEC 240.4(B))

The breaker and the conductor are a paired system — the breaker protects the conductor by tripping before the conductor overheats, and the conductor must be sized to carry the load without exceeding its insulation rating. NEC 110.14(C) + 240.4(B) govern the pair. The pairing table below maps typical NEC 240.6(A) breakers to the smallest 75 °C-rated copper conductor that satisfies both rules:

How to pair the breaker with the conductor

Once the calculator returns the OCPD size, jump to the Wire Size Calculator to size the conductor. The pairing flow is bidirectional — wire-size calculators should reference back to the recommended breaker (NEC 240.6(A) standard OCPD), and breaker-size calculators should reference the conductor that pairs with the OCPD. Pairing both directions is what makes a permit drawing inspector-friendly.

For motor branch circuits, the conductor is sized per NEC 430.22 (125 % of motor FLA — a different calculation from the OCPD-sizing path), so the breaker and conductor decisions are independent. Confirm both with the calculator pair.

FAQ

Seven common questions an electrician or homeowner asks on the permit desk — answers cite the NEC Article so the citation line is copy-paste-able onto a drawing.

What size breaker do I need for 50 amps?

A 50 A continuous load requires a 50 A standard OCPD on a conductor rated at least 50 A (NEC 240.4). For a 50 A non-continuous load (a typical subpanel feeder or RV outlet), pair it with 6 AWG copper (75 °C ampacity 65 A) and a 50 A breaker — direct match, no round-up. For a 50 A continuous load, NEC 210.20(A) requires OCPD ≥ 125 % of load = 62.5 A, which round-ups to the next standard OCPD = 70 A under NEC 240.4(B). Most 50 A subpanels are non-continuous, so the 50 A breaker is correct — verify with the actual load profile before pulling the permit.

Can I use a 60 amp breaker on a 50 amp circuit?

Yes — but only under specific NEC 240.4(B) conditions. The “next-larger standard OCPD” rule permits a 60 A breaker on a conductor whose ampacity does not correspond to a standard OCPD — for example, a 6 AWG copper conductor at 75 °C ampacity = 65 A, which has no standard OCPD match (50 A and 60 A bracket it). In that case, 60 A is the code-compliant choice. The rule does NOT apply if a standard OCPD (50 A) already matches the conductor ampacity; using 60 A there would be an over-size violation. For continuous loads, NEC 210.20(A) still requires the 125 % multiplier first — a 50 A continuous load still needs a 70 A OCPD.

What is the NEC 240.4(B) round-up rule?

NEC 240.4(B) — Overcurrent protection of conductors — states: “The next higher standard rating of overcurrent device … shall be permitted for protection of conductors … where the ampacity of the conductors does not correspond to a standard ampere rating …” — but the next-larger OCPD is capped at 800 A. In practice: round up to the next standard OCPD from NEC 240.6(A) only when no standard OCPD exactly matches the conductor ampacity. Conductor sized at 65 A (6 AWG / 75 °C) → 70 A OCPD. Conductor sized at 50 A → 50 A OCPD (direct match, no round-up). This rule is the most common source of inspector pushback — verify with the AHJ before permit submission.

What size breaker for a Level-2 (40-50 A) EV charger?

For a Level-2 EV charger (NEC 625 + 210.20(A) — EV charging is a continuous load, defined as running ≥ 3 hours): 32 A continuous → 40 A breaker (32 × 1.25 = 40 A direct match), 40 A continuous → 50 A breaker (40 × 1.25 = 50 A direct match), 48 A continuous → 60 A breaker (48 × 1.25 = 60 A direct match). Pair the breaker with our Wire Size Calculator to size the conductor: a 60 A EV charger typically needs 4 AWG copper (75 °C ampacity 85 A, but verify with the charger nameplate and AHJ). Common 11 kW Level-2 units (Tesla, ChargePoint, JuiceBox) draw 48 A continuous — use the 60 A EV charger preset in our calculator for a one-click answer. Verify the charger’s actual demand with our EV Charging Power Calculator.

How do you size a breaker for a motor?

Motor branch-circuit OCPD sizing follows NEC 430.52 — fundamentally different from resistive loads. The formula: OCPD ≥ FLA × multiplier, where FLA (full-load amps) comes from NEC Table 430.250 (3-phase) or Table 430.6 (single-phase), and multiplier depends on NEMA/IEC design letter: Design A or B (most common) = 250 % of FLA, Design C or E = 150 % of FLA, Design D = 150 % of FLA (verify against manufacturer OCPD list). Example: 460 V 3-phase 25 HP Design B motor → FLA = 34 A → 34 × 2.5 = 85 A → next standard OCPD = 90 A. Round-up to next-larger standard OCPD is permitted under NEC 430.52(C)(1) Exception 1. For motor feeder overcurrent protection, see NEC 430.62.

What is NEC 240.6(A)?

NEC 240.6(A) — Standard Ampere Ratings — lists the 27 standard OCPD sizes recognized by the National Electrical Code: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800 A. These are the only OCPD sizes you can specify on a permit drawing — anything else (e.g., “55 A breaker”) is non-standard and will be rejected by the Authority Having Jurisdiction (AHJ). Our calculator always returns a value from this list, with NEC 240.4(B) round-up logic applied when the load exceeds the largest direct match below 800 A.

What’s the difference between NEC 210.20(A) and 215.3?

NEC 210.20(A) governs branch-circuit OCPD sizing — the breaker on a single load circuit (a 20 A kitchen receptacle, a 30 A dryer, a 60 A EV charger). NEC 215.3 governs feeder OCPD sizing — the breaker on a subpanel feeder or service-entrance conductor (a 100 A subpanel feed, a 200 A service). Both articles apply the 125 % continuous-load rule identically: OCPD ≥ 1.25 × non-continuous load + continuous load. The distinction matters for which conductor you’re sizing — branch-circuit conductors per NEC 310.16 + 110.14(C) terminal ratings; feeder conductors per NEC 215.2 + 220 (demand factors may apply for feeders only). For a single-family residence, the 125 % rule is the same on both sides; the difference shows up in commercial / multi-family demand-factor math.

What breaker for a hot tub (NEC 680)?

A 240 V hot tub typically draws 30 A continuous (per nameplate) → NEC 210.20(A) + 680 require OCPD ≥ 30 × 1.25 = 37.5 A → next standard OCPD = 40 A. Conductor: 6 AWG copper (75 °C ampacity 65 A) on a 40 A or 50 A GFCI breaker (50 A is also common for hot tubs with larger heaters). The GFCI protection requirement is NEC 680.44 — must be a GFCI breaker (not just a GFCI receptacle) for spa/hot tub installations. Verify with the manufacturer’s installation manual — some 240 V hot tubs draw up to 50 A continuous and require a 60 A breaker + 4 AWG copper.

Related Tools

• Wire Size Calculator — pair the breaker with the conductor per NEC 110.14(C) + 240.4(B).
• Three-Phase Power Calculator — kW → FLA pre-step for NEC 430 motor branch circuits.
• EV Charging Power Calculator — verify the actual EV charger demand before sizing the OCPD.

⚠️ Reference only. This calculator and its accompanying copy apply the 2023 NFPA-70 (National Electrical Code) formulas described above and are intended for educational and reference use. Confirm all breaker, conductor, and GFCI sizing decisions with a licensed electrician and your local Authority Having Jurisdiction (AHJ) before pulling a permit or energizing any circuit. NEC adoption and amendments vary by jurisdiction — the AHJ’s interpretation is the final authority on your installation. All math runs 100 % client-side; your load data never leaves the browser.