What is overcurrent protection and why does NEC Article 240 require it?

Overcurrent protection devices (fuses and circuit breakers) protect conductors from thermal damage caused by excessive current. NEC 240.4 requires conductors to be protected at their rated ampacity per NEC Table 310.16 — meaning the OCPD must trip or clear a fault before the conductor's insulation is damaged. Without proper protection, overloaded conductors can overheat, melt insulation, and start fires.

What are the NEC 240.4 conductor protection rules?

NEC 240.4 states conductors shall be protected at their ampacity per NEC 310.15. Three key sub-rules: 240.4(B) permits rounding up to the next standard OCPD size when conductor ampacity doesn't exactly match a standard size (for devices ≤ 800A, except multi-receptacle branch circuits). 240.4(C) requires conductor ampacity to equal or exceed the OCPD rating for devices over 800A — no rounding up. 240.4(D) caps OCPD for small conductors: 14 AWG Cu = 15A max, 12 AWG Cu = 20A max, 10 AWG Cu = 30A max.

How do I use this overcurrent protection selector?

In Max OCPD mode, select your conductor size, material (copper or aluminum), and terminal temperature rating. The calculator looks up the conductor ampacity from NEC Table 310.16, applies the 240.4(B) round-up rule where permitted, checks 240.4(D) small conductor caps, and returns the maximum OCPD size with a full step-by-step NEC citation chain. In Min Wire mode, select your desired OCPD size to find the minimum conductor that satisfies NEC 240.4.

When can you round up to the next larger breaker or fuse per NEC 240.4(B)?

NEC 240.4(B) permits using the next higher standard OCPD size when three conditions are all met: (1) the circuit is not a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads, (2) the conductor ampacity does not correspond exactly to a standard OCPD size, and (3) the next higher standard size does not exceed 800 amperes. If any condition is not met, the OCPD must not exceed the conductor's actual ampacity.

Why is 14 AWG copper limited to a 15-ampere breaker per NEC 240.4(D)?

NEC 240.4(D)(3) caps the maximum OCPD for 14 AWG copper conductors at 15 amperes regardless of the conductor's actual ampacity. Although 14 AWG copper has a 75°C ampacity of 20 amperes, the 15-ampere cap applies because the round-up rule of 240.4(B) would otherwise allow a 20-ampere breaker — which is too close to the conductor's limit to provide a meaningful safety margin for residential wiring.

How is the overcurrent protection sizing formula calculated step by step?

Step 1: Look up conductor ampacity from NEC Table 310.16 at the terminal temperature rating per NEC 110.14(C) — use 60°C or 75°C column. Step 2: If ampacity exactly matches a standard OCPD size, use that size. If not, round up to the next standard size per NEC 240.4(B) provided the result is ≤ 800A and it's not a multi-receptacle branch circuit. Step 3: Apply NEC 240.4(D) cap for 14/12/10 AWG copper. Step 4: For devices over 800A, conductor ampacity must equal or exceed the device rating (NEC 240.4(C)).

Frequently asked questions about NEC fuse and breaker sizing

Common NEC fuse sizing questions: 12 AWG copper can only use a 20A maximum breaker (NEC 240.4(D)) even though it has 25A ampacity at 75°C. Aluminum conductors follow general ampacity rules since 240.4(D) only specifies copper limits. For continuous loads, the OCPD must be rated at 125% of continuous load per NEC 210.20(A), meaning the maximum continuous load is 80% of the OCPD rating. Standard fuse and breaker sizes are defined in NEC 240.6(A).

⚡ Electrical Calculator

NEC Fuse Sizing Calculator

Overcurrent Protection Selector · NEC 240.4 · Round-Up Rule (240.4(B)) · Small Conductor Caps (240.4(D)) · Standard Sizes (240.6(A))

⚙️ Conductor Selection

Conductor Material

Conductor Size (AWG / kcmil)

Terminal Rating (NEC 110.14(C))

Circuit Type (affects round-up eligibility)

Multi-receptacle branch circuits cannot use the round-up rule per NEC 240.4(B)(1).

Load Type

Continuous Load

Operating 3+ hours (NEC 210.20(A) — max load = 80% of OCPD)

Please fill in all required fields.

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Conductor Ampacity

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Max OCPD

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Continuous Load Max

📊 Step-by-Step — NEC 240.4 Logic

📋 NEC 240.4 Quick Reference — Max OCPD by Conductor Size

Conductor	Cu Ampacity (75°C)	Cu Max OCPD (General)	Al Ampacity (75°C)	Al Max OCPD (General)

240.4(D) cap shown in amber — applies to 14, 12, and 10 AWG copper only. General circuit (not multi-receptacle). Terminal rating 75°C. NEC 240.6(A) standard sizes applied.

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Need NEC Article 240 on the job?

VoltageLab's AI Code Lookup has every NEC Article 240 section — overcurrent device sizing, conductor protection, fuse and breaker ratings, and the small conductor rules — referenced and explained instantly. Stop hauling a codebook to every panel.

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Ampacity from NEC Table 310.16 (conductors in raceway or cable, not more than 3 current-carrying conductors, 30°C ambient). Terminal temperature limits per NEC 110.14(C).
Round-up rule: NEC 240.4(B) — permitted for general circuits where result ≤ 800A and conductor ampacity does not correspond to a standard size. Not permitted for multi-receptacle branch circuits.
240.4(D) caps apply to copper only: 14 AWG = 15A max, 12 AWG = 20A max, 10 AWG = 30A max. Standard OCPD sizes per NEC 240.6(A).
Results are indicative. Always verify with a licensed electrical engineer. Local codes may impose additional restrictions.

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