Overcurrent Protection Selector

Select the standard circuit breaker or fuse size per NEC §240.6(A). Applies the 125% continuous load factor, conductor ampacity protection, and §240.4(B) next-size-up rule.

NEC edition

Inputs

Load current (calculated)

Load type

Continuous loads: OCPD must be rated at ≥ 125% of load per §210.20

Conductor size

Used to verify the OCPD does not exceed conductor ampacity

Conductor material

Aluminium ampacities and the §240.4(D) caps differ from copper

Termination rating

§334.80 requires NM-B be sized from the 60°C column even though the conductors are 90°C rated

Apply next-size-up rule?

§240.4(B): OCPD may be next standard size above conductor ampacity if ≤800A and not a supervised industrial installation

Result

Calculated result

20 A OCPD

⚠️ ⚠️ §240.4(B) does not apply: #12 Copper has an ampacity of 25 A at 75 °C, which is itself a standard device rating — rounding up is permitted only where the ampacity does not correspond to a standard rating.

⚠️ ⚠️ §240.4(D) small-conductor rule: #12 Copper may not be protected above 20 A regardless of its 25 A table ampacity.

⚠️ ⚠️ The largest OCPD this conductor may take (20 A) is below the 40.0 A the load requires — upsize the conductor.

Code references

NEC §240.6(A) Standard ampere ratings for fuses and fixed-trip circuit breakers
NEC §240.4(B) Next-size-up rule — only where ampacity is not itself a standard rating
NEC §240.4(D) Small conductors — 15 A / 20 A / 30 A caps on #14, #12 and #10
NEC §210.20 Overcurrent protection — continuous loads at 125%
NEC §334.80 NM cable ampacity — sized from the 60°C column

Load current 40 A

Load type Non-continuous

Continuous factor N/A

Required OCPD rating 40.0 A minimum §240.6(A)

Conductor ampacity #12 Copper @ 75°C = 25A Table 310.16

Ampacity is a standard rating? Yes — §240.4(B) round-up not permitted §240.4(B)

Small-conductor cap 20 A §240.4(D)

Selected OCPD 20 A (standard size) §240.6(A)

⚡ Test your knowledge

NEC Article 220 was renumbered in the 2026 code cycle. What is the new article number?

A. Art. 100
B. Art. 120
C. Art. 200
D. Art. 230

💡 NEC 2026 renumbered Article 220 (Branch-Circuit, Feeder, and Service Calculations) to Article 120.

Energy Management Systems were moved from Article 750 to which article in NEC 2026?

A. Art. 100
B. Art. 125
C. Art. 130
D. Art. 150

💡 NEC 2026 moved Energy Management Systems from Article 750 to Article 130.

Which NEC standard size overcurrent device comes after 90A?

A. 95A
B. 100A
C. 110A
D. 105A

💡 Per NEC §240.6(A), the standard sizes are …90, 100, 110, 125… There is no 95A standard size.

How it works & the NEC rules behind it

Overcurrent Protection Devices (OCPDs), including circuit breakers and fuses, protect electrical wiring from dangerous overloads and short-circuit fault conditions.

Core NEC Sizing & Selection Rules:

Standard Ampere Ratings (§240.6(A)): Fuses and inverse time circuit breakers must be chosen from the standard sizes recognized by the NEC: 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, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 Amperes.
The Next-Size-Up Rule (§240.4(B)): For circuits rated 800 Amps or less, you are permitted to use the next standard higher OCPD rating above the conductor’s allowable ampacity, provided the conductor does not serve multiple receptacle branch circuits.
Above 800 Amps (§240.4(C)): For circuits rated over 800A, the conductor ampacity must be equal to or greater than the OCPD rating (next-size-up is NOT permitted).

Formulas & equations

Minimum OCPD Rating = (Continuous Load × 1.25) + Non-Continuous Load Standard Breaker Selection = Next Standard Size from §240.6(A) ≥ Minimum OCPD Rating

Next-Size-Up Eligibility (§240.4(B)): Allowed if OCPD ≤ 800A AND conductor ampacity does not correspond to a standard OCPD size.

NEC reference table

Standard OCPD Sizes (Amperes) — NEC §240.6(A)	Standard Wire Pairing (75°C Cu)	Continuous Load Limit (80%)
15 Amps	#14 AWG Cu	12.0 Amps
20 Amps	#12 AWG Cu	16.0 Amps
30 Amps	#10 AWG Cu	24.0 Amps
40 Amps	#8 AWG Cu	32.0 Amps
50 Amps	#8 AWG Cu (75°C) or #6 AWG	40.0 Amps
60 Amps	#6 AWG Cu	48.0 Amps
100 Amps	#3 AWG Cu (or #1 AWG Al)	80.0 Amps
125 Amps	#1 AWG Cu (or #2/0 Al)	100.0 Amps
200 Amps	#3/0 AWG Cu (or 250 kcmil Al)	160.0 Amps

Worked example, step by step

Example Calculation: 68A Continuous Commercial Lighting Feeder

Scenario: You are protecting a commercial feeder with 68 Amps of continuous LED lighting using 75°C Copper conductors.

Step 1 (Apply 125%): 68A × 1.25 = 85.0 Amps minimum required OCPD rating.
Step 2 (Select Standard Breaker): Looking at §240.6(A), 85A is not a standard size; next standard size is 90 Amps.
Step 3 (Conductor Selection): Under Table 310.16 (75°C Cu), #4 AWG is rated 85A. Under the §240.4(B) next-size-up rule (≤ 800A), a 90A breaker is permitted to protect #4 AWG (85A) conductors.

Frequently asked questions

Under NEC §210.20, standard circuit breakers are rated for 100% duty only on non-continuous loads, but limited to 80% on continuous loads (operating 3+ hours). Only special "100%-rated" circuit breakers with listed assemblies can carry their full nameplate rating continuously.

If the ampacity of a conductor does not match a standard fuse or breaker rating listed in §240.6(A), you may use the next higher standard OCPD size, provided the rating is 800A or less and the circuit is not a multioutlet branch circuit supplying receptacles.

Inputs

Take this toolkit with you on the job

Studying for your electrical exam?

How it works & the NEC rules behind it

Core NEC Sizing & Selection Rules:

Formulas & equations

NEC reference table

Worked example, step by step

Frequently asked questions

Why are breakers usually loaded to only 80% capacity? −

What is the next-size-up rule in NEC 240.4(B)? +

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