Phase Academy — OCPD Sizing Calculator

Scope & Limitations — read first

Educational tool only. This calculator models general branch-circuit and feeder OCPD sizing for the selected, adopted NEC edition — the AHJ and the adopted edition always control. It does not verify or design for:

Available fault current, AIC/interrupting rating, SCCR, series ratings, or selective coordination
Voltage drop, arc-flash / incident energy, or short-circuit conductor withstand (I²t)
Equipment-specific articles, terminals/lugs/listing details, parallel conductors
Tap rules (240.21), transformer secondary conductors, PV/ESS/battery rules
HVAC nameplate MCA/MOCP
Motor/special-equipment circuits — except the dedicated NEC 430 branch-circuit module here, which covers only the individual motor branch circuit (not group installations per 430.53, motor feeders per 430.62/430.24, or circuits supplying a motor plus other loads)
Ambient correction factors are entered by you — the tool does not verify that the factor came from the correct temperature/insulation column of Table 310.15(B)(1)

Also note: the termination-temperature selector offers 60°C and 75°C only. An all-90°C-listed system is rare in practice and is not modeled; 90°C is available as an insulation column for derating, which is the normal use of that column.

Load-to-Device Path

How the calculation flows from raw load to selected standard device.

Calculated Load

—

amperes

→

Continuous Factor

—

on continuous portion

→

Minimum Rating

—

amperes

→

Selected Size

—

amperes

At-a-Glance Comparison

Where everything sits relative to the conductor's final ampacity.

Raw Load (as entered) Minimum OCPD (calculated) Selected Device Conductor Final Ampacity

Standard Size Ladder — NEC 240.6(A)

Every chip is evaluated by the same engine. Green = compliant · orange = permitted next-size-up (240.4(B)) · red = not adequate. Selected device is outlined. Click any chip to "select" it and see what would happen.

Continuous vs Non-Continuous Multiplier Impact

How the 125% factor changes the minimum required rating for the same load.

Noncontinuous (×1.00) Continuous (×1.25) 100%-Listed Assembly (×1.00)

Illustrative Time-Current Sketch

Illustrative only — NOT a manufacturer time-current curve. A cartoon of "device trips before the conductor is damaged." It does not verify NEC compliance, conductor damage protection, selective coordination, AIC adequacy, arc-flash, or short-circuit withstand.

Device (sketch) Conductor (sketch) Calculated Load Selected OCPD

Safety Margin Gauges

Independent margins from the shared evaluator. Load coverage and conductor protection must both be satisfied for a compliant install.

Load Sweep — How Min Rating Scales

Holding the continuous/100%-listed settings constant, this is the minimum OCPD rating curve as load varies. Your current calculated load is marked.

Headroom Donut

Of the conductor's final ampacity, how much is consumed by the base load, the continuous-load adder, and the unused margin.

Per-Size Compliance Heatmap

Every standard size evaluated by evaluateOcpdDesign(). Green = passes load coverage and conductor protection · orange = permitted 240.4(B) next-size-up · red = below load, small-conductor cap violation, over conductor, over-800 next-size-up violation, or 240.4(B) ineligible.

Step-by-Step Calculation Trace

The path the math took to arrive at your selected device — straight from the shared engine.

Conductor vs OCPD

Interpretation Notes

Standard Size Table — Closest Picks

Size (A)	vs Min	vs Conductor	Status

Assumptions

NEC Code Logic — References

Paraphrased explanations for learning. Always consult the actual published NEC edition adopted by your AHJ.

240.4Conductor Protection. Conductors must be protected against overcurrent according to their ampacity after correction/adjustment. The OCPD rating generally cannot exceed conductor ampacity. Exceptions live in (B) through (G).

240.4(B)Next Standard Size Up. When the conductor ampacity does not correspond to a standard rating, the next higher standard rating is permitted — but only when (1) the circuit does not supply more than one receptacle for cord-and-plug portable loads, (2) the ampacity isn't already a standard rating, and (3) the next rating is 800 A or less. It never overrides 240.4(D).

240.4(C)Over 800 A. Where the OCPD exceeds 800 A, the conductor ampacity must be equal to or greater than the OCPD rating. Next-size-up does not apply.

240.4(D)Small Conductors. Copper 14/12/10 AWG cap at 15/20/30 A; aluminum & copper-clad aluminum 12/10 AWG cap at 15/25 A. These hard ceilings apply after correction/adjustment and are not overridden by next-size-up.

240.6(A)Standard Ampere Ratings.

210.20(A)OCPD — Continuous + Non-Continuous Load. The branch-circuit OCPD rating ≥ non-continuous load + 125% × continuous load. The 1.25 factor disappears only where the assembly, including the OCPD, is listed for 100% continuous operation.

210.19(A)(1)Conductor — the Two-Part Test. The conductor test is two tests, not one. (a) the minimum conductor size has an allowable ampacity ≥ non-continuous + 125% × continuous, using the ampacity before ambient and bundling adjustment; (b) the minimum conductor size has an allowable ampacity ≥ the load, at 100% with no multiplier, using the ampacity after adjustment and correction. Both must pass. This tool evaluates and displays each sub-test separately in the Calc Trace. 215.2(A)(1)(a)/(b) states the same rule for feeders.

240.6(B) / (C)Adjustable-Trip Breakers. The rating of an adjustable-trip breaker is its maximum possible setting — not the setting you dialed in. The setting counts as the rating only where access to the adjusting means is restricted per 240.6(C): removable and sealable covers, bolted equipment enclosures, or locked doors accessible only to qualified persons (the 2023 edition adds (D) for password-protected electronic adjusting means). Listed fixed nonstandard ratings are permitted directly by 240.6(A).

215.2 / 215.3Feeder Conductor & OCPD. Same non-continuous + 125% continuous rule, applied at the feeder level.

110.14(C)Termination Temperature. The ampacity used for sizing cannot exceed the ampacity at the lowest-rated termination temperature (commonly 60°C or 75°C), even if a 90°C column is used for derating.

430 Part IVMotor Branch-Circuit Protection. Motors have their own table-driven rules. Short-circuit/ground-fault OCPD is sized as a percentage of FLC from NEC tables (not nameplate) per Table 430.52(C)(1), and the percentage depends on the motor type as well as the device:

Motor type	NT fuse	DE fuse	Inst. trip	Inverse time
Single-phase · squirrel-cage (other than Design B EE) · synchronous	300%	175%	800%	250%
Squirrel-cage, Design B energy-efficient	300%	175%	1100%	250%
Wound rotor	150%	150%	800%	150%
Direct current (constant voltage)	150%	150%	250%	150%

Running overload protection is separate (430.32) and is based on the motor nameplate current rating (430.6(A)(2)).

430.52(C)(1) Exc. 1 & 2The Two Motor Exceptions Stack. Exception 1 permits the next higher standard rating whenever the Table 430.52 value is not itself a standard rating — for non-time-delay fuses, dual-element fuses, and inverse-time breakers. This round-up is unconditional. Exception 2 then applies "where the rating specified in Table 430.52, or the rating modified by Exception No. 1, is not sufficient for the starting current": it permits a further increase up to 400% (non-time-delay fuse ≤ 600 A or time-delay Class CC fuse), 225% (dual-element fuse), or 400%/300% (inverse-time breaker at FLC ≤ 100 A / > 100 A). 430.52(C)(3) Exc. 1 allows an instantaneous-trip setting up to 1300% (1700% for Design B energy-efficient) where an engineering evaluation shows the need. The Exception-2 percentages are a maximum for an optional upsize, never a cap on the Exception-1 round-up, and the device must never end up below the Table 430.52 value.

Standard Ratings — Current Mode

Why OCPD Sizing Matters

The overcurrent protective device (OCPD) does one job: open the circuit when current exceeds a threshold. Everything downstream — the wire, the equipment, the building — depends on that threshold being set correctly.

Too low: nuisance tripping, dissatisfied occupants, and a strong temptation to "upgrade" to a bigger breaker for the wrong reasons.

Too high: the wire carries more current than it can dissipate as heat, insulation degrades, and the consequences begin. The OCPD protects the conductor, not the equipment, and not your feelings about nuisance trips.

Core Formula

Load_calc = I_{non-continuous} + 1.25 × I_continuous
OCPD_selected = smallest standard size ≥ Load_calc (240.6(A); 240.4(B) next-size-up ≤ 800A when eligible)

The 1.25 multiplier becomes 1.00 only when the assembly (including the OCPD) is listed for 100% continuous operation — a device-only listing is not sufficient.

Common Mistakes

"Round up to a bigger breaker if the load grows."

The breaker protects the wire. Upsizing the breaker without upsizing the wire removes the wire's protection. The wire didn't get bigger.

"125% is just a safety pad."

It's a thermal accommodation. Devices carrying rated current for hours build heat. The 125% is the minimum adder, not a margin of comfort.

"Next-size-up is automatic."

240.4(B) is conditional: ≤ 800A, ampacity not already standard, not a multi-receptacle portable branch, and not excluded by 240.4(D). 12 AWG can't be on a 25A breaker just because it's "the next size up."

"A 100%-rated breaker removes the 125%."

Only if the whole assembly — enclosure, terminations, and OCPD together — is listed for 100% operation. A device-only listing does not qualify.

"Motor circuits use the same OCPD math."

Motor branch circuits use 430 Part IV. Short-circuit/ground-fault OCPD is a percentage of table FLC that depends on both the device and the motor type (squirrel-cage: 175% dual-element fuse, 300% non-time-delay fuse, 250% inverse-time breaker, 800% instantaneous-trip — but wound-rotor and DC motors use 150% rows). Sizing "for the load" gives wrong answers.

"430.52 Exception 2 caps how far you can round up."

Backwards. Exception 1's next-standard-size round-up is unconditional. Exception 2 only comes into play when the table value or the Exception-1 round-up still will not carry the motor's starting current, and it permits going higher still. A device below the Table 430.52 value is never the right answer — it will open on starting.

"An adjustable breaker's rating is whatever I set it to."

Per 240.6(B) the rating is the breaker's maximum possible setting. The dialed-in setting only becomes the rating where access to the adjusting means is restricted per 240.6(C)/(D) — a sealed cover, a bolted or locked enclosure, or password protection for qualified persons.

Knowledge Check — Quiz

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Correct of answered

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Design Challenge — Apply What You Know

Real-ish scenarios with hidden NEC traps, graded by the same evaluator the dashboard uses. Pick the OCPD and conductor, then submit for evaluation.

Passed

Partial Credit

Failed

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For education only · verify against codes

For education, training, and preliminary study only. Verify every result against the applicable codes and standards, manufacturer data, project specifications, AHJ requirements, and qualified engineering judgment. This tool does not imply guaranteed compliance.