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Fuse & Breaker Sizing

Overcurrent protection per NEC / IEC.

Electrical

Inputs (SI units)

Isc×1.25 for PV strings, or load current for AC

From module datasheet — 0 to skip check

Corrected ampacity of the protected conductor

Used for DC device voltage rating check

DC or AC rated voltage of the fuse/breaker/isolator

Results

Selected device rating
20 A
pv fuse
Required rating (In, theoretical)
18.72 A
Cable ampacity (Iz)
24 A
Voltage rating check
PASS
Breaking capacity check
PASS
Cable protection (In ≤ Iz)
PASS
✓ PASS — Device voltage rating (1,000 V) covers the coldest-condition string Voc (950 V).
✓ PASS — Breaking capacity (500 A) covers the prospective fault current (300 A).
✓ PASS — Selected/required rating is within the module's maximum series fuse rating.
✓ PASS — Selected device rating (20 A) ≤ cable ampacity (24 A) — cable is protected.
⚠ WARNING — Ensure the device is explicitly DC-rated for PV use — standard AC breakers cannot safely interrupt DC arcs.

Engineering Recommendations

  • →Select a 20 A pv fuse — meets rating, voltage, breaking-capacity and cable-protection requirements.
  • →Use gPV-rated DC fuses for PV source/output circuits — never substitute gG (AC) fuses.
  • →Engineering-grade preliminary calculation. Final design must be verified against project-specific site conditions, manufacturer datasheets, applicable standards and utility requirements.

Detailed Calculation Log

DC-rated device required (arc-suppression rated for DC)

Save & Load Project

Designs are stored privately in this browser — nothing is uploaded.

Engineering Formula

  • I_ocpd ≥ 1.25 (continuous) × 1.25 (PV irradiance) × I
  • = 1.56 × I for PV source circuits

NEC 690.9 mandates 156% multiplier for PV. Non-PV continuous loads use 1.25×. Round up to next standard rating.

Step-by-step Calculation

  1. 1.Design currentIsc×1.25×1.25 (PV) or load current (AC)12 A
  2. 2.Protected-cable ampacity (Iz)from cable-sizing engine24 A
  3. 3.Required device rating (In)I × 1.5618.72 A
  4. 4.Voltage rating checkDevice Vrating ≥ String Voc(cold)1,000 V ≥ 950 V → OK
  5. 5.Breaking capacity checkDevice breaking capacity ≥ prospective fault current500 A ≥ 300 A → OK
  6. 6.Next standard device ratingnext catalogue size ≥ required In20 A
  7. 7.Cable protection checkIn(selected) ≤ Iz20 A ≤ 24 A → OK

How to use this calculator: Fuse & Breaker Sizing

Overcurrent protection per NEC / IEC. The example below is calculated by this page's real engine from the displayed inputs.

  1. 1Confirm that the Fuse & Breaker Sizing matches the quantity or design check you need.
  2. 2Enter Design/continuous current, Module max series fuse rating, and Cable ampacity (Iz) using the units printed beside each field.
  3. 3Select the applicable Device type options; these choices change the calculation method or factors.
  4. 4Calculate, then follow the substituted equations in the worked example and compare the result with any stated limit.
  5. 5Read the assumptions, warnings and cited references before using the result for a financial, medical or engineering decision.

Input guide and example values

Use values from the same measurement basis and time period. Conditional fields appear only when the related option is selected.

InputExample valueWhy it matters
Device typePV string fuse (DC)Select the option that matches the real installation or scenario.
Design/continuous current12 AIsc×1.25 for PV strings, or load current for AC
Module max series fuse rating20 AFrom module datasheet — 0 to skip check
Cable ampacity (Iz)24 ACorrected ampacity of the protected conductor
String Voc at coldest condition950 V DCUsed for DC device voltage rating check
Device voltage rating1000 VDC or AC rated voltage of the fuse/breaker/isolator
Prospective fault current300 AMeasured or known prospective fault current used by the calculation engine.
Device breaking capacity500 AMeasured or known device breaking capacity used by the calculation engine.

Formula inputs & variables for Fuse & Breaker Sizing

These are the named quantities used by this calculator. When the source formula does not define a mathematical symbol, OneCalcApp keeps the real input label instead of inventing one.

Variable / inputUnitMeaning in this calculation
Device type—Select the option that matches the real installation or scenario.
Design/continuous currentAIsc×1.25 for PV strings, or load current for AC
Module max series fuse ratingAFrom module datasheet — 0 to skip check
Cable ampacity (Iz)ACorrected ampacity of the protected conductor
String Voc at coldest conditionV DCUsed for DC device voltage rating check
Device voltage ratingVDC or AC rated voltage of the fuse/breaker/isolator
Prospective fault currentAMeasured or known prospective fault current used by the calculation engine.
Device breaking capacityAMeasured or known device breaking capacity used by the calculation engine.

How the Fuse & Breaker Sizing works

The Fuse & Breaker Sizing uses Device type, Design/continuous current, Module max series fuse rating, Cable ampacity (Iz), String Voc at coldest condition, Device voltage rating, Prospective fault current, and Device breaking capacity to calculate Selected device rating, Required rating (In, theoretical), Cable ampacity (Iz), Voltage rating check, Breaking capacity check, and Cable protection (In ≤ Iz). Its engine applies I_ocpd ≥ 1.25 (continuous) × 1.25 (PV irradiance) × I; the worked values below come from that same live calculation rather than a separately typed example.

With Device type PV string fuse (DC), Design/continuous current 12 A, Module max series fuse rating 20 A, Cable ampacity (Iz) 24 A, String Voc at coldest condition 950 V DC, Device voltage rating 1000 V, Prospective fault current 300 A, and Device breaking capacity 500 A, the main worked-example result is Selected device rating = 20 A.

How each Fuse & Breaker Sizing input is used

Device type

The Fuse & Breaker Sizing worked example selects “PV string fuse (DC)”. Available choices include PV string fuse (DC), DC breaker (combiner/array), DC isolator/disconnect, and AC breaker. This selection may change the method or factor used by the engine, so choose the option that matches the real case.

Design/continuous current

The Fuse & Breaker Sizing worked example uses Design/continuous current = 12 A. This value is passed directly into the calculation, with an allowed minimum 0.1. Isc×1.25 for PV strings, or load current for AC

Module max series fuse rating

The Fuse & Breaker Sizing worked example uses Module max series fuse rating = 20 A. This value is passed directly into the calculation, with an allowed minimum 1. From module datasheet — 0 to skip check

Cable ampacity (Iz)

The Fuse & Breaker Sizing worked example uses Cable ampacity (Iz) = 24 A. This value is passed directly into the calculation, with an allowed minimum 0.1. Corrected ampacity of the protected conductor

String Voc at coldest condition

The Fuse & Breaker Sizing worked example uses String Voc at coldest condition = 950 V DC. This value is passed directly into the calculation, with an allowed minimum 1. Used for DC device voltage rating check

Device voltage rating

The Fuse & Breaker Sizing worked example uses Device voltage rating = 1000 V. This value is passed directly into the calculation, with an allowed minimum 1. DC or AC rated voltage of the fuse/breaker/isolator

Prospective fault current

The Fuse & Breaker Sizing worked example uses Prospective fault current = 300 A. This value is passed directly into the calculation, with an allowed minimum 0.1.

Device breaking capacity

The Fuse & Breaker Sizing worked example uses Device breaking capacity = 500 A. This value is passed directly into the calculation, with an allowed minimum 0.1.

Fuse & Breaker Sizing formulas and result interpretation

Formula 1: relationship used

In the Fuse & Breaker Sizing, I_ocpd ≥ 1.25 (continuous) × 1.25 (PV irradiance) × I. The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Formula 2: relationship used

In the Fuse & Breaker Sizing, = 1.56 × I for PV source circuits. The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Selected device rating

For the displayed Fuse & Breaker Sizing worked example, Selected device rating is 20 A. pv fuse Verify Device type, Design/continuous current, and Module max series fuse rating and their units before relying on this output.

Required rating (In, theoretical)

For the displayed Fuse & Breaker Sizing worked example, Required rating (In, theoretical) is 18.72 A. Verify Device type, Design/continuous current, and Module max series fuse rating and their units before relying on this output.

Cable ampacity (Iz)

For the displayed Fuse & Breaker Sizing worked example, Cable ampacity (Iz) is 24 A. Verify Device type, Design/continuous current, and Module max series fuse rating and their units before relying on this output.

Voltage rating check

For the displayed Fuse & Breaker Sizing worked example, Voltage rating check is PASS. Verify Device type, Design/continuous current, and Module max series fuse rating and their units before relying on this output.

Breaking capacity check

For the displayed Fuse & Breaker Sizing worked example, Breaking capacity check is PASS. Verify Device type, Design/continuous current, and Module max series fuse rating and their units before relying on this output.

Cable protection (In ≤ Iz)

For the displayed Fuse & Breaker Sizing worked example, Cable protection (In ≤ Iz) is PASS. Verify Device type, Design/continuous current, and Module max series fuse rating and their units before relying on this output.

Fuse & Breaker Sizing accuracy, checks and limitations

  • Fuse & Breaker Sizing units check: confirm Device type, Design/continuous current (A), Module max series fuse rating (A), Cable ampacity (Iz) (A), String Voc at coldest condition (V DC), Device voltage rating (V), Prospective fault current (A), and Device breaking capacity (A) before calculating.
  • Fuse & Breaker Sizing result check: compare Selected device rating, Required rating (In, theoretical), Cable ampacity (Iz), Voltage rating check, Breaking capacity check, and Cable protection (In ≤ Iz) with the substituted formula steps and the displayed rounding precision.
  • Fuse & Breaker Sizing: Use this result for preliminary design and cross-checking. Confirm the applicable code edition, manufacturer data, site conditions and qualified-engineer approval before final design or installation.

Understanding the result

Read the main result together with supporting checks, assumptions, limits and intermediate values.

For a manual check, repeat the first equation, confirm the units and change one input at a time.

Common mistakes when using Fuse & Breaker Sizing

  • Do not mix units for Design/continuous current (A), Module max series fuse rating (A), Cable ampacity (Iz) (A). A unit mismatch changes the input magnitude even when the typed number looks reasonable.
  • Do not leave Device type on the default choice unless that choice matches the real scenario; the selected option can change the calculation path or factor.
  • Do not replace the displayed I_ocpd ≥ 1.25 (continuous) × 1.25 (PV irradiance) × I relationship with a different convention without also changing the underlying assumptions; compare like-for-like methods when checking the result.
  • Do not treat Selected device rating = 20 A from the worked example as a universal answer. It belongs to the displayed example inputs and must be recalculated for the actual case.

When the Fuse & Breaker Sizing is useful

Fuse & Breaker Sizing is designed for cases where Device type, Design/continuous current, Module max series fuse rating, Cable ampacity (Iz) are known and you need Selected device rating, Required rating (In, theoretical), Cable ampacity (Iz). The page keeps the live calculator, calculation method and worked example together so the result can be checked instead of treated as a black-box number.

Use the calculator for the scope described by its inputs and notes. The displayed method is I_ocpd ≥ 1.25 (continuous) × 1.25 (PV irradiance) × I. If the real project or decision needs factors that are not represented here, treat the result as an estimate and add the missing checks separately.

Device type and Design/continuous current: what changes the answer

The worked example uses Device type = PV string fuse (DC), Design/continuous current = 12 A, Module max series fuse rating = 20 A, Cable ampacity (Iz) = 24 A. With those values, Selected device rating is 20 A. Changing an input should be interpreted according to that field's unit, range, option and hint rather than by the number alone.

For this calculator, the main input roles are: Device type: Select the option that matches the real installation or scenario. Available choices include PV string fuse (DC), DC breaker (combiner/array), DC isolator/disconnect, AC breaker. Design/continuous current (A): Isc×1.25 for PV strings, or load current for AC Module max series fuse rating (A): From module datasheet — 0 to skip check Cable ampacity (Iz) (A): Corrected ampacity of the protected conductor

How to sanity-check a Fuse & Breaker Sizing result

Start by confirming the entered values and units, then compare the substituted working with the displayed formula or calculation steps. Pay particular attention to Selected device rating, because it is the first worked-example output shown by the live engine.

Finally, compare the result with the assumptions, warnings and related calculators on this page. A nearby calculator can be useful as a cross-check when it measures the same workflow from a different input or output direction.

Next logical calculator

Continue with Sq mm Wire Current Capacity Chart

Sq mm Wire Current Capacity Chart covers the same practical workflow from a related calculation angle, making it a useful cross-check after Fuse & Breaker Sizing.

Open Sq mm Wire Current Capacity Chart

Design Assumptions

  • • Ambient below 40 °C (else derate 0.9× per 10 °C).

Engineering Tips

  • ◆Match string-fuse to module Ifuse rating (typ. 15–25 A).
  • ◆Use gPV fuses for DC PV — NOT gG.

Warnings

  • ▲AC breakers cannot interrupt DC arcs — never substitute.

Standards & References

NEC 690.9IEC 60269-6 (gPV fuses)IS 13703

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