Fuse Size Calculator

A fuse size calculator starts with the circuit load current and the fuse factor applied by the reviewed design method. It produces an entered reference fuse current—the arithmetic current target used to compare a documented selected fuse rating.

This number is commonly used during branch-circuit or feeder review before related design work is finalized. The selected overcurrent protective device affects conductor ampacity review, equipment terminal limitations, available fault-current and interrupting-rating checks, selective coordination, and sometimes motor-circuit protection analysis. It may also affect panel schedules and material takeoffs.

The worksheet does not select a fuse from a manufacturer table or determine whether the selected fuse is acceptable for the installation. It records entered values and shows the numerical relationship between the load current, entered fuse factor, and selected fuse rating.

Calculation Inputs

The calculator uses four input fields:

Input	Electrical use
Load current (A)	The current value being reviewed for the circuit or equipment load
Entered fuse factor (%)	The percentage factor used by the reviewed sizing method
Selected fuse rating (A)	The ampere rating represented by the selected fuse record
Source reference	The record identifier or source supporting the entered load and selected rating

Load current (A) should represent the current basis used for the protection review. Depending on the circuit, that may be a calculated noncontinuous load, a continuous-load-adjusted value, an equipment nameplate value, a motor calculation value, or another documented design current. The worksheet does not determine which current basis applies.

Entered fuse factor (%) is the factor already chosen for the reviewed method. For example, entering 125 applies a 125 percent multiplier. The source for that factor should be retained in the design record because different equipment and circuit conditions can use different sizing rules.

Selected fuse rating (A) is not generated from a standard fuse-size list. It is the rating entered to represent the selected fuse record. A positive margin means the entered selected rating exceeds the calculated reference fuse current; a negative margin means it is below that reference current.

Fuse Sizing Formula

The calculator performs the following arithmetic:

\(\displaystyle \text{Entered reference fuse current} = \text{Load current} \times \frac{\text{Entered fuse factor}}{100}\)

It then compares the entered selected rating to that calculated current:

\(\displaystyle \text{Rating margin} = \text{Selected fuse rating} - \text{Entered reference fuse current}\)

\(\displaystyle \text{Rating margin percent} = \frac{\text{Rating margin}}{\text{Entered reference fuse current}} \times 100\)

The output is a comparison value, not a final overcurrent-protection determination. It does not round to a standard fuse size, select a fuse class, or evaluate whether a particular fuse will carry normal load current without nuisance operation.

Calculation Example

Assume the worksheet contains the following entered values:

Field	Entered value
Load current (A)	24 A
Entered fuse factor (%)	125%
Selected fuse rating (A)	35 A
Source reference	fuse-record-a

First, calculate the entered reference fuse current:

\(\displaystyle 24 \text{ A} \times \frac{125}{100} = \text{30 A}\)

Next, compare the selected fuse rating:

\(\displaystyle 35 \text{ A} - 30 \text{ A} = \text{5 A}\)

Finally, express that margin as a percentage of the reference current:

\(\displaystyle \frac{5 \text{ A}}{30 \text{ A}} \times 100 = \text{16.6667\%}\)

The resulting worksheet values are:

Result	Value
Entered reference fuse current	30 A
Selected fuse rating	35 A
Rating margin	5 A
Rating margin	16.6667%
Record comparison	Source recorded

A 35 A selected fuse rating is therefore 5 A above the entered 30 A reference current. The calculation alone does not establish that 35 A is the correct or permitted protective-device rating for the circuit.

Use in Circuit Design Review

The entered reference fuse current is a useful documentation point when coordinating protection selection with the rest of the electrical design.

For a branch circuit or feeder, the selected overcurrent protective device should be reviewed alongside:

• Conductor AWG or kcmil size and allowable ampacity
• Adjustment factor and correction factor applied to conductor ampacity
• Number of current-carrying conductors in a raceway or cable
• Insulation temperature rating and equipment terminal rating
• Raceway fill and conductor installation method
• Calculated voltage drop under expected load conditions
• Equipment listing, fuse class, and fuse time-current characteristics
• Available fault current and the fuse interrupting rating
• Required coordination with upstream and downstream overcurrent protective devices

A fuse rating cannot be evaluated from conductor ampacity alone. For example, an ampacity calculation may involve conductor temperature limitations, ambient conditions, and bundling adjustments, while the acceptable fuse selection may also depend on the equipment being protected and the applicable installation rules.

Motor and HVAC circuits require particular care because their overcurrent protection can be based on motor-specific or equipment-specific rules rather than a general load-current multiplier. A selected fuse may be intended for short-circuit and ground-fault protection while overload protection is provided separately.

Field Verification

Use the Source reference field to identify the load calculation, one-line diagram, equipment schedule, manufacturer documentation, or reviewed fuse record that supports the entries. The displayed record comparison confirms that a reference has been entered; it does not validate the source or the electrical design.

Verify the final installation separately with the project documents, equipment instructions, applicable code requirements, and the authority having jurisdiction (AHJ). Confirm the actual fuse class, voltage rating, interrupting rating, equipment listing, conductor protection, coordination requirements, and available fault current before installation or approval.

Boundary: This is an entered-value worksheet only. It does not evaluate fuse class, interrupting rating, coordination, listing, conductor protection, or a final protection result.