Fuse Sizing Calculator

What this fuse sizing calculator does

This page keeps the fuse sizing basis visible. It does not hide all code paths behind a single "recommended fuse" label. Instead, it shows the calculated ampere value and separates the standard-size review step so you can confirm whether the adopted NEC path allows the next higher standard size.

Supported sizing screens

• General branch or feeder load: a 125% planning screen from the entered current basis.
• Motor branch fuse: a motor branch short-circuit and ground-fault fuse screen using 175% for time-delay families and 300% for non-time-delay style review.
• Transformer primary fuse: a primary fuse planning screen from transformer primary current, with primary-only and primary-plus-secondary protection arrangements kept separate.
• Capacitor bank boundary: a 135% capacitor-current basis for conductor and disconnect planning, with a clear warning that the actual fuse must be selected from manufacturer data, inrush behavior, and the adopted code path.

How to read the output

The calculator returns two standard-size values:

• Standard size at or below calculation: the largest common standard fuse ampere rating that does not exceed the calculated value.
• First standard size for review: the next common standard rating. This value is not automatically approved; it is the first value to check if the specific code path allows a next higher standard size.

Motor example

A motor branch circuit is screened with 65 A as the motor FLC basis and a dual-element time-delay fuse family.

• Sizing percentage: 175%
• Calculated fuse value: 65 A x 1.75 = 113.75 A
• Standard size at or below calculation: 110 A
• First standard size for review: 125 A

This page does not size motor overload protection. Motor overload protection, conductor ampacity, available fault current, SCCR, fuse class, fuse holder, and manufacturer instructions still need their own review.

Transformer example

A transformer primary is screened with 90.2 A of primary current.

• Sizing percentage: 125%
• Calculated fuse value: 112.75 A
• Standard size at or below calculation: 110 A
• First standard size for review: 125 A

Whether that first standard size is acceptable depends on the actual transformer protection arrangement, voltage class, primary current threshold, and adopted transformer rule. The calculator deliberately leaves that decision visible.

When the transformer arrangement includes both primary and secondary protection, the primary-side screen uses a 250% review basis. That does not size the secondary overcurrent protection or secondary conductors.

Capacitor bank boundary

The capacitor-bank mode is not a one-number fuse recommendation. It shows the 135% capacitor current basis used for conductor and disconnect planning, then marks the fuse decision as a separate review. Capacitor fuses must be kept as low as practicable while still handling capacitor inrush, harmonic current, available fault current, the fuse class, the holder, and the manufacturer data.

Fuse class is not the same as fuse ampere rating

Class J, RK1, RK5, CC, and T fuses have different physical rejection features, interrupting ratings, time-current behavior, and current-limiting performance. Those features matter for equipment compatibility and SCCR, but they do not remove the need to verify the ampere rating against the correct code basis.