Fuse Size Calculator

Q: How do I size a fuse for a motor?

For motors, use 250% of full-load running amps (NEC 430.52) to allow for starting surge current. Select the next standard fuse size up from that calculated value.

Q: What fuse size for a general power circuit?

For general resistive or non-motor loads, the fuse rating should be 125% of the full-load current for continuous loads, rounded up to the next standard size.

Q: What are standard fuse sizes?

Common standard fuse ratings: 1, 2, 3, 5, 7, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 125, 150, 200 amps. Always select the next size up from your calculated minimum.

Select the correct fuse rating for motor, general, and lighting circuits. Enter your load current and circuit type to get the recommended standard fuse size.

This calculator provides estimates only. Always consult a qualified electrician and verify against local electrical codes before installation.

Calculate Fuse Size

Full Load Current (Amps)

Load Type

Result

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Recommended Fuse (A)

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Minimum Required (A)

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Multiplier Used

Formulas

General / Lighting (continuous loads):

Minimum fuse = Load current × 1.25
Select next standard size ≥ minimum

Motor circuits:

Minimum fuse = Full-load amps × 2.5
Select next standard size ≥ minimum

The 250% factor for motors accommodates the high inrush current at startup (typically 6–7× running current for induction motors). Time-delay (dual-element) fuses are recommended for motor circuits.

Load Type Guide

General / Resistive: Heaters, incandescent lighting, resistive heating elements. No startup surge. Use 125% multiplier.
Motor loads: Pumps, compressors, fans, conveyor drives. High inrush at startup. Use 250% multiplier with time-delay fuses.
Lighting (fluorescent/LED): May have ballast or driver inrush. Treat as continuous load — 125% multiplier. Use fast-blow fuses.
Welders and transformers: High inrush — consult equipment nameplate for fuse recommendations, typically 200–300% of rated current.

Standard Fuse Sizes

Standard IEC and NEC fuse ratings (amps):

1 · 2 · 3 · 5 · 7 · 10 · 15 · 20 · 25 · 30 · 35 · 40 · 45 · 50 · 60 · 70 · 80 · 90 · 100 · 125 · 150 · 200

Always select the next standard size up from your calculated minimum — never round down to a smaller size as this would not protect the circuit properly.

Frequently Asked Questions

How do I size a fuse for a motor?

Use 250% of the motor's full-load running amps as your minimum fuse size, then select the next standard size up. This accounts for starting inrush current. Always use time-delay (dual-element) fuses for motor circuits — standard fast-blow fuses will blow on every start.

What fuse size for a general power circuit?

Multiply full-load current by 1.25 (125%) and round up to the next standard size. For a 16A load: 16 × 1.25 = 20A minimum, so use a 20A fuse. This 125% rule applies to continuous loads running more than 3 hours.

What are standard fuse sizes?

Common standard ratings are: 1, 2, 3, 5, 7, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 125, 150, 200A. In the UK, plug fuses are typically 3A or 13A. In continental Europe, domestic fuses are often 6, 10, 13, or 16A.

Should I use a fuse or a circuit breaker?

Both protect against overcurrent, but fuses are single-use (replace after blowing) while breakers reset. Breakers are preferred for residential and most commercial applications for convenience. Fuses are still common in industrial panels, motor control centres, and applications where fast clearing time is critical.