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Fuse and Circuit Breaker Sizing Guide
Fuses and circuit breakers are designed to protect electrical circuits from overcurrent conditions that could cause damage or fire.
Key considerations:
- The protection device must be rated for the circuit voltage
- The current rating should be slightly higher than the normal operating current
- The interrupting capacity must exceed the available short-circuit current
- The device must protect the smallest conductor in the circuit
The National Electrical Code (NEC) specifies requirements for overcurrent protection:
- Continuous loads (3+ hours) require protection rated at least 125% of the load current
- Motor circuits have special rules based on motor full-load current
- Transformers require protection based on primary and secondary currents
- Conductors must be protected according to their ampacity
Fast-Acting Fuses: Open quickly on overcurrent, good for sensitive electronics
Time-Delay Fuses: Allow temporary inrush currents, good for motors and transformers
HRC Fuses: High rupturing capacity for circuits with high fault currents
MCB (Miniature Circuit Breaker): Common in residential/commercial for branch circuit protection
MCCB (Molded Case Circuit Breaker): Higher current ratings for industrial applications
RCD/GFCI: Protection against ground faults
AFCI: Protection against arc faults