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Fuses

Fuses are another method of protecting circuits against the damages of overloads and short circuits. A fuse is defined by NEMA as "a device which protects a circuit by fusing open its current-responsive element when an overcurrent or short circuit passes through it."

"AIC" means "Amperes of Interrupting Current" and is sometimes shortened to "IC," (also called the "interrupting capacity"). This is the rating assigned by the manufacturer of the fuse or circuit breaker after careful testing. Fuses are available with interrupting ratings up to 300,000 amperes. There are two general types of low-voltage fuses - plug and cartridge.

Article 240-6 in the National Electrical Code covers the Standard Ampere Ratings for Fuses and Circuit Breakers as follows: 1, 3, 6, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amperes.

Following is the Fuse Tabulation Summarized:

  AC Voltage  Amperes  Interrupting Rating 
Fuse Class  Rating  Rating  RMS Symmetrical Amperes 
300 to ground  0-60  100,000 
600  0-600  200,000 
250; 600  0-600  50,000; 100,000; 200,000 
600  601-6,000  200,000 
250; 600  0-600  10,000 
250; 600  0-600  200,000 
300; 600  0-1200  200,000 
Plug and Type S  125  0-30  10,000 

Class G, J, K, L, R and T are current limiting fuse classifications. Class H can have renewable links. All fuses must be designed so that they cannot be physically interchanged with a different class fuse in their mountings. Restrictions also apply between ampere ranges of the same class fuse. This prevents over fusing or using an incorrect fuse when replacement is required. The manufacturers' literature should be consulted for actual electrical and physical fuse characteristics.