Fuse Ratings and Choosing the Correct Fuse

Choosing the right size, right type fuse with the correct voltage and current ratings is one of the most important aspects of system sizing. This short article is a summary of fuse types, ratings, and characteristics.
Class T, CC, H and G types of fuses
Class L, RK1, RK5 types of fuses
Class J types of fuses
Class K5 types of fuses
Class T, CC, H and G types of fuses
Class L, RK1, RK5 types of fuses
Class J types of fuses
Class K5 types of fuses

One of the most important aspects of electrical wiring of photovoltaic systems is fusing.  Fuses provide integral protection against overcurrents that could otherwise damage your valuable PV equipment.  Furthermore, using an incorrect fuse can be extremely hazardous! 

When sizing the fuse, the most common calculation method is to multiply the continous load/feed current of a branch by 1.25, and using the closest rated fuse that is greater than your result. However, there are exceptions to this calculation method.

We have observed that a very common mistake is to use a fuse that is rated for 600 VAC on a DC disconnect that is rated for 600 VDC. At first sight, the cable size, the current rating may seem correct, however, the voltage rating (a small description on the fuse) is even more important than the current rating in some circumstances. AC rated fuses SHOULD NOT be used in DC voltage circuits unless DC application ratings are provided by the fuse manufacturer.

The characteristics and certification of the fuses are usually defined on the fuse label. UL and CSA are the most common fuse certifications used in North America. The fuse label may provide information on the applicable AC or DC voltage, the maximum current rating and other information such as “interrupting rating”, “current limiting”, “time-delay” and “fast-acting”. The definitions of these specifications are explained below. 

Please refer to NEC code Article 240 Overcurrent Protection when sizing the fuses for your application.

  • Interrupting rating: Interrupting rating is the current that a fuse, circuit breaker, or other electrical apparatus is able to interrupt without being destroyed or causing an electric arc with unacceptable duration.
  • Current limiting: A current-limiting device is one that reduces the peak let-thru current to a value substantially less than the potential peak current that would occur if the current-limiting device were not used.
  • Time-delay: A fuse in which the burnout action depends on the time it takes for the overcurrent heat to build up in the fuse and melt the fuse element.
  • Fast-acting: A fuse that opens on overload and short-circuits very quickly. Fast-acting fuse is not designed to withstand temporary overload currents associated with some electrical loads.
Fuse Type Max. Current Rating ACVoltage Rating Classification General Use UL Notes
Class L 601-6000A 600 VAC

DC Optional
-Non-renewable,

-Current-limiting,

-Bolt-in

-Time-delay
Service switches, Switchboard mains and feeders, Bolted pressure contact switches, Motor control center mains, Large motor branch circuits, UL Listed series-rated protection for molded case circuit breaker, panelboards and loadcenters, Primary and secondary protection for transformers, Protection of power circuit breakers UL 248-10
Class RK1 600A 250/600 VAC,

DC Optional

-Non-renewable

-Current limiting

-Time-delay

All general purpose circuits, Motors, Transformers, Solenoids, Fluorescent lighting, All system components with high in-rush currents UL 248-12
Class RK5 600A DC circuits, All general purpose circuits, Motors, Transformers, Solenoids, Fluorescent lighting, All system components with high in-rush currents
Class C 1200A 600 VAC,

DC Optional

-Non-renewable

-Time-delay

  UL 248-2
Class CC (Midget) 30A 600 VAC,

DC Optional

-Non-renewable

-Current limiting

-Fast-acting

CCMR series fuses are specifically designed to withstand sustained starting currents of small motors, Provide short-circuit protection for motor branch circuits, Use with IEC- and NEMA-rated motor controllers and contactors, General purpose circuits up to 60 amp UL 248-4
Class T 1200A 300/600 VAC,

DC Optional
-Non-renewable

-Current limiting

-Fast-acting
Class T fuses can be used in applications that require fastacting protection, such as equipment containing variable speed drives, rectifiers, and other surge-sensitive components. Main switches containing Class T fuses may be used to provide protection to individual electric services and meter stacks. Molded case circuit breaker load centers and Panelboards also will have increased interrupting ratings when “series rated” with Class T fuses. UL 248-15
Class G 21A / 60A 480/600 VAC,

DC Optional
-Non-renewable

-Current limiting

-Time-delay
  UL 248-5
Class H (Renewable) 600A

250/600 VAC,

DC Optional

-Non-renewable

-Current limiting

-Fast acting
Circuits having relatively low levels of available short circuit current, Industrial and commercial applications having frequent outages where a renewable type fuse is desired UL 248-7
Class H (Non-Renewable) -Non-renewable

-Current limiting

-Time-delay
UL 248-6
Class J 600A 600VAC,

DC Optional
-Non-renewable

-Current limiting

-Fast acting
Fused combination motor controllers to provide IEC Type 2 (“No Damage”) motor branch circuit short-circuit and ground fault protection, Motor control centers, Transformer protection, Protection for UL Listed series rated molded case circuit breaker panels, General purpose circuits — mains, feeders and branch circuits — especially when space is limited UL 248-8
Class K 600A 250/600 VAC,

DC Optional
-Non-renewable

-Current limiting

-Fast acting
  UL 249-9

 

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Comments

Keep this info coming, Leventbas!  It's been mighty helpful on a few jobs for me.

Could you explain how you tell when a fuse has blown?  Is there any notification?  Do you need to look at it?  Thanks.

Thanks!

In fact you would tell instantly when there is no current coming through the circuit. You could also tell by visual inspection. Here is a picture:

Also, here is an image from one of the documents I mentioned as a reference:

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