Posted by Roger Miller on December 11, 2013 at 12:00 AM UTC

Where installation manuals specify the use of a User Supplied fuse, the installer will normally use an appropriately rated automotive style fuse in conjunction with a compatible fuse holder.  Automotive style fuses are typically available in the following current ratings:

Mini (APM & ATM), often called Mini ATO: 2A30A

Standard (ATO): 1A40A

Maxi (APX): 20A to 120A

It should be noted that as the current rating gets higher in each fuse size group, the fuse holder is often the weakest link.  Great care must be taken to ensure that the fuse holder is rated for and capable of operating continuously at the required current.

For example, even though ATO size fuses are available in 30A, 35A & 40A sizes, I know of no commercially available fuse holder that is capable of operating continuously at these currents.  Similarly, I know of no Maxi Fuse holder that is will withstand continuous use at 70A through 120A. Many of the commonly available fuse holders will simply overheat and melt.   This overheating can also cause damage to other components and wiring within the vehicle.  

When the continuous current requirement for a given circuit even remotely approaches the maximum rating for the fuse holder being considered, it’s wise to switch to the next larger size fuse style and choose a compatible fuse holder that is rated for continuous use at 150% to 200% of the fuse current rating that you intend to use.

Good installation practice requires that the installer consider all of the above factors in combination with the requirements of the products for each installation.  Using a non-contact Infrared Thermometer to run a final check for unusual heat rise in fuses, terminal blocks and wire terminals is one extra step toward ensuring a high quality installation.   
Fuses and Fuse Holders are available from the below listed internet vendors among others.

Code 3, Inc. - 2013

Code 3 Guidance for Installers on User Supplied Fuses

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Guidance for Installers on User Supplied Fuses


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