Location: Derating a Fuse

Derating, the Math to Choose the Proper Fuse

A fuse will blow if the circuit sustains an amperage at the rating of the fuse. Derating helps you choose the right fuse. A fuse will blow if the circuit sustains an amperage above the rating of the fuse. Quite simply the fuse rating, or the number on the fuse is the ideal max operating amperage of the circuit, aka do not operate the circuit that operates that voltage. Under select circumstances though, a fuse can blow when operating at the amperage on the sticker. To choose the proper fuse we need to simply size the amperage and voltage to operate normally. Electricians manuals suggest that one should derate the circuit by 20% to facilitate hot operating conditions.

A fuse will blow under ambient temperatures with the following current applied to it:
%of amp rating   	Time to Blow Fuse
   110% 	                4 hours minimum
   135% 	                1 hour maximum
   200% 	                5 minutes maximum

So to be safe, and allow the toys to run, we oversize the fuse by just a bit. Most fuses happily accept a 20% derating, or are designed to operate efficiently at 80 percent of their break point. This statement applies to electronics, not motors. Motors use more power to start up than they need under operational load. This spike of power consumption makes most fuses a very poor selection for current protection with this application. See the breakers vs fuses page for more info on motors and breakers. There are fuses designed specifically for these loads that look like springs, but we don't carry those as they aren't a good investment in our eyes. There are pertinent applications, but not many in our arena. Therefore, we are going to ignore these very slow blow fuses on this page.

This page is dedicated to non-inductive loads, predominantly electronics, and protecting them with fuses. In other words, this series of equations covers computers, stereos, laptops, televisions, microwaves, and other things without motors in them. For inductive load fuse sizing, go to the inductive fuse rating page here. Again this page is designed to teach you to find the info, do the word problem, and then derate for the fuse that is right for your non inductive applications, like electronics. (If you want to protect very expensive electronics, choose a smaller fuse, but expect it to blow every so often. This set of equations allows you to push your toys to their limits.)

From the previous BD Fuses page, you should have surmised the voltage and amperage for your application. The basics are as follows.
1 Ensure the voltage is withing the range
2 Derate the fuse by 20%
3 Ensure the fuse holder is designed to handle the current


Let's do some math to prove the simplicity.
We are going to use some previous customer's circuit designs, and derate them accordingly.

Example 1: What equipment?
1000 watt inverter, running on 24 volt battery bank, with shore power.

Inverter peaks to 1000 watts from battery bank,
another 1000 watts from shore power when available for peak
Fuses between battery and inverters are needed.

So we know that the maximum amount of power we are going to need is 1000 watts.
We have a 24 volt battery bank. SO
1000 watts / 24 volts = 41.667 amps
For this application we typically derate by 20% as the inverter is the limit.
41.667 amps x 20% = 8.33 amps
So we want a fuse that is 8.33 amps bigger than 41.667 amps so it won't blow.
Shortcut math:
(41.667 amps x 20% ) + 41.667 amps = 41.667 x 1.2 = 50 amp fuse
Typically we suggest the bussman fuses for the 12-32 volt bracket.

Example 2: What equipment?
CPAP Machine manufacturer rating is for 14 amps at 12 volts per manual

This one is easy, the manufacturer tells you the parts directly.
14 amps x 1.2 derating= 16.8 amps.
They make 15 and 20 amp fuses, select the 20, not the 15 or you will blow it.
Typically we suggest ato/atc mini fuses for cost reasons in the 8-16 volt bracket.

Example 3: What equipment?
1957 Hyster Forklift, 36 volts, 2 hp electric drive motor.

This one is harder, the manufacturer tells you the size of the motor but nothing else. The equipment is aged so there is not adequate online documentation. All you have to go on is the new motor you want to install...tricky, tricky...
Let's start by deciding the size of the draw from that motor. A horsepower can be converted to kilowatts with a factor of .7456. So a 2hp motor will use 1.491 kilowatts, or 1,491 watts. To determine the amps we then divide by the volts.
Watts = volts x amps
1491 watts = 36 volts x ____amps
1491 watts/36volts = 41.42
41.42 x 1.2 derating = 49.7 amps.
They make a 50 amp fuse so we would select that one.
Typically we suggest ANN or ANL fuses for 24-56 volt applications.

Now that you know the math, let's look at the different types of fuses, their construction, and so on.

BD Fuses is operated by BD Batteries. Please Click for BD Batteries - Battery Home Page or Click here for BD Batteries available fuses for sale. If you wish, you are welcome to call our phone number (303.800.4725) during business hours with any questions or to order any products we offer! Thanks for taking the time to visit BD Fuses and we look forward to serving you in your fuse and electrical needs.

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