Fuse sizing question for a newbie

Thread Starter

risingfish

Joined Dec 31, 2017
3
Hi, first time here and boy am I happy I found this forum. I have a question regarding fuse sizing I was hoping someone could enlighten me on. I know that fuse amp sizing should about 20% higher than normal load in most cases. In my case I am running a 1000W hot tub heater to heat a 300 gallon fish quarantine tank for a sick fish (long story here). The circuit this heater is plugged into is 20A/115V. Based on the 1000W heater rating the draw at a full power will be 8.7 amps. The closest size fuse I could find in a 10A fuse which is about .5A lower than the 20% buffer. I'm using AGC fuses.

My question is is only a 15% buffer ok? I'm actually fine with it blowing a little sooner in a short situations as long as I'm not having to replace the fuse every week because of power fluctuations.
 

dl324

Joined Mar 30, 2015
9,126
Welcome to AAC!
My question is is only a 15% buffer ok? I'm actually fine with it blowing a little sooner in a short situations as long as I'm not having to replace the fuse every week because of power fluctuations.
You shouldn't require any buffer.

From Littlefuse documentation:
upload_2017-12-31_19-38-47.png

A fuse will carry 75% of it's rated current indefinitely (at 25C). The first column is for automotive, second column is what you want.
 

tranzz4md

Joined Apr 10, 2015
310
For your purely resistive load, 100% will give you the best protection! I'd say for an AGC fuse 10A is close to perfect!

And I've been fusing loads waaaay longer than there's been web based forums like this!
 

Thread Starter

risingfish

Joined Dec 31, 2017
3
Thank you for all the replies! It sounds like the 10A fuse should be fine, and if for some reason the 10A doesn't handle it going up to a 12A is an option. Out of curiosity, why does the resistive load of the heating element make ~100% a better number to shoot for?
 

tranzz4md

Joined Apr 10, 2015
310
Because a purely resistive load has no inrush, or power factor (where the current and voltage aren't in phase) components. So, the only reason you'd want any difference or margin, was if you had conductor, termination, or heating problems, or issues with the quality or tolerances of the fuses themselves.

If your load element were very expensive to replace (including the labor to do it), you'd want to intentionally size your overcurrent protection as close as possible without nuisance tripping.
 

Thread Starter

risingfish

Joined Dec 31, 2017
3
OK, thank you for the explanation. Even with my rudimentary understanding of electronics, that actually makes sense.
 
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