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What fuse size for disconnect

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Full Member level 3
Jan 24, 2018
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Hey guys, I am working on an application, where I have 3 motors with a full load of 9.5 A and the rest of the load is 38.6 A. Would it be fine to use 70 A fuse for the disconnect? The motors and other loads already have fuses or motor protector devices connected to them with the appropriate sized branch protected devices installed for each load, but the disconnect fuse has a larger amperage, would think is is okay?

* Motors commonly draw current surge at power-up. Run tests to see if maximum current surge exceeds 70A. It depends on whether you allow all motors to start simultaneously, or in staggered sequence.

* There's also the decision whether to select slo-blow or fast-blow fuse.

* And details such as whether you wish to protect certain circuitry with fast-blow fuses.

* Also whether you wish to protect components which are costly, or difficult to desolder and replace, etc.
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I want to add:
Please read fuse datasheets and application notes.
They give additional information.

You need to learn what "holding current" and "trip current" is and the trip behavior with time.


As recommended in post above, you should find datasheet to your fuses. There also good practice to firstly use an "rude" equation like Itrip=Inominal*a,
where a is in range 1.2...1.5. Remember! This coefficient is application-specific, as example, some motors or capacitive loads can consume a large current in start point. Also, look for trip time in datasheet for your fuse. 10A-rated fuse can hold 10 A for hours but trips at 20 A in milliseconds e.t.c. Also, fuses have a temperature dependence of their parameters. In some application it can be critical. Sometimes it is good to use separate fuse for each power trace.

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