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Author Topic:   Selecting Fuse Size
Tom W Clark posted 04-06-2015 04:05 PM ET (US)   Profile for Tom W Clark   Send Email to Tom W Clark  
What size fuse should one use if they are installing this USB Power Outlet device onboard a vessel with 12 volt power?

http://www.polyplanar.com/product/ usb-pm-waterproof-dual-port-usb-charger

The manufacturer offers these specifications:

Input Voltage: 12V DC
Output Voltage: 5V/3A

...but offers no advice about protecting this device with a fuse at the electrical distribution panel.

padrefigure posted 04-06-2015 05:54 PM ET (US)     Profile for padrefigure  Send Email to padrefigure     
Well, you could back into a fuse size by taking the 5V/3A
and assuming the capacity of the component is 15W, divide by 12v, 1.25A, so to with a 2A fuse.
DVollrath posted 04-06-2015 06:02 PM ET (US)     Profile for DVollrath  Send Email to DVollrath     
Hi Tom,
I'd use a 2A fuse, depending upon the sizes of the fuses available for your box. Waving hands, this would assume a (5V*3A)/(12V*2A) = 62.5% conversion efficiency for the device, which is reasonable. Assuming an intra-console run, wire it with 20AWG or larger.

Please be aware that upon retirement last week, all electrical engineering goo was excised from my brain, so you get what you pay for :)

Go Badgers!

Dennis

timf posted 04-07-2015 02:31 AM ET (US)     Profile for timf  Send Email to timf     
As others have said, 2 amp is likely fine. Unlikely, but if the thing is pretty inefficient in how it reduces voltage, a 5A fuse could work just as well if you have that available.

In reality for a device like this, the fuse if mostly protecting your boats wiring more than anything else.

Tom W Clark posted 04-07-2015 03:33 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Thank you. I appreciate the practical and applicable advice
jimh posted 04-07-2015 08:42 PM ET (US)     Profile for jimh  Send Email to jimh     
We don't know the efficiency at which the 12-VDC power is converted to 5-VDC power. If the conversion is just by a simple voltage regulator, to create 3-Amperes of power at 5-VDC may require a current of 3-Amperes at 12-VDC. If that is the case, fuse the 12-VDC branch circuit at 3-Amperes.

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