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Author Topic:   Selecting Fuse Size

Tom W Clark posted 04-06-2015 04:05 PM ET (US)
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)
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)
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)
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)
Thank you. I appreciate the practical and applicable advice
jimh posted 04-07-2015 08:42 PM ET (US)
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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Author	Topic: Selecting Fuse Size
Tom W Clark	posted 04-06-2015 04:05 PM ET (US) 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) 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) 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 (5V3A)/(12V2A) = 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) 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) Thank you. I appreciate the practical and applicable advice
jimh	posted 04-07-2015 08:42 PM ET (US) 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.