I want to install a 12V power supply on my boat dash for GPS, etc, but the power supply plug did not come with any sort of fuse holder and since this is a new install, I need to know if I need an inline fuse and what size (amp?) fuse? Can anyone shed some light on this please? Thanks!

Well you need to size it based on the devices you are going to power. If the GPS is 5 amps, you need at least a 5 amp outlet and wiring. Each device is different, thing like cell phone chargers, ipods etc and very low current. Your wifes hair dryer would be somewhat higher . Look on the back of the GPS, it should say how much current it draws.

Decide what amperage fuse you need; it will be in the GPS instruction manual. It will be quite a low ( e.g. my Navman chartplotter has a 1A fuse).

Then find your main fusebox; if you're lucky, there may be an unused terminal and slot for a fuse. There was on mine, as I had a few things I wanted to hardwire in I used that terminal to feed a small waterproof auxillary fuse box which then supplies the radio, GPS and fishfinder, all with their individual, appropriate fuses. Doing it that way, all the fuses are together, easy to find if there is a problem. The fuse I used on the main fusebox to feed the auxillary fusebox has a rating of the sum of all the other fuses on the auxillary box. If you just want to wire in the GPS, you could feed it directly from the main fusebox if there is a spare slot, with the appropriate fuse fitted.

Otherwise, you're going to need to either take a feed from the input to the fusebox to an inline fuse for the GPS, or take the feed to that fuse from somewhere like the auxillary 12V socket, (which is already fused at the main fusebox, but with quite a high rating of 20A if I remember correctly).

Graham

I am a bit confused in that you are putting a 12VDC power supply in a boat that has 12VDC battery power available?

But regardless, the purpose of a fuse is to protect the (down-stream) wiring, not necessarily the device connected to the wiring. Each device itself should have its own fuse to protect the device.

In the scheme of things, the fuse should be sized to protect the smallest unprotected wire in the circuit (say a wire otherwise unprotected by a branch circuit fuse).

I guess I didn't word that correctly; I'm adding what amounts to a cigarette lighter plug as the boat does not currently have one. Something to power my GPS, maybe an inflator for tubes and such, or a spotlight if needed. The boat doesn't have a fusebox, so I'll need to come straight off the battery to the hot lug and then ground off the other end. The kit I purchased did not come with an in-line fuse holder and I'm sure I need to add that in. I just want to have a big enough fuse so as not to keep replacing or damage the boat.

What gauge wire is on the socket going to the battery or does it have wire yet? You don't want it's wire to burn up before the fuse blows. That is the determining factor. I would think the kit has some sort of current rating on it. Does the GPS plug have a fuse in it?

Those plugs should be fused for 10 amps. It's kind of an industry standard on cigarette lighter outlets.

I would've thought so too, but no mention of any rating anywhere. The GPS does have a fuse in it's plug, but like you say, I don't want smokin' wires~! I think I'll go with Dave R's suggestion of 10 amps, with a few spares in the glove box. Thanks everyone!

On my Four Winns the 12V receptacles on the dash are connected to +12V as as standard by a 20A fuse. Both of their cables are protected by the same fuse, plus that fuse protects the electric engine hatch lifter cable/ switch. It still means though that if only one circuit is being used, it's a 20A rating fuse that protects it. Cables are 14 AWG according to the schematic.

If putting in a new circuit, it makes sense to protect it by putting in as low a rated fuse as possible (that doesn't blow under normal operation). Then you get maximum protection should something go wrong with what is connected (or if there is a short). No need to use a 10 A fuse if 1A or 3A is adequate, even if the cable used has an enormous current carrying capability.

Graham

I'd size the fuse for use with a spot light, just in case you need to plug one in.

If you are wiring directly to the battery, the Coast Guard requires circuit protection (i.e. fuse) within 7" of the + battery post.

Fuse?

How about one or two of these--



They come in different ratings, seem to work well, and you never have to buy fuses...

(I have a box full of old ones, that were in the Sun Runner--but I don't know the ratings)

As long as they are ignition rated (if they are put into the engine room), they would work OK.

Carling makes those, as I am sure a lot of different companies do as well. If you could find the part number, you could probably figure out the rating.

My old breakers used to have a paper stuck to one side which probably had the rating on it.
Molded in are "E - T - A" /series 1658, made in usa, pat pend...no indication of rating. I bought Carlings as replacements, the ratings were printed on them. Not sure what brand that is in the photo, got it from the West Marine website. I think a better breaker would be suited near the battery, but these are great for in-dash use.

That is correct, Same if you are connecting to a bus bar or terminal strip that is fused for heavier wire than what you are using for the branch circuit.



It has been posted, but I'll repeat: The fuse protects the wire, not what you may connect to the wire. The above statement should read "I'd size the wire and fuse for use with a spot light, just in case you need to plug one in."

You should make sure the outlet itself can handle the current drawn by the spotlight.

They would work if they are ignition protected, but they can be difficult to mount within seven inches of the battery.

Rather than connect anything (other than a bilge pump, CO detector, or stereo memory) directly to the battery, connecting to the load side of the battery switch is often a better idea. You then have a place to mount circuit breakers or fuses and have the advantage of being able to turn everything off with one switch.

The only Power Outlets I have ever seen, and the one I put in Lowrider several years back, was rated 10A on the packaging, and (I would have to look) MAY say so on it.
My max purpose was spotlight, and anything else I desired.

Wired 12-gauge, and fuzed 10A, which is what I would suggest as well.