Fuses

[WesS]

I am looking for a fuse holder that holds less than 10 AMP fuses. I have a 2.5 Amp and 5 Amp Im needing, HELP???

[jehardiman]

???

Fuse holders are sized for a given SIZE of fuse (1AG, 2AG, AGC, 5x20mm, etc) and a MAXIMUM amperage ( 10, 15 etc) and Voltage (32, 125, 250, etc), the fuses of that size can be bought in many different amperages (.01, .5, 1, 5, Etc), voltages (as above), and speeds (fast, medium, or slow blow).

Look here for examples http://www.action-electronics.com/fuses.htm

[Landlubber]

Nice reply jehardiman.....that should sort it all out.

[WesS]

Thanks guys for the help...however, I still need help. I have a switch panel from Seasense- A LED 6 switch with 15 Amp breakers that cannot be changed out. The breakers say: 250VAC 50VDC 125 VAC 15A. I have these accessories:
1- Fishfinder- doesnt say amp on it but fuse size is 1 AMP
2- Bilge pump- doesn't say AMP but fuse size 2.5 AMP
3- Head lights- they are 15AMP and have a fuse already
4- Run lights- ?
5- Anchor lights- ?
6- Floor Lights- .034 AMP but doesn't say fuse size
7- 12V plug- ? (I don't want this on a switch if I can help it because I only have 6 switches

I am guessing all 18G AWP wire except the headlights which already have wire.

The boat is wood and epoxy/fiberglass with conduit run inside the pontoons. I am in construction and understand home wiring... but I just don't want my new baby to burn. Thanks for any help you guys can give me.

[jonr]

Depends on the standard, but here is one calculator you can use:

http://www.csgnetwork.com/directwireinfocalc.html

[jehardiman]

If your breakers are 15 amps and your wire is 18G, then you are already in trouble as 18G is too small for for 15 amps at 12 VDC in a useful (~25 foot) run. While AWG 18 is permitted to carry up to 20 amps maximum at zero length, a chafe short at the end of a long run would melt the wire before the breaker went due to power drop.

Google up "ABYC amp wire table" and check out some of the Tables and calculators.

FWIW, breakers are to protect the wire, fuses are to protect the unit. Most units should be self fused. If not, what you may want to try is have the breaker feed a fuse block, and then fuse each circuit for the end unit. The wires to the unit would still have to be calculated against the breaker for safety.

[Luckless]

Originally Posted by jehardiman

FWIW, breakers are to protect the wire, fuses are to protect the unit.

Where did you get that idea? Both fuses and Breakers do the exact same thing, and offer similar protections.

Fuses can be made more sensitive, faster reacting, and cheaper than breakers, but nearly all are a one time use item and require replacement. Many devices have built in fuses, but often the choice of a fuse over a breaker is simply because they're cheaper. (Some electronic devices favor the fast blow of fuses for safety reasons.)

You should never require both a fuse AND a self-tripping breaker on the same circuit, a fuse and a simple switch will work fine for if you want to be able to open the circuit to replace fuse.

[jehardiman]

Originally Posted by Luckless

Where did you get that idea? Both fuses and Breakers do the exact same thing, and offer similar protections.

Marine damage control requirements. Fuses, bus ties, and breakers actual do different things with respect to speed and single use, and are often selected in marine applications based upon those differences.

[TollyWally]

From a fairly pragmatic point of view based on working commercial vessels I'd say fuses and breakers respond the same way but serve different purposes.

Breakers protect the wiring from overheating through overloading

Fuses protect the devices from nasty gremlins

Switches which haven't been mentioned, turn devices on and off. It is a mistake in my opinion to use breaker style switches as switches to turn on devices. You are putting excessive wear on a part of your safety system instead of wiring in dedicated switches. For things seldom used it hardly matters, for things switched on and off often, I think it does over the long term.

Wires should be as short as possible, leaving a little extra for future fiddling about and to relieve strain. Wires should err on the fat side. Fat wires cost little more by the time all is said and done, run cooler, and make downstream devices happier.

This is my opinion based on my experience, worth approximently .02, but that's the way I roll.

[Fanie]

Breakers protect the wiring from overheating through overloading

Correct !

Fuses protect the devices from nasty gremlins

You mean supposed to ! You get various types of fuses, from slow blow to ultra fast. The types of fuses are used for different types of applications.

Excessively thick wires are heavy (and expensive).

Wes, I'm sure the auto spares shop can give you a nice fuse box. Find something with a veriety of fuse ratings and look for one that you can access the inside of for waterproofing either with something like silicone but preferably conformal coating, a good grade. Don't coat the contact areas
Buy spare fuses.



Keep in mind you get something like a polly fuse as well. A polly fuse goes 'open circuit' (actually draws very little current) when it exceeds it's current rating and heats up. These devices are available from electronic spares shops and their ratings vary from 20mA up to 9Amp.

You can increase the current level by parallelling them, but you have to use a heat transfer method between them so they maintain the same temperature. It is also advisable to schrink heat schrink sleeving of the correct size around them, it allows the polly fuse to maintain the correct heat without outside interference. If you cool the polyfuse down 'artificially' ie by pressing ice against it it will begin to conduct again.

The terminals and wiring distribution I'm making for my boat will be all polyfuses. The reason is simple. If there is a problem and you get a short, all you have to do to correct the circuit is to remove the short or the problem that draws too much current. You don't blow things up, and you don't have to fiddle around with finding fuses and have to replace with the wrong fuse because you don't have anything else. I like the fuses to be maintenance free !

So en light of this, you can seal the box where the fuses goes into water proof, and a simple LED and series resistor (2k2 1/4W) across the multifuse indicates if it has gone open circuit.

Sealed, maintenance free, self replacing fuse with over current indication
I almost forgot to mention, they will probably cost less than conventional fuses with their fuse holders.

[jonr]

It's important to differentiate between the ampacity that will cause a fire risk and the resistance and length that will cause excessive voltage drop. The latter doesn't apply here.

18 gauge wire with 15 amp breakers is on the edge for ABYC standards - depends on how hot the area gets. 16 gauge is recommended.

[WesS]

Talked with a friend and he said (in lamens terms for me):
Bulb watt/12= AMPS
AMP x 2.5%= fuse size
use 16G wire
Accessories fast blow, motors slow blow

Is this correct as well?

[Fanie]

The accessories should have their own fuses rated at some current and will be the type (slow to fast) required.

On the supply side in your fuse box you can have them all slow blow.

[CDK]

Quite a lot of fuse talk here, and mostly true.

I like to add that if the text on a fuse says 1 Amp, the manufacturer states that the fuse can carry 1 Amp without blowing for a certain length of time, say at least 30 minutes, so virtually continuously. Loaded at 125%, even a fast fuse will keep conducting a long time.

And one more remark:
I have replaced most of the cylindrical fuses in my boat with automotive, knife type fuses because fuse holders tend to corrode in salty air, causing all kinds of intermittent contact problems. The automotive fuses fit 6.3mm faston crimp terminals with much more contact pressure than a fuse holder can provide.
The only round fuses I kept are the ones supplied by equipment manufacturers for radio, fishfinder etc. because they are watertight rubber ones installed directly under the instruments.

[jehardiman]

Originally Posted by jonr

It's important to differentiate between the ampacity that will cause a fire risk and the resistance and length that will cause excessive voltage drop. The latter doesn't apply here.

Actualy they are quite tied together. The internal resistance of a wire is what causes it to heat up (increasing it's resistance at therefor amp draw) and this resistance is what causes voltage drop over the circuit. Under increasing amprage, wire temp and voltage drop rise. Voltage drop requirements are designed to limit this heating to something manageable by the wire insluation, which is rated by the tempature that it can maintain (typically 90C for home wiring, but should be 105C for marine applications). For 100% service requirements, ABYC requires that the voltage loss should be limited to 3% throughout the run. For intermitent service, the ABYC limit is 10%.