Determining fuse sizes

[francene]

I am wiring up the 12 volt system now. So how do you determine the fuse size needed for each circuit? I will be using the glass-type fuses.

The Shurflo pump specified a 10 volt slow-blow fuse, so I've got that in place.

Now I've got a bunch of lights and a 12v socket left to fuse.

Thanks for advice
Fran

[Bud Kennedy]

The fuse rating in amps, not volts, is in the curcuit to protect the wiring, not the appliance.

The fuse should be rated no larger than the current capacity of the smallest wire in the circuit.

Ideally, ALL the wires in a circuit would be the same size, but this is not always the case as circuit extensions are commonly added by the user who sometimes uses what he has on hand.

If you have a camper that has wiring added by a previous owner, you may have to play detective to confirn that the added wiring is of sufficient size.

If you presume that the wire size is sufficient, then the fuse should be sized a little over the anticipated load. For instance, a 12 or 15 amp fuse for a 10 amp load.

Some appliances pull a lot more current on start up than during normal running. In this case a slow blow fuse is used so that the fuse dosn't blow when the unit is turned on. This is usually not a problem but something you should be aware of.

In most cases, what blows a fuse is that there has been a short in the circuit. When this happens there is a HUGE increase in the current draw that will blow most any fuse. Therefore, any size is better than none.

[Kevin K]

This should help.

[DonDeutsch]

Kevin, I have done some residential wiring and 14 gauge solid wire is 15 amps, 12 gauge is 20 amps. So where is your list from? Is multi-strand different or are vehicles different from residential?

[GeorgeR]

The fuse rating in amps, not volts,..........

Agree, however fuses are also rated for maximum AC or DC voltage. So using automotive fuses usually rated at 32V in 120V application is not good. Blowing a low voltage fuse in a high voltage application will not necessarily open the circuit.

George.

[Kevin K]

Found the info here
http://www.oznium.com/forum/topic15761

[Bill K]

I have never seen a 32 volt fuse, all my fuses are 240 volt.

Bill K

Agree, however fuses are also rated for maximum AC or DC voltage. So using automotive fuses usually rated at 32V in 120V application is not good. Blowing a low voltage fuse in a high voltage application will not necessarily open the circuit.

George.

[GeorgeR]

I have never seen a 32 volt fuse, all my fuses are 240 volt.
Bill K

As far I know the maximum voltage rating standard for most automotive fuses such as ATC, ATM and MAX is 32 volts.
http://stevenengineering.com/Tech_Support/.../06AUTOFUSE.pdf
George.

[Bill K]

Guess I am showing my age, I was talking about glass tube fuses. Sorry about that.

Bill K

As far I know the maximum voltage rating standard for most automotive fuses such as ATC, ATM and MAX is 32 volts.
http://stevenengineering.com/Tech_Support/.../06AUTOFUSE.pdf
George.

[Roger C H]

Here is a good discussion of fuses:

http://en.wikipedia.org/wiki/Fuse_(electrical)

Don't forget that when you are dealing with electrical circuits you are trying to do a task which professional electricians spend years learning how to do correctly.

Every task looks simple until you get into the nitty-gritty of all the ramifications of every action.

[Darwin Maring]

DC Wiring: The type of fuse that George showed are the one's in my converter box and they lite up if the fuse is BLOWN and I think that is far superior to the old glass fuse that is somewhat hard to tell without taking out and sometimes testing with a ohm meter.

On the glass fuse: Some are SLOW Blow meaning they will accept a higher START UP load without blowing, The regular ones will blow immediately if the load is exceeded during startup.

Me, I would install the heaviest wire (10 or 12 Gauge) and then fuse for the load. With the heavy wire you will cut the voltage loss and never have to worry about you wire melting the insulation with it fused at a lower amperage.

15Amp seams to be the most common. I guess you could have one of those 12DC Coffee pots and that may take a larger fuse and If you installed (10 or 12 Gauge) wiring you could fuse up to accommodate it and still be safe by not exceeding the wire capability.

[francene]

Thank you to all who responded. Sounds like we're here to learn a thing or two from eachother.

My fuse terminal block uses the glass fuses, so that's what I'm working with.

So say I use 12g wire for a circuit, but the light I am using has skinny wires (18g) in the fixture itself. Is this difference relevant to anything?

Also, how do you join 12g wire to 18g wire? Those crimping type connectors are sized for only one gauge.


Fran

[Darwin Maring]

Wire Nut.

[Art VanDelay]

You really need some electrical assistance. By asking this question it tells me that you should purchase a really good "How To Do It" book and solicit someone with the experience so you will not hard boil your egg and/or kill yourself.

[John Jesse]

Thank you to all who responded. Sounds like we're here to learn a thing or two from eachother.

My fuse terminal block uses the glass fuses, so that's what I'm working with.

So say I use 12g wire for a circuit, but the light I am using has skinny wires (18g) in the fixture itself. Is this difference relevant to anything?

Also, how do you join 12g wire to 18g wire? Those crimping type connectors are sized for only one gauge.


Fran

If the light has 18 gauge wire there is no need to run 12 gauge to those connections unless there will be other loads on that circuit. If you do plan to connect 12 gauge to 18 gauge DO NOT use wire nuts, they are not designed for that type of connection and should never be used on these type of 12 volt circuits. Neatly solder and heat shrink the connections, to make the connection watertight shrink a few layers of tubing over the small wire to make it similarly sized to the bigger wire so the final heat shrink tube that goes over the solder joint will be tight on both sides. You could also use splicing tape, it looks like thick electrical tape, you wrap it really tightly around the connection and it self fuses into a watertight connection.

[Bud Kennedy]

I disagree, you will get less voltage drop whenever you run a larger wire. so if you run #12 for 15 feet and then connect to 10 iches of #18, you are better off than running #18 the whole way. ----- besides the #18 appliance might not be the only thing on the circuit. Now or in the future.

I also recomend wire nuts when the sizes are dissimular, it works just fine. Just be sure you strip plenty of insulation off so that you get a good wrap of the strands.

[GeorgeR]

If the light has 18 gauge wire there is no need to run 12 gauge to those connections unless there will be other loads on that circuit. If you do plan to connect 12 gauge to 18 gauge DO NOT use wire nuts, they are not designed for that type of connection and should never be used on these type of 12 volt circuits. Neatly solder and heat shrink the connections, to make the connection watertight shrink a few layers of tubing over the small wire to make it similarly sized to the bigger wire so the final heat shrink tube that goes over the solder joint will be tight on both sides. You could also use splicing tape, it looks like thick electrical tape, you wrap it really tightly around the connection and it self fuses into a watertight connection.

In the mobile environment electrical connection are exposed to vibration and very variable humidity. I agree with John J. that a watertight soldered connection is a foolproof solution. A self fusing electrical tape or shrinkable tubing is my preferred ways to seal a connection. I use wire nuts at home only.
George.

[Darwin Maring]

Wire Nuts:
Using the proper wire nut and taping it to the wire once it has been twisted properly should remain serviceable without any problems however if you really want to do it right, there is a crimp connector that looks like a top hat where you insert both wires in one end and then crimp the other end and everything is contained inside the connector. The connectors are semi transparent with a copper crimp ring inside. Go to the Home Depot or Lowes and look for them in the electrical department. (See Site Below)

Note: If you use in a damp location, it is advisable to fill the wire nut and/or top hat crimp with a dialectic grease first (B4) you twist tight and/or crimp. This keeps the elements from reaching the bare wires inside.

The rule for crimping is:
Crimp once. You never make a second crimp because the second crimp may loosen up the first crimp.

Crimp Tool:
The BEST crimp tool is a Ratcheting Crimping Tool, one that must be squeezed all the way down before it will release. (See Site Below)

WHY:
Because using a cheep tool that does not ratchet may not crimp all the way and you may loosen your grip and the tool may move and you wind up with a less than perfect crimp.

The Crimp Tool
http://www.harborfreight.com/cpi/ctaf/disp...temnumber=97420

The Crimp Connector:
http://www.idealindustries.com/prodDetail....insulated_ideal

[Harry Young]

Francene,

These are low volt glass fuses that glow when they blow so you can see which fuse is bad right away as mentioned earlier...these are the glass type you will use.



Happy Camping, Safe Trails.

Harry

http://outlet.crutchfield.com/S-QMgYnde8YIn/p_082SFE0001/Littelfuse-Smart-Glow-SFE-Glass-Fuses.html?cc=07

[Raya]

There are crimp connectors that "step" so that you can join two different wire sizes, in case that's what you'd like to use. If you choose to use them, I recommend high-quality, heat shrink connectors, such as those made by Ancor or 3M.

Raya