HELP! What size fuse?

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HELP! What size fuse?

Postby sHoootR » Sun Jun 18, 2006 8:13 am

Getting started on my wiring. Just don't know how to pick fuse SIZE for each circuit..Any help greatly appreciated. Is there a simple formula for figuring fuse size? :?
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Postby GeorgeTelford » Sun Jun 18, 2006 8:30 am

Hi

You could make a serious study of fuseology or for the simple circuits in a tear select a fuse of only marginally higher ampage than that circuit will use max

ie lets say you have 2 16 Watt lights on one circuit, if both were switched on the current draw would be 2.6666r so a 5 Amp fuse would be ample. but in this situation you are only protecting the cable from overload so a 10 Amp would not be great problem.

with electronics the fuse will be specified (or even included!) but even so If you are running a length of cable to that device protect it with a fuse slightly over the max draw.

The above is really simplistic, but will generally cause you no problems.

If you want to understand a little more here is a basic fuseology link

http://www.circuitprotection.ca/fuseology.html
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Postby GeorgeTelford » Sun Jun 18, 2006 8:36 am

If you just wanted to get slightly more technical (here is a basic note I kept (it is from a webpage but I do not have the link)

Fuse Selection: In general fuse values should be selected so that the normal continuous circuit current is 75% of the fuse's value. So a 1 amp fuse should be selected for a circuit with a normal continuous current of 750mA. Fuses are rated for a ambient temperature rating of 25oC [ambient temperature around the fuse, not the room]. So under normal operating conditions a fuse should be de-rated 25% [normal current / .75] while operating at 25oC. Another rule of thumb to protect a circuit from excessive current is to select the fuse value for 125% [normal current x 1.25] over the normal circuit load. To protect against a direct short, select the fuse value as 150% above the normal circuit current. Fuse current is de-rated based on the fuse type and ambient temperature of the air. In general de-rate the current rating 5 to 10% per 10oC of temperature increase, check the manufacturer for derating information. Circuit designs having high in-rush or turn-on current may require the use of a 'slow-blow' fuse to allow the momentary current spike to pass with out blowing the fuse. Power supplies and motors are two devices that have large in-rush currents.
The voltage rating of the fuse does not indicate the operational voltage of the fuse. The voltage rating determines the maximum voltage that will not jump the gap between the elements after the fuse has already blown. So the fuse will operate at the rated voltage and any voltage below the rating.
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Postby Cutterpup » Sun Jun 18, 2006 11:14 am

George with all your knowledge of electricity you always state an answer only a college professor at the master’s level could understand. People ask simple question that are very similar in nature and want a simple answer. I have never read any of your answer that was simple.

In this last post once I got thru all your smarter than everybody else answer there it was use a fuse that is slightly larger rated capacity than the most power that you will use. I will never understand why with these simple trailers you have got to answer with an explanation that only confuses people and must cause you great pains from laughing so hard at the confusion that you caused.
:laughing1: :laughing1:

Really George simple answers are the best way to help us poor amatures. :scratchthinking: :scratchthinking:
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Postby Kevin A » Sun Jun 18, 2006 11:42 am

Cutterpup wrote:George with all your knowledge of electricity you always state an answer only a college professor at the master’s level could understand. People ask simple question that are very similar in nature and want a simple answer. I have never read any of your answer that was simple.

In this last post once I got thru all your smarter than everybody else answer there it was use a fuse that is slightly larger rated capacity than the most power that you will use. I will never understand why with these simple trailers you have got to answer with an explanation that only confuses people and must cause you great pains from laughing so hard at the confusion that you caused.
:laughing1: :laughing1:

Really George simple answers are the best way to help us poor amatures. :scratchthinking: :scratchthinking:
Dan

Dan,

To be fair to George, I think you missed a great post he made two posts above your's. He provided a basic answer to the fuse question and supplied a link to a more detailed basic explanation of fuseology for those who wish to delve further into the topic. Let's keep the conversations friendly and try to avoid goading fellow posters. :shake hands:
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Postby GeorgeTelford » Sun Jun 18, 2006 11:55 am

Hi Dan


Read my first answer again, it was simple and to the point and whats more it was not in the least bit rude either.

Only the subsequent posts gave more detail if anyone wanted to delve further.
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Postby Cutterpup » Sun Jun 18, 2006 12:23 pm

George I never said you were rude just your answers are just a bit over most people needs.

As for answering the question in the first post just what is a 2.6666r

ie lets say you have 2 16 Watt lights on one circuit, if both were switched on the current draw would be 2.6666r so a 5 Amp fuse would be ample. but in this situation you are only protecting the cable from overload so a 10 Amp would not be great problem


Is's not W for watts

A for amperage

V for volts

so what is "r"

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Postby madjack » Sun Jun 18, 2006 1:06 pm

Kevin A wrote:
Cutterpup wrote:George with all your knowledge of electricity you always state an answer only a college professor at the master’s level could understand. People ask simple question that are very similar in nature and want a simple answer. I have never read any of your answer that was simple.

In this last post once I got thru all your smarter than everybody else answer there it was use a fuse that is slightly larger rated capacity than the most power that you will use. I will never understand why with these simple trailers you have got to answer with an explanation that only confuses people and must cause you great pains from laughing so hard at the confusion that you caused.
:laughing1: :laughing1:

Really George simple answers are the best way to help us poor amatures. :scratchthinking: :scratchthinking:
Dan

Dan,

To be fair to George, I think you missed a great post he made two posts above your's. He provided a basic answer to the fuse question and supplied a link to a more detailed basic explanation of fuseology for those who wish to delve further into the topic. Let's keep the conversations friendly and try to avoid goading fellow posters. :shake hands:


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Postby GeorgeTelford » Sun Jun 18, 2006 1:06 pm

Hi Dan

r = reccuring

I said my answer wasnt rude, ie unlike your post and I quote

Quote "In this last post once I got thru all your smarter than everybody else answer"

Not only is this rude, but you followed it up with inaccurate point

Quote Cutterpup I have never read any of your answer that was simple.

amazingly this was your follow up to probably the simplist explaination possible of how to select a fuse

select a fuse of only marginally higher ampage than that circuit will use max

Really Dan it doesnt get more simple and accurate than that.
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Postby Elumia » Sun Jun 18, 2006 1:44 pm

George,

I think it is also important to note the size of the wire chosen. Wire size will determine the maximum load. Fuses and circuit breakers are designed to protect the wires not the devices connected to those wires.
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Postby GeorgeTelford » Sun Jun 18, 2006 2:02 pm

Hi Elumia

At first I thought you were trolling here, repeating something that I often say.

But you are expressing it incorrectly, because Max load determines the wire size used, not the other way round.

Fuses protect wires, circuit breakers protect allsorts

Of course wire selection is important, but it is a totally seperate issue, wire or cable should always exceed the load by a good margin and a fuse must blow before the cable (or otherwise what is the point)

Sorry this is rushed but next World cup match as just kicked off

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Postby bdosborn » Sun Jun 18, 2006 4:33 pm

Wire Gauge / Maximum Ampacity
14 / 15
12 / 20
10 / 30
8 / 45
6 / 65

The fuse is there to protect the wire. Size the fuse to the ampacity of the wire as shown above, i.e. a #14 awg wire should be protected with a 15A fuse. To size the wire just add up all the amps of the devices you want to connect on a single wire (circuit)and add 25%. To find amps from watts just divide by 12.
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Postby GeorgeTelford » Sun Jun 18, 2006 4:58 pm

Wire size is all well and good, but the wire is selected to match the load that it will carry not the other way round.

Or does anyone just run random wires around their teardrop and then find appliances that match the random wiring? This really would be the tail wagging the dog.

What I am saying is that we have an appliance to fit, we run wire that will easily carry the max load required with the minimal voltage drop, we then add a fuse that will cope with the max load (and sometimes we have to allow for slow blow due to surge (inrush Currents)

I do not hold with the chart you published there Bruce, at 12v 14 Guage is good for 15 Amps but at max distance of 1 Meter. If you wanted to have 15 Amps over a distance of 5 Meters you would need 7 Guage to minimise voltage drop.

POP quiz, let us say that we have 5 Meters carrying a max of 15 Amps, so we would need to use 8 Guage wire to minimise voltage drop, according to the chart you just published we would fit a 45 Amp fuse, but clearly it would be ridiculous to use a 45 Amp fuse on a 15 Amp max circuit.

Yes the fuse is protecting the wire, but it does not have to be rated at same current carrying capacity to do that.
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Postby madjack » Sun Jun 18, 2006 5:06 pm

...we made it real simple (KISS ya know) and ran 14ga wire w/10amp fuses for all 12v circuits...maximum run was about 6' long....
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Postby bdosborn » Sun Jun 18, 2006 7:13 pm

George,

Good catch on voltage drop but if you'll review my post you'll see I suggested oversizing the wire by 25%. I didn't explain it but that's what its for. Since most people are designing a 4' or 5'x8' trailer voltage drop shouldn't be an issue. The original poster asked for a simple way to size fuses and using wire ampacity is the simplest.
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