Fuse question

[Hazwan]

Is it okay if I replace 40A fuse with 2x20A fuse wired in parallel to each other?

[A18A]

i dont see why not, but someone will probably have a reason against it

[Hazwan]

Yeah I'd like to make sure first before I blow something up lol

[2oodoor]

I think that would still ony be 20 amp protection.

You can connect two 12 volt batterys in paralell and get 24 volts
You can connect two 12 volt batterys in series and get 12 volts
But if you connect a voltmeter to either battery it will only be 12 volts

A fuse is a circuit interupter , a small piece of wire designed to burn as load increases (in this case appx 20 amps) resistance to ground. No matter how you connect those fuses it will pop if it"s rating is overloaded , just a matter of which (positive side or earth ground) will blow first.
If you need a 40 amp fuse, get a maxi fuse like they use in new model cars. They make separate fuse holders for those, I get them from emergency equipment suppliers.

http://www.autoshop101.com/ this link is to free online training, for better understanding of series and parallel circutis click "online instruction" then "electrical"

[Hazwan]

You can connect two 12 volt batterys in paralell and get 24 volts
You can connect two 12 volt batterys in series and get 12 volts


I think you got it backwards?

Anyway, won't the current flow into both path (2 fuses) and divides the current between the 2 fuses?

Thanks for the link, I haven't read them yet though.

[2oodoor]

I think you got it backwards?

Anyway, won't the current flow into both path (2 fuses) and divides the current between the 2 fuses?

Thanks for the link, I haven't read them yet though.

yes ass bakkerds. My point still is there.
The link is really good for explaining this.
If a overload or short blows the first 20amp fuse, and the condition still exists a fraction of a second afterwards, the second one will blow too would't it? So what would be the point. it still is not capable of a 40 amp load.
What is it you are trying to do exactly?

[2ndGenGuy]

Think of electricity like water. It follows the path of least resistance. The fuses burn by creating resistance across them. If you send 30 amps down the line, and it hits one of those 20 amp fuses, the electricity should wind up flowing down through the other fuse before any heat or anything has a chance to build up. Essentially, I believe it would self balance the load between those two fuses and the 30 amps would split between the two putting a 15 amp load on each. Once you hit 41 amps, one fuse should blow, whichever sees the most resistance first, then the entire load flows through the other one blowing it up. So yeah, I am pretty sure two 20 amp fuses will act as a single 40.

But that's just my educated guess.

[2oodoor]

A fuse is nothing more than a short length of wire designed to melt and separate in the event of excessive current. Fuses are always connected in series with the component(s) to be protected from overcurrent, so that when the fuse blows (opens) it will open the entire circuit and stop current through the component(s). A fuse connected in one branch of a parallel circuit, of course, would not affect current through any of the other branches.
Fuses are primarily rated, as one might expect, in the unit for current: amps. Although their operation depends on the self-generation of heat under conditions of excessive current by means of the fuse's own electrical resistance, they are engineered to contribute a negligible amount of extra resistance to the circuits they protect. This is largely accomplished by making the fuse wire as short as is practically possible. Just as a normal wire's ampacity is not related to its length (10-gauge solid copper wire will handle 40 amps of current in free air, regardless of how long or short of a piece it is), a fuse wire of certain material and gauge will blow at a certain current no matter how long it is. Since length is not a factor in current rating, the shorter it can be made, the less resistance it will have end-to-end.

credit to http://openbookproject.net/electricCircuits/DC/DC_12.html
permission http://openbookproject.net/electricCircuits/DC/DC_A3.html

[steveko]

Current flows through the path of least resistance. As 2nd GenGuy said. This is true.Although if you wired 2 in parallel all the way too the load. Being 2 separate wires it would only be fused by one. I'm sure you knew this.
But I take it you meant in series/ parallel

[2ndGenGuy]

Right, but as roodoo said, resistance across the fuse builds heat which causes it to blow. Before the resistance should get high enough to blow, some of the electricity should flow across the other fuse.

[2oodoor]

Think of electricity like water. It follows the path of least resistance. The fuses burn by creating resistance across them. If you send 30 amps down the line, and it hits one of those 20 amp fuses, the electricity should wind up flowing down through the other fuse before any heat or anything has a chance to build up. Essentially, I believe it would self balance the load between those two fuses and the 30 amps would split between the two putting a 15 amp load on each. Once you hit 41 amps, one fuse should blow, whichever sees the most resistance first, then the entire load flows through the other one blowing it up. So yeah, I am pretty sure two 20 amp fuses will act as a single 40.

But that's just my educated guess.

I am confused, you just agreed with me then in the last sentence ..no agree
IF the overload pops the first 20 then the second will pop too which means it is only 20 amp protection.
Maybe even less. Draw it up on a paper and look at it.

[2oodoor]

courtesy of crayola....
http://i162.photobucket.com/albums/t280/roodoo2/1207698685.jpg

[dlr1989]

In an ideal circuit where two fuses are paralleled the current would be divided up between the two paths through the two fuses. If this ideal circuit had exactly the same resistance in the both of the parallel paths, the current would be equally divided between the two paths. In reality the two paths will not have exactly equal resistance. Therefore the current through each path will be different.

If an overload or short circuit occurs most likely one fuse will open up before the other. Therefore the protection for the circuit will not happen as quickly as a single fuse would allow. Depending on what is being protected by the fuses this may or may not be a problem.

If this is an existing circuit that is being modified other questions come to mind. For example is the wire conductor sized large enough to handle the increased current?

If this is a new circuit, the cost of a properly sized fuse and fuse holder is a relatively small cost and is most certainly a better wiring practice than the two fuse setup. Another random thought is that it is always better to have fewer points of potential failure. I can't say that I would never parallel two fuses, but it would not be my first choice.

[88Accord-DX]

Ohms law. Two fuses wired like that diagram isn't going to work. Spend the money & get the right fuse.

[Hazwan]

Thanks, I think I'll just go and buy the proper fuse.

BTW this is for my dad's car where the previous owner thought it would be a good idea to get rid of the main fuse near the battery and replaced with regular wire and I'm 100% its not fusible link.

The main fuse is divided into 2, 40A and 30A fuse, which is kinda weird since I never see such setup. And yes, I'm really sure as I've already refered to the service manual.

[2ndGenGuy]

courtesy of crayola....
http://i162.photobucket.com/albums/t280/roodoo2/1207698685.jpg


In your drawing, the load is going to disperse across both fuses. It's not just flowing through the first fuse. When it hits 21 amps total load, 10.5 amps are going to go across one fuse and 10.5 amps across the other fuse. If all 20 amps were to go through the first fuse, then no power at all would be going through the second fuse.

Lets say you could divide the load up into 1amp chunks and isolate each ampere from each other across the fuses. You put 1amp through the circuit, the first fuse would take it. So you have 1amp on one fuse and 0amps on the other. Now lets say you added another 1amp to the circuit for a total of 2amps. There is going to be currently less resistance across the second fuse. That second amp you added is going to go through the second fuse and divide the load equally. Each carrying 1amp across a total 2amp circuit.

Also think about this. You now have double the amount of material for the electrical current to flow across. Instead of having X amount of fuse material, you've now got 2x the fuse material. Not in length either, in thickness.

[steveko]

Re: Fuse quest.

If an overload or short circuit occurs most likely one fuse will open up before the other. Therefore the protection for the circuit will not happen as quickly as a single fuse would allow. This statment contradicts itself. Posted by dlr1989.

It will actually blow faster.

[ghettogeddy]

Ohms law. Two fuses wired like that diagram isn't going to work. Spend the money & get the right fuse.

isent in ohms law that you have to have a pos and neg to change the ohms

I = V/R

where I is the current in amperes, V is the potential difference between two points of interest in volts, and R is a circuit parameter, measured in ohms (which is equivalent to volts per ampere), and is called the resistance. The potential difference is also known as the voltage drop, and is sometimes denoted by U, E or emf (electromotive force) instead of V.[1]

The law was named after the physicist Georg Ohm, who, in a treatise published in 1827, described measurements of applied voltage and current passing through simple electrical circuits containing various lengths of wire. He presented a slightly more complex equation than the one above to explain his experimental results. The above equation is the modern form of Ohm's law.

The resistance of most resistive devices (resistors) is constant over a large range of values of current and voltage. When a resistor is used under these conditions, the resistor is referred to as an ohmic device because a single value for the resistance suffices to describe the resistive behavior of the device over the range. When sufficiently high voltages are applied to a resistor, forcing a high current to flow through it, the device is no longer ohmic because its resistance, when measured under such electrically stressed conditions, is different (typically greater) from the value measured under standard conditions (see temperature effects, below).

Ohm's law, in the form above, is an extremely useful equation in the field of electrical/electronic engineering because it describes how voltage, current and resisitance are interrelated on a macroscopic level, that is, commonly, as circuit elements in an electrical circuit. Physicists who study the electrical properties of matter at the microsopic level use a closely related and more general vector equation, sometimes also referred to as Ohm's law, having variables that are closely related to the I, V and R scalar variables of Ohm's law, but are each functions of position within the conductor. See the Physics and Relation to heat conduction sections below.

[2oodoor]

Gheddy did you quote that from the link{s) I provided? there were very good explanations of Ohm's law in the autoshop 101 and other one as well.
As for polarity being required for OHM's law to apply, I have an urge to question that since you can measure resistence on spark plug wires with no power source or polarity involved , unless the meter serves as opposite pole.

I am going to agree to disagree here that the diagram I drew to simulate Hazwan's proposed fuse setup demonstrates suffecient protection at 40 amps, I think it would actually be 20 or less actually>>>The paralell circuit , The series circuit is twenty amps and the second fuse is totally useless.
I suppose either it is or isn't could be tested.
If you are talking DC in a vehicle, a 40 amp circuit is important enough to use the right stuff to prevent fires and protect electronics.
If it were an AC circuit ?? hrmmm

[2ndGenGuy]

Series circuit would be 20 amps, that I'll agree with. But a parallel series of fuses gives more path for electricity to travel across, so why would it blow sooner?

A 40 amp fuse is going to have twice the conductive material in it of a 20 amp fuse (theoretically, in a perfect world), so that more juice can flow across it before the resistance gets so high that the fuse blows...

If you took that 40 amp fuse, and physically split it in half, the total amount of conductive material stays the same, but you instead have two fuses that can carry 20 amps each. Now just because you've split it in half, would it suddenly not be able to carry the same amount of electricity? There will, of course be small inconsistencies and imperfections that would bring down the efficiency, but it would be able to handle more than 20 amps.

Not trying to argue, just discuss. I really want to try testing this out with a little faux breadboard and a spare car battery and some lights... :cheers:

[dlr1989]

Unless both fuses open at exactly the same instant, the total time for the two fuses in parallel to protect the circuit will be longer because the total time is the time it takes fuse1 to open plus the time for fuse2 to open. Both fuses must open to clear the circuit so the total time is from the time the first fuse starts to open until the time the second fuse is completely open. As opposed to a single fuse circuit where the total time would be the time it takes for a single fuse to open. If the fuses were in series not parallel I would agree with your correction.


Re: Fuse quest.

If an overload or short circuit occurs most likely one fuse will open up before the other. Therefore the protection for the circuit will not happen as quickly as a single fuse would allow. This statment contradicts itself. Posted by dlr1989.

It will actually blow faster.

[steveko]

--------------------------------------------------------------------------------

Unless both fuses open at exactly the same instant, the total time for the two fuses in parallel to protect the circuit will be longer because the total time is the time it takes fuse1 to open plus the time for fuse2 to open. Both fuses must open to clear the circuit so the total time is from the time the first fuse starts to open until the time the second fuse is completely open. As opposed to a single fuse circuit where the total time would be the time it takes for a single fuse to open. If the fuses were in series not parallel I would agree with your correction. Ijust meant the one fuse would blow faster in the parallel fuse diagram.Mabey I misunderstood what you were saying.

This discussion is getting long winded with different views. Including mine of course

[ghettogeddy]

Gheddy did you quote that from the link{s) I provided? there were very good explanations of Ohm's law in the autoshop 101 and other one as well.
As for polarity being required for OHM's law to apply, I have an urge to question that since you can measure resistence on spark plug wires with no power source or polarity involved , unless the meter serves as opposite pole.

I am going to agree to disagree here that the diagram I drew to simulate Hazwan's proposed fuse setup demonstrates suffecient protection at 40 amps, I think it would actually be 20 or less actually>>>The paralell circuit , The series circuit is twenty amps and the second fuse is totally useless.
I suppose either it is or isn't could be tested.
If you are talking DC in a vehicle, a 40 amp circuit is important enough to use the right stuff to prevent fires and protect electronics.
If it were an AC circuit ?? hrmmm

it was in the ohms law wiki

[2oodoor]

I just wanted to be the last one to post , hahahahaaaa

no just kidding. Steve it is equally difficult to tell who is pro and con.. lol

[Hazwan]

I really want to try testing this out with a little faux breadboard and a spare car battery and some lights... :cheers:

Yeah I'm curious on seeing how this will work out. I have few extra fuses... hmmmm :idea:

[steveko]

--------------------------------------------------------------------------------

(I just wanted to be the last one to post , hahahahaaaa

no just kidding. Steve it is equally difficult to tell who is pro and con.. lol )

That' True roodoo, LOL. Maybe,Hazwan's experiment will be the True test for him that's all that matters.

[ghettogeddy]

no matter how its put it will only protect to 20 amps no more

[dlr1989]

For those who want to know more, google is your friend. A couple of interesting links. No matter if you agree or disagree they still make very interesting reading.

http://books.google.com/books?id=V84ZHMmdNmYC&pg=PA316&lpg=PA316&dq=%22parallel+fuses%22&source=web&ots=9ZpTvy-_5Z&sig=C2O9mwMWDjmJWmMAHTZEmwcqNlw&hl=en

http://www.ecnmag.com/Dual-Fuses-Protect-High-Current-Board-Level-Circuits.aspx?menuid=580

Just remember, youth and enthusiasm is always trumped by age and treachery.

[2oodoor]

http://www.autoshop101.com/
http://openbookproject.net/electricCircuits/DC/DC_12.html

I already put these up on this thread old treacherous one:) both links can be followed back to other chapter tables for some very education... err interesting reading. As are the ones you provided.. that second one nails the subject, good find!

[carotman]

Fuses do not blow instantly, it tales a couple of milisecond fot it to blow. I'm sure it's going to work.

[88Accord-DX]

I guess that will work in parallel if wired right. Seems more work than it's worth with spade terminals, soldering & so forth for the set up. Anyway, 40 amp fuse-able link sounds like less work.

I was leaning towards 20 amp protection with the crayon thing there.