PDA

View Full Version : How to select the proper fuse size

lotas
05-26-2005, 01:19 AM
There seems to be a lot of questions regarding fuses and fuse sizes lately.
Here are some helpful hints in the process to find the right fuse for you.

1.) Determine your wire size's max. amp capacity. Refer to http://www.the12volt.com/info/recwirsz.asp to find your wire. A lot of people run 1/0 so we will use this as an example. The max capacity is 350amps. This means that if more than 350amps are running through the wire it will burn. The point of a fuse is to have the fuse open before this wire burns.

2.) Next, you have to understand the point of the fuse as explained earlier, so obviously you want to make sure the fuse is not larger than the max. capacity of the wire, as we want the fuse to open (or blow/break are other terms) before the wire burns. So right now we are looking at fuses under 350amps.

*By the way... When you are dealing with a fuse size up around 300amps or so, you will probably be looking at ANL fuses (the long ones horizontally, with a sort of spade terminal on each end), not AGU (the cylinder ones) or MAXI (the colored plastic ones found in you car/truck fuse box). *

3.) Next we have to make sure that your equipment will not draw too many amps to open the fuse. **IF YOU DO NOT HAVE A HIGH WATTAGE SYSTEM YOU CAN SKIP OVER STEP 3** For this step, we have some calculations to do. Let's say for arguements sake we are running a 1500watt Class D mono amp for our subwoofer, and a 300watt Class A/B amp for our components. To calculate the estimated amps that each of these will require, here is a quick calculation.

A.) Take each amp, one at a time, and take the amps RMS and (too be safe) divide by 12. (More realistically, your car runs at about 13.8volts, but to be safe, use 12. You could calculate for 12volts and 13.8 and average the two, but we are just looking for a ballpark figure).

1500(watts rms)/12(volts) = 125amperes (or amps) current
300(watts rms)/12(volts) = 25 amperes (or amps) current

B.) Next you want to account for class efficiency. Class D amps are about 80% efficient, so take your figure from your class D amp, 125amps, and multiply it by .2 (or 20%) to add efficiency.
125amps * .2 (class D efficiency) = 25amps
Now add this figure to the original
125amps(from the total RMS/12 of the class D amp) + 25amps (the class D efficiency) which gives you 150amps. So your 1500watt class D amp will draw around 150amps. Already you can see what could go wrong if you used a 150amp fuse, for just this amp alone is drawing around 150amps, which could open our fuse.

Now for the 300watt A/B amp. A/B amps are about 60% efficient, so multiply the original figure by .4 (40%)
25amps * .4 (class A/B efficiency) = 10amps
As we did with the Class D, add 25amps (total RMS/12 from the A/B amp) and add the 10amp (Class A/B efficiency) = 35amps

C. This gives you 150amps for you Class D amp, and 35amps for your A/B amp. Therefore your system will pull around 185amps, at full RMS, at 12volts (although when your alternator is running, you are at more around 13.8volts, which would give us a smaller figure than 185, more around 150 or so)

4.) So as we know our fuse must be rated ABOVE what our system will draw, we now know approximately what our system WILL draw, which is 185amps. Therefore our fuse must be under 350amps, and over 185amps. A 200amp or 250amp fuse would be fine for this situation, most people go with a 250amp fuse on 1/0gauge.

I am no electrical engineer, so please, feel free to chime in on anything you feel is wrong and should be corrected, or any questions. I will gladly edit and make changes. Thanks for your time, I hope this can help a few people out.

lotas
05-31-2005, 12:55 AM
bump...this should help some people

94CamryBumpin
05-31-2005, 01:20 AM

05-31-2005, 01:24 AM
Just a quick note.

You should use the Peak Wattage values of your Amplifiers, Not the RMS values.

Peak is always higher than RMS, so when your Amps reach their Peak Output, they will blow a fuse rated to handle the RMS value

Acidburn
05-31-2005, 01:34 AM
Just a quick note.

You should use the Peak Wattage values of your Amplifiers, Not the RMS values.

Peak is always higher than RMS, so when your Amps reach their Peak Output, they will blow a fuse rated to handle the RMS value
you = idiot
that is all

jsloan10
05-31-2005, 02:04 AM
A usfull tidbit for those who don't know, a fuse or breaker can actually handle much more than its rated amperature in short bursts so using peak wattage ratings is not going to give you ideal protection, you need to establish the nominal current your amp will consume then choose a fuse/breaker that will allow for it.
For example-
In one of my systems I had 2 amps, the sub amp had four 30amp fuses on it and the four channel had four 25amp fuses on it. Thats a total of 220 amps of fuses yet I only had a 100 amp breaker under the hood that only tripped once in over a year.
Why?- the amps had a nominal usage under 100amps, they would draw peak amperage over 100amps in very short bursts the didn't linger long enough to trip the breaker. The one time the breaker did trip was a very hot day and I was really pushing the systems limits, a very hot amp will start drawing more current until it finally goes into protection, blows its onboard fuses or blows your underhood fuse/breaker, the underhood breaker tripped before the amps reached their critical point.
But lets not forget the first and formost reason for having an underhood fuse, if your main power wire shorts out on the chassis of your car it can start a fire and burn your car down to the ground.

05-31-2005, 02:14 AM
you = idiot
that is all

Please explain how my post makes me an idiot.

**An amplifier with an RMS output of 120 watts has a Peak output of ~180 watts.** There is no way around this.

Using the formulas given above:

120 Watts / 12 volts = 10 Amps
180 Watts / 12 volts = 15 Amps

All of a sudden, Our system has the ability to draw up to 50% more power than we originally accounted for.

Now using the Amplifiers described above:

1500 Watts RMS = ~2250 Watts Peak
2250 Watts / 12 Volts = 187.5 Amps
187.5 Amps @ 80% efficiency = 234.375 Amps Total

300 Watts RMS = ~450 Watts Peak
450 Watts / 12 Volts = 37.5 Amps
37.5 Amps @ 60% efficiency = 62.5 Amps Total

234.375 Amps + 62.5 Amps = 296.875 Amps MAX Current Draw

POP goes the 250 Amp Fuse.

Granted I will admit that this only applies if you run your System at it's maximum possible output for long periods of time - Something which I hope none of us do. I will also admit that the fuses are there to protect the WIRE, not the Amplifiers.

** I used 120 and 180 here because these are numbers that I know to be within 2% accuracy**

jsloan10
05-31-2005, 02:28 AM
That 296 is an instantanious peak, the 250 amp fuse will handle the short bursts without blowing, the fuse/breaker needs to heat up before it fails, the rate it heats up is in accordance to how many amps are flowing through it.
For example-
Say the flesh on your fingertip is rated to handle 70 celcius (canadian) but you can quickly touch a hot surface over 70C without getting burned, as the temperature gets hotter the amount of time it takes your skin to burn shortens.
Like a hot potato to a burning peice of coal, you can touch the hot potato for a short time and possibly if your quick enough tap the burning coal without getting burnt all the while your skin is rated for a much lower constant temperature.

See what I mean?.

lotas
05-31-2005, 05:13 PM
That 296 is an instantanious peak, the 250 amp fuse will handle the short bursts without blowing, the fuse/breaker needs to heat up before it fails, the rate it heats up is in accordance to how many amps are flowing through it.
For example-
Say the flesh on your fingertip is rated to handle 70 celcius (canadian) but you can quickly touch a hot surface over 70C without getting burned, as the temperature gets hotter the amount of time it takes your skin to burn shortens.
Like a hot potato to a burning peice of coal, you can touch the hot potato for a short time and possibly if your quick enough tap the burning coal without getting burnt all the while your skin is rated for a much lower constant temperature.

See what I mean?.
that is correct

I believe the fact is that a fuse can handle 200% of its rating for 5seconds. So I wouldnt worry about the max rating.

bump for more help

94CamryBumpin
05-31-2005, 05:23 PM

jsloan10
05-31-2005, 06:33 PM
Aw shucks guys :blush:

Insomniac119
07-16-2005, 10:07 PM
Please explain how my post makes me an idiot.

**An amplifier with an RMS output of 120 watts has a Peak output of ~180 watts.** There is no way around this.

Using the formulas given above:

120 Watts / 12 volts = 10 Amps
180 Watts / 12 volts = 15 Amps

All of a sudden, Our system has the ability to draw up to 50% more power than we originally accounted for.

Now using the Amplifiers described above:

1500 Watts RMS = ~2250 Watts Peak
2250 Watts / 12 Volts = 187.5 Amps
187.5 Amps @ 80% efficiency = 234.375 Amps Total

300 Watts RMS = ~450 Watts Peak
450 Watts / 12 Volts = 37.5 Amps
37.5 Amps @ 60% efficiency = 62.5 Amps Total

234.375 Amps + 62.5 Amps = 296.875 Amps MAX Current Draw

POP goes the 250 Amp Fuse.

Granted I will admit that this only applies if you run your System at it's maximum possible output for long periods of time - Something which I hope none of us do. I will also admit that the fuses are there to protect the WIRE, not the Amplifiers.

** I used 120 and 180 here because these are numbers that I know to be within 2% accuracy**

But if you never run your amps past the point at which it starts to distort, than you'll have no problem (with the exception of SPL competitors). If the fuse blows because the current that the amps peak at is flowing threw it than the fuse did it's job. Sure most amps have internal fuses, and the main job the the inline fuse is to protect the wire, I'd still want the extra protection.

Lets look at your example. Your math shows that the MAX current draw is ~300amps, yet the wire's current flow can't excede 350amps. The problem occures when only 300amp and 350amp ANL fuses are avalable. 300 would "pop" and yet 350 would be not enough protection from fire (not enough for my comfort). The obvious solution would be a bigger wire, but all is beside the point.

Most people are going to benifit best from relying on RMS calculation and not peak. Like said earlier, only SPL competitors are going to drive their amps to such limits and then they can get the 2/0, 3/0 wire. But for the average intaller there is no need to run peak current.

Sorry to resirrect an old thread, and no disrespect, I just wanted to clearify something and bump a very helpful thread back to the top :D.

chinny
07-16-2005, 11:55 PM
sticky?

Insomniac119
07-17-2005, 11:49 AM
sticky?
x2

jacko
07-17-2005, 01:52 PM
hey... this has 1/2 of what i wanted in that sticky from awhile ago :-P

RandyJ
07-17-2005, 06:24 PM
sticky?

its common knowledge that can be found by those who are not clued in on the field by a simple one word search, "fuse". For that reason, there is no need. There was a topic the other day for new stickys and it was determined that with such common questions like this, there really is no reason for it because the people asking will not take the time to read the stickies.

Also, for the arguement that peak ratings shoudl be used, acidburn said you are an idiot because peak ratings are never sustained for more then a few seconds. Even when you areplaying at max volume, that doesnt mean you are getting peak figures. Peak figures on an amp are kind of like the peak ratings for a fuse. You will never see the "2000watts" from that amp rated at MAX figures, simply because if it does put out anythingnear that, it is only when the caps are charged and there is energy to spare on a big burst of signal, which usually last for something like .75seconds.

RandyJ
07-17-2005, 06:28 PM
Also, one thing i noticed whilst reading your sticky. You kind of made it sound like the amp in question, the class d one, will be pulling 150A all the time. This isn't the case for those of you who don't know. The amp will only pull that at near max lvls.

xshort_ex
08-09-2005, 02:26 PM
so can you use a 150 amp fuse for the 1500 watt amp???
*sorry if its a noob question*

Silver
08-09-2005, 03:04 PM
yes sir.

Great topic! Alot of info.

As for the underhood fuse. Isn't it required to run it atleast 12in away from the fuse box or battery?

PRIVATEpastry
08-12-2005, 05:34 PM
sticky?
x3

powerhouse604
11-07-2005, 02:35 AM
hmmm i have a pg 150a, would that be good enuff for a 2500d?

02-28-2008, 07:05 PM
First off I would like to thank the contributors of this thread greatly.

Second, I had a question about how to calculate the fuse size required for the distribution block.

Background:

I followed the steps to calculate my total system current draw and my final number is:
171amps

JL Audio 1000.1 = 101amps
Kicker KX600.4 = 70amps

0/1 gauge wire:
http://www.knukonceptz.com/productDetail.cfm?prodID=KFX0R
300 amp max
Used from battery to distribution block

In-Line Waterproof ANL Fuse Holder w/ 250amp Fuse
http://www.knukonceptz.com/productDetail.cfm?prodID=FH-20
Used within 18 inches of battery terminal

4 gauge wire:
http://www.knukonceptz.com/productDetail.cfm?prodID=KFX4R
150 amp max
Used from distribution block to amps

Distribution block:
http://www.knukonceptz.com/productDetail.cfm?prodID=KNF-12

My question is how do i figure out what fuse size I need for the distribution block?
the options are 30amp-150amps

Also if anyone could recommend other sites that sell the above/comparable materials for less that would greatly be appreciated.

I will also be doing the big 3 using the same 0/1 gauge wire

I believe one of the big 3 requires an inline fuse between positive battery post and alternator.

Could I use another In-Line Waterproof ANL Fuse Holder w/ 250amp Fuse for this step? or should the fuse be a different size? If so how would I figure out the correct fuse size required?