What size fuse to use?
Printed From: the12volt.com
Forum Name: Car Audio
Forum Discription: Car Stereos, Amplifiers, Crossovers, Processors, Speakers, Subwoofers, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=85752
Printed Date: May 10, 2025 at 2:04 AM
Topic: What size fuse to use?
Posted By: kencat21
Subject: What size fuse to use?
Date Posted: November 16, 2006 at 11:59 PM
I need help deciding what size fuse to use for my system.
i have a 72x4rms @4ohms amp, powering 4 speakers and i have 2 svc 12" 4 ohm subs wired in parallel @ 2 ohms to get around 400 watts rms from a 1 channel amp. Plus i'll have a 1.5 farad capacitor.
any help will be greatly appreciated.
I plan on wiring it like this pic plus i'll have the second sub.
Replies:
Posted By: kymadan
Date Posted: November 17, 2006 at 3:30 AM
Use Ohms Law.........Watts divided by Volts = Amperes Take the total amount of system wattage....... Divide it by 12....... And that will equal the Amperage of the fuse needed. The fuse needs to handle the power that will flow through the cable under maximum operating conditions (full power) and no more (ideally). Like if you get 58 amps you would go with a 60 amp fuse. If your blowing fuses and it has been sized correctly, there is a short circuit that needs to be addressed. If you came between fuse sizes, you could go to the higher amp rating fuse (Which is what I would do.) for less of a chance of accidentally blowing the fuse, or you could go with the lower size that will have better short circuit protection.
Posted By: haemphyst
Date Posted: November 17, 2006 at 6:04 AM
If you are asking what size fuse to put in your fuse holder, at the battery, in the red line on your diagram, then we need to know what size the power cable is.
If using a #4, fuse it at 150A, a #2 at 225A, and a #0 at 350A. That fuse is there to protect the car and the power cable in the event of a catastrophe, not for the amplifiers.
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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."
Posted By: xtremej
Date Posted: November 17, 2006 at 7:54 AM
Make sure to fuse the power cable at the distro block either a fused distro block or fuse holders right after it. Just a a reminder, I didn't see it in your diagram.
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Posted By: 1lowgalant
Date Posted: November 17, 2006 at 2:25 PM
nice diagram.  knowing wire size would indeed help. as haemphyst said, power wire fuse is to protect the vehicle and the wire, not the amps, but as mentioned, a fused distrobution block would be a good idea. ------------- Whatever you do, DO NOT let the white smoke leak out of the wires.....
Posted By: aznboi3644
Date Posted: November 17, 2006 at 5:46 PM
kymadan wrote:
Use Ohms Law.........Watts divided by Volts = Amperes
Take the total amount of system wattage....... Divide it by 12....... And that will equal the Amperage of the fuse needed. The fuse needs to handle the power that will flow through the cable under maximum operating conditions (full power) and no more (ideally).
Like if you get 58 amps you would go with a 60 amp fuse. If your blowing fuses and it has been sized correctly, there is a short circuit that needs to be addressed. If you came between fuse sizes, you could go to the higher amp rating fuse (Which is what I would do.) for less of a chance of accidentally blowing the fuse, or you could go with the lower size that will have better short circuit protection.
Ohm's law??? fuse is to protect the wire
Posted By: kencat21
Date Posted: November 17, 2006 at 7:34 PM
thanks for the responses, i will be using 8 gauge wire until it reaches the power block and then 4 guage after that until the battery. Haemphyst are you sure it would be 150? I was thinking more like 100 can you show me the math you used?
Im still not sure on how to mathematically do it because i read somewhere that you get the total rms power of both amps, multiply by 2, then divide by 13.8. But do i use the 4 0hm rating or however many are used. I dont know if this is right, but let me know if it is or not.
72x4=288
288+400=688
688x2=1376
1376/13.8=99.7
so wouldnt i use a 100amp fuse?
and at the power block use a 60amp fuse for the 4channel amp and a 40amp fuse for the mono amp?
thanks for the help.
Posted By: haemphyst
Date Posted: November 17, 2006 at 10:01 PM
Here...
No math required, all you need to know is the safe current capacity of the wire you are using, and fuse for THAT capacity. You are making it FAR harder on yourself than you need to... ------------- It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."
Posted By: kencat21
Date Posted: November 18, 2006 at 3:13 AM
so ur saying i should use a 150a fuse at the battery and two 100amp fuses at the power block?
Posted By: xtremej
Date Posted: November 18, 2006 at 9:45 AM
Yes although imo you generaly can fuse it to a low rating ie if something does happen it will pop a little sooner. As it is to protect the wire and not the amps is something that is understood, but I know several installers that use the theory of going 10 to 20 amps higher than the combined total of the amps fuses for fuse sizing of the power cable.. I hope that made sense  . -------------
Posted By: wible22
Date Posted: November 18, 2006 at 10:08 AM
if it was me i would just go with an 80 watt fuse thats what i use no matter how many watts my system is and it hasnt hurt it at all but if a fuse pops i check your wireing cuz if you wire a power cap wrong its like a bomb
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wible
Posted By: xtremej
Date Posted: November 18, 2006 at 10:31 AM
Where do you buy 80 watt fuses at? haha Thats an interesting theory hope you haven't done that to anybody elses car.
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Posted By: 1lowgalant
Date Posted: November 18, 2006 at 12:35 PM
xtremej wrote:
Where do you buy 80 watt fuses at? haha Thats an interesting theory hope you haven't done that to anybody elses car.
ditto that. 80 no matter how many watts, huh?................... ------------- Whatever you do, DO NOT let the white smoke leak out of the wires.....
Posted By: master5
Date Posted: November 18, 2006 at 12:59 PM
First off fuses do not rate by "watts"..they rate by amperes.(amps) and voltage (in this case we are discussing 12v of course) Secondly, the MAIN fuse you will use near the battery is to protect the wire. If you use a fuse that is too small it will blow. If you use a fuse that is "too large" this in itself will NOT "cause" any damage per say. In other words the system is only going to draw as much current as it needs to operate..no more..no less. At times a "surge" may cause it to draw more current for a short period of time which is why going over the rating is generally reccomended, within reason. Here is what the fuse will do. If there is a short (a power wire touches ground) or lets say a component goes bad causing excessive current draw, the fuse will blow. This will prevent wires from burning if it is shorted. However if the excesive draw is due to a faulty piece of equipment the fuse will not save it. The fuse did not cause the damage regardless but it could help in a sense that further damage to the component will be minimized due to circuits burning up. However, the equipment still needs to be serviced, using a larger fuse will not fix the problem. So it's really simple, no need to read too much into it. With amplifiers generally they will have a fuse or fuses included. So lets say for example the amp has 2-30a fuses. That means the amp is not designed to draw more then 60a under ANY circumstance. So using an 80a fuse would be perfectly safe. If they made a 70a fuse this would be ok too but generally they run 30, 40, 50, 60,80, 100...etc. now you could probably use a 60a but as stated earlier a "surge" could cause it to blow and using a slightly larger fuse would prevent this without any problems under normal use. Another option is a breaker. This is more expensive BUT it serves the same purpose as a fuse. The advantage is it can be reset. Many of them have a reset switch as well so you can kill the main power to the system easily during service. Kind of nice. I see a few replies discussing wire gauge. IMO this is only for making sure you use wire large enough to meet the current demands of the system. However you still want to use a fuse slightly larger then what the amp can pull at max power, and that rating is simply the fuse(s) that come with the amp. No rocket science needed at all. If the amp for some strange unbeknownst reason has no fuse or current rating you can use the ohms law with the rated max power output to determine current, and use a fuse slightly larger. -------------
Posted By: haemphyst
Date Posted: November 18, 2006 at 1:01 PM
kencat21 wrote:
so ur saying i should use a 150a fuse at the battery and two 100amp fuses at the power block?
Yes, fuse the primary wire for 150A, then at the distro block, you could use 80A fuses, yes, but only if your amps are fuse protected, themselves - i.e. they have their OWN fuses in the end of them. If they are not fuse protected, then put in the distro block the size fuse recommended by the manufacturer, as long as it is SMALLER than the maximum safe current capacity of a #8 wire (coincidentally, 80A...).
BTW, master5, that was very well put. ------------- It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."
Posted By: brad h.
Date Posted: November 19, 2006 at 5:42 AM
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