Using Fuse Size To Determine Amp Wattage

Hello all,
   I have seen past posts about calculating theoretical wattage for an automotive amplifier. The posts were all misinforming. Your first consideration to look at is this.... The manufacture always fuses 20% over the rms rating of an amplifier. This is to protect heavier loads that may be created by low voltage. This happens when young hammerheads think they could add 5000 watts in a car with the stock alternator, and no extra batteries, oh and one stiffening cap...lol

    Say your fuse size is 25 amps. The max amperage will be 25 amps in a dangerous situation for the amp. If the voltage is correct during peaks, and transients, it should never exceed 20 amps. Efficiency does not have to be calculated into this, because they run these amps to their max at the factories, and fuse accordingly to the running of many of the same amps.The even overdrive the amps to see their stability. Wattage will change with their loads, but will not exceed fuse rating in the correct environment (with correct voltage and feed wire sizes).

     You will take the 20 amp rating.. this is the max load at the lowest impedance that is your amp is allowed to run at. Take the average test voltage, usually 13.4 or 13.8 volts. I have seen these up to 14.4 volts. Multiply it x 20, and you'll get your maximum wattage output at the max load. (Max load is the manufacturer's spec such as 2 ohm capable, 4 ohm, whatever it is. Also if it is 2 ohm mono capable, and runs in stereo, the stereo rating is most likely going to be a reciprocal of that....2 reciprocal plus 2 reciprocal= reciprocal, then you get your answer. So if you have two diiferent impedance drivers, this is how you figure the average impedance of drivers in parallel that are two different rating. If they are the same impedance, we all know that in parallel, this splits the number in half.

I also have seen in other posts posts about oversizing power fuses due to power cables.... The maximum you want in a power distribution block is the same size as your fuses in amps totaled. They are already oversized 20%. If you oversize fuses on a 1/0, you are going to cause a fire in your car if it shorts to ground, or a battery explosion.. You do not need to oversize if your voltage stays in the correct range. Wire resistance creates a voltage drop. Running too much amperage through a conductor burns the wire and creates resistance. This creates a voltage drop, and when voltage goes down, amperage climbs up to the unknown. It is a dangerous situation for amplifiers componentry. So, correct wire size is necessary. Look it up in wire size charts. Be sure to use a chart that includes wire length. This is vital to determine the resistance, and voltage drop it is going to produce. Most installers will use a wire 20% larger than necessary, sometimes 50% to correct voltage drops cause by mechanical connections that are terrible, because antioxidant are never used for the connections. Oxidized connections cause resisitance, and again form a voltage drop, and increase in amperage. Correct additions of batteries, alternators, and stiffening caps are required to keep this up to par in some large amperage situations.

Most installations are running under voltage that I have seen. This raises hell with vehicle computers, and amplifiers power supplies, as well as the output stage in an amp. Amps should never run hot. If it gets hot, you are brutalizing the outputs, or the power supply.

If anybody has questions, please reply to this article. I've been in this audio thing now since 1989, and I do have a ton of answers, as well as correct suggestions!
Marc Belanger
Formerly of Scary Car Stereo, Waterford, CT

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Marc Belanger

Nowhere did I see anything about the class of amplifier.   Is this formula accurate for both class a/b and class d?????

belangers wrote:

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I also have seen in other posts posts about oversizing power fuses due to power cables.... The maximum you want in a power distribution block is the same size as your fuses in amps totaled.

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This is not correct.  The purpose of the power cable fuse is to protect the vehicle from failure or overload of the cable.  The fuse on the power cable should be sized for the cable; no larger than the current rating of the cable you use.  It really doesn't matter what the loads are pulling as they will each have their own fuses (as you state) or the installer will individually fuse them.

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i am an idiot wrote:

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Nowhere did I see anything about the class of amplifier.   Is this formula accurate for both class a/b and class d?????

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This.

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That's what I said. It does not need to be the recommended fuse size. It can be smaller, never larger. However, if it is not the recommended size for the cable, you must measure the resistance to determine if it is causing a voltage drop. The fuse at the battery is simply to protect the battery, and the vehicle. A power distribution block does not need 100 amps more capability than what your amps are using. If something very thin got across the power distribution block in the back of your vehicle that would burn instead of a monstrous fuse, guess what, you have yourself a fire or red hot object in a trunk , or under a seat, etc. Instead of blowing too big of fuse under the hood . I've seen this happen, where the amps were drawing 85 amps and the fuse by the battery was fused at 200 amps. An object made of stainless in the poor guys trunk smashed the plastic covers on the power distribution blocks, and acted as a heater element. The fuse at the battery didn't blow, and his ampboard and back seat burst into flames.

So, be careful what you say. Overriding in any way is never even close to being a clever idea..(anything that's not a good conductor , like stainless steel, is a big resistor, and heats up like crazy if its thin enough, instead.of causing a massive draw.)

I know the instance is rare. But please don't doubt my suggestions. I have more experience in this field than most, and have seen it all.

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Marc Belanger

All classes of amps should be pretty accurate. It really depends on manufacture engineering standards. If the engineer proposed 15% over Rms for their amps, or 20%, you will notice the difference in your measurements.. 20% over is standard fusing in all types of electrical circuits. It is a NEC standard as well as an IEEE standard. The other method is to put an amprobe (clamp on) around the power wire, get your amperage, and multiply times the present voltage. This will get you theoretical wattage no matter what class it is. No device can be more efficient than 100%, and most are 50-65% efficient. Metering would use a potential transformer and a current transformer to achieve the same results, and thee over 96% accurate. There isn't any way to tell exactly what type of wattage is going to your speaker without proper instrumentation (wattage meter , which is big money). Also, the efficiency determines how much of your wattage is dissipated as heat, instead of music. The amp still draws the same current, and generates the same wattage. The efficiency will determine where it goes. To the speaker, or to the cooling fins.


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Marc Belanger

One of my post says overriding instead of overfusing.

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Marc Belanger

belangers said:
"20% over is standard fusing in all types of electrical circuits. It is a NEC standard as well as an IEEE standard"

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Careful how you state this. I'm well aware of the NEC. What you really mean to say is the a circuit is usually overside by 125% of the current needed by the equipment, or you are only using 80% of the breaker or fuse rating. This is the rating for a continuous load, which the NEC defines as on more than 3 continuous hours. You never use a higher rated fuse/breaker than what the wire is rated for. It is simple fire protection.

Simple example you have 100 amp wire, you fuse at 100 amps. 80% of 100 amps is 80 amps. You can draw 80 amps all day long without tripping the breaker from heat build up. Or if your equipment draws 80 amps and will be running continuous. 80 amps needed x (times) 125% = 100 amps. Then use 100 amp rated wire and fuse/breaker. Now this is true for AC home/building/machine wiring, I can't say it holds true for DC Automotive Stereo wiring. I just wanted to clear up the example "belangers" gave when referring to the NEC.

Never use a main fuse or breaker with a larger rating then the wire is rated for.

belangers wrote:

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I have more experience in this field than most, and have seen it all.

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You might want to check the profiles of some of our regular members here before you start making statements like that.  :)  Also, you may have been an installer for a few years, but on this forum you're still a noob.

It is never good advice to just add up the ampacity of your loads and use that for main power fuse rating.  There are many other factors in play, the most important one being the size of the power cable.  There is absolutely nothing wrong with fusing 0-AWG cable with a, say, 300A fuse and then using amplifiers that only utilize 100A. 

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Class A/B amps run between about 40 and 60% power efficiency.  Class D or G amps can run as high as 90%.

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belangers wrote:

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The manufacture always fuses 20% over the rms rating of an amplifier.

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Interesting information. How have you came to this determination? Is there some sort of agreement between EVERY company that manufactures amplifiers? Why 20% and not 21%?

If the fuse is not 20% over the rms rating of the amp is there a fine or penalty that holds the manufacturer accountable for their improper sizing?

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Kevin Pierson

DYohn] wrote:

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belangers wrote:

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I have more experience in this field than most, and have seen it all.

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You might want to check the profiles of some of our regular members here before you start making statements like that.  :)  Also, you may have been an installer for a few years, but on this forum you're still a noob.

 

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Dyohn: Just in case...

Since 1973, sound system and loudspeaker designer and engineer for several audio and video related companies and manufacturers, including Oberheim, Harbinger, Sony, Ampex, Bag End, Creative Labs, Lucasfilm/THX, Precision Audio, and Tyco. Also worked as a video producer and sound designer for Columbia Pictures, Arista Records, A&M Records, MTV and The California Music Channel. Worked in several capacities for broadcast radio and television stations, and for ten years owned my own video production company. Current owner of Four Winds Audio in San Jose, CA.

Education: PhD in Cybernetic Systems from the University of California. Also a BSEE in acoustics and an MA in Broadcast Communications. University Professor in Radio and TV and in Communications at San Jose State University and Santa Clara University in California from 1990-1996.

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M.E.C.P & First-Class
Go slow and drink lots of water...Procrastinators' Unite...Tomorrow!

Noting that the thread title is about determining amp wattage for the fuse size...

And that almost all fuses will operate all day at a 110% overload...


But that "80% rule" is pretty much right. (I sometimes write 70%, but that's more for general wiring and loads, else just wrong.)


I only glance quickly at the previous replies and I suspect some cases of omission, taking too literally, and rewording in retrospect.

Fusing is to protect the battery and the vehicle and the downstream wiring (that's included in "the vehicle").

Modern vehicles are set for up to 14.4V. 14.2V is probably the norm. Long gone are the days of 13.8V.

I can see the engineering -cum- economic reason for matching distribution sizes, but I also see the engineering -cum- design reason for overrating the distribution - ie, to minimise voltage drops to the load. I prefer the latter.
(But I cannot understand how a 3V drop is considered "the max acceptable" the typical things we like to modify.)

And not that I understand yet why so many SMPS amps behave like resistive loads. (I haven't yet deciphered my Kicker SMPS configuration, but it uses a typical SMPS chip.)
But that's another story. I would probably never use such a primitive system if I were into performance amplification (ie, the SMPS integral to remote amps).


This could be an interesting discussion...