Fuse Question!

mrblob · Post by **mrblob** » Tue Sep 25, 2007 7:53 pm

Hi, I got a new 1/0 awg wiring kit by PG and it has a battery terminal inc. The battery terminal has basically 2 places for 2 kf style fuses each rated @125A.

However, the fuses are basically put in parralel. If I put those 2 125A fuses on, does that mean I have 125A protection? Or 250A -- or less. It just won't wrap around my mind --

I will have a rsd 1200.1 and a rsd 500.4. What should I rating fuses do I need or is what the package came with perfect?

Post by **AVICJR** » Tue Sep 25, 2007 8:11 pm

You should have 250 A of fusing. What size fuses does it recommend in the manual for the amps? Go by that.

theburb · Post by **theburb** » Tue Sep 25, 2007 8:14 pm

125a is what it will take to pop the fuses...i dont think you need anything over 80amps for both of them....80 at the source and 60-60 for each at the power dist block....i hope that helps.

mrblob · Post by **mrblob** » Wed Sep 26, 2007 12:37 pm

So are you saying two 60amp fuses at the battery terminal in paralel is equal to 120A overall protection (at the battery terminal) ?

Post by **Francious70** » Wed Sep 26, 2007 12:51 pm

Fuse for the wire size, not the amps power draw. So 250A would be about right.

Post by **bdubs767** » Wed Sep 26, 2007 2:49 pm

simple math...see how much current your amps will draw.

Amps=watts/voltage

So your trying to figure out how many amps your system will draw. I dont know what ohm load your going to show your amps but Lets say your going to run both at full power. So thats around 1350 watts for the RSD1200.1 and 550 watts for the RSd 500.4.

Second I dont know what volts typically your car runs at but lets say at ideal 12 volts.

SO...

Rsd500.4

Amps=(550/12)(2)
times 2 because class A/B are usually 50% efficient
91.66 Amps of current drawn.

RSd1200.1

Amps=(1350/12)(1.2)
1.2 because class D are 80% efficient

135 amps

S0 226.66 possible amps that can be drawn.
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Post by **stipud** » Wed Sep 26, 2007 2:55 pm

Brandon, you are forgetting amplifier efficiency. An A/B amp is pretty much only 50% efficient, so you need to double the current draw to produce the same number of watts.

Rule of thumb is simply adding up the fuse ratings from all of your amps. Note that this is probably excessive, because to draw that amount of current, all of your amplifiers would have to be playing 0 decibel sin waves all at once.

However, the fusing at the battery is there to protect the car, not your amps, so it's not a huge deal if you have a little overkill in your fusing. This is why, at the front of the car, you fuse for the wire... not necessarily for the amplifier draw (though that never hurts). Just ensure you get the proper size fuse in the back near the amplifiers.

Post by **bdubs767** » Wed Sep 26, 2007 2:58 pm

stipud wrote:Brandon, you are forgetting amplifier efficiency. An A/B amp is pretty much only 50% efficient, so you need to double the current draw to produce the same number of watts.

, that I did forget lol. Let me fix it.

Toms right on avg class a/b is about 50% and class D is 80%

dedlyjedly · Post by **dedlyjedly** » Wed Sep 26, 2007 3:57 pm

You guys are certainly right, the fuse under the hood protects the car. But nonetheless, I would recommend against going overboard on the amperage rating on that fuse (or fuses). Does anyone here know how much current it takes to blow a 250A fast blow fuse?

stipud wrote: Just ensure you get the proper size fuse in the back near the amplifiers.

Which leads me to ask the baited question, "What does the fuse at or on-board the amp protect?"

Post by **stipud** » Wed Sep 26, 2007 4:26 pm

dedlyjedly wrote:Which leads me to ask the baited question, "What does the fuse at or on-board the amp protect?"

It protects your amp to a degree, but if you're going to blow that fuse, odds are a few things in the amp will go as well. I guess it can protect against overcurrent situations as well (i.e. too low of a load or overdriving the amp).

Post by **Francious70** » Wed Sep 26, 2007 5:21 pm

It protects if you get a dead short. it cuts the power so you don't burn your car down.

dedlyjedly · Post by **dedlyjedly** » Wed Sep 26, 2007 8:14 pm

Francious70 wrote:It protects if you get a dead short. it cuts the power so you don't burn your car down.

Good work Paul. An on-board fuse does NOT protect the amplifier from potential operating problems! Properly designed amps protect themselves. In fact the only thing a fuse protects the amp from is reverse polarity, and amps that experience that tend to need a whole lot more than a fuse to protect them from their owners!