# How do figure out how many amps my amplifier is drawing form my cars electrical system?



## 619 (Mar 29, 2004)

the amp has a 20amp external fuse. Does this mean it will have a max draw of 20amps @ 12volts? If it exceeds that the fuse will blow right?


----------



## Pat @ Pitt Soundworks (Nov 14, 2008)

Depends.... fuses burn up because of heat. Most can support more current draw than the rated amount for very short periods of time. But the fuse ratings are typically a good place to start.


----------



## 619 (Mar 29, 2004)

OK Pat help me with an argument here.. I have an industrial electrician friend who knows nothing about car audio. he asked me why I didnt just use the 12v power outlet for my 200wrms sub amp since i never plan to use it. I said cause its not even close to the wire thickness the amp needs. He used some basic electric formula to figure out that my amp was only using 9.9 amps @ 12volts to get 200 wrms. based on this figure the wire going to the 12v output in the trunk is plenty for the 200w amp as it has a 20 amp fuse.

This argument is over my head but I know hes wrong.. right?


----------



## harmankardon35 (Sep 16, 2009)

id say maybe....if it has a 20A fuse then obviously the total constant current draw is less than 20A, and most cigarette lighter/12v power outlets use a 20A fuse from the factory. It may not be ideal, but with an amp that size it could be done, but then again if you got a few spare minutes just run a thicker power wire


----------



## 619 (Mar 29, 2004)

For the record Im using 8 gauge to the amp. He saw it and chuckled like "what the 8 gauge for?" And the conversation ensued.

This has got me looking around the house at things and like the home audio subwoofer, a 10" 200w powered polk audio sub and the power wire to the wall isnt any thinker than any other house hold appliances.

A blow dryer runs 12.5 amps of current, microwave 12.7, space heater 15 amps ect. I know we're comparing apples to oranges here but if we're comparing current draw vs wire thickness he has a good point.


----------



## Pat @ Pitt Soundworks (Nov 14, 2008)

You could run it off the cig adapter in the back, although I wouldn't. Wire increases resistance as the length i
increases. I wouldn't spec any wire for an install under 8 gauge, which is good for about 50-60a at 15ft. You never know when you're going to upgrade, either.

As for home electronisc, most home circuits are fused at 15 or 20a. Ever try to run a microwave and a fridge or hair dryer on the same circuit at the same time? It doesn't work. Running high powered electronics like that are just at th edge of what the wire is capible of handeling.


----------



## seroki (Jan 25, 2010)

You also have to know that anything you plug into the wall in your house is AC, not DC like in a car. That polk audio sub only needs 1.5A to pull that 200 watts for its power. AC power runs anywhere between 115-125 V and significantly less amperage than a car battery. Most car batteries may only be 12 volts, but the cold cranking amps are in the high hundreds to thousands of amps. Most people dont think thats much since every single danger sign says something about high voltage, never amperage. A human can survive several thousand volts parsing through their body, but if 100 milliamperes runs through your body even if its 10v thats enough to stop a persons heart. 

If he's an industrial electrician, He works with AC current almost indefinately. AC and DC power sources are vastly different based on frequency modulation and phase modulation. DC you will need the larger wire due to the larger amount of cold cranking amps that will run through the wire at start up which is why most auto electronics use fuses about double the power handling. Been a while since my Electronic Circuits classes but thats pretty much the gist of it from what i remember.


----------



## harmankardon35 (Sep 16, 2009)

yes, 12v is a lot more current that 120V, this is why my entire house is powered off a pair of 4 gauge cables from the utility pole, yet a winch on my truck needs minimum 4 gauge to run properly. More voltage = less resistance, this is why very high voltage will arc (spark plugs, tazers, lightning...it actually travels over air!)


----------



## SMkVGTI (Jun 14, 2007)

P = I x V

200 watts = (12 volts) x (I), I = 16.67 amps
200 watts = (14.4 volts) x (I), I = 13.89 amps

The above current calculations assume 100% amplifier efficiency (impossible!) and full power (head unit at full, undistorted volume and amplifier gain properly set); however, class D amps can be ~75-80% efficient, so multiply the above current values by 1.333 to get a rough feel for true current draw, If it's a normal class AB amp, multiply the above current values by 2 to get a rough feel for true current draw. Remember, most music is not constant duty, so the current draw is momentary. A 50 Hz tone at 0 dB reference level will be constant duty, so expect a loud volume to require ~30 amps of current at 12 volts with a normal class AB amplifier rated for 200 watts.

Proper wire thickness is based upon current draw (amps), not voltage. As current passes through a wire, the wire heats up. Increasing a wire's temperature also increases its resistance. Smaller gauge wire has less strands to conduct the current. Thicker gauge wire has more strands to spread the current delivery from point-to-point.


----------



## harmankardon35 (Sep 16, 2009)

^ this also goes to show you how a fuse rating is quite vague, as it can change with the voltage its being used at (12 V ignition off, or 14.4 engine running)


----------



## BassNotes (Mar 16, 2005)

seroki said:


> You also have to know that anything you plug into the wall in your house is AC, not DC like in a car. That polk audio sub only needs 1.5A to pull that 200 watts for its power. AC power runs anywhere between 115-125 V and significantly less amperage than a car battery. Most car batteries may only be 12 volts, but the cold cranking amps are in the high hundreds to thousands of amps. Most people dont think thats much since every single danger sign says something about high voltage, never amperage. A human can survive several thousand volts parsing through their body, but if 100 milliamperes runs through your body even if its 10v thats enough to stop a persons heart.
> 
> If he's an industrial electrician, He works with AC current almost indefinately. AC and DC power sources are vastly different based on frequency modulation and phase modulation. DC you will need the larger wire due to the larger amount of cold cranking amps that will run through the wire at start up which is why most auto electronics use fuses about double the power handling. Been a while since my Electronic Circuits classes but thats pretty much the gist of it from what i remember.


You've got a few things mixed up, bro.


----------



## BassNotes (Mar 16, 2005)

If you want to find out how much current the amp draws, pull its fuse and connect an ammeter in its place. Make sure it can handle more than 20A, or even 30A if possible.

Start the engine and run your stereo like you normally do. Read how much current is going through the ammeter.


----------



## Non_Affiliated (Feb 12, 2002)

BassNotes said:


> If you want to find out how much current the amp draws, pull its fuse and connect an ammeter in its place. Make sure it can handle more than 20A, or even 30A if possible.
> 
> Start the engine and run your stereo like you normally do. Read how much current is going through the ammeter.


Most DMM's will only read up to a 10A continious rating, maybe a short 20A in-rush for a few seconds if it unfused.

For your average person, it really is a point of diminishing returns to try and measure accurately the exact current draw of a piece of gear. Accruacy being the key term. How accuratly do you need? I for one say your fine running 8 ga. is it overkill, most likely. But overkill isn't going to melt that wire if it's properly installed.

Your already money ahead, by just not worring about what your buddy thinks. I have to laugh when a, by trade, electrician tries to tell me how electronics work. Or someone ask if I can help them wire up something their house, I by trade am a electronics technician.

Electronics and electricians are to vastly different trades and mindsets, while they share alot of the same electric fundimentals. Anyone who tells you they are the same is a fool.

Not saying a Electrician can't know anything about electronics and vice versa. But most trade electricans can't tell you how a LM555 Monostable Ocillator works, while most electronics techs caouldn't tell you difference between power Generation, Transmission, and Distrobution lines.


----------



## jimtunes (Mar 29, 2010)

I'm really surprised no mentioned this before. Look in the documentation that came with the amp or on the Mfr web site. That being said I'd run a new line to the amp. The lighter circuit has several junctions as the current passes from the battery to the lighter. It's a recipe for picking up noise.


----------

