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View Full Version : How to figure Amp Current Draw on Stereo


KS X-10
02-12-2008, 12:02 PM
Can someone explain how I figure out how much current draw I will have on my system with running 3 rockford Fosgate Amps. the Amps are: T400/2, T600/2, T600/4.

I have had 2 differ answers....
One person said take the total output watts RMS of all 3 amps divided by 12 (12volt system) = Total amps of current draw pulling from batt.

1200w / 12 = 100A current draw on system ??

Another person said to take half the amount of the Fuse rating for each amp to get what it will actually draw on constant basis.

T400/2 100A Rating/ 2 = 50A Draw
T600/2 150A Rating/2 = 75A Draw = 200A Current draw on system
T600/4 150A Rating/2 = 75A Draw

So Is one of these right? I'm confused!!

kskib046
02-12-2008, 07:32 PM
with one of these...

http://buy1.snapon.com/catalog/item.asp?P65=&tool=diagnostics&item_ID=56127&group_ID=1368&store=snapon-store&dir=catalog

BTW... I have T600/2 and T600/4 both running through a single 75amp stinger breaker and it never popped. The local stereo shop said I would be good doing that.

kskib046
02-12-2008, 08:01 PM
also both of those T600s say they need a 150amp fuse each... LOL!

sk8salomon
02-12-2008, 11:55 PM
are you installing all this yourself? there are a ton of things you need to consider if so. 75+ amps is more amp draw than anything factory on your boat combined. most factory alternators aren't rated for anything over 60 or so amps so upgraded alternator, dual battery config & stiffening caps are a for sure thing. if not, you will toast a battery and/or alternator every year or so. go big with your wiring! your system is only as good as the wiring so spend the extra dough for heavy gauge wire and quality components. use things like distribution blocks for multiple amp hookup. run a single larger fused leg off the battery say 4ga into the block then branch out each amp separately with lighter fuse. do your homework 1st and you will save yourself tons of headaches later, believe me bro!

Leroy
02-13-2008, 12:00 AM
You should see some max current/max watts for each amp if you look on the back. The output watts algorithm will be close but depending on the amp may be 10 - 15 percent low. Fuse ratings is not a reliable method.

KS X-10
02-13-2008, 11:51 AM
are you installing all this yourself? there are a ton of things you need to consider if so. 75+ amps is more amp draw than anything factory on your boat combined. most factory alternators aren't rated for anything over 60 or so amps so upgraded alternator, dual battery config & stiffening caps are a for sure thing. if not, you will toast a battery and/or alternator every year or so. go big with your wiring! your system is only as good as the wiring so spend the extra dough for heavy gauge wire and quality components. use things like distribution blocks for multiple amp hookup. run a single larger fused leg off the battery say 4ga into the block then branch out each amp separately with lighter fuse. do your homework 1st and you will save yourself tons of headaches later, believe me bro!


Thanks for info... Yes, I am installing myself. And I do have 200A Circuit breaker after Aux. batt. and 1/0ga. Pwr. and Grnd running from batt. to fused Dist. bocks. then 4 ga. to each amp. 12ga. wire to all speakers. Just not sure about the battery capacity i need and charging by Alternotor if it will keep up?

I had Bill from TopSide Marine (all he does is upgrades to ski/wakebrd boats) tell me that i take 1200w / 12.5v = 96A current draw @ MAX. use. But he said not many are pushing to max 100% of time. probably more like 60-70amps draw on a constant basis.
so he said i may get by with my stock 70amp alt. but suggested i upgrade to a 120A Alt. in near future.

He said the wires going from alt. to batt and such would be ok at what gauage they are now...
Is there any of the engine wiring harness/Alt. wiring that should be bigger as well?

Eagle Lake Rebel
02-13-2008, 10:59 PM
KS X-10 sounds like you are going about it right using a distribution block. To help out your electrical system, I'd add a large capacitor (10 farad) in your 1/0 supply line. The cap will keep your system voltage up aroud 14.4v and keep the alt from seeing big power demands. I added one to my system and it makes a big difference when the tunes are turned up. When sizing your fuses make your selection based on your wire size. Yes, the fuses should be sized to help protect your amp but it's more important to protect the wiring. One other thing to note is that most amp output wattage ratings are based on 14.4v not 12v. As the voltage goes down so goes the output power and your amps tend to heat up. If you add a cap make you sure you charge it properly before connecting it directly to the batt. Don't put the cap in the batt switch circuit it needs to stay charged and it will tend to drain your batt down so keep the batt on a trickle charger while the boat is not in use.

sk8salomon
02-14-2008, 07:21 PM
10 FARAD??? holy moly!!!!!!!!!!!!!!! that's enough to knock you down, possibly kill you! are you sure you don't mean 1.0 farad? if i remember correctly, they used to recommend 10,000 microfarads per 100 watts (e.g. 1 farad = 1,000 watts) or something like that.

JimN
02-14-2008, 08:57 PM
KS-X10- you'll want to err on the side of being careful, so oversize the charging system. The P=I(E) is correct. The reason for taking 1/2 of the fuse size is that that's a a pretty good average for continuous draw if the system is run loud on a regular basis. I'm not going to agree that a stiffening cap will do what you want- it won't. Sure, it's OK for stabilizing voltage ripple but it doesn't work on a continuous basis under heavy load. The reason is that the charging system needs to recharge it and if that's totally taxed, the devices drawing the most current will get what they need first.

As far as the power and ground- you want the breaker within 12" of the battery positive terminal. You want the power and ground to be equal size. If you're running a lot of different pieces from those cables, use a distribution block on both- it makes connecting them a lot easier. If you can, twisting the power and ground cables along their length will minimize "common mode" noise. Don't run power/ground parallel to signal wires or speaker wires. Connect the head unit's ground wire to the case of the head unit and run another ground wire from that point to the negative distribution block. You'll tend to have less noise issues if you do this.

If you plan to crank this system a lot, beef up the alternator and charging lead. It's not worth being stranded by a dead battery or alternator. It's even worse to have a fire.

KS X-10
02-14-2008, 10:33 PM
KS-X10- you'll want to err on the side of being careful, so oversize the charging system. The P=I(E) is correct. The reason for taking 1/2 of the fuse size is that that's a a pretty good average for continuous draw if the system is run loud on a regular basis. I'm not going to agree that a stiffening cap will do what you want- it won't. Sure, it's OK for stabilizing voltage ripple but it doesn't work on a continuous basis under heavy load. The reason is that the charging system needs to recharge it and if that's totally taxed, the devices drawing the most current will get what they need first.

As far as the power and ground- you want the breaker within 12" of the battery positive terminal. You want the power and ground to be equal size. If you're running a lot of different pieces from those cables, use a distribution block on both- it makes connecting them a lot easier. If you can, twisting the power and ground cables along their length will minimize "common mode" noise. Don't run power/ground parallel to signal wires or speaker wires. Connect the head unit's ground wire to the case of the head unit and run another ground wire from that point to the negative distribution block. You'll tend to have less noise issues if you do this.

If you plan to crank this system a lot, beef up the alternator and charging lead. It's not worth being stranded by a dead battery or alternator. It's even worse to have a fire.


JimN ,
Thanks for the Tips! I think i will stay away from Cap. I have just heard more Neg. than pos. about them..
I do have Dist. block for both pwr and Grnd. and I'm running them seperate from speaker and RCA wires. so I'm good there i hope. I'll tell ya whats more confusing than the stereo setup is the whole issue of the dual batts...Isolator....perko.... alternator...charging them correctly...etc...... I'm still trying to figure out the best setup for me and how to actually get it wired correctly... Thanks for your help.!!

sk8salomon
02-15-2008, 05:54 AM
Stiffening caps are not bad, just most of the time people don't understand how they work and what they do. I highly suggest a .5~1 farad cap for the amp your subs run off of. The unstable voltage that JimN was talking about happens most frequently with a subwoofer because it shocks the system as the bass hits. That's when unstable amperage/voltage may peak and blow a fuse. Your amps for midss & highs probably don't need one.

Eagle Lake Rebel
02-15-2008, 01:00 PM
The going rule of thumb is 5 farads/1000w rms. Since this seems to be a large set-up I threw the 10 farad cap out there. No doubt you have to be careful with them, these things will knock you into next week because they are quite big. You can do each amp and drop to the 1 to 2 farad size, but then you have to be careful with all these shock monsters in the boat. They do a great job of leveling out system voltage when high current demands are required by the amps. If you are not used to working with them I agree don't take the DIY approach. I was a bit leary about using caps till I started putting them in big systems the difference they make with the sound and electrical system surges is quite good. Good luck & rock on!

sk8salomon
02-15-2008, 08:43 PM
The going rule of thumb is 5 farads/1000w rms.

not trying to start anything here man, but you are going to get somebody hurt. that info not correct! i got the below info from searching about caps!

Along with an adequate basic power supply, a stiffening capacitor for your amp is the simplest way to be sure your system always gets every bit of current it needs. From the smallest 0.5 farad, to the 3 farad model by Legacy, there's a capacitor suitable for your installation. Mount them as close to the amplifier as possible, and choose at least .5 farad (500,000 microfarads) per 500 watts RMS of power output. You can use as many farads as you wish, but just the number indicated by the formula above is all that is necessary.

Eagle Lake Rebel
02-17-2008, 01:43 AM
sk8 I don't disagree with your reasearch. There seems to be a wide range of sizing recommendations. The caps I use are made by Tsunami and they make some jumbo caps. Check them out at Crutchfield.com. I'm running around 1000w rms and tried some "smaller" caps (1&3 farad) and for the $ the 5 farad did a much better job. I don't disagree when caps are normally sized in microfarads that dealing with caps rated in farads are in a whole other league and must be properly charged and discharged when necessary or dammage can occur. I stick labels on all the amps to alert others and remind me that a large cap is in the circuit and that the power supply is hot until cap is discharged. I brought up using a cap because I see lots of systems built that demand way more out of the charging system than it was ever designed to do and all kinds of electrical problems pop up later from low voltage high amperage demands. Again, when working with caps you have to be careful and I don't take issue with your concern that they can hurt you if mishandled.

JimN
02-17-2008, 08:52 AM
"Along with an adequate basic power supply,"

Right there, the need and size of a stiffening cap can be minimized. If the charging system is beefed up so it can handle everything connected, a smaller cap can be used, and then only to minimize a bit of ripple. Using a cap to reduce the cost of upgrades is a dangerous way to do this. Also, it's a good idea to use a heavy-duty relay or switch when the vehicle is not running. A hot cap is dangerous but it takes a lot less time to discharge than a battery.

We could debate this all day long but a second battery stores far more energy than a stiffening cap and will work just as well, if not better, for smoothing the supply voltage. A cap does have it's uses but generally they're only used to get rid of voltage ripple after voltage has been rectified and while a vehicle's charging system is an example of that, using the cap as a major source of current will eventually damage the charging system, especially if it isn't upgraded. Using a 90 amp alternator and trying to run a 1000W stereo in addition to the vehicle's original accessories will work, but not forever and when it fails, some of the vehicles electronics can be damaged by unrectified voltage when the bridge fails.

Eagle Lake Rebel
02-17-2008, 10:41 AM
I brought up using the cap as a helper not a replacement for 2 batts, 90a alt and proper wiring. I have all the above and then added the cap to help out and yes I've added a switching circuit with a 10ohm 20watt charge/discharge resistor to mannage the cap. I like what it did for my system. It has its pros and cons. Just passing on my experience with them as an avenue to explore not a cure all. Like anything posted here or on other boards I would hope folks take what they read and follow up on it and draw their own concluesions if it's right for them.

JimN
02-17-2008, 11:23 AM
A 90A alternator is squat, when a huge audio system is being powered. The car or boat will draw 2/3 of that on a regular basis if all of the accessories are on and at idle, the voltage isn't 14.4, it's about 13V- less if those accessories are on and the alternator is close to being maxed out. I did 12V audio/security for a long time, including competition systems. Never had a charging system failure in well over 5000 vehicles.

CRAIGTHEMAN
02-17-2008, 01:32 PM
i have the same problem right now too..im puttin in a new stereo in my 2000 x star and new some help on the wiring part..im running three amps..a MTX 1000d amp which is 1500watts rms and 2 clarion amps which are 320watts rms a piece..so all together im have a little over 2000watts going..curently i have 2 deeps cycle blue tops im putting in the boat that im going to isolate but my question it is that enough??? or do i need to do some upgrading..do you guys think i should cap it or maybe beef up the alternator??????????

JimN
02-17-2008, 02:11 PM
Look, your alternator is what has to replenish any batteries and run the motor at the same time. Assuming 100% efficiency (impossible), 2000W/14.4Vdc = 138.9A and at 12Vdc, it's drawing 167.66+Amps. Your alternator doesn't put out 14.4V at anything less than about 2000RPM and at idle, you'd be lucky to see 10V with the motor running and the stereo cranked. If you want to run that off of a 90A alternator, go ahead but don't expect it to last long. Even with extra batteries and stiffening caps, at some point, the charging system is going to puke and you're going over a waterfall.

If you want your amplifiers to develop 2000W, don't expect them to draw 2001 Watts from the charging system. Most amps are just over 50% efficient, regardless of what the spec sheets say, so 2000W of output will mean they'll draw somewhere near 4000W. Now, double the current numbers I posted above. Ylu still think your factory charging system will handle it? Amplifiers get very hot and the heat is a great indication of how much power it took to get the output at the speaker terminals. If you want to do the conversion from BTU to Watts, 33,000 BTU= 1 HP and 746 Watts = 1HP.

Getting to the current capacity of the power cables- 4AWG is good for 60A on a 16' length, which is fairly typical for a car or many boats. Doubling the gauge makes it 2 AWG and 120A, 1/0 AWG for 240A, etc. 2/0 AWG is the minimum I would recommend for 2000W of draw, with or without a cap or spare battery.