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Thread: Isolator what size do i need?

  1. #1
    Newbie jfxwave's Avatar
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    Isolator what size do i need?

    I have a 250amp alternator and 2 battery's. I also have 4 amplifiers with 6800w and if i add the fuses up on all the amplifiers it adds up to 140 amps.

    Question: I have a 120amp rated isolator, will this work or will i need to use a higher isolator to keep up with the amplifiers?

    I know the amplifiers will not draw nearly 140 amps but i would rather be on the safe side.

    Any help will be appreciated.

  2. #2
    FLAC PhilG's Avatar
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    It's hard to say because, first off, with 6800 watts of amps you would be running 1000 amps worth of fusing and 6+ runs of 2ga. wire so something is not adding up.
    I am only running 2 Infinity amps (657w mono & a 4 channel) and they are fused at 120amps combined so something is incorrect. What amps are you running and what are the amps fused at?
    My 2007 Ford F350 Work Log located HERE

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    Newbie jfxwave's Avatar
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    Opps you are correct i was looking at the manuals and it said 50 x 3 but i counted 50 not 150 so i have 2 amps @ 50 x 3 = 300 and 1 amp @ 50 x 2 = 100 plus 1 amp @ 2 x 40 = 80 so i have a whopping 480amps. Plus i was reading and all my amps are class D (the new smaller and needs less power then the traditional amps) I use Soundstream.

    so once again:

    I have a 250amp alternator and 2 battery's. I also have 4 amplifiers with 6800w and if i add the fuses up on all the amplifiers it adds up to 480 amps.

    Question: I have a 120amp rated isolator, will this work or will i need to use a higher isolator to keep up with the amplifiers?

    I know the amplifiers will not draw nearly 480 amps but i would rather be on the safe side.

    Any help will be appreciated.

  4. #4
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    Something still isn't adding up. You say you have 6,800 watts of amplifiers. The calculation is Watts = Volts X Amps. When you substitute 6,800 = 12volts X Amps you get Amps = 566. Since you have a 250 amp alternator and don't mention your battery going dead, I'm going to guess you are quoting the peak wattage value. What you want is the RMS (root mean squared) value of each amp. This is more representative of the operating amperage.

    With a 120amp isolator, you can safely handle: Watts = 12 volts X 120amp which yields 1,440 watts.
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    Newbie jfxwave's Avatar
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    I still have these in the box new, i just know thats the watts (peak not rms) and the fuse it takes and i do think Soundstream knows what size fuse to use. I also have a 760w (peak) xplod amp i'm using now and it take just 1 40amp fuse. ( I do understand that the RMS on the amp might not be high as PhilG Infinity amps thus the smaller amp fuse )

    With a 120amp isolator, you can safely handle: Watts = 12 volts X 120amp which yields 1,440 watts.
    I'm not going to work with the RMS because it is the AVERAGE for the power an amplifier can continuously create but who does that so i will just work with the fuse they use in the amps so i'm sure not to over power the isolator. I'm i wrong in doing it that way?

    I guess what i'm asking is does a isolator have to be higher rated in amps then the amps your pulling from the second battery with your system because at some time your amps will pull though that isolator to charge and feed the second battery and the amps.

    Or should i just shutup LOL

  6. #6
    Raw Wave
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    The 6800W is probably the usual ratings crap - not RMS.
    480A of fuses implies a max output of about 7000W (for 4 amps @ 14.4V).


    But therefore, the isolator should probably be rated for at least 500A.
    That assumes the amps can run at 480A +10% all day (ie, 530A; but some fuses handle 120% of rating for ages).

    Add to that the recharge current that the 2nd battery is taking.... (assuming it is near the amps - but where else would it be?)
    And that the potential (no pun) supply for that link is the 250A alternator plus whatever the first battery can supply....


    That's were battery interlink protection (sizing; fusing) gets so tricky. [ I have a link intended for a 10A max load. I use 50A self-resetting breakers because 30A fuses would blow. The 2nd battery - a 38AH AGM - can have initial charge currents well over 50A. ]


    I'd try a 500A relay in this case.
    It can be controlled by the 120A isolator. (That means triggered/controlled - NOT paralleled!)
    Or controlled by other means....
    Last edited by OldSpark; 02-23-2011 at 04:33 PM. Reason: spelink

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    Newbie jfxwave's Avatar
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    Thanks Oldspark.

    Now how would i wire that up? Would the wire feeding the second battery from the isolator be changed to feed a 500a relay coil? And the relay will feed the second battery?

  8. #8
    Raw Wave
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    The 500A rated contacts (relay/isolator #30 & #87) sits between the 2 batteries.

    There are also 2 fuses or breakers to protect the cable and the relay mounted as close to each battery as possible - ie, battery#1 +12V thru fuse thru cable-relay-cab;e thru fuse to battery#2 +12V.
    The fuses breakers MUST be rated lower than else equal to the cable and relay rating (whichever is the lowest - ie, the fuse/breaker mus have the lowest or equal lowest Amperage rating).

    The new relay can be controlled by the original isolator. IE - if it was a Smart Isolator (LOL) - ie, voltage sensing - then just use that to power the 500A relay's coil (#85 & #86).
    If it is an ordinary relay, then it can be swapped with the 500A relay assuming the 500A relay's coil current is ok (probably a few Amps).


    Just make sure you adequately beef up the ground pah. Ground connection should at least equal the size/capacity of the +12V path. I practice they are often larger (or doubled up) as it is a cheap way or reducing (or halving) the ground path resistance. (Doubling the +12V path to halve the path resistance is usually far more expensive, and impractical).

  9. #9
    FLAC PhilG's Avatar
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    I am assuming by your avatar, you are working on a diesel Ford? Is so, I will also assume you are talking about isolating one of the 2 factory batteries? If this is the case, this is not a good plan. The truck needs those two batteries paralleled for CCA for the diesel and they both need to remain fully charged all the time. If I'm wrong and you have added a third battery and that's the one you are isolating, then fill your boots!
    A couple of questions:
    What are you running for quantity and guage of power wire to the distribution blocks for the amps?
    Who wound the 250 amp alternator for you or is it actually the available dual alternator factory option on the truck?
    My 2007 Ford F350 Work Log located HERE

  10. #10
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    Quote Originally Posted by jfxwave View Post
    Question: I have a 120amp rated isolator, will this work or will i need to use a higher isolator to keep up with the amplifiers?

    .

    When in doubt, Overspec.

    Oddly enough when I ran a 250 amp isolator, I had all kinds of weird issues.

    Im running a 500 amp RAC isolator, 2 redtops & a 250 amp alt. no problems.......smooth, quiet, and no issues.
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