Funny story - Thousand Amp circuit...

Discussion in 'OnBoard Electronics & Controls' started by AndySGray, Jan 1, 2015.

  1. AndySGray
    Joined: Jun 2014
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    AndySGray Senior Member

    Few years ago, I had an email from a friend asking how to find the wire size for a marine application...

    I passed on a couple of links and told him to Divide the power in watts by 12 volts to get the current in amps.

    Then use the table and the length of the run to get the cable size.... easy

    The guy called me when he had the email, and said it was no good as the table didn't go that high?

    Course it does!

    No, the biggest cable is 4/0...

    What? How many amps?


    Holy Cr@p, thats absurd - you're an idiot, Divide not multiply...

    I did,

    Look, a thousand amps is insane, I have a BIG welder with only 200A - you're doing something wrong

    It says on the side 12kW...

    On the side of what?

    I got a used 12 kW radar unit...

    Hmm, I've never thougth about the 'power' before?

    I'd helped fit a similar sized radar and remembered it was about a 14 gauge cable and a 15A fuse, but he was right - a 12 kW device, 12,000 watts, on 12 volts MUST draw 1000 amps :confused:

    But even an 8D marine battery can only manage that for a matter of minutes... :!: Its 12 times more than a big microwave oven - cook a Seagull in 4 seconds... :D

    So the 12,000 watts had to be wrong?

    I took the model number and searched the internet finally finding an install guide.
    180 watts, 12 gauge cable and a 15A breaker - more like it.

    But I was still confused, the 12 kW had to be a lie...

    I read the entire thing and finally found the answer - it IS 12 kW, but it is a pulse with a duration of a few micro seconds - A thousand Amps, Yes, but for Millionths of a second - the average over a second is so small as to be nominal, it's actually the motor turning the thing that draws almost all the 180 watts!

    I suspect they should be using a different unit as the 12kW implies a sustained output, and I was damn confused for about half an hour :eek:

    I guess thats the issue with used items - can't RTFM. ;)

    The flip side (and a question which prompted this post) is that doubling your radar might not require 'doubling' the power supply circuit, bigger batteries, bigger alternators $$$$$$ - if the drive motor is a similar size, it may be a much simpler upgrade - it won't draw twice the amps (Unless you really do want to microwave seabirds)

  2. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Nice story!
    "Volts times Amps is Power" is still valid, but it applies only to simple DC circuits. For most other cases the outcome must be multiplied by a power factor or duty cycle to obtain the average power consumption.
    The radar transmitter, car ignition and electric fence are special cases because the output power is obtained from discharging a capacitor. The word 'power' is meaningless without the addition of the type (music, average, pulse, peak, consumption etc.)
  3. Carioca
    Joined: Aug 2005
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    Carioca Junior Member

    Just as an aside:

    'RMS' (root-mean-square) is another method for estimating 'power' and other physical entities, for example, voltage, current, displacement etc.

    For example, the average (mean) value of a sinusoidal wave is 'zero', although it´s 'RMS' value can be shown to be it´s peak value divided by 'square-root-of-2'.

    When a spec somewhere says 127 V AC, it is the 'RMS' value of the voltage waveform; similarly, for current draw and power consumption.

    RMS can be thought of as the equivalent DC value - of an AC wave - that produces identical heating power.

    'RMS' is also the preferred (formal) method for estimating random, non-repetitive waveforms.
    Last edited: Jan 3, 2015
  4. AndySGray
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    AndySGray Senior Member

    Yes 'RMS' is a good example,

    Squaring a number and then 'un-squaring' it seems pointless 'till you realise that a squared number will always become positive...

    take 3, square it giving 9, find the root = 3 (back where you started, what a waste of time!)

    but if you start with -3 the square is 9 not minus 9 so the root is 3 even though you started with -3.

    Of course you need to ensure you compare like with like.

    There was a 'con' several decades ago with music amplifiers;-

    All the quality brands were using RMS as a fair and sensible measure of output, then the market was flooded with cheap imported units claiming high outputs. Some of the 'old guard' were knocked off the top spots temporarily but the trick was revealed as using PMPO - peak music power output.

    a 25 watt (pmpo) could not sustain the same output as a 20 watt (RMS) system but in a shiny box on the shelf one was cheaper AND appeared to be more powerful 'till the public wised up (that they sounded like cr@p was the other reason for the rapid downfall).

    CDK's comment really does sum it up, power figures are meaningless on their own, and the radar is a great example - 12kW (pulse) would have been much less confusing.
  5. Poida
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    Poida Senior Member

    That's interesting -3 squared = 9 but -3 to the third = -27

  6. Rurudyne
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    Rurudyne Senior Member

    Years ago I read a book about a guy who defected with a Mig 25. One of the tidbits about the plane was, IIRC, that it's radar was considered too dangerous to use near the ground.
  7. Petros
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    Petros Senior Member

    That would have been Victor Belako, who defected with is than most advanced soviet Mig-25. His book is called "Mig Pilot", I met him once at a book signing. An interesting read that shows the contrast between the than communist government based on control, and the free market system and an all voluntary military in the west. Reader's digest had a shortened version of his story, I think it was in the '80's some time.

    It is a great read.

  8. Rurudyne
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    Rurudyne Senior Member

    I still tend to think the whole pooping in some other guy's boots on a cold night thing sums up communism rather nicely.
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