Power and Ampacity of cable

Discussion in 'General Electronics Chat' started by mlkcampion, Jul 19, 2007.

  1. mlkcampion

    Thread Starter Active Member

    Nov 16, 2006
    59
    0
    Hey,
    I have a 360W @ 24Vdc device which draws 15amps. So at 240Vac that is
    1.5amps.
    When selecting the cable to carry both these currents, i can use something like 1.5mm for the 240Vac mains and i then use something like
    2.5mm to carry the DC current.
    My question is how come both cables can carry the same power in Watts ignoring the current for a moment?

    Cheers
     
  2. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    Wire is rated with a resistance, such as ohms/foot (or kilometers, or whatever).

    So lets say you were using 20 feet of 16AWG wire, which has a resistance of 0.004 ohms/foot. You would then have a total resistance of

    0.16 ohms = 2 * 20' * 0.004 ohms (note: 2 * 20' = 40' up & back)

    So with 15 amps, you would have:

    2.4V = 15A * 0.16 ohms
    36W = 2.4V * 15A

    Note the above calculations indicate the wire would be dissipating 36W in heat, and you would drop (or lose) 2.4V from one end to the other.

    It all just depends upon the wire type: solid vs. stranded, size & number of strands, type of metal (copper/aluminum), etc.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I like to use analogies when talking about stuff like this.

    Think of voltage as pressure. Pressure doesn't move.
    Think of current as a volume of "stuff", like water, or electrons. Water moves, so do electrons.

    For this example, think of tree leaves as watts. They're on your sidewalk, and you want to remove them. Your choices are to use a hose with a small nozzle on it connected to high-pressure water, or dumping a trash can full of water on the sidewalk.

    Since the high pressure will forcefully squirt the leaves off the driveway quickly, you won't need much water, or current.

    Since the trash can full of water contains very little pressure, you'll need a lot of water to get the leaves off the walk.

    The trash can represents a large conductor, whereas the hose nozzle represents a small conductor.
     
  4. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
  5. techroomt

    Senior Member

    May 19, 2004
    198
    1
    the current is not ignored. please realize the ampacity of a wire, or it's abillity to carry current, is based on several criteria determined from it's physical characterestis - wire material (copper aluminum,etc,), style (solid, stranded), and insulation type. the ampacity value is that amount of current that specific wire can carry without exceeding a given (safe)temperature.

    as far as a wiring installation, the engineer/designer/installer must be also aware of IR drop, or voltage drop, of a certain length of wire, i believe the desired < 2% (max) drop. as nomurphy stated, wire has an ohms/foot value - the longer the wire the more the resistance. in other words a larger gauge of wire (less ohms/foot) will be required if the length of wire causes the voltage drop to exceed 2% of the applied voltage.
     
  6. alim

    Senior Member

    Dec 27, 2005
    113
    1
    Hi the POWER in both cables would be the same because the POWER consumed by the secondary side has to be equal to the POWER delivered by the primary side. Put another way you cannot consume more nor less power than that supplied by the primary.
     
  7. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    I am assuming you mean how can both wires deliver the same amount of power to the load. The answer is they do not and it is the reason AC is used to transmit power. The ability to step up voltages reduces the current in the wire and minimizes resistive losses. In your example the 240V (AC) line could potentially deliver more "real" power to the load (assuming PF is corrected) since the average current is much lower --less power dissipated by the wire resistance ( 36W vs 3.6W using the numbers nomurphy kindly provided). So actually both wires do not "carry" the same power-- the 24V/15A one drops some on the way ;)

    John
     
  8. mlkcampion

    Thread Starter Active Member

    Nov 16, 2006
    59
    0
    Hey,
    Thanks to everyone for your reply!
    Between analogies (just out of curiosity when did you come up with that one SgtWookie? :) ) and examples i understand where my thinking was wrong.


    Cheers
     
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