reg voltage drop...

Discussion in 'The Projects Forum' started by prakashkumr, Jul 2, 2010.

  1. prakashkumr

    Thread Starter New Member

    Jun 2, 2010
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    hi guys.. we are modifing fence lighting around the plant around 7 kms, power supply is like this, transformer(4160/480v - 75kva) to distribution panel(MCCB - 40A) to local control panel(we are using contactor to control the supply).. in local control panel there is H-O-A switch, we are using photocell for controlling in auto mode..there is total of six control panel and one master control panel..the same H-O-A is in master control panel.. we are taking cables from the local control panel to master control panel(nearly 3000 meters), we are using AWG # 14(7 core x 2.5mm2).. my doubt is how we make control from the master control panel.. if we make supply from the master to local control panel how much voltage drop it is?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    What voltage and VA from the master to the local panel?

    Are you certain that you mean 3 kilometers? Using AWG-14 wire?

    Unless you're using a step-up transformer at the master end and a step-down at the local end, you will have quite a voltage drop.

    A calculator is on this page:
    http://www.stealth316.com/2-wire-resistance.htm

    A current of 1 Ampere through the wire will cause a 25.35v drop across the wire.
    Don't forget that you have wire going both ways. Your total voltage drop will exceed 50v per Ampere.
     
  3. prakashkumr

    Thread Starter New Member

    Jun 2, 2010
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  4. SgtWookie

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    Jul 17, 2007
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    From the 75KVA secondary to the distribution panel you will need minimum AWG-OOO copper wire. The conductor measures 0.4096" or 10.4mm in diameter, and has a resistance of 0.202704 Ohms per km.

    From the distribution panel to the other panels and lighting you will need a minimum of AWG-5 copper wire, which is 0.1819" or 4.62026mm in diameter, with a resistance of 1.027624 Ohms per km. Even at that larger gauge, with a 40A current, you will have about a 130V drop across 3km of wire, or 260v across 6km wire. Since you're using 480V on the secondary, you won't have much left. Even AWG-5 is inadequate. Using AWG-14 wire would be like trying to pull an elephant out of a mudhole with a piece of spaghetti.

    If you use aluminum (aluminium) wire, you will have to use much larger gauge wire.

    The only thing I can think of offhand is to use a 3-phase step-up transformer at the distribution panel, and get the voltage back up to somewhere between 4kv to 20kv, and then to use local step-down transformers in the vicinity of the lights. This would greatly reduce the conductor size requirement; for a given VA, the higher the voltage, the lower the current.

    For example, if you used 20kva 1:40 transformers, 480v @ 40A becomes 19,2kv @ 1A. Over a 3km run, AWG-14 would have a voltage drop of 25.35v; this is practically of no consequence compared to 19.2kv; you'd still have 19.175kv left. Use step-down transformers in the vicinity of the lamps.

    However, I am NOT an electrician, and I have absolutely no idea of what the electrical codes are in SA. What I am suggesting is merely theoretical. You need the advice of a professional electrician/electrical engineer who is approved/licensed to work on such electrical projects in the area for which it will be used.
     
    Last edited: Jul 3, 2010
  5. prakashkumr

    Thread Starter New Member

    Jun 2, 2010
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  6. SgtWookie

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    Jul 17, 2007
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    Oh, you're just running signal lines from the master panel to the distribution panels?

    Why don't you use a 4-20 control loop? That way, the distance doesn't matter as much.

    The power to energize the contactors at the distribution panel might be taken from the input side of the contactors.

    However, as I've said - I'm not an electrician. You need one THERE, who is intimately familiar with the SA regulations and equipment available.
     
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