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What size cable and cable run

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  1. #1
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    Default What size cable and cable run

    Hi guys,
    I know this is a hard question to answer but I just want to get a rough idea of what it will end up costing me.
    I have a bungalow out the back which will be converted into living quarters. By that I mean, kitchen, bathroom, AC, washer, dishwasher, etc. I have to run a single phase supply to it. I worked out that in total it will be sucking up a maximum of 80 amps. I need to run the cable about 35 meters from the switchboard the main house. I guess the cable will be running under the main house in conduit all the way and then straight under a decking and into the bungalow. The cable won't be buried anywhere and won't be in sunlight either. Does anyone know what thickness cable will I will need so that I can estimate this cost?
    Thanks, Tricks.

  2. #2
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    Only for estimate sake 16mm times two plus 6mm earth would be a safe rough estimate.
    Why 80amp most likely you will only have access to 63amp at best.

  3. #3
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    Hi Applied,
    80A is what I worked out. Seems a lot to me too.
    AC: 15A
    Oven: 15A
    Power point circuits x 2: 40A
    Lighting circuit: 10A
    Does this seem right??
    Tricks

  4. #4
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    In a word it's wrong there is a formula for working it out but you need to know all the factors.
    I doubt you would ever draw more than 40amps ever you would probably be fine with 10mm but it depends on the size of your mains from the street if you have excessive voltage drop you will need bigger cable.
    I would cost it using 16mm active and neutral single double insulated strands with 6mm earth if it's cheaper due to down sizing that's a bonus.

  5. #5
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    OneFlashAnYrASH is right. Your sub-mains size is calculated using maximum demand on the basis that it is unlikely that everything will have its maximum load connected at once. AS3000 "Wiring Rules" Appendix C has the rules for calculating and several examples. For example, for up to 20 lights the load is calculated at 3A, and for a Range 50% of its rated load etc.

    On your figures, your maximum demand would be in the order of just over 40A, so 10mm twin and earth in conduit would do (rated at 47A for round and 50A for flat, less allowance for voltage drop). It is not a bad idea to overcater a bit to allow for future needs, so if you worked on 16mm (63A/66A) you should have plenty of margin of safety. Talk to you local friendly licensed electrician.

  6. #6
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    To answer your question, the cable coming in off the street is 16mm twin. The main house has a 80A main breaker. Therefore, will the sparky most likely put two main breakers in the one switchboard (1 main for the house and 1 main for the bungalow) or will the bungalow's main run after the main breaker of the house? I'm trying to work this out without having to change the main consumer cable coming in off the street. Can he bridge the main wire to two 80A breakers is what I am trying to say?

  7. #7
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    You will have a main switch at the house and then a 63 or less breaker after that for the sub board in the bungalow the bungalow will have it's own main switch in it's sub distribution board.
    Essentially if the fire department come they need one switch that kills all the power.
    You will most likely find there is a 63amp fuse in the 80a fuse holder, you need to consult a paid electrician if you want more specific advice.

  8. #8
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    Thanks Applied. I'll have to get a quote...I hope I don't p*** off the sparky if I turn down the job. If its too exey, I can't do it.

  9. #9
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    One way to save a few bucks is to negotiate some of the donkey work. Sparkies HATE digging trenches, which works well for DIYers who don't like paying people to dig!

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