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Discuss Rusty on cable calcs in the Commercial Electrical Talk area at

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  1. fattony

    fattony Regular EF Member


    I’m a little bit rusty on cable calcs it’s not something I’m involved with that often for various reasons, ie promodantly do fire alarms and the small works we do are incredibly small and we tend to install 4mm to be safe.

    I remember seeing on here a formula which calculate the maximum length of run you could install a cable carrying a certain current

    At the moment I don’t have the design current but what I do know is

    1) Confirm current Ze
    2) work out design current
    3) confirm current supply is capable of supplying old and new circuits
    4) Apply correction factor
    5) Select the cable
    6) Confirm volt drop
    7) using resistivity calculate R1 + R2
    8) Calculate Zs from info above

    I’m cautious because from the bus bar chamber to the new DB is approx 150m, this is proposed to be a 160A supply

    From this DB a further 20m will be a 63A supply protected by a Type D mcb, my concern is getting the cable selection right (and not go over the top) to ensure the Zs at the end of the 63A supply is low enough to comply

    Could anyone offer advice on the formula?

  2. Spoon

    Spoon Forum Mentor

    • Agree Agree x 1
  3. fattony

    fattony Regular EF Member

    Pretty handy, thanks
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