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Thread: supply cable

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    supply cable

    i am busy completing a project which involved and upgrade from 60 to 80 amps 3 phase.

    i did my claculation and due to other considerations...worked out...that it would be in my best interest to install a 25mm x 4 core ecc cable...so i fit everything and wait for the supply authority to connect the main supply cable...shortly after the connection is completed...my customer confronts me about the size of the cable...because apparently according to the electrician who installed the supply cable a 16mm x 4 core...indicated to my customer that i had ripped them off because i had installed a 25mm cable and should have only installed a 16 mm cable which according to him is capable of carrying 60-100 amps and the 25 mm cable can carry 100-125 amps...i am still waiting for the calculations to backup his theory...

    just something to note...we have a serious volt surge and drop issue on site...which has been reported to the supply authority on numerous occasion...they have increased the transfomer tap settings (which has caused even more headaches due to voltage surges)...i had a PQA connected to the supply for a week...because of the clearline trip connect 3 unit i installed...it keeps dropping out the contactor...the PQA indicates that the voltage varies from 179 volts to 254 volts...the span set on the trip connect 180-250...where do you stop opening up the span?

    the reason i installed a trip connect unit was because the thieves keep stealing the main sub cable...and for some reason they always manage to cut the neutral first...and you know what happens on a 3 phase system...at least with the trip connect it drops the contactor out protecting the servers and other sensitive electronic devices from the voltage surge caused when the neutral is broken...the only problem is it keeps switching off the power every time the voltage drops below 180 volts or in the evening when it jumps to over 250 volts.

    the supply authourity...indicate that the volotage problem is due to expansion in the area and the extremely old electrical infrastructure (still got old lead cables)...which they have no control over...mmmm...maybe they should upgrade the infrastructure?

    so heres a tip for anyone wanting to move to your bussiness to a new location...

    1st check that there is adiquate power ie...100...125...250 amps or whatever you require.

    the voltage is stable

    and the infrastucture can handle the exsisting load and future addtions.

    you thought you just had to worry about the illegal cocs...dream on.

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    Diamond Member AndyD's Avatar
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    There's many different variables the could affect the acceptable cable size such as;
    • the exact type of cable, is it Cu or Al,
    • what insulation type is the cable
    • method of installation ie is it buried in the ground, is it running alongside other cables
    • is any part of the cable going to be subjected to higher temperatures
    • what length is the cable
    • what is the PF of the load being supplied



    The list is endless, hell if it's a teflon insulated cable six inches in length running in free air inside a blast freezer you could probably use 6mm for your 80Amp supply

    There are circumstances where a 16mm cable would be acceptable for an 80Amp supply assuming you don't want to future-proof the installation but without the nitty gritty info I can say.

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    Lots of additional info is needed before we could confirm your or the "other" guys calcs, but in my mind you aren't ripping off a customer by quoting and providing a higher spec (for growth, security, or whatever reason). You only rip off if you don't supply what you quoted or have no remote justification for size - You did your calcs, and that is your recommendation based on your knowledge and safety level - 5 months down the line when the customer manages to draw a peak higher than expected through the cable for whatever reason and the 16mm causes problems, I am sure there would be a complaint or even worse and investigation as to why you supplied so close to the limits.

    AndyD's superconductors aside of course

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    Dave A (20-Apr-12)

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    Diamond Member AndyD's Avatar
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    Maybe the other electrician was using a fusewire table instead of supply cable current ratings table

    Fuse wire rating (A) Cu Wire diameter (mm)
    3 - - - - - - - - - - - - - - - - -0.15
    5 - - - - - - - - - - - - - - - - -0.20
    10 - - - - - - - - - - - - - - - - 0.35
    15 - - - - - - - - - - - - - - - - 0.50
    20 - - - - - - - - - - - - - - - - 0.60
    25 - - - - - - - - - - - - - - - - 0.75
    30 - - - - - - - - - - - - - - - - 0.85
    45 - - - - - - - - - - - - - - - - 1.25
    60 - - - - - - - - - - - - - - - - 1.53
    80 - - - - - - - - - - - - - - - - 1.8
    100 - - - - - - - - - - - - - - - -2.0
    Last edited by AndyD; 19-Apr-12 at 10:47 PM. Reason: BB Code fight broke out
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    looking at it from another angle...according to table 6.8...the 16 mmx 4core cable buried under ground...which the supply authority installed is under spec'd no matter how you look at it...as i mentioned we have huge voltage issues...and i am told by the supply authority it is due to sudden expansion....yeah right....maybe they should start by using the same electrical standards...the rest of the electrical industry works according to...maybe then we wouldnt have all these electrical outages...electrical fires...total blackouts...load shedding...do i need to continue...

    "notice on the tele" telling everyone the useage is high switch off...etc...etc...bend over consumers of SA we coming thru...

    and if you earn more than a poor person or live in a house with a value of more than...we will squeeze you for eveything you have...be it food...petrol...water...electricity...rates...pric e increases...taxes...road tolls...until you loose everything...the banks will offer you more money in loans...bonds...credit cards...to a point that you just cannot keep up with the cost of living...and fall...i can see this happening everyday.

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    Site Caretaker Dave A's Avatar
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    Quote Originally Posted by AndyD View Post
    Last edited by AndyD; Yesterday at 10:47 PM. Reason: BB Code fight broke out


    The [table] code by any chance?
    The trouble with opportunity is it normally comes dressed up as work.

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    i would normally use a 25mm cable for 80 amps even for shorter runs.tell the customer a domestic supply is usually 60 amps on a 16mm cable.you dont have to mention that its limited by eskom. a 1 phase cable can be loaded more than 3 phase in any case.

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    Diamond Member AndyD's Avatar
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    Quote Originally Posted by Dave A View Post


    The [table] code by any chance?
    Yeah, the 'table' tags were giving me some uphill and I ran out of patience
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