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Thread: brain teaser

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    brain teaser

    if you have to fit an addtional motor to an exsisting installation...what size droppers from the main busbar to the fuse holder and what size cable from the soft starter to the motor and why?

    motor 400 volt...200 kw (365 amps) with a soft starter which requires 700 amp semi conductor fuses.
    main supply 1000 kva transformer feeding the main control panel.
    the panel where the motor will run has a 1200 amp main switch rated at 25/35 ka...just to put a spenner in the works the transformer will be increased in size to a 2000 kva at some stage not finalised yet.
    the motor will be 20 metres away from the control panel.
    cables will be in paralel and touching if 2 or more are selected.
    ambient temp. 55 dgrees C.
    motor is connected in delta...400 volts or star if 660 is required.
    last but not least is a COC required if it forms part of a machine?

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    Email problem BigRed's Avatar
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    Pass

    I'll stick to wiring my toaster

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    tec0 (11-Feb-10)

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    just some more info...the main busbars are 63x10 mm...if i need to drop from the main busbars to a 5 amp fuse for control wiring in the plc bucket...what is the min. size cable required for the controls.

    if i decide to go with 3 cables what would the spacing required for the cables on the cable rack in open air so that there is no derating of the cable?

    where is the fault free zone in a type tested panel?

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    Diamond Member AndyD's Avatar
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    Quote Originally Posted by Title of post
    'brain teaser'


    This isn't your average teatime brainteaser, the procedure for arriving at the installation specifications are quite involved. More info about type of motor, duty, start and run power factors, installation altitude, method of running cables (ducted, tray, free air or combination) would also be needed.

    The COC shouldn't be necessary if the installation is confined to the control panel which should be classed as part of the machine, not part of the building electrical installation.

    I can see you going with parallel cables, the 400mmsq conductors I'm guessing this motor will require make for difficult installation with bending radius etc.

    Is this a serious project or is it just for fun? If it's serious I'll dig out my college notes, ampacity and derating tables when I've got a spare hour.
    Last edited by AndyD; 10-Feb-10 at 08:50 PM.

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    its for real...one of the more complicated projects i am busy with...what has amazed me is how many different asnwers...speaking to various supply companies and engineers.

    i have had 3 x 35 mm cables
    2 x 70 mm
    1 x 185mm

    some engineers have indicated that 3 x 35 mm are not advisable...others have indicated it not even an option.

    some have indicated the best is to use a 185mm and some have indicated a 2x 70 would be the best option

    and others have indicated 2 x 120 mm would be the best option

    so my question is what is the correct answer and how and is there a formula or program which a person could use to get an accurate answer?

    for years i have just worked out the cable by the current rating of the motor and never had a problem...well it looks like this is not the best way to work it out because there are more factors to take into consideration before a decision can be made as andy has indicated.

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    this topic has become a real interesting one...especially for people who work with this type of installation...it is also interesting because a 3 phase installation electrician who signs over COC should be able to asnwer this question with ease.

    a clue...the min. size cable allowed for this application is 185 mm....not 2 x 95...or 3 x 35...even though they are capable of carrying 360 amps after being derated for grouping...temperature...locked rotor current...duty cycle...etc.

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    Diamond Member AndyD's Avatar
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    Quote Originally Posted by murdock View Post
    its for real...one of the more complicated projects i am busy with...what has amazed me is how many different asnwers...speaking to various supply companies and engineers.

    i have had 3 x 35 mm cables
    2 x 70 mm
    1 x 185mm

    some engineers have indicated that 3 x 35 mm are not advisable...others have indicated it not even an option.
    The volt drop across 3 x 35 may be too high and termination is a problem.

    Quote Originally Posted by murdock View Post
    some have indicated the best is to use a 185mm and some have indicated a 2x 70 would be the best option

    and others have indicated 2 x 120 mm would be the best option

    so my question is what is the correct answer and how and is there a formula or program which a person could use to get an accurate answer?
    There are many installation specific variables that need to be known. Designing in an office from plan is one thing but there's no substitute for the real geography. At least one full site survey during the design process is essential and often multiple visits are necessary. There are computer applications available to specify but most of the applications I've come across are based on the IEEE, BS and NEC regulations. I usually set up spreadsheets for our commonly required specs but this is just for timesaving, they're no more accurate than the tables in the regulations. No matter the method, the end result is only as good as the specifiers ability.

    Quote Originally Posted by murdock View Post
    for years i have just worked out the cable by the current rating of the motor and never had a problem...well it looks like this is not the best way to work it out because there are more factors to take into consideration before a decision can be made as andy has indicated.
    It's even more tricky nowadays with VFD's and soft starters. There's often a certain amount of art to specs when combinations of components of different manufacturers are in the same circuit, especially with overcurrent protection. The 700 semi conductor fuses you mentioned are protection for IGBT outputs but are non compliant with Regs10108 for overload protection.

    Quote Originally Posted by murdock View Post
    this topic has become a real interesting one...especially for people who work with this type of installation...it is also interesting because a 3 phase installation electrician who signs over COC should be able to asnwer this question with ease.
    The cable current carrying capacities have so many variables depending on installation method, spacing, bundling, load pf and even the physical construction of any walls they might run inside I could never remember all the values. There's at least half a dozen things that might warrant deratings being applied also so off the cuff calculations are dangerous. I have no problem with reliance on the tables, I've never seen it as a weakness.

    Quote Originally Posted by murdock View Post
    a clue...the min. size cable allowed for this application is 185 mm....not 2 x 95...or 3 x 35...even though they are capable of carrying 360 amps after being derated for grouping...temperature...locked rotor current...duty cycle...etc.
    A single cable is usually better for volt drop. I haven't done any calcs but 185mm sq might be a little shy depending on exact installation method of the cable and the motor start and run cos Ø. If it's a short run I wouldn't write of 2 x 120mm sq as a possible option.

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    i am waiting for the curve tables for the semi conductor fuses...this will change certain aspects of the design...the fuses are more for short cicuit protection...to protect the soft starter and reduce the fault level slightly.

    the soft starter has thermal protection.

    the cables as per the suppliers spec indicate we cannot use a cable less than 185 due to the fault level at the motor terminal as indicated by the program we use for design... by improving the response time if there is a fault to .5 of a second...we could use smaller cables.

    the 3 x 35 mm are within spec for volt drop but cannot handle the fault level.

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    here is an issue which has just shown its ugly head ...

    i purchase a 200 kw soft starter from ABB...in the technical data sheet on page 27 of their installation and commissioning manual for the soft starter...it states that you need to use a semi conductor bussman fuse amps 700 and type 170M5013....so i order the soft starter with the fuse holder...i get a 400 amp fuse holder with the starter...so i query this...i am told they issued the in correct fuse holder...so they replace it with an 800 amp fuse holder...i install the fuse holder trusting that the technical staff at ABB know what they are doing ...

    i take the fuse holder to the local specialist fuse supplier to get the fuses...they tell me that the fuse holder is not designed to fit a semi conductor fuse...so i contact ABB and ask them to supply me with the semi conductor fuses...they tell they DO NOT supply semi conductor fuses...yet they specify that you need to use them...


    while this whole fiasco is going on i try contact ABB reps...head office...sharecall and 24 tecnhnical support number 0861 488 488 they all go to answering services...sorry nobody available to assist please leave your telphone number ...hell of a thing having a 24 hours support service and nobody to man the telephone.


    3 fuse holders later and my panel looking like in a bomb blast with all the holes trying to fit the replacement fuse holders...my control panel is still not working.

    Cant say ABB will be the prefered supplier for the next 3 x 200 kw motor control stations....and to think someone recomended this product...i am sure the poor fella wont be recomending this product in a hurry again.

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    Diamond Member AndyD's Avatar
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    ABB will be very quick to tell you that they are just a manufacturer / retailer, not a specifier. They are usually not qualified to advise you beyond a certain point on the installation of the kit they're selling. In this scenario that you're in I try to find a known layout with specific products that are proven to play nicely together and fulfill the requirements of the regulations and if necessary make as few as possible deviations from that spec to make it work for the current project. Specifying from the ground up is a real pain for exactly the reasons you're encountering.

    The ironic thing is that once you've gone through the headaches of this installation and worked around all the issues, your easiest way ahead for the next control stations will be to duplicate your hard earned solution. So it will be back to ABB with gritted teeth unless you're willing to cut off your nose to spite your face that is.

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