brain teaser

[murdock]

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?

[BigRed]

Pass

I'll stick to wiring my toaster

[murdock]

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?

[AndyD]

'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.

[murdock]

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.

[murdock]

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.

[AndyD]

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.

[murdock]

trust me when i tell you ABB will not be my first choice for the next project...when they receive my R22000.00 bill for all the alterations required to remove modify and replace the fuse holders...panel doors...busbars...backing plates...etc because they supplied the incorrect fuse...i dont think they will want to sell me any more product...i have 5 more projects lined up to install...the next one is a 315 kw motor...even bigger challenge...and how i love a challenge...i look forward to it...i will start planning for the next project as soon as i have completed the one i am busy with...and this time make sure who ever supplies the equipment has a better idea of what they are selling and why...in fact one of the companies have offered a 3 day course free of charge on VSDs soft starters etc...just dont know when i will get a chance to spend 3 days in a lecture room.