Cable calculation challenge.

[Bobster]

Trainees, I realise this will probably be a little over what you will have covered (or in fact will cover) in college. I'm hoping that it may open up a new route of thinking how to plan bigger installs.

Mentors feel free to have a bash.

Here's the scenario. (It's from a real job I undertook a few months ago in Russia)

Ok so its for a 640Kw motor, starting method is a soft start inside delta.
FLC is 1100A (I've done the math here for you)
Voltage out there is 380/220v
It's run is clipped direct to tray, with a grouping factor of 0.78
Ze is 0.003
Protected by 1600A MCCB (this is providing short circuit and earth fault protection). The Zs required for the mccb is 0.015.
The Soft start has a built in thermal overload that will be set at 0.58*FLC.
Length of run is 70 meters.
XLPE SWA is the preferred cable type.


I will explain later on how a soft start works inside delta. Treat it as a star delta starter, regarding wiring from the control panel to the motor.

Tony, I know your in here now and you may remember me discussing it with you. Don't give away the answer.

Oh, and no there wasn't any MV on site suitable.


Up for grabs on a correct answer: The electricians bible on motors by Brooke Crompton (there now as rare as rocking horse s***). Some of the older members on here will have been given them when they started their [FONT=helvetica, arial, sans-serif]apprenticeship. [/FONT]

 

[Paul.M]

Wow another prize! Nice one Rob.

 

[Bobster]

Wow another prize! Nice one Rob.

Someones got to get it right first

 

[Knobhead]

The Brook Crompton Installation and Maintenance of Electric Motors hand book used to be issued to all industrial apprentices, it’s often referred to as “the electricians bible”.
I always have it to hand, it’s about 2’ from where I’m sat at the moment.

Well worth having a try for.

 

[Bobster]

Trainees please don't let this over face you. I will answer any questions and explain anything you need.

Mentors rs you also have a chance. I'm not offering help to you lot though.

 

[jimmyray]

Hi Rob,

I'm probably being a bit dumb (few England match beers!), but what do you want us to work out? Correct size of SWA? Cheers.

Jim

 

[Bobster]

You can use any cable install you want. I just want to know the CSA of cable you would use, multiple if necessary.

Preferably xlpe swa, as I have my calculations to hand. If you use another method that's fine.

 

[pennychew]

I'll have a crack at this later on, answers by PM so everyone can have a go?

 

[Knobhead]

Up for grabs on a correct answer: The electricians bible on motors by Brooke Crompton (there now as rare as rocking horse s***). Some of the older members on here will have been given them when they started their apprenticeship.

I’ve just looked at the frontice page of my “bible” that I was issued with in 1974.
At that point 190,000 copies had been given out. Try and get hold of one today, no one will part with them. They are that well respected. When I kick the bucket there will be another copy up for grabs.

I had a third year apprentice (Dave) with me on a 300HP compressor motor change, it’s on a plant that RoB knows.
One of the weird things with this company, if it’s an electric motor, the electricians have to fit it.
As usual I let Dave take the lead. We got the motor in place with assistance of an OHET crane. No problem so far, in fact I was more than happy with things. The motor was on slide rails that were horizontally level with the compressor so no problem with horizontal alignment.
Then I was the rotten barsteward. “What is the next step?” Lateral and angular alignment. At this point I sent him to get “the bible” out of my cupboard and left him to it for a while, I went back and he was 90% done. He’d followed the instructions in “the bible”.
As for the rest of the job it turned in to a disaster movie. It was bloody funny all the same.



Back to the question RoB has set, he’s given an important clue about the supply.

 

[pennychew]

I'm nowhere near with this yet! i've spoken to Rob via PM and I'm slowly getting there! need to watch a few youtube vids on motor overloads and star delta starters today then i'll have another crack at the hotel tonight.

 

[Bobster]

Well gents there's been a few getting close, so keep at it.

I'm off to China tomorrow to work with a 59MW motor, so if I'm not on here/don't reply to PM's it's more than likely I've no internet or no free time.

 

[pennychew]

Well gents there's been a few getting close, so keep at it.

I'm off to China tomorrow to work with a 59MW motor, so if I'm not on here/don't reply to PM's it's more than likely I've no internet or no free time.

Ive been mega busy but i'm off til saturday now so chilling out, you'll have my final answer in about 30 mins!