Generator Supply

**welshmadness** · 29-06-2009

Hi all.
Going to be doing a standby generator installation soon and i need to work out a minimum duct diameter for the cables i intend to use.
The generator has a 630Amp merlin Gerin MCCB, i was going to use 2*300mm 4core s.w.a. cables, until i went and paid a visit to have a look at the generator, and was amazed at how little cable, glanding and termination space there was. So we done some re-calcs and now decided to use unarmoured singles (2*185mm tri-rated per phase)
Now, ive worked out that there will be 8*185mm and one 240mm earth cable going through this duct, each 185mm cable has a diameter of 32mm, and the 240mm had a diameter of 40mm.
Can anyone help? Been looking through the regs but cant find anything specific for what im trying to achieve

Thanks

**montybaber** · 29-06-2009

What rating is the generator out of interest?

**welshmadness** · 29-06-2009

Its got a 630A 4pole mccb, 150Kva if memory serves me right, big bugger too.

**Lenny** · 29-06-2009

Hi there.

Tri rated singles, deffo the best way to go. Done loads of geney systems upto 1MW and always used them.

What are you supplying/how far. Any risk of damage??

As for the duct I'm guessing your going underground??? if so just use the biggest you can get in there, makes life so much easier when drawing through.

**montybaber** · 29-06-2009

Must be bigger than 150kva surely? wouldnt that only require the cable to carry 210ish amps per phase.

You could get away with 120mm 4core SWA in a duct

Then again im a duffer so i prob got it wrong lol

**welshmadness** · 30-06-2009

Yeah, sorry it is bigger than 150Kva, its a 410Kva.
Lenny, we are supplying a MCC for a pumping station, cable run is about 25meters, the duct (stainless steel) will run from the generator, through a concrete plinth and into a sub-basement where the singles will jump onto some ladder racking then into a mains/generator auto changeover unit.
The duct will be earthed as i see it as being an exposed conductive part.
There wont be much chance of damage to the cables as they will be mechanically enclosed, Generator vibration wont be a major issue.

I am thinking of a minimum 16inch diameter duct, not done one of these before so im a bit worried incase i spec the wrong size.

Thanks for the replies so far guys.

**montybaber** · 30-06-2009

Hi there, thought that seemed small

AFAIK there is no regs (17th) with regard to space factor but after reading around apparently 15th edition quoted 45% which is what i'd go with as a minimum if I were in your position.

Just some scribbles but........

32mm diameter x 0.5, squared and multiplyed by pi = approx 804mm(squared) csa

40mm diameter x 0.5, squared and multiplyed by pi = approx 1256mm(squared) csa

8 x 804 = 6432 + 1256 = 7688mm(squared)

7688 / pi = approx 2447, sqrt of that is near as damn it 50, multiply this by 2 to give total diameter (of cables and insulation) = 100mm I would multiply this by 2 to allow for the gaps between each cable and then multiply this by 2.2 to allow space factor so this gives

440 mm diameter which is 17.3"

Whatdya reckon? I have no idea if this is ok BTW

**darkwood** · 30-06-2009

Just be cautious if paralleling your cables up when it comes to glanding them especially for a genie this size you need to avoid eddie currents forming within the gland plates usually achieved with used of a non ferrous metal or non metallic gland plate, depending on the load characteristics of the load; overheating, arcing, induced noise, rf issues etc are all major problems if done incorrectly. Appologies if im teaching you to suck eggs just gave impression you havent done this type of system before.

**welshmadness** · 01-07-2009

Thanks Montybaber.
I wasnt far off with my guesstimation of 16inch diameter, but was good to see some calcs from someone else.
I never did the 15th as i was still in school at the time so wasnt sure, so cheers for looking.
I plan using a 12mm thick pvc board from RS to be a substitute gland plate to eliminate eddy currents. Done a few genny sets before, but were only small 100A backup sets, most of which you could terminate S.W.A. into without much hastle..........until now lol.

I would post pics of the geny set on here, but no idea how as im still a virgin forum user

Thanks again for your replies.

**montybaber** · 01-07-2009

You would have to host your pics on photobucket or the like... its really easy even I can do it lol

Just join photobucket, upload the pic and then copy the img text and paste it onto here and it appears

ps I dont remember 15th either lol im 29