15kva Transformer

eddybs23 · Aug 3, 2011

Been asked to run a feed for a 15kva transformer over about 40 meters for a site transformer. SWA Cable will be underground any suggestions on sizes and protection? Also would it be best to drop the transformer out and put a board in instead (as I have a spare three phase board hanging around my yard somwhere?

Knobhead · Aug 3, 2011

More information needed, sorry.

johnboy6083 · Aug 3, 2011

ive just done some v basic calcs. im guessing at a PF of 0.8, and arrive at at a figure of 45A from your tranny. I would install a 16mm SWA, and run it in ducting, to facilitate replacement, and to provide a new cable route. I would use a bs88 fuse, as it will comply with Zs, where if you use a MCB D type to 40A, it will not, and you will have to up your cable size to 25mm2

THE ABOVE IS WRONG, I messed up the calcs!

johnboy6083 · Aug 3, 2011

Bear in mind that the above was based on a TNC-S system , with a Ze of 0.35. in reality, your Ze will probably be lower, and you may get away with a D type MCB,. but please check first.

eddybs23 · Aug 3, 2011

... it's a TNCS system Ze = 0.11 ohms - 40 meter run - it's on a bulding site and will be used to power 110v accsessories until the build is finished. I was wondering if i can take it straight out of the head into a rotary isolator with outlet to the transformer? If so what protection and where? Would 10mm be okay?

MarkWysepower · Aug 4, 2011

On 10KVA 3 phase transformers I use 4mm 4Core SWA on a 32A type D.

The fourth core is soley for added earthing security as more often than not transformers take a bit of punishment on site and glands are regularly pulled out.

Bear in mind this must also have RCD protection.

Hope this helps

Knobhead · Aug 4, 2011

As you say the cable is to feed the transformer the FLC will be 20.8A @ 415V so work from there. As has been pointed out expect damage to occur so protect accordingly.
I installed a temporary transformer in a steel stock ground a while back it was reduced to scrap within two days despite having a girder frame around it

johnboy6083 · Aug 4, 2011

My calcs are well out! Trying to see what I did wrong! Any ideas?John

johnboy6083 · Aug 4, 2011

Just clicked. I multiplied the 15kva by 0.8 and forgot to divide by 1.732 vos it's 3ph. Schoolboy error.

MarkWysepower · Aug 4, 2011

I would also be mindful that you can allow a fair amount of diversity even though it is a single point circuit. I have carried out regular load testing on a variety of 10Kva transformer circuits ( up to 3 on radial ) and the highest loading i have seen is around the 10-12A per phase mark.

As I said before, this MUST have rcd protection if on a building site.