electricalforum.nz

Hi Guys,

I have a job coming up for 20 units/workshops/offices. They range in size from 60 square metres up to 200 square. At this stage they will all have a small office, toilet and mezzanine floor so I will be just installing the switchboard, a few lights, roller door power etc.

The plan is for a central supply with metering cabinets for each unit in a body corp storeroom. Each unit will be independently owned.

The plan is to supply each unit with a 3phase 16mm cable but possibly only connect single phase just so they have provision to upgrade the supply at a later date without digging the concrete.

The question I have is how do we go about calculating the supply cable size from the boundary to the central supply when I have no idea what is to be in each unit? They could just be storage units but at the same time they could be a workshop, bakery etc. If I know what we are dealing with I can do a maximum demand calculation but it's just a guess at this point. This is in Christchurch on the Orion network if that changes anything.

The majority of my work is residential so I'm a wee bit out of the loop but we take what we can get work-wise at the moment!

Thanks,

Nick

Taking your suggested 3 x 16mm2 as the basis; this suggests 3 x 63A for each as maximum.
But since unlikely all 20 units will be fully loaded; I'd suggest mains to allow min 3 x 20 A for each unit,
which would come out at 400 A per phase as a starting point.
And maybe consider going up rather than down.
There's no "right" way to make this sort of judgement; it's all balancing cost now versus what may be wanted in future.
But there's no point aiming at the bottom end.

Thanks for the input. I was concerned there may have been a rule/regulation to follow but couldnt find anything. It may take some convincing with the developer to oversize everything but i guess it will stop any issues down the track.

Don't forget to calculate voltage drop. You haven't mentioned distances in your post, but for 20 units, there must be a reasonable length on some of them.

Have a discussion with developer.
They never want to spend money, but they need to be able to sell /lease the properties.

Chances are that not all will be workshops needing 3 phase 63 A
but equally they won't all be simple storage.
With 20 units, you can allow quite a bit of diversity factor.

And where access for later upgrade is easy, there's less need to future-proof now.