Electrical supply to a portacabin

jimshall · Oct 21, 2021

As trustee of a school I'm helping them with adding a 2nd hand portacabin situated adjacent to an existing one. An electrician has advised the best way to supply the new portacabin, made up of 6 modules, is from the incoming supply to the existing portacabin, to dig a trench and install six 10mm swa cables to connect to the 6 terminal boxes situated on the outside of each module.
I can't help thiinking it would be better and less expensive to bring a single 25mm swa cable to the mid-point of the new portacabins (approx 20m) and distribute from there to each module terminal box.
Has anybody advice/ experience to offer?

Strima · Oct 21, 2021

What is the intended use of the cabin and expected maximum load demand? Type of heating/cooling systems, lighting, sockets etc will all have an impact.

Mikegh · Oct 21, 2021

Based on what I read you're electrician is right

jimshall · Oct 21, 2021

Strima said:
What is the intended use of the cabin and expected maximum load demand? Type of heating/cooling systems, lighting, sockets etc will all have an impact.

It's for classrooms. There are electric radiators on the wall about 5kW in each of the 6 modules , strip lights, sockets for light appliances, maybe a kettle also. We thought 10mm cable would give more than enough capacity.

westward10 · Oct 21, 2021

Does each module have its own distribution board.

jimshall · Oct 21, 2021

The internal wiring circuits are connected through the wall to the boxes that are situated on the outside just below roof height. The wiring then routes above the false ceiling. Maybe I need to open up one of the boxes and see if inside it has RCD/mcb's.

Spoon · Oct 21, 2021

jimshall said:
An electrician has advised the best way to supply the new portacabin, made up of 6 modules, is from the incoming supply to the existing portacabin

jimshall said:
There are electric radiators on the wall about 5kW in each of the 6 modules

So, for heating alone, then you are looking at about 22A per module. 6 modules = 132A. I presume that all 6 modules will be used at the same time so I wouldn't be keen on using selectivity, as all the heaters will be turned on at the same time. Yes, it will be a lot better once the modules are up to temp.
Then you have all the additional power.

If the intension is to wire to the existing portacabin then what size cable is powering that and what size breaker is it wired to?
I'm presuming it's also a 3 phase supply.

jimshall · Oct 22, 2021

The existing portacabin is fed directly from a nearby transformer via an underground cable. I don't know the capacity or if it's single or three phase but is connected only to one phase.

westward10 · Oct 22, 2021

Does each module have a separate distribution board.

jimshall · Oct 22, 2021

I need to go into the school and check. Will also take some photos

pc1966 · Oct 22, 2021

Also how long is the total run from the main DB to the 'new' Portacabins?

jimshall · Oct 22, 2021

So there are 5 modules each with a distribution box supplying a wall heater (2.5kW) a ring with sockets and lighting. The electrical supply to the original portacabin is 3 phase, it looks like one is used. The incoming cable from underground is huge and in place to cater for future expansion of the school..

Mikegh · Oct 22, 2021

I suppose I was working off the assumption that the supply to existing portacabin is adequate

With everything adjacent you would distribute separately to each module

Wiring to a midpoint and distributing makes no sense to me anyhow