What size 3 phase cable and protective device.

[Ste69]

Bit nervous about putting this on the open board, normally stick to the trainee section.

Hope i can get some help. I have recently qualified so dont be to harsh if im not 100% correct.

Ive been asked to help an older spark to pull a a 3 phase SWA cable. Ive been asked to design the circuit i,e correct size cable and protective device.

This is the info i have. We are using a 3 phase 4 core SWA cable, we are coming from the main DB, run of 35 metres to a kitchen, into another DB (this is going to be used for the new kitchen equipment) the maximum load will be 10kw. We are clipping direct to walls and it will not be bunched with any other cables. I will be getting more info tomorrow.

This is my thought path so far.

Ib=P/1.732xVl, Ib=10000/1.732x400, Ib=14.43A (unsure if i should use correction factor cg? to get In, as there are 4 cores in the cable is this classed as bunched or is it classed as one cabel? sorry if that is dumb) i decided not to use for now. so using Ib. ( i also realise if i were designing a bigger circuit i would need to use power factor?)

i then looked in my on-site guide, table F5 on page 147(green), Ref method c lets me use 1.5mm cable. (this seems way to small?)

next i worked out the volt drop mvxibxl/1000, so looked in the on site guide, table F4 on page 146 i used 25 mv/a/m, so 25x14.43x35/1000 = VD 12.62 so it is good to go as it is under 20VD.

conclusion is i could use 1.5mm cable and a 16 amp 3 phase MCB type C? ( i would think using a 2.5mm cable with 16 amp mcb may be better as the vold drop would be lowered by quite a bit or even 2.5mm with a 20 amp mcb)

This is as far as i have got not knowing more info. Am I on the correct path here? any advice? very much appreciated

I am not going to do anything with this, this is not my job, i am just helping but i want to know how to do this for future. What info should i ask for tomorrow?

any constructive criticism welcome! any crap just stay oot! cheers.

I really need to buy the new regs!

 

[Marvo]

You say you're running a 4 core so I'm assuming there's a neutral. Is the load 230v or 400v?Is the load linear ie just heating elements etc or are there motors? Is it a balanced load or are there single phase appliances that may be running independently of each other?

 

[Ste69]

You say you're running a 4 core so I'm assuming there's a neutral. Is the load 230v or 400v?Is the load linear ie just heating elements etc or are there motors? Is it a balanced load or are there single phase appliances that may be running independently of each other?

Cheers Marvo.

Im not 100% sure as i have not been to the job yet, boss just told me the above and to have a look after work. I think the load will be both 230v and 400v. Pretty sure its linear with heating elements. dont think its running any motors.

 

[Marvo]

There's some considerations about the load you need to accommodate, you can't just use the 10kW figure off the bat. Firstly you need to figure out the current draw on the highest loaded phase. If you have mixed load such as sockets and lights and 3-phase loads you need to work out how they can be best balanced ie maybe an RCD and socket circuit on each phase. Then look at the type of load and consider starting currents and power factor for motors and also possible diversity then work out the current requirement per phase. Consider future provision as well so you might want to oversize 25% for example. Then finally look at cable size and you might need to use 230v in the calculations if it's predominantly single phase sockets.

 

[Ste69]

Right the circuit is for a new food counter in a cafe, overhead light for display unit, merry chef oven, under counter fridge, coffe machine, bottle fridge and a general purpose socket outlet. All single phase so no 3 phase equipment. Works out at just under 10kw....I'll do my working out later once I'm home and put up what I get. We are just pulling the supply cable in, the kitchen fit company are wiring from the kitchen db to the equipment/commando sockets.