Submain calc

[MUFC1999]

Right, I'll try to keep this simple.

I have a job coming up that involves installing a couple of storage heaters. The 2 rooms are on the 2nd floor of an old building and the DB is in the basement. They also require a ring main to give power to the 2 rooms.

There is no way of running cables back to the DB as this would create a lot of extra work which they can't have done as the premises is in constant use.

My plan is to wire the neccesarry circuits on the their own DB and run a Submain back to the original DB down the side of the building and into the basement.

How do I calculate what size cable I need for this. It's a while since I did this. I've worked out my cable calcs for the necceassary circuits.

I'll have 2x 16A for the heaters and 1x32A for the sockets.

 

[MUFC1999]

Sub Main calc

Right, I'll try to keep this simple.

I have a job coming up that involves installing a couple of storage heaters. The 2 rooms are on the 2nd floor of an old building and the DB is in the basement. They also require a ring main to give power to the 2 rooms.

There is no way of running cables back to the DB as this would create a lot of extra work which they can't have done as the premises is in constant use.

My plan is to wire the necessary circuits on the their own DB and run a Sub Main back to the original DB down the side of the building and into the basement.

How do I calculate what size cable I need for this. It's a while since I did this. I've worked out my cable calcs for the necessary circuits.

I'll have 2x 16A for the heaters and 1x32A for the sockets.

 

[kevyn]

Calculate the maximum load that is going to be on thses two rooms including the heaters. Then chose your cable size for the submain. I would use a 16mm T+E it might sound a bit overkill but it will give you plenty of scope should there be any more added to that con/unit.

 

[JUD]

How big are the rooms and what are they going to be used for?

Do you really need a 32A socket circuit for 2 rooms. Wouldn't a 20A radial suffice?

 

[MUFC1999]

How big are the rooms and what are they going to be used for?

Do you really need a 32A socket circuit for 2 rooms. Wouldn't a 20A radial suffice?

A 20A radial probably will do to be honest. I'm just rough planning for it at the moment so didn't pay too much attention to that.

 

[JUD]

Do you know how much power the storage heaters actually use?

How long is your sub-main feed going to be? Important for Voltage Drop calculation etc.

What cable are you using for the sub-main? If I've understood your post, you're running it outside.

 

[Andyfm]

Do bear in mind that the storage heaters will not be in powered during normal office hours (unless they`re on economy 10 or 11 tariffs), so you dont need to be too generous in sizing your submain

 

[JUD]

Do bear in mind that the storage heaters will not be in powered during normal office hours (unless they`re on economy 10 or 11 tariffs), so you dont need to be too generous in sizing your submain

If he's running them off the same CU as the sockets I would presume they will have power available all the time, so they would either have to be switched on and off manually or run off a timer of some sort.

 

[Jabbajaws]

Re: Sub Main calc

There are several things that need to be determined here MUFC1999.

The sub main needs to be capable of carrying the tabulated currents of all circuits on the new DB. The new DB must now have RCD protection as required by BS7671.

The length of the sub main cable is very important because of voltdrop, which is now 3% for lighting and 5% for all other circuits. (correct me if im wrong anyone).

If you are taking the sub main capable outside the premises then l suggest that you use SWA cable. It may be expensive but its recommended for outdoors.

What wattage are the storage heaters, MUFC1999?

 

[scotsparky]

Re: Sub Main calc

jabberjaws.
The length of the sub main cable is very important because of voltdrop, which is now 3% for lighting and 5% for all other circuits. (correct me if im wrong anyone).


you are correct with that satement however that is total drop from origin (meter). If its sub mains i reccomend doing the calcs for 1% or less for sub mains as it gves more leaway when doing the final circuits. and it may only increase the swub mains cable by 1 size .

 

[Jabbajaws]

Re: Sub Main calc

Thanks for that helpful tip, Scotsparky. It may come in handy in the future...

 

[Spudnik]

Threads merged

 

[MUFC1999]

The storage heaters are 2.55KW.

I'm fully aware that I'll need RCD protection and I will be running the Sub Main in SWA. I've not measured it yet but I reckon 20 meters will do it.

 

[JUD]

Assuming a design current of around 43A (20A for sockets and just over 22A for the storage heaters), I would go with 10mm² 3-core SWA (CCC Method C = 67A) using one of the cores as cpc.

Voltage drop for 20 meters = 4.4 x 43 x 20 ÷ 1000 = 3.8V leaving you 7.7V to play with for your final circuits.

Would depend if any lights are going to be fed from sub-main obviously.

 

[MUFC1999]

That's what I'd worked it out at just needed a second opinion. Like I said it's a while since I last did cable calcs.

Cheers for the help.