Help please....

[sythai]

Just doing some calcs for extension supply.... 100A after diversity - run is 25m away !!

Some how dont think 16mm T+E will be sufficient for this one.

Would 25mm SWA do it...?

Having probs doing my volt drop, can someone help me please ?

 

[JUD]

What's the install method?

 

[sythai]

Running through joists in free air, clipped along a wall, final 3m through 100mm thick insulation ......... think I've managed to re-do me figures, can get it down to 88a as can put a couple of circuits on existing supply. And also may be able to cut run down to 25'ishm.

So if I've got my sums right a 2core 16mm swa and seperate 10mm earth hopefully should be ok...? Going to be a real struggle if it's 25mm

 

[JUD]

Having probs doing my volt drop, can someone help me please ?

Volt Drop (16mm² 70°C PVC SWA) = 2.8 x 25 x 88 / 1000 = 6.16V(2.68%)

Volt Drop (16mm² 90°C XLPE SWA) = 2.9 x 25 x 88 / 1000 = 6.38V (2.77%)

Doesn't leave you much leeway if you're supplying any lights

 

[Guest123]

You cant use diversity when selecting cable sizes and switchgear.

 

[sythai]

If that is the case, then I think my only option is going to be to run 2 x 16mm core xple and have 2 consumer units in extension. 1 x for hob and ovens = 75a and the other to cover the remainder lights, ring, immersion, shed, garage = 77a....

 

[Guest123]

We seem to be a bit confused.

What I was saying was you cant factor in diversity when calculating the Ib for a circuit. Take a cooker rated at 9KW for example - you would calculate Ib to be 39 odd amps so you would select a 10mm T&E and a 40A MCB. You wouldn't take the 39A apply diversity to it and then select the cable and MCB or you'd end up with something like a 2.5mm on a 20A.

As for your install.....seems like a hell of an additional load, what type and rating of supply is there at present???

Obviously when selecting the sub-main cable you can carry out a max demand calc applying diversity and as long as the CCC of the cable is sufficient for OCPD rating then it'll be fine. Just be sure not to over engineer it too much as the last thing you want is for the sub to start overloading and there to be nothing you can do about it as the cable isn't large enough to uprate the OCPD.

 

[sythai]

Ah I see making a bit more sense now thanks Lenny for helping me out.... always learning.

Basically supply in at present to existing property is 100a single phase:

5 bed house - 12 way CU - 4 x rings, 4 x lights, 1 x cooker, 1 x Immersion, 2 x spare

Extension going on - new circuits as follows (before diversity):

hob 51a
double oven 23a
immersion 13a
ring 32a
lights 10a
sub to garage 20a
sub to summershouse 20a

CCC of the cable is sufficient for OCPD rating then it'll be fine

could you explain what CCC and OCPD stand for please.


And definately wont be under sizing the cable, thats why I'm making sure I get my calculations right eventually...

 

[keniff]

Current carrying capacity, and over current protective device I think mate.

 

[Guest123]

CCC = current carrying capacity. OCPD = overcurrent protective device (MCB/fuse etc)

Well at least there aren't any electric showers, what type of heating will there be in the extension & summerhouse???

That is quite a bit of additional load, obviously you will size OCPD's to those ratings i.e cooker 50A, oven 25A or 32A etc, but you can apply diversity to those figures to get a max demand figure and a rough idea for sub-mains.


Is there any way the garage and summer house can be supplied form the existing house DB to cut down on the demand of the extension DB as you say there are 2 spare ways in it???

 

[sythai]

Thanks chaps... as you can probably tell I havent done that many jobs of this size... nice to get the work really cant complain. Just getting my head round the figures.

No electric showers going in or present, heating is rads through out just got to put an extra immersion in for new megaflow, summerhouse is just a 2 way CU supplying a double socket. Same again for garage plus 4ft flurescent.

 

[ezzzekiel]

thought you had a max demand of 100a just for summer house, thought it seemed a tad high

 

[sythai]

thought you had a max demand of 100a just for summer house, thought it seemed a tad high

Nop thank fully not this time

 

[malcolmsanford]

The DNO guys when calculating the load for anything use the 0.4 method on working out diversity. They add up the load of the protection devices and then multi ply that by 0.4.

The OSG in appendix 7 gives the diversity factors for all your circuits if you want to work it out that way.

I'm wondering though about the 2 x 20amp supplies to the garage and summer house. Will these be feeding CU in those locations? as I'm sure that supply will logically be divided into lights and power.

You then will be dealing with a whole new lot of problems. If the supply to the house is PME will the DNO let you export earths out to these areas.

How far are these out buiildings as you will need to do a calc for theses as well ?

Are these out buildings having any water or gas services in them as if they are and you can export the earth you will have to run a 10mm^ bonding cable right back to the incoming main MET.

Will it be better then to TT these out buildings.

As the DNO supply is 100amps this extension will can nearly be classed as another small home. Is that 100amps supply enough?

This is quite a big undertaking for you, as you said yourself, will you be ok with it

 

[sythai]

Is there any way the garage and summer house can be supplied form the existing house DB to cut down on the demand of the extension DB as you say there are 2 spare ways in it???

Not really would be hell of a run, Already moved a 1 x ring and 1 x lighting circuit on to there from extension to get my figures down.

I'm wondering though about the 2 x 20amp supplies to the garage and summer house. Will these be feeding CU in those locations? as I'm sure that supply will logically be divided into lights and power.

Thats right they will be feeding a 2way CU in each location. And Both will be TT as existing property being upgraded to PME. Summerhouse is 25m away and will have 1 x 15a radial running from CU supplying single socket and garage which is 10m away will have 1 x 15a radial supplying 1 x single socket and FCU to supply light.


Right think I've got it under control now, panic over....

Spoke to customer informed them that there is possibility of that there existing 100a supply may not be adequate (informed them of worst case scenerio and what may be involved etc..), they are going to have a word with DNO when they come out to upgrade the PME and see what calcs they apply and if it is required.

Got my figures sorted out now:

Submain to cu 1: 10mm 3c SWA to feed : hob,ovens,immersion = 50a max after diversity (30m run ref D)

Submain to cu 2: 16mm 3c SWA to feed : ring,lights,s.house,garage = 69a max after diversity (30m run ref D)

And my be able to get runs down to 25m....

Thanks for all your help,

Sy