Design current

[Jimmyb]

What you base the design current of standard 3-4 bedroom house with electric cooker/hob and shower ? the reason i ask is im trying to check vd on a 50m cable.

Thanks

 

[trev]

Design current is based on the load mate. What load is proposed and the relevant correction factors dictate what cable and protective devices are installed.
Can't you check the vd in the OSG?

 

[JamesBrownLive]

I've always calculated VD on an individual circuit basis, why would knowing the Ib of the whole house help with that? Can you be more specific?

Cheers

 

[Jimmyb]

basically i have a cable from the service head that is 50meters long then into the house, i calculated the house to have a average Ib of 70amp and checked vd on the exisiting 16mm cable and it came out at 9.8v so my thought is that now we are adding a extension with 3 additional rooms the cable would need to be upgraded and i keep getting told that its not often the load is going to be that high and cable is fine and just to add addtional circuits to it, so i asked what load would you base the cable and no-one will give me any info at all, tbh im a little unsure on how to do it myself hence why im asking on here.

 

[JamesBrownLive]

50m meter tails, wow! Are you saying they are 16mm? Don't suppose the DNO would extend the service head into or closer to the house for you? Would certainly make things easier.

What do you get as VD on a worse case scenario?, i.e based on In.

 

[Jimmyb]

sorry it a farm so its an swa cable from barn to house, what do you mean worse case scenario ?

 

[telectrix]

he's saying add up the MCB ratings to get an absolute max. current. however, that would never happen. your 70A is about right for a worst case scenario, IMO, and your VD should not be too badly affected by your additions, unless you've got some heavy heating loads. whast csa is the 50m of SWA?

 

[Jimmyb]

totally mcb's are 160amp and then i applied dev. and came to 70a, the cable is fused at 60a so based on that they must of designed it at 60 because anything less is just not right and anything more is bigger than the fuse, the cable is 16mm 2core swa and the total vd at this load is 8.4v and this is over the 3% for lighting, now i reliase that this is not very often going to be this load but i thought u had to allow so much as it would be reached but even if i base it on a 40 ib the vd is 5.6 which only leaves me 1.3 for lights which is alot really.

I'll be honest im really lost as when i mentioned about upgrading the cable and the cost the owner said the others that qouted can not be doing this as it not in the quote and also your pirice go from the cheapest to well over once you've priced the upgrade in, i ask him to check with the others if they was happy with the cable and they all said it is sized ok. im lost ??

 

[GATEHOUSEN]

i hate working on farms ............allways a nightmare!!!!! what ever u do stay in regs or as close as poss....i refused to carrie out anymore work on a farm as he was wanting things i was unable to do and feel ok. so he got an unqualified mong to do it.

 

[telectrix]

as long as that 16mm is fused correctly, i would carry on with it if it means you have to to get the job. if he then blows the fuse with overloading, then you can say " i told you so" and price for upgrading the SWA.

 

[GATEHOUSEN]

maybe get it in writting that thats what they want ...then they cant blame u if its wrong.

 

[JamesBrownLive]

I'll be honest im really lost as when i mentioned about upgrading the cable and the cost the owner said the others that qouted can not be doing this as it not in the quote and also your pirice go from the cheapest to well over once you've priced the upgrade in, i ask him to check with the others if they was happy with the cable and they all said it is sized ok. im lost ??

Maybe you're competing with cowboy types who don't really give a damn and that's why they haven't mentioned upsizing the swa, or maybe they're hoping to slip it in later when work has already started.

Those figures that you mentioned are they with the new loads taken into account? Is the swa the 70 or 90 degree type?

 

[Jimmyb]

as long as that 16mm is fused correctly, i would carry on with it if it means you have to to get the job. if he then blows the fuse with overloading, then you can say " i told you so" and price for upgrading the SWA.

Yes but i have to sign it off and comply with voltdrop so want it right and my light circuit would not comply but i do know what you mean

 

[Jimmyb]

Yes that will enclude the new load and swa is 70c type.

What size would put in if this was a new install because i would want 25mm min but would use 35mm

 

[JamesBrownLive]

Well you're looking at a usable rating of 99A buried underground for the 25mm or 119A for the 35mm (Table 4D4A) BRB.
If you're gonna go to the trouble of all the digging that's going to be involved, might as well add a proviso for the future.

VD drop looks to get a little more complicated for 25mm + as there seems to be 3 differnt figures in table 4D4B, 'R' 'X' and 'Z'. Not sure what they mean, so time for someone with more knowledge to step in :uhoh2: