Help needed for choosing a cable to feed a top floor flat.

[arturas]

Hi there guys, just to be sure I wanted to double check with someone more experienced. Usually all my work is re-wiring of properties. Now I need to feed up the top flat. The run will be roughly 45-50 meters, with a 60a fuse.
Now when calculating the voltage drop, as there will be lighting circuits it has to be 3%. I'm thinking of 25mm three core swa. It will be run on the tray next to other cables feeding other flats. And when calculating the voltage drop I'm using column z of a table 4d4b which is 1.75 per amper per meter: 1.75x60x50/1000= 5.25 which is < then 3% of 230v. And now when trying to determine a cable size I'm using a formula: In/CgCaCsCdCiCfCc . Because it's above the ground in free space no thermal insulation and the ref method is e, all of the factors =1 except the grouping factors which is 0.72. In that case I get 60/0.73= 83 which is less then maximum current carrying capacity of 25mm cable. Am I right?thanks for any help!

 

[Richard Burns]

You are planning for the worst case scenario there with a design current of 60A but this will mean that you are very OK with that supply cable and this would allow for some leeway should the fuse be uprated to 80A. If the customer will accept it then go for it; remember to earth the armour!

So long as your description is accurate then the cable is fine.

 

[arturas]

Thanks for a quick reply! It is the worst case scenario, but the fuse is 60 amp and it will hold a bit more before blowing so the cable has to as well.

 

[Spoon]

As Richard has said. You cant go any smaller in the cable size as 16mm has a VD of 8.4V which is 3.6%
If they upgrade the fuse to 80A, as mentioned above by Richard, then the VD gets really close... 7V or 3.043% but you can only do the calcs to what is there now and not what may possibly happen in the future.

 

[Spoon]

Just out of interest.. where is the cable tray being routed to go to the top floor?

 

[arturas]

Cable tray is visible hanging of the ceilings, it's a big landlords aeria with a factory look, so everything is visible,metals,trays, plumbers pipes and etc

 

[arturas]

As Richard has said. You cant go any smaller in the cable size as 16mm has a VD of 8.4V which is 3.6%
If they upgrade the fuse to 80A, as mentioned above by Richard, then the VD gets really close... 7V or 3.043% but you can only do the calcs to what is there now and not what may possibly happen in the future.

I understand that, I can't see a need to upgrade a main fuses, 60amp for 2 bed flat is plenty enough, however I will install a 60amp fused isolator so even if they would, the cable would still be protected.

 

[Badged01]

This is the way your DONT want to do it, cram all those 16mm cables off 80A landlords fuses into a bit of trunking and start running them up 4 storeys to 8 conveted flats throughout the building with everything run of electric (kitchen, heating & water) ..... and while you're at it put in a CU for the communal areas/fire alarms/emerg lites and dispense with the protection around it!

Picture quality not that good as the camera phone was shaking a bit at the time - I was just starting to get my first overall initial impressions before completing an EICR post the local council refusing to re-register the property.

Not my install I hasten to add but done by a reputable "electrical contractor" who pride themselves on 'time served' electricians!! (In fairness I think the consumer unit had been damaged after the install)

 

[arturas]

I wasn't even thinking of running 16mm I wanted to be sure that 25mm is big enough. Wow that install is on another level! I would like to book an yearly assesment on that one for a joke, I think 500£ would be worth seeing guys face!

 

[micknew]

This is the way your DONT want to do it, cram all those 16mm cables off 80A landlords fuses into a bit of trunking and start running them up 4 storeys to 8 conveted flats throughout the building with everything run of electric (kitchen, heating & water) ..... and while you're at it put in a CU for the communal areas/fire alarms/emerg lites and dispense with the protection around it!

Picture quality not that good as the camera phone was shaking a bit at the time - I was just starting to get my first overall initial impressions before completing an EICR post the local council refusing to re-register the property.

Not my install I hasten to add but done by a reputable "electrical contractor" who pride themselves on 'time served' electricians!! (In fairness I think the consumer unit had been damaged after the install)

Saw work like this on a farm, it was done by a Polish 'electrician'.