Advice on main supply please

[chappers]

Hi all,

I'm looking for some advice on a main supply if anyone could help please.

I'll explain the layout: supply from substation 100A fuse on 16mm 3-core SWA, armour earthed at sub end, not connected at other end. This supply feeds into four 63A MCBs, each one supplying a CU in a little apartment in the block. Each CU has a 32A ring and 6A lighting circuit. The feed cable from each 63A is 6mm T+E running through the attics under deep insulation.

It's immediately clear the 6mm T+E needs changing. Interestingly, back in 2008 I see it passed periodic inspection and testing like this.

I'm concerned about the feed from substation to apartment block as well, and want to get my facts straight before advising the site owner to get the installation upgraded. I'm not very familiar with main supplies from subs and how the supply rating is calculated. I've always noticed that the main supply is protected at less than the total load possible from the submains it feeds, so assume that diversity is applied on the assumption that it's unlikely that each apartment in a block, for instance, will be pulling maximum load and at the same time. How is this calculated for main supplies?

Also, it would appear that the 16mm SWA (buried underground) is undersized anyway as it's protected by a 100A fuse.

Any advice most welcome, thanks.

 

[Darkwood]

Welcome to the forum chappers from the staff

 

[amlu]

Hi all,

I'm looking for some advice on a main supply if anyone could help please.

I'll explain the layout: supply from substation 100A fuse on 16mm 3-core SWA, armour earthed at sub end, not connected at other end. This supply feeds into four 63A MCBs, each one supplying a CU in a little apartment in the block. Each CU has a 32A ring and 6A lighting circuit. The feed cable from each 63A is 6mm T+E running through the attics under deep insulation.

It's immediately clear the 6mm T+E needs changing. Interestingly, back in 2008 I see it passed periodic inspection and testing like this.

I'm concerned about the feed from substation to apartment block as well, and want to get my facts straight before advising the site owner to get the installation upgraded. I'm not very familiar with main supplies from subs and how the supply rating is calculated. I've always noticed that the main supply is protected at less than the total load possible from the submains it feeds, so assume that diversity is applied on the assumption that it's unlikely that each apartment in a block, for instance, will be pulling maximum load and at the same time. How is this calculated for main supplies?

Also, it would appear that the 16mm SWA (buried underground) is undersized anyway as it's protected by a 100A fuse.

Any advice most welcome, thanks.

That cable from substation is that before the meters? Then it will be DNO business how big it is, not yours. They have different rules.
6mm distribution in the building - any signs of overloading, thermal damage? With 38 amps maximum load and diversity applied it does not sound that bad. Possibly the deep insulation in the attic was not there when it got installed.

 

[chappers]

amlu, the substation cable is after the meters.

There is no sign of overloading or thermal damage, and I have been told that the insulation thickness was doubled years after the installation was carried out. I personally don't think it represented a danger, I was just surprised that the cables had been protected by a 63A MCB. Even a 40A would have been too much for the cable under the original thinner insulation.

The only reason it's come to my attention is because each of the four apartments is having an 8.5kW electric shower installed on a 40A MCB. That 6mm T+E is going to have to come out. I'm thinking 16mm SWA.

It's the additional loading of 4 electric showers that made me concerned about the 100A supply fuse and 16mm2 SWA protecting it. I know that it's unlikely that all four showers will be used at the same time, and that they will not be on for very long when used; it's just whether or not the 100A supply fuse might end up blowing (the apartments house elderly people who don't react well to not having juice) and whether it'd pass inspection and testing.

 

[amlu]

yeah, with 4x electric shower it will need an upgrade...