Relocating Consumer Unit to new location

richardg · Mar 15, 2021

A builder has asked for a quote for rewiring a domestic property all very straight forward until he mentioned relocating the consumer unit to another part of the house with a run of 10m to the new location. TNC-S at orgin.

Looking at options to make this as painless as possible. Thoughts on the different solutions?

Option 1 (no mechanical protection of cable run, in plaster and under floor boards)
Small metal clad unit with DP 100A main switch, 80A 100maRCD 2 pole with 16mm Double insulated meter tails to new consumer unit. Separate 16mm earth. Current rating 87A Consumer unit fitted with RCBO type A

Option 2
Small metal clad unit DP 100A main switch, 80A MCB with 16mm 3 core SWA -banjo on supply end, Current rating 94A. Consumer unit fitted with RCBO type A

So far these are the 2 options i'm looking at to provide necessary protection to the extended tails -
are there any options to use 25mm cable as I don't think 25mm fits into most MCBs and neutral bars unless you know of a brand that would.

Many thanks

pc1966 · Mar 16, 2021

Before heading off for the night, was going to add a last comment that if you ever find no reasonable option other than a MCB feeding a sub-main, say it is coming out of an existing DB or similar, then you generally want the supply MCB to have as high a trip point as you can get away with.

Typically that means going to a D-curve breaker and as high a current as you can meet either both:

sub-main overload protection (if load not otherwise limited)
end of sub-main Zs for 5s disconnection

Appendix B of the on-site guide is your friend here, as you can see 5s Zs for MCBs on page 131, or for a fused switch on pages 128 (industrial BS88-2 style fuses) or 129 (domestic style fuses BS88-3 similar to BS1361 on following page). Fuses are usually better as well for this, for example, some Zs values:

63A BS88-2 fuse = 0.62 ohm
63A BS88-3 fuse = 0.55 ohm
63A D-curve MCB = 0.28 ohm

You don't see 5s values for C or B curve MCBs as they reach the "instant" trip before the thermal curve gets down to 5s. While selectivity varies a lot from make to make and model to model, taking a couple of examples from the Hager commercial catalogue Jan 2020 that I happen to have on hand:

63A BS88 fuse feeding 32A B-curve RCBO fault limit = 1.5kA (page 133)
63A D-curve MCB feeding 32A B-curve RCBO = 0.80kA (page 136)
63A C-curve MCB feeding 32A B-curve RCBO = 0.51kA
63A B-curve MCB feeding 32A B-curve RCBO = 0.27kA

So you see the fuse has both twice the selectivity of the D-curve MCB and usable with double the Zs!

Also you see for the B and to some extent C-curve MCB feeds the fault level needed to trip it is easily managed by a typical RFC or modest length radial circuit.

richardg · Mar 18, 2021

pc1966 said:
Before heading off for the night, was going to add a last comment that if you ever find no reasonable option other than a MCB feeding a sub-main, say it is coming out of an existing DB or similar, then you generally want the supply MCB to have as high a trip point as you can get away with.

Typically that means going to a D-curve breaker and as high a current as you can meet either both:

sub-main overload protection (if load not otherwise limited)

end of sub-main Zs for 5s disconnection

Appendix B of the on-site guide is your friend here, as you can see 5s Zs for MCBs on page 131, or for a fused switch on pages 128 (industrial BS88-2 style fuses) or 129 (domestic style fuses BS88-3 similar to BS1361 on following page). Fuses are usually better as well for this, for example, some Zs values:

63A BS88-2 fuse = 0.62 ohm

63A BS88-3 fuse = 0.55 ohm

63A D-curve MCB = 0.28 ohm

You don't see 5s values for C or B curve MCBs as they reach the "instant" trip before the thermal curve gets down to 5s. While selectivity varies a lot from make to make and model to model, taking a couple of examples from the Hager commercial catalogue Jan 2020 that I happen to have on hand:

63A BS88 fuse feeding 32A B-curve RCBO fault limit = 1.5kA (page 133)

63A D-curve MCB feeding 32A B-curve RCBO = 0.80kA (page 136)

63A C-curve MCB feeding 32A B-curve RCBO = 0.51kA

63A B-curve MCB feeding 32A B-curve RCBO = 0.27kA

So you see the fuse has both twice the selectivity of the D-curve MCB and usable with double the Zs!

Also you see for the B and to some extent C-curve MCB feeds the fault level needed to trip it is easily managed by a typical RFC or modest length radial circuit.

Thank you for the very in depth answer appreciated ?