Multiple CUs with 25mm and 16mm tails. RCD specs

Robbot · 31 Oct 2025

I have 100amp DNO fuse feeding three CU's.

One of the CU's is fed with 16mm tails and contains an 80 amp RCD. The tails are around 1.5m in length and go through the cavity wall with batten insulation. Do I have a potential issue with this set up, or should the 80 amp RCD be replaced with a 60 amp item?

AndyPRK · 31 Oct 2025

RCDs don’t limit current, so that’s not going to make a difference

pete01 · 31 Oct 2025

Do you mean fuse not rcd?

trojanhawrs · 31 Oct 2025

If it is indeed a RCD (people seem to use that to refer to any consumer unit switch) 80A just means it can handle 80 amps safely.

Tails going through cavity walls is debatable, if it's straight through I wouldn't be concerned.

Murdochcat · 31 Oct 2025

You are worrying about a problem you don’t have

Robbot · 1 Nov 2025

pete01 said:
Do you mean fuse not rcd?

The fuse in the meter box is 100 amp, the RCD in the CU fed by 16mm tails is 80amp. I should add that the other two CUs are fed by 25mm tails and have 63amp RCDs. Hence why I asked.

But looking through the posts, its looks nothing to worry about

pete01 · 1 Nov 2025

Well it's not a major worry but 16mm tails shouldnt be on a 100 amp fuse.

Murdochcat · 1 Nov 2025

pete01 said:
Well it's not a major worry but 16mm tails shouldnt be on a 100 amp fuse.

Not totally sure I agree with that. The OP has 3 x CUs and as long as the CU the 16mm tails are feeding has a load of less than 80A I’d say it’s ok.

A single CU would be a different matter

pete01 · 1 Nov 2025

Murdochcat said:
Not totally sure I agree with that. The OP has 3 x CUs and as long as the CU the 16mm tails are feeding has a load of less than 80A I’d say it’s ok.

A single CU would be a different matter

Yes I agree

Robbot · 1 Nov 2025

Murdochcat said:
Not totally sure I agree with that. The OP has 3 x CUs and as long as the CU the 16mm tails are feeding has a load of less than 80A I’d say it’s ok.

A single CU would be a different matter

It feeds an EV charger and the garage. Maximum draw on a high use day would be circa 55amps at most. Average around 32amps.

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Multiple CUs with 25mm and 16mm tails. RCD specs

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