Working out the length of a ring finals circuit

[timhoward]

Regulation 433.1.204 seems kind of dumb where it states the 30 / 32 amps on a rfc

I don't disagree. Bring back the 14th edition!

So would a ring on 20amps be coded as a c3 or a c2 seems hard to justify if its safe but not a generic circuit design

My opinion: If otherwise compliant and appearing fit for purpose, C3 if I thought there was no reason it couldn't be 32amp, otherwise I might just put a Note to document that I'd noticed it.

 

[Tetris1985]

I don't disagree. Bring back the 14th edition!
View attachment 116556

My opinion: If otherwise compliant and appearing fit for purpose, C3 if I thought there was no reason it couldn't be 32amp, otherwise I might just put a Note to document that I'd noticed it.

So in this case where it is compliant on a 20 amp, compliant in terms or ZS and VD but not reg 433.1.204 it would be a c3.

Would it be a c3 or c2 if it was on 32 amps complied with ZS but VD was 7.5% in your opinion?

 

[timhoward]

Would it be a c3 or c2 if it was on 32 amps complied with ZS but VD was 7.5% in your opinion?

I don't think there's a generic answer. We're not robots! EICRs are a judgement on safety and risk based on experience (I'm very much still learning and will be until I retire). It would depend on how it's being used or is likely to be used, as discussed above.

 

[Tetris1985]

I don't think there's a generic answer. We're not robots! EICRs are a judgement on safety and risk based on experience (I'm very much still learning and will be until I retire). It would depend on how it's being used or is likely to be used, as discussed above.

Ah that Makes sense, I would of thought it would be cut and dry with regards to exceeding VD, that's what im struggling with

....for a RFC is the design current always 32 or (26) or could you have a design current of 25 but still have it on a 32 amp mcb or rcbo?

 

[timhoward]

For clarity, assessing an existing installation is a very different ball game to designing, installing and initial verification of a new installation - in the latter case I would ensure VD rules are met.
Your questions keep flipping between the two scenarios!
Just because you judge something safe on an EICR doesn’t mean you’d copy it!

 

[westward10]

For an EICR you are not expected to verify volt drop however if you do suspect it is detrimental to the circuit then it could be included but it is likely to be an extreme case.

 

[timhoward]

The other thing - does it seem likely there is 155m of cable from the CU to these bedrooms and back?
What sort of property is this?
Was it a calibrated tester you used?

 

[pc1966]

Regulation 433.1.204 seems kind of dumb where it states the 30 / 32 amps on a rfc

Being more specific that rule is an exception to the usual case where the protection against overload has to match the cable. In the RFC case you are allowed 50% over due to the two 'legs' of the ring largely sharing the current but they pin it down to this case.

The earlier regs allowed this 50% margin (or the cable CCC is 67% of OCPD) as a general rule, later it became an RFC is 30/32A but I suspect the real reason for changing it was to prevent "creative" designs of a 45A ring on 4mm cable, etc, being created.

dropping the mcb rating does not make it less safe, does it? I mean you can get 25amp mcb now

I feel like it should mean you can use 32 amps on a ring wired in 2.5mm even though the cable is rated lower than 32 amps

So would a ring on 20amps be coded as a c3 or a c2 seems hard to justify if its safe but not a generic circuit design

No, there is nothing unsafe about a lower MCB and it is something that is not too uncommon as I have seen a couple of RFC that were on 20A rewirable BS3036 fuses. I think in some cases if the supply Ze was originally high then it was to meet ADS times on the fuses.

there is like four sockets per room i think

It seems no real risk of VD being a problem then!

 

[davesparks]

Regulation 433.1.204 seems kind of dumb where it states the 30 / 32 amps on a rfc

Why?

dropping the mcb rating does not make it less safe, does it?

No, it just makes the ring pointless, it might as well be a radial circuit if you're using a lower rated MCB.

So would a ring on 20amps be coded as a c3 or a c2 seems hard to justify if its safe but not a generic circuit design

It wouldn't be coded anything as it is not unsafe and there is no safety improvement to be recommended.

 

[davesparks]

So in this case where it is compliant on a 20 amp, compliant in terms or ZS and VD but not reg 433.1.204 it would be a c3.

No, EICRs are not assessing absolute compliance with BS7671, they are only concerned with safety for continued use. On an EICR this would not be mentioned at all.

Would it be a c3 or c2 if it was on 32 amps complied with ZS but VD was 7.5% in your opinion?

Neither, volt drop on final circuits is not something you'd normally consider on an EICR.

 

[davesparks]

Ah that Makes sense, I would of thought it would be cut and dry with regards to exceeding VD, that's what im struggling with

It is cut and dry, you design circuits which comply with volt-drop limits to ensure that they function correctly.

On an existing installation you'd only assess volt-drop when planning additions to the installation or when investigating problems in the installation.

 

[Pretty Mouth]

you would not be concerned with the at 32 amps?
is that because the VD is only a few percent over ? or becasue you doubt they would ever be loaded that much to reach 32 amps?

A combination of all. Here's how I look at it:

A supply voltage of 216V (lower limit that is permitted to be supplied) supplying a circuit with a 5% voltage drop, gives a voltage of 205.2V at its furthest point when under load - quite low. This is perfectly acceptable according to regulation.

A supply voltage of 245V (common in my experience, much more likely to come across this than 230V) supplying a circuit with a 10% voltage drop gives a voltage of 220.5V at its furthest point under load. Twice the permitted voltage drop as a percentage so non-compliant with regs, yet the working voltage is 15V higher than the previous, compliant example, and is much closer to the 230V the equipment is designed for.

So the working voltage in the 2nd example is 'better' than that in the 1st, despite it being non-compliant.

The downside to the second example is there will be more power wasted as heat through the cables, when under load. For a circuit that is expected to be fully loaded for long periods of time (eg. EV charger), this would be a costly, wasteful problem. However, the average ring final is unlikely to be fully loaded often, if ever. Even if it is, it is unlikely that it will be fully loaded for any length of time, nor is it likely that the bulk of the load will be concentrated at the mid-point of the circuit.

In short, it's unlikely to happen, and if it does, the consequences are likely to be negligible. I'd say the cost of rewiring the circuits will probably outstrip any savings made due to reduced cable losses, many times over throughout the life of the circuit.