Working out the length of a ring finals circuit

[Tetris1985]

r1: 1.15
r2: 1.80
rn: 1.15
r1r2 0.69

To work out the length of a ring in 2.5mm twin would this be correct

0.69/(19.51/1000) = 35m

or would you use the r1r2 before you divide it by 4?

 

[timhoward]

why cant you comply with the regs if a ring is not on 30 or 32 amps seems strange

In brief, a ring final circut is an exception to the normal rule than the cable can carry at least as much current as the protective device.
The one and only reg (look up 433.1.204) sets out the conditions you can break the normal rule, and part of those conditions are a 30 or 32 amp device.
It's basically saying that if the load current is unlikely to exceed the CCC of the cable for long periods, and the cable can carry 20 amps, it is deemed to comply.

(Much older regs versions had more flexibility with formula to apply the same idea to a wider range of circuits, but those days are long gone)

 

[nicebutdim]

Regulation 433.1.204

Edit: too slow.

 

[timhoward]

I have started working with my dad who is QS and old spark and he brought a new property and has let me test it to get some real world experience before he tests its. ( its empty I cant hurt anyone )

That sounds like a great and fun opportunity - real world testing is certainly different to the college rig!

im just asking realistically in the real world is that a problem where you would rewire in 4mm

In the real world I'd be looking at the actual loadings. If I was very worried, I'd probably be looking for a way to get a distribution circuit with very low resistance nearer the bedrooms and supply the bedroom circuits from there.
I doubt this would actually be needed though.

 

[Tetris1985]

That sounds like a great and fun opportunity - real world testing is certainly different to the college rig!

In the real world I'd be looking at the actual loadings. If I was very worried, I'd probably be looking for a way to get a distribution circuit with very low resistance nearer the bedrooms and supply the bedroom circuits from there.
I doubt this would actually be needed though.

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

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

there is like four sockets per room i think

 

[Tetris1985]

The breaker is there for overload and fault protection. The only relation the breaker has with design current, is that the design current should be no greater than its rating. Lowering the breaker rating doesn't lower the design current however - you design the circuit and protective device(s) based on the design current, not the other way round.

In this case, you're dealing with existing circuits. @pc1966 and @SparkyChick have covered this nicely in the last few posts, the only things I'll add are:

It is unlikely that the circuits will be heavily loaded for any length of time, if ever.
Even if they are, the supply voltage, in my experience, usually measures 240V or more, which is a large margin of error over the lower acceptable limit of 216V.

Personally, I wouldn't be concerned with these circuits.

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?

 

[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.