Calculating It

[JohnW2]

I've now got;
It - 32/(1*1*0.78*1) It - 41A So It > In - All Good?
But then using table 4D1A And Reference Method B - It has to be 6/10mm cable - Obviously not right.

For a start, assuming you are using T+E cable (not singles) you should be using either 4D2A or, better (since it's slightly kinder!) 4D5!

Is there a rule for sizing cables in a RFC? Two legs, reduction in CSA for instance? (I'm really not sure and struggling with this! )

Yes. You're really doing this back-to-front. The requirement (per 433.1.204) that for a ring final protected by an MCB with an In of 32A, the CCC of the cable should be at least 20A. By my reckoning, if it's Method B and you have a de-rating factor of 0.78, then 4mm ² cable would be fine (and 2.5mm² "would not be far off" - not that that is much consolation!). That 'dispensation' applies only to ring finals. For any other (radial) circuit, the CCC of cable in a circuit protected by a 32A MCB has to be at least 32A, not 20A.

KInd Regards, John

 

[TommyB123]

For a start, assuming you are using T+E cable (not singles) you should be using either 4D2A or, better (since it's slightly kinder!) 4D5!

Yes - My Mistake!

Yes. You're really doing this back-to-front. The requirement (per 433.1.204) that for a ring final protected by an MCB with an In of 32A, the CCC of the cable should be at least 20A. By my reckoning, if it's Method B and you have a de-rating factor of 0.78, then 4mm ² cable would be fine (and 2.5mm² "would not be far off" - not that that is much consolation!). That 'dispensation' applies only to ring finals. For any other (radial) circuit, the CCC of cable in a circuit protected by a 32A MCB has to be at least 32A, not 20A.

KInd Regards, John

Sorry John , Where am i going back-to-front? Should i be trying to work out It at all? Or just using the values given in 4D5 and taking the suitability of the cable from there?

So Iz = It*Ca*Ci*Cg*Cf

Iz =21*1*0.78*1*1 (Assuming It = the value given in 4D5 for 2.5mm Reference method 102)

Iz = 24.96

Which would then confirm 2.5mm in this situation would suitable for a circuit with In of 32A?

 

[ban-all-sheds]

For any other (radial) circuit, the CCC of cable in a circuit protected by a 32A MCB has to be at least 32A, not 20A.

Not necessarily.

 

[TommyB123]

The requirement (per 433.1.204) that for a ring final protected by an MCB with an In of 32A, the CCC of the cable should be at least 20A.

I can't find reference to 433.1.204 anywhere in the regs?

433.1.103 states -"Ring final circuits protected by a 30 or 32A CPD are deemed to meet the requirements of Reg 433.1.1 If the CCC (Iz) of the cable is not less than 20A and if under the intended load conditions of use, the load current in any part of the circuit is unlikley to exceed for long periods of time the CCC(Iz) of the cable"

Meaning my last calc (If it is correct!) would allow the use of 2.5mm?

 

[JohnW2]

Sorry John , Where am i going back-to-front? Should i be trying to work out It at all? Or just using the values given in 4D5 and taking the suitability of the cable from there?

Maybe "back-to-front" was not quite the right phrase. What I was getting at was that, for a ring final circuit with an OPD of In=32A, what you need to do is ensure that the CCC of the cable, after allowing for installation method and any/all other 'de-rating factors',is at least 20A (thereby satisfying 433.1.204 {aka 433.1.101 if you don't have Amd3}).

So Iz = It*Ca*Ci*Cg*Cf
Iz =21*1*0.78*1*1 (Assuming It = the value given in 4D5 for 2.5mm Reference method 102) .... Iz = 24.96 ... Which would then confirm 2.5mm in this situation would suitable for a circuit with In of 32A?

I don't understand your arithmetic - 21x0.78 is surely 16.38, not 24.96, thereby (since 16.48 is less than 20) demonstrating that 2.5mm² cable would not be adequate. IF Method 102 is applicable, and IF that 0.78 is your only 'de-rating factor', then you would have to use 4mm² cable (It=27A, 27x0.78=21.06, which is greater than 20).

Remember that all this only applies because it is a ring circuit. If it were a radial sockets circuit with a 32A MCB, Iz would have to be at least 32A - which, with Method 102 and a de-rating of 0.78 would require a cable of at least 10mm² !!

Kind Regards, John

 

[ban-all-sheds]

I can't find reference to 433.1.204 anywhere in the regs?

Your copy is out of date.

 

[JohnW2]

I can't find reference to 433.1.204 anywhere in the regs? 433.1.103 states -"Ring final circuits protected by a 30 or 32A CPD are deemed to meet the requirements of Reg 433.1.1 If the CCC (Iz) of the cable is not less than 20A and if under the intended load conditions of use, the load current in any part of the circuit is unlikley to exceed for long periods of time the CCC(Iz) of the cable"

Indeed. 433.1.103 in the 'green book' (Amendment 1) has become 433.1.204 in the 'yellow' one (Amendment 3, published 5th Jan 2015) - but the wording hasn't changed.

Meaning my last calc (If it is correct!) would allow the use of 2.5mm?

It would, IF your calculation were correct - but, as I've just written, I don't think it is With your parameters, I believe you would need 4mm² cable.

Kind Regards, John

 

[JohnW2]

For any other (radial) circuit, the CCC of cable in a circuit protected by a 32A MCB has to be at least 32A, not 20A.

Not necessarily.

I presume you are thinking of circuits for which it can be argued that overload protection is not required, If it is required, then any circuit (other than a ring final) protected by a 32A OPD requires a cable CCC of at least 32A, doesn't it?

Kind Regards, John

 

[ban-all-sheds]

I presume you are thinking of circuits for which it can be argued that overload protection is not required,

You presume correctly.

If it is required, then any circuit (other than a ring final) protected by a 32A OPD requires a cable CCC of at least 32A, doesn't it?

If it is, yes.

But it might not, which is why I said "not necessarily".

 

[JohnW2]

I presume you are thinking of circuits for which it can be argued that overload protection is not required,

You presume correctly.

If it is required, then any circuit (other than a ring final) protected by a 32A OPD requires a cable CCC of at least 32A, doesn't it?

If it is, yes. But it might not, which is why I said "not necessarily".

No-one could fault the correctness of what you say.

In reality, I think it would be extremely unusual to find a circuit in 'fixed' domestic wiring in which the designer had invoked the "overload protection not required" argument. AFAIAA, that argument is usually only invoked in relation to bits of flex connecting that 'fixed wiring' to an appliance.

Kind Regards, John

 

[TommyB123]

In a situation where the cables were to be run between a ceiling and floorboards of the next floor, where no insulation is present, would a de rating factor be applied?

 

[JohnW2]

Ha! - Yes i seemed to have c*cked up there! ... Fair enough, my circuit needs a 4mm cable. Am i right in thinking that unless there are no de-rating factors 2.5mm rings dont follow the regs?!

Well, if (as is most commonly the case) it is Method C (CCC=27A), then you can have up to about 0.74 worth of de-rating factors and still have an Iz of at least 20A.

Kind Regards, John

 

[JohnW2]

In a situation where the cables were to be run between a ceiling and floorboards of the next floor, where no insulation is present, would a de rating factor be applied?

Installed by what method between floor and ceiling? For example, if it were buried in plaster/masonry ('clipped direct', Method C), then 2.5mm² cable would have a CCC of 27A, hence 21.06A after application of your 0.78 de-rating factor. Since 21.06A is greater than 20A, in that situation 2.5mm² cable would be adequate for a ring final protected by a 32A MCB.

Kind Regards, John