Help with CPC size

[Gigsy]

Hi I am training to become an electrician. I have a problem to solve to work out the size of the CPC. My calculations say I would use 6mm and 1mm CPC. This can't be right, can it?

Please refer to attached document for details.

View attachment 35629

View attachment 35630

 

[Dave OCD]

You may be overthinking this as it seems the question is asking for minimum cable size and as clipped direct it will be T&E, therefore having determined that a 6mm T&E is adequate the 2.5mm integral CPC will be adequate also as with the low Ze and type of OCPD deployed automatic disconnection under fault conditions will be achieved well within 'spec'.

 

[Wilko]

I think it was a good pick by Sintra at #4 ... Using the graphic supplied we can only go to 0.1 sec. As Risteard says in #8 it will likely discon quicker, but it varies by manufacturer and mcb model. And using 0.1 sec leads us to the practical cpc of 2.5mm answer too

Reading the report, a couple of suggestions :

• If Ib is 36.96A and In >= Ib, then In can't be 36A ...
• The non impacting correction factors aren't 0, they must be ...

Hope that helps, and good luck Gigsy

 

[Gigsy]

You may be overthinking this as it seems the question is asking for minimum cable size and as clipped direct it will be T&E, therefore having determined that a 6mm T&E is adequate the 2.5mm integral CPC will be adequate also as with the low Ze and type of OCPD deployed automatic disconnection under fault conditions will be achieved well within 'spec'.

You could be right, I may have over thought it. Still it was a good exercise I did learn allot.

 

[Gigsy]

I think it was a good pick by Sintra at #4 ... Using the graphic supplied we can only go to 0.1 sec. As Risteard says in #8 it will likely discon quicker, but it varies by manufacturer and mcb model. And using 0.1 sec leads us to the practical cpc of 2.5mm answer too

Reading the report, a couple of suggestions :

• If Ib is 36.96A and In >= Ib, then In can't be 36A ...
• The non impacting correction factors aren't 0, they must be ...

Hope that helps, and good luck Gigsy

You are right, the protective device must be greater than the current flowing. Ib has to be 37. There is no 37 breaker so I guess it should be 40?

Thank you for pointing this out

 

[Gigsy]

I think it was a good pick by Sintra at #4 ... Using the graphic supplied we can only go to 0.1 sec. As Risteard says in #8 it will likely discon quicker, but it varies by manufacturer and mcb model. And using 0.1 sec leads us to the practical cpc of 2.5mm answer too

Reading the report, a couple of suggestions :

• If Ib is 36.96A and In >= Ib, then In can't be 36A ...
• The non impacting correction factors aren't 0, they must be ...

Hope that helps, and good luck Gigsy

The none impacting factors must be.....
Must be what?

The graphic supplied does goto 0.01, I drew a green line to indicate the value I was using, I may have covered over the writing with the green line.

 

[Gigsy]

You may be overthinking this as it seems the question is asking for minimum cable size and as clipped direct it will be T&E, therefore having determined that a 6mm T&E is adequate the 2.5mm integral CPC will be adequate also as with the low Ze and type of OCPD deployed automatic disconnection under fault conditions will be achieved well within 'spec'.

Also I think every electrician would just use 10mm2 cable anyway. That way there is room for a future upgrade to maybe a 9kw device

 

[Wilko]

Think you'll find the fat black lines stop at 0.1 sec, with the little box telling us to consult manufacturers data just below.
One is the loneliest number that I ever knew (a song ...)

 

[Gigsy]

I think it was a good pick by Sintra at #4 ... Using the graphic supplied we can only go to 0.1 sec. As Risteard says in #8 it will likely discon quicker, but it varies by manufacturer and mcb model. And using 0.1 sec leads us to the practical cpc of 2.5mm answer too

Reading the report, a couple of suggestions :

• If Ib is 36.96A and In >= Ib, then In can't be 36A ...
• The non impacting correction factors aren't 0, they must be ...

Hope that helps, and good luck Gigsy

I get it now, anything times 0 is 0. I guess you just don't factor them in or they must be 1?

 

[Gigsy]

Think you'll find the fat black lines stop at 0.1 sec, with the little box telling us to consult manufacturers data just below.
One is the loneliest number that I ever knew (a song ...)

You have a sharp eye for detail.

 

[DavidSpark]

Hey!

I noticed the question alludes to the cable being T+E which I'm sure BS 7671 has listed in Table 4D5 with all the relevant reference methods. It seems you have used the values from Table 4D2A.

If I have given any incorrect information, someone please correct me!

 

[Gigsy]

Hey!

I noticed the question alludes to the cable being T+E which I'm sure BS 7671 has listed in Table 4D5 with all the relevant reference methods. It seems you have used the values from Table 4D2A.

If I have given any incorrect information, someone please correct me!

I don't think I could use a 2.5mm CPC so, I used table 4D2A, 2 core cable.

There is not much difference anyway and it does not change my results. 47A flat cable with CPC and 46A 2 core cable.

 

[Gigsy]

I think I have completed the question. Can anyone find fault with this?

 

[DavidSpark]

In is the rated current or current setting of protective device, so that would be 40A BS EN 60898. Therefore your tabulated calculation is slightly off as it uses 37A as opposed to 40A. You could use Ib in this calculation if overload protection was NOT required.

The picture highlighting your first R1+R2 values also show 6mm plus 2mm, however your calculations take the values of 6mm and 4mm.

The final calculation for your S value also does not quite add up either.

 

[Dave OCD]

On page 3 you've circled 2.5 in green mate when working out suitable CPC but quoted the resistance value for 4mm.

 

[Gigsy]

In is the rated current or current setting of protective device, so that would be 40A BS EN 60898. Therefore your tabulated calculation is slightly off as it uses 37A as opposed to 40A. You could use Ib in this calculation if overload protection was NOT required.

The picture highlighting your first R1+R2 values also show 6mm plus 2mm, however your calculations take the values of 6mm and 4mm.

The final calculation for your S value also does not quite add up either.

Thank you, I think I have corrected the errors, apart from final S value, it comes out the same every time?