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How do you apply de-rating factors to cable and protective devices?
If you had a de-rating factor of say 0.5 ??
If you had a de-rating factor of say 0.5 ??
Applying de-rating factors to cable and protective devices?
- Thread starter Newboy1996
- Start date
H
holmslaw
..
De-rating factors are applied to the rated current of your chosen protective device, not the cable.
Divide the rated current of the protective device (In) by any applicable rating factor for ambient temp (Ca), further divide for rating factor of thermal insulation (Ci) then further divide by the applicable rating factor for the type of protective device or installation condition (Cc).
Divide the rated current of the protective device (In) by any applicable rating factor for ambient temp (Ca), further divide for rating factor of thermal insulation (Ci) then further divide by the applicable rating factor for the type of protective device or installation condition (Cc).
H
holmslaw
...
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Seems to be a little disagreement here?? 
Given by BS7671:2008, page 256, section 5.1.
Seems to be a little disagreement here??
The text I typed above isn't far off word for word.
Come on now guys, it's very basic maths. You can re-arrange the formula depending on what you want to find out.
If you have a specific size of cable subject to a set of de-rating factors, you can determine the rating of the protective device needed to protect that cable installed in that situation.
More usually, you know what the max load on the circuit will be and therefore what size protective device you need, so you calculate the required size of cable for the protective device to protect it in that situation.
It's the same equation re-arranged and with a different variable as the unknown. The regs use one form, but you can easily get the other from it.
If you have a specific size of cable subject to a set of de-rating factors, you can determine the rating of the protective device needed to protect that cable installed in that situation.
More usually, you know what the max load on the circuit will be and therefore what size protective device you need, so you calculate the required size of cable for the protective device to protect it in that situation.
It's the same equation re-arranged and with a different variable as the unknown. The regs use one form, but you can easily get the other from it.
????
H
holmslaw
..
No, the rating of the protective device will stay the same.
Have you read section 5.1, p256 yet?
Have you read section 5.1, p256 yet?
H
holmslaw
..
Explain why this is the worst bit of advice ever given then, considering the very same is in BS7671:2008 and BS7671:2001.
H
holmslaw
..
As I said, I typed the method employed by BS7671:2008 where de-rating factors are applied against the rating of the protective device - not the cable.
It's not bad advice at all, in fact, it's not even advice but merely and extract from BS7671.
RCBO's all over again.....
It's not bad advice at all, in fact, it's not even advice but merely and extract from BS7671.
RCBO's all over again.....
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