cable calcs question

[hodaire]

i'm doing the 2391 course not sure about a way we have been shown with regards to rewirable fuses (not quite how i remember 20 years ago) heres the example -

7000 watt heater to be covered by a rewirable fuse

current is 30.43 amps, fuse will need to be 45a

now he has applied the correction factor to this (not to the design current)

so

45 / .725 = 62.07 amps, cable needs to be 10 mm (twin)

is this right? i always thought we applied fuse correction to design current

so

30.43 / .725 = 42 amps, still go with a 45 amp fuse but using 6mm cable

i know i'm probably wrong, just want to make sure

 

[kung]

i'm doing the 2391 course not sure about a way we have been shown with regards to rewirable fuses (not quite how i remember 20 years ago) heres the example -

7000 watt heater to be covered by a rewirable fuse

current is 30.43 amps, fuse will need to be 45a

Why 45a ? mabe 32a ?

 

[hodaire]

going with the sizes stated in the regs 30 or 45 amps

 

[Guest123]

Hi there.

so 7000W/230V gives you a current rating of - 30.43A

As it'sa re-wireable fuse to BS 3036 a correction factor of 0.725 must be applied.

The correction factor is applied to the CCC of the cable you intend using and the factor is mulitplied not divided.

I dont have ol' red to hand but you would need to find the CCC of 6mm and check it with the correction factor to start with.

Hope this helps.

 

[Spudnik]

32A re-wireable?

Maybe plug in MCB?

 

[baldsparkies]

Kung, he is applying correction factors to rewirable not mcb's, hence the jump from 30 to 45A.
Your calcs need to cover the cable in relation to the overcurrent device, not the design current of the connected load so the way factors are applied in your example are correct.That said, in the real world your open circuit voltage would be nearer 240v than 230v so with 7kw connected load you will still be above 230 and you only need to be around 233 to be within the fuse rating of 30A. Its almost a trick question when compared with real world scenario, but then they know that, thats why they applied such close call figures.

 

[Guest123]

I also found this -


"If the circuit concerned is protected by a semi-enclosed (rewirable) fuse the cable size will need to be larger to allow for the fact that such fuses are not so certain in operation as are cartridge fuses or circuit breakers. The fuse rating must never be greater than 0.725 times the current carrying capacity of the lowest-rated conductor protected."

"In effect, this is the same as applying a correction factor of 0.725 to all circuits protected by semi-enclosed (rewirable) fuses. The ambient temperature correction factors of {Table 4.3} are larger than those for other protective devices to take this into account."

 

[kung]

Sorry bald thought it was bs88 re-wirables ! gave up smoking 5 weeks ago after 35yrs of smoking ! heads all fuzzy at the mo lol.
Sorry m8.

 

[baldsparkies]

Sorry bald thought it was bs88 re-wirables ! gave up smoking 5 weeks ago after 35yrs of smoking ! heads all fuzzy at the mo lol.
Sorry m8.

Lost my hair when I gave up smoking mate

 

[hodaire]

Hi there.

so 7000W/230V gives you a current rating of - 30.43A

As it'sa re-wireable fuse to BS 3036 a correction factor of 0.725 must be applied.

The correction factor is applied to the CCC of the cable you intend using and the factor is mulitplied not divided.

I dont have ol' red to hand but you would need to find the CCC of 6mm and check it with the correction factor to start with.

Hope this helps.

multiplying the ccc by .725 or dividing the design current by .725 would surely achieve the same results, suppose its which way round you do your calcs

the question was do you apply the correction factor to design current or breaker rating

can see me getting more confused here

 

[baldsparkies]

multiplying the ccc by .725 or dividing the design current by .725 would surely achieve the same results, suppose its which way round you do your calcs

the question was do you apply the correction factor to design current or breaker rating

can see me getting more confused here

Overcurrent device (Its not a breaker, its a rewirable fuse)

Your applying the correction factors in relation to whats protecting the cable, not the design current of the connected load, hope this helps.

 

[Guest123]

The fuse rating must never be greater than 0.725 times the current carrying capacity of the lowest-rated conductor protected

Fuse rating.

 

[hodaire]

so the correction factor is applied to the breaker rating of the fuse

thanks for your help

 

[baldsparkies]

so the correction factor is applied to the breaker rating of the fuse

thanks for your help

Spot on, the fuse rating of the overcurrent devices breaker thingy