And another mad question ("minimum current")

[rich3009]

28. Which one of the following is not considered when calculating the minimum current carrying capacity of live
conductors of a cable?
a Ambient temperature.
b Grouping.
c Type of protective device.
d The maximum permitted volt drop.

 

[Amp David]

The term ‘calculating’ is the key here

 

[Wes1000]

Think about Ib<In<Iz and your derating factors
One of those answers isn’t used in those calculations.

 

[rich3009]

why minimum and not maximum

 

[Wes1000]

why minimum and not maximum

I assumed it’s a typo.

 

[rich3009]

in that case its volt drop which you would check by length having selected cable

 

[Wes1000]

That’s how I see it.

 

[rich3009]

Good stuff find device rating divide by factors, look up increased calculated current against correct table and check install method column choose nearest highest value. Then worry about volt drop

 

[Deleted member 26818]

Has anyone ever calculated the minimum CCC of a conductor?

 

[Deleted member 26818]

Type of protective device would have no bearing on the CCC of a cable.

 

[Wes1000]

Type of protective device would have no bearing on the CCC of a cable.

The way I read the question is you would select the cable on the size of protective device so it would have a bearing on it. Ib<In<Iz

 

[Deleted member 26818]

The way I read the question is you would select the cable on the size of protective device so it would have a bearing on it. Ib<In<Iz

So with a 20A radial, what effect would the 20A protective device have on the CCC of the cable used to wire the circuit?

Better still, consider designing a cooker or a shower circuit.
What is the first thing you need to know?

 

[Wes1000]

But ask yourself why do you put the 20A MCB on that radial??
First you need to know the total design current for the circuit. For arguments sake say 7A of lights on a radial. You would then choose your protective device based on this calculation. So say you choose a 10A 60898MCB, forgetting derating factors for now you would then choose a cable with a current carrying capacity higher than the protective device chosen.
The only thing not needed in the OP question for CCC calcs is VD

 

[Richard Burns]

You take a protective device of current rating C, the minimum current carrying capacity of a live conductor connected to this protective device is C (or 0.725C for semi enclosed fuses), however the cable is grouped with other cables so limiting the current that can be taken on the live conductors so now the minimum CCC is C/cg, but the cable is in a high ambient temperature so the minimum CCC of the cable is now C/cg*ca.
The minimum to which the current carrying capacity of the cable can be taken is C/cg*ca therefore you have calculated the minimum CCC of the live conductors by taking into account the type of protective device, the ambient temperature and the grouping.
The volt drop does not enter into the CCC calculation, it may be very relevant for compliance but not for current carrying capacity.

 

[Deleted member 26818]

Actually, thinking about it, there are correction factors for different types of protective devices.

 

[Wes1000]

I’ve read your post Richard but surely the very fact it is the current carrying CAPACITY implies a maximum. And the cables capacity must be above that of the protective device.
It just doesn’t work as a minimum?

You take a protective device of current rating C, the minimum current carrying capacity of a live conductor connected to this protective device is C

Surly if a protective device is C then the CCC of cable has to be above C but as for the minimum, it could be anything you want just so long as the protective device will trip before the maximum is reached.

Maybe we are getting at the same thing and I’m missing your meaning? Maybe it’s time for another glass of red. Now where did I put the other bottle.

 

[Deleted member 26818]

Whilst the rating of the protective device should be no greater than the CCC of the cable, the tripping current of the device should also not be greater than 1.45 times the CCC of the cable.

 

[Wes1000]

Too late I’ve opened the 2nd bottle now. This makes no sense any more.

 

[Richard Burns]

The current carrying capacity of the cable must be greater than or equal to C, e.g. if C is 10 then CCC could be 10, 20, 540, 2340. However the minimum value is 10. It cannot be lower than this.

 

[rich3009]

Ohhh I see so by minimum they are talking what's the minimum that will do the job, the wording is bad it should say calculate the minimum required capacity live conductor current for the circuit.

 

[Wes1000]

The current carrying capacity of the cable must be greater than or equal to C, e.g. if C is 10 then CCC could be 10, 20, 540, 2340. However the minimum value is 10. It cannot be lower than this.

Yes, on a sober mind this morning it all becomes clear. I completely agree.

 

[Wes1000]

@Amp David why do you disagree? You need to know the size of protective device so you can select a cable that will carry a current capable of tripping the device safely if your going to design a circuit.