Wiring exam question.

[adiewadie247]

This is a old question from C&G 2330 which I have answers to but the explanations given refer to the old IEE regs. (pre 17th edition) so I am not confident that the things I have done are correct. Any help would be gratefully received.

A 230v, 6 kW single phase load is supplied by single core pvc cables in high impact pvc conduit fixed to the surface of a brick wall over a distance of 20 metres. The circuit is protected by a type C circuit breaker. The temperature is 35 degrees Celsius.

Determine

a. the design current
b. the installation method
c. the circuit breaker rating
d. the correction factor
e. the minimum cable rating
f. the minimum cable size.

I have the design current as 26A, and reference method B, circuit breaker at 32A but then I get confused as I don't think I need a correction factor for reference method B in this situation...

 

[Paul.M]

Not going to tell you the answer so I'll just push you towards looking at the question again.

"ambient temp is 35C" ??? Hint hint.

 

[JUD]

Second ^^^^

 

[Brad]

Ca......

 

[Strima]

Forget this post, beers goggles kicking in, I didn't read the first post fully.

 

[RISElectrical]

The only way you wouldn't need a correction factor for the 35 degrees is if ref method b encorporated some kind of cooling system hint hint

 

[adiewadie247]

Well I think that for an ambient temperature of 35 degrees using 70 degree thermoplastic sheath the rating factor would be 0.94 which then gives me a minimum cable rating of 34A (32/0.94) and then using table 4D1A I need a 6mm2 csa as that is the smallest cable which is rated as larger than 34A.

 

[JUD]

If it is a fixed load and you don't need to provide overload protection (as with a socket circuit) then you can work your cable sizes out on the design current.

Iz = Ib ÷ (Ca x Cg x Ci x Cc) so Iz = 26 ÷ (0.94 x 1 x 1 x 1) = 27.7A. Single-core 4mm² has a CCC of 32A Ref Method B.

 

[adiewadie247]

I agree but the question as stated does provide a protection device so the calculation is

Iz = In ÷ (Ca x Cs x Cd x Ci x Cf x Cc ) = 32 ÷ (0.94 x 1 x 1 x 1 x 1 x 1) = 34.04A

 

[JUD]

I don't think you're with me.

For a fixed load such as an oven or a shower where you know they won't be overloaded (i.e. the protective device isn't providing overload protection, only fault protection) you can work your cable sizes on the design current.

A socket circuit, on the other hand needs overload protection as you have no idea what the load will be because it depends how much stuff is plugged in at one time.

So for a fixed load you can use Iz = Ib ÷ (Ca x Cg x Ci x Cc)
For circuits that require overload protection you would use Iz = In ÷ (Ca x Cg x Ci x Cc).

 

[adiewadie247]

I suspect that the issue here is the lack of detail in the question set. We have no knowledge whether the load is an oven or shower or some other type of load. The question states a type C circuit breaker which suggests to me that it might be a motor and that the designer has planned for tripping at 5 to 10 times full load current which sounds like an overload to me.

Thanks for your input. It has really forced me to thumb through my Big Green Book and think about the whole process ;-)

 

[Silly Sausage]

A 230v, 6 kW single phase load

I would take that as implying it is a fixed load.

 

[mickys86]

Give me ten minutes, I have this question in my school bag. I guarantee you this will come up in the exam where you will get 2 marks for each item. You need to know it

 

[mickys86]

I can pretty much guarantee you that this will be the question you will get. My tutor told us we will get this question and we did.
A 230v 6kw load is supplied via a bs1361 fuse using single core 70*c thermo plastic PVC cables in conduit over a distance of 20 metres. The ambient temperature is 35*c.

Determine:
1 design current
2 minimum fuse rating
3 correction factor value
4 minimum cable rating
5 minimum cable size
6 cable volt drop

 

[mickys86]

Not gonna do ya homework because no one will do mine but a few pointers.
1 Ib = design current how many amps is the circuit going to use?
2 In what size fuse is appropriate
3 correction factor read page 123/124 of the red osg (sorry dont have the green book yet)
4 read page 123
5 what cable complies with what you are trying to do
6 what is the actual volt drop?