cable calculations exercise 2

[markymark2406]

design current Ib = ______________

an 8kw load at 230v is required to be fed by a pvcswa cable on perforated cable tray touching other cables in a single layer, calculate:



overcurrent protective device . type:- _____ rating:-____

method of installation:-______________________

Current rating table:- _______ column:-________

correction factors (if any) Ca:-____ Cg:-___ Ci:-___

minimum current rating of cable= ___ = ____

.
. . cable size:- _____ mm2

 

[RF Lighting]

Which bits have you worked out so far?

 

[Taylortwocities]

Here's a first clue.

if P = V x I then
I = P / V

you know P (its in the question) you know V (its what comes out of the cable). So, what is Ib ?

 

[ban-all-sheds]

im starting 17th edition training friday

Which means you are supposed to be doing the learning, you are supposed to understand these questions and you are supposed to be able to do them yourself to show that you do understand it and can do the calculations.

Handing in things which have been filled in by people here is really not the way to go about it - if you're struggling then your lecturer needs to know.

I am not that good at working out formulas and the math so will be posting a few things here for some help in the next couple of days.

But you've chosen a career where you need to be good at it.....

 

[Taylortwocities]

You will not need to do a cable calculation in the 17th edition exam.
You'll need to know where the current carrying capacity table is and how to read it though.

 

[ban-all-sheds]

You will not need to do a cable calculation in the 17th edition exam.

You don't have to show that you recognise, can find, and can use correction factors?

 

[Taylortwocities]

Yes, in the regs exam you'll need to know where the relavent information is but you will not be expected to do the calculations. There isnt time.

You will be expected to do cable calculations if you are doing 2330 or 2391 exams. Marky is talking specifically about the 17th edition.

Here's a sample of the type of cable calc questions he would face:

Untitled

• Taylortwocities
• 11 Oct 2008
• 1

You could argue that he would be better off knowing all about cable calcs to answer the questions fully but its a mickey mouse multiple guess exam.

 

[ban-all-sheds]

I could answer 60 questions like that in about 20-30 mins....

 

[jockspark]

done my 17th 60 questions 32 mins looked in the regs book once

 

[Taylortwocities]

I could answer 60 questions like that in about 20-30 mins....

And that is why Marky doesn't need to know all about cable calcs just for his 17th regs exam.

He'd getter a better appreciation iof what's going on if he did though....

 

[ColJack]

and yet in the mock exams I've been given, and in the book, there are volt drop and cable CSA questions which require calculation..

 

[trey]

What are the answers to the questions in that image (up the thread). Are they D, C, B ?

 

[ColJack]

no, D,D,B

Q: A multicore 70°C thermoplastic cable with 2.5mm² conductors supplies a single phase load of 20A at 230V a.c. over a distance of 22 meters. The voltage drop in the cable will be?

A. 6V
B. 6.6V
C. 7.92V
D. 9.2V

Q: A single phase load of 13A is supplied via a 15A BS1361 fuse using a single-core 70°C thermoplastic copper cables installed to method 4 ( reference method B ). The rating factor for grouping is 0.7 and for ambient temperature 0.87, and overload protection is to be provided. The minimum acceptable size cable would be?

A. 1mm²
B. 1.5mm²
C. 2.5mm²
D 4mm²

 

[jockspark]

hi i got 2 calcs on my test my tutor got 5 on his some of the guys got none
it would be better if he understood the calcs but could easy pass without
before anybody jumps on me i am not saying thats a good thing

 

[ban-all-sheds]

no, D,D,B

According to my copy of BS 7671 it's D,D,C....