Help with some questions

stekel · Jan 31, 2015

Hi everyone can anyone help me with some questions im stuck on please

1. Find the CSA of a single copper cable 500m long, which carries a current of 2.5 amperes and has a volt drop of 3.58 volts. Take the resistivity of copper to be 1.72 x 10[SUP]-8[/SUP] ohm/metre[SUP]3[/SUP].

1. An immersion heater takes a current of 13A and is fed through a twin cable, each core having a CSA of 2.5mm[SUP]2[/SUP]. If the conductors are made of copper and have a resistivity of 1.72 x 10[SUP]-8[/SUP] Ω/m[SUP]3[/SUP], calculate the greatest length of cable which may be used, if the cable voltage drop is not to exceed 11.5 volts.

1. A motor takes 45A from a 230v supply. A twin aluminium cable 40m in length feeds the motor, each core having a CSA of 16mm[SUP]2[/SUP]. Calculate the voltage at the motor terminals. Take the resistivity of aluminium to be 2.65 x 10[SUP]-8[/SUP] ohm/metre[SUP]3[/SUP].

Marvo · Jan 31, 2015

Now you have access I've moved this to the Trainee Forum for you.

stekel · Jan 31, 2015

Hi all. thanks marvo. I must say this site is amazing. THANKS

stekel · Jan 31, 2015

Any help with the above questions would be a great help

happysteve · Jan 31, 2015

Hi,

Generally, folk learn stuff easier if they have a punt themselves, so what are your initial thoughts?

Saying that, you're not helped by the fact that the units of resistivity (Greek symbol "rho", ρ) is actually Ω.m, not what is stated in your questions. (The value is correct, though.)

Ask yourself: do you know any equations that link resistivity, length, area, and resistance? And if you don't know one of those values, can you get it by using other information you're given in the question, and a really well known equation that all trainees should know?

Have a think, have a crack at it, show us your calculations, then I'm sure we'll be able to help some more.

pennychew · Jan 31, 2015

Theres a good book on electrical calculations that you can download from one of the sticky posts.

I'll get back to you with the name as i cant remember it at the moment but it helped me with the resistivity formulas

telectrix · Feb 1, 2015

you have 2 formulae to use here. 1 is in BS7671 and is the formula to use to calculate volt drop ( transpose as necessary), and the other is A =Πr²

stekel · Feb 1, 2015

Thanks for the help guys. okay ill give them a crack

1. R=V/I=3.58/2.5=1.432Ω A=Πxr²=3.142x1.432²=6.44A

stekel · Feb 1, 2015

Im not doing too well with question 2

stekel · Feb 1, 2015

3. R=V/I=230/45=5.1 V=IxR=45x5.1=229.5V

happysteve · Feb 1, 2015

telectrix said:
you have 2 formulae to use here. 1 is in BS7671 and is the formula to use to calculate volt drop ( transpose as necessary), and the other is A =Πr²

I don't think he needs the second one, Tel? (area of a circle).

stekel, you got the first bit spot on in post #8, you correctly used Ohm's Law to determine the resistance of the 500m cable - good stuff!

Now you need to relate resistance ("R") with resistivity ("ρ"), length ("l") and cross sectional area ("A"). You now know 3 of these values, you need to work out the 4th.

Good luck, let us know how you get on

stekel · Feb 1, 2015

Morning steve. Thanks for the imput. question 3 i have tried also my answer is on post 10#. any feedback on that would be great.

Thanks
Steve

telectrix · Feb 1, 2015

stekel said:
3. R=V/I=230/45=5.1 V=IxR=45x5.1=229.5V

no. you need to calculate R from the resistivity and the cable length. you have the resistivity in ohms/cu,m. this you need to convert to 16mm² for 1m, and then x the length.

Simonslimline · Feb 1, 2015

stekel said:
Hi everyone can anyone help me with some questions im stuck on please

1. Find the CSA of a single copper cable 500m long, which carries a current of 2.5 amperes and has a volt drop of 3.58 volts. Take the resistivity of copper to be 1.72 x 10[SUP]-8[/SUP] ohm/metre[SUP]3[/SUP].

1. An immersion heater takes a current of 13A and is fed through a twin cable, each core having a CSA of 2.5mm[SUP]2[/SUP]. If the conductors are made of copper and have a resistivity of 1.72 x 10[SUP]-8[/SUP] Ω/m[SUP]3[/SUP], calculate the greatest length of cable which may be used, if the cable voltage drop is not to exceed 11.5 volts.

1. A motor takes 45A from a 230v supply. A twin aluminium cable 40m in length feeds the motor, each core having a CSA of 16mm[SUP]2[/SUP]. Calculate the voltage at the motor terminals. Take the resistivity of aluminium to be 2.65 x 10[SUP]-8[/SUP] ohm/metre[SUP]3[/SUP].

Been long time since i did any of these so i am definitely rusty. I get 6.0mm2 for the 1st question. I am not 100% sure if this is correct though to be honest!