Volt Drop Calculation

[easymart]

Appendix 4 in the regs has tables for Current-Carrying Capacity and Voltage drop.

I've used these many times with no problems, but i've never had to calculate a cable higher than 25mm.
when you look at cables 25mm and higher you see three values for mV/A/m (r, x, & z).

Using the calulation

mV/A/m x Ib x L = VD
1000

Which value do I use for mV/A/m? r, x or z? What are the differences between the values? do I have to add them together or do another calculation first?

any help would be cool, cheers guys

 

[Mac]

Appendix 4 in the regs has tables for Current-Carrying Capacity and Voltage drop.

I've used these many times with no problems, but i've never had to calculate a cable higher than 25mm.
when you look at cables 25mm and higher you see three values for mV/A/m (r, x, & z).

Using the calulation

mV/A/m x Ib x L = VD
1000

Which value do I use for mV/A/m? r, x or z? What are the differences between the values? do I have to add them together or do another calculation first?

any help would be cool, cheers guys

Voltage Drop Calculator JavaScript
Hope this helps

 

[scotsparky]

ok got this written in my BRB.

R = Resistive load of cable
X =Reactive load of Cable
Z = Imedance of cable.


It start to get into the power factor correction of the cable its self not 100% sure but the Electrical installation Design guide page 58 says if the power factor of the load is not known use the impedance value of the cabe (z)

 

[easymart]

ok got this written in my BRB.

R = Resistive load of cable
X =Reactive load of Cable
Z = Imedance of cable.


It start to get into the power factor correction of the cable its self not 100% sure but the Electrical installation Design guide page 58 says if the power factor of the load is not known use the impedance value of the cabe (z)

Thats helped loads, thanks.
I've got the design guide, so the page number was great, had a read through the section and it all makes sense now, cheers.

 

[Gtbadboy1]

cable calculation

can anyone tell me how to do cable calculation

 

[scotsparky]

mV/A/m x Ib x L = VD
1000


mv/A/m- get info from the tables for cable type BRB appendix 4
ID is the circuit desighn current.
L lenght of run.


As you are using mv devide all by 1000 to get true volt drop.

 

[easymart]

Re: cable calculation

can anyone tell me how to do cable calculation

You first have to work out the design current (Ib) of the circuit, then you can choose the size of protective device (In) then you have to work out the size of cable (It) needed using the correction factors

Ib / Ca x Ci x Cg x Cr = It

Ca = correction factor for ambiant temp
Ci = correction factor for thermal insulation
Cg = grouping factor
Cr = correction factor for BS3036 fuse

Remember that It>In>Ib

you use the tables in appendix 4 to choose the type and size of cable to use ensuring that the current carrying capacity of the cable is higher than or equal to It.

you then check for voltage drop using the previously mentioned equation.

You'll need to use your regs for all the correction factors and current carying cappacities the cables.
Also check the regs to make sure my info is correct, this is all off the top of my head so i may have forgotten something, or got something wrong. I dont have mine to hand.
hope this helps.

 

[Gtbadboy1]

thanks mate