How to determine Nominal current and size of cables?

[nazoom]

Hi

I am trying to determine the nominal setting for protection, I have two question:

After calculating Design current (Ib) which table and on what page of IEE do i use to find the Nominal current (In)? I understand that the (In) must be equal or greater than (Ib) but unsure how to find this (In) value!

I know that I can use (In) to find Tabulated current (It), which then I can use another table to find out the size of cables. Which table and what page is this on in the IEE? Is it table 4DA1 page 274 of IEE?

Any help.

Many thanks

 

[sedgy34]

100% for effort

 

[mechelec]

Osg ?????????????????????

 

[ackbarthestar]

Have you got a copy of BS7671?
If so then 433.1.1 and take note of 433.1.100-103, then table 41.3

 

[nazoom]

I have OSG, IEE.

 

[JUD]

In is the rated current or current setting of a protective device i.e your MCB rating should be greater than or equal to your design current (In ≥ Ib)

 

[Guest123]

http://www.electriciansforums.net/e...al-regulations/47059-cable-calc-template.html

 

[JUD]

1. Work out your design current -- I = P ÷ V

2. Choose a protective device with a rating ≥ your design current -- In ≥ Ib

3. If your circuit is for a fixed load (e.g. an oven) then the calculations for cable size can be done using your design current (Ib) although you'll need to use the adiabatic equation to determine what your minimum cpc size should be. If your protective device will be providing overload protection (e.g. for a socket circuit) then cable calculations will be done using your protective device rating (In).

4. Choose a cable with a current carrying capacity (Iz/It) ≥ In/Ib in it's installed environment (Iz = In/Ib ÷ (Ca x Cg x Ci x Cc)). Ca is the correction factor for ambient temperature other than 30°C (Table 6A OSG). Cg is the correction factor for grouping (Table 6C OSG). Ci is the correction factor for thermal insulation (Table 6B OSG) and Cc is for the type of protective device or installation condition (If using a BS 3036 fuse Cc = 0.725. If cable installation method is in a duct in the ground or buried direct Cc = 0.9. If both apply Cc = 0.725 x 0.9 = 0.653)

5. Work out the Voltage Drop for the circuit (VD = (Ib x L x mV/A/m) ÷ 1000). 6.9V (3%) for lighting, 11.5V (5%) for everything else (mV/A/m Table 6D2 OSG)

6. Work out Zs (Zs = Ze + R1+R2). R1+R2 can be worked out using the resistance table in Appendix 9 of the OSG.

7. If your circuit is for a fixed load then the adiabatic equation is used to determine the minimum cpc size needed. S = √(I²t) ÷ k

 

[nazoom]

For step 2 do i need to use a table? Where do i find this In value that is bigger than Ib?

 

[JUD]

For step 2 do i need to use a table? Where do i find this In value that is bigger than Ib?

You choose a fuse/circuit breaker with a rating greater then your design current i.e. If your design current is 27A you would use a 32A breaker.

There's no tables involved.

 

[nazoom]

Very Handy steps. Many thanks for all the effort and help JUD.

Thanks for all the help.

 

[nazoom]

One more question: Can you give me more examples of fixed load and examples of protective device providing overload protection?

Thanks.

 

[nazoom]

Sorry friends I am full of questions today!

Another question: For step 4, after calculating (It/Iz) which table do i use to find the corresponding size of the Cable? on the OSG?

Thanks.

 

[JUD]

One more question: Can you give me more examples of fixed load and examples of protective device providing overload protection?

Thanks.

A fixed load is anything where you know it's power consumption (oven, immersion heater, shower etc).

A socket circuit could be overloaded by plugging too many appliances in at the same time, hence you need to provide overload protection.

Sorry friends I am full of questions today!

Another question: For step 4, after calculating (It/Iz) which table do i use to find the corresponding size of the Cable? on the OSG?

Thanks.

Tables 6D1 and 6E1.

There's more tables in the Big Green Book. Do you have a copy? If not, you should probably get one.

 

[nazoom]

I have the Red book IEE and OSG. however I was quite lost!!

Many thanks again.