OSG Conventional Circuits

[Budweiser]

Please can somebody help?

Does anyone know how the Conventional Circuits in the OSG are calculated.

Please can someone explain how the OSG Red Book Table 7.1 has been calculated for Max Cable Length considering volt drop.

Looking at the conventional lighting circuits, 5 amp BS3036 and 6 amp Type B CB I’m obviously missing something.

Is it telling me that the maximum length of 1mm2 T+E cable that can be installed, protected by a 5 amp BS3036 semi enclosed fuse is 71 meters and for a 6 amp Type B CB it’s 59 meters.

If so why when I do a simple volt drop calc the lengths arn't permitted. Can anyone explain where I’m going wrong?

Thanks in advance

 

[Spark123]

You may find that they are taking into account that all the load isn't used at the remote end of the circuit, the limiting factor being the earth fault loop impedance.
I'll have a peep at the OSG later and see if I can figure it out.

 

[Budweiser]

Would really appreciate your help Spark123.

I only got as far as volt drop and realised I must have something wrong.

I’ve tried various calcs but I’m obviously missed something. Would be grateful for any pointers in the right direction if you find the time.

Thanks again

 

[Spark123]

It is based on the volt drop equation however not all the current is utilised at the remote point, it is distributed along the circuit so in the case of the 6A MCB the first leg may be carrying the whole 6A, the next leg 4.8A, next leg 3.6A and so on, hence the amount of volt drop decreases as you progress along the circuit.