Voltage drop on a ring?

[CRAIGNEWHAM]

Morning,
Just a quick question?
In the OSG (2008) (page 44), it says that you can have 106 metres of 2.5 T&E with a 32A type B cb. Any ideas how they are working this out?
My understanding of vd on a ring is mvam x half of length x half of ib / 1000
So 18 x 53 x 16 / 1000 = 15.2 v??
I assume they take some sort of diversity into account??
Or am I missing something?
Are you supposed to do your own calcs and use whichever is the lowest between those and the table?
Cheers

 

[malcolmsanford]

How can you work out the Ib on a ring, do you know what is being plugged into you? what you have there is the In

If you go pg 54 of the OSG you will see that it tell s you the assumed load of a ring will be 20amps at the furthest point and balance for the protective is evenly distributed. I know that it says that for a 32amp PD it is 26amps

But refer back to regulation 433.1.5 where it tells you that to meet the requirments of 433.1.1 that a cable having an Iz larger than 20amps which 2.5mm^ is, or it's not likely to exceed the Iz for long periods. the circuits are deemded to have met the requirements. So with that your formula is

18 X 20 X 106/100= 38.16/4 = 9.54.

I may be a little rusty on Ring finals as I don't use them often now, but I think you'll find that right. I'm always open though to be corrected.

 

[Chr!s]

Divide the length by 4 = 26.5 x 18 x 26/ 1000 = 12.402 v

Bs 7671 pg 258, 6.1 correction for operating temp.

This calculation give you a ct of .923, x 12.402 = 11.45v

or

4 x 11.5 x 100/26/18 = 98.3 / .923 = 106.5

It assumes 20 amp at the furthest point and 12 amp distributed evenly around the ring so,

average = (32 + 20)/2 = 26


Chris

 

[CRAIGNEWHAM]

Thank you both

Chris could you explain a bit how you used that equation to get the .923?

Thanks
Craig

 

[millwall ken]

Thank you both

Chris could you explain a bit how you used that equation to get the .923?

Thanks
Craig

Would that be 'NEWHAM' in East London by any chance mate?

 

[CRAIGNEWHAM]

Afraid not, its my surname, although I normally get called Newman

 

[millwall ken]

Afraid not, its my surname, although I normally get called Newman

Just wondered mate, as Newham, in East London is where I hail from.

 

[Engineer54]

What area is your multi storey shopping centre in Ken?? ....London or Essex??

 

[Chr!s]

Re: Voltage drop on a ring?

Thank you both

Chris could you explain a bit how you used that equation to get the .923?

Thanks
Craig

The equation allows an adjustment to the volt drop due the the operating temp being less that the 70 C.

For the ring circuit they assume 26amp, so 13 down each leg, it is taken as 20 amp and tp 70 C, put those figures into the equation for the .923