Voltage Drop issue

[danny124]

hi guys

trying to get the right table in the BS7671 book and the On site guide, regarding wroking out my CCC and VD....

I worked out my design for my shed, its going in twin and earth flat, in round conduit. Looking in the BS7671 it looks to me that is a Method B.

There isnt a method b, on the flat cable in either book. I thinking its going to be multicore cable PVC.
so been looking at table 6D1 OSG & 6D2 But when working out VD on using 2.5mm T&E, using 32amp MCB, and is only 20m cable to be used.
When working out the VD im getting weird answers and getting 5% dead on volt drop.

my calculation for my ring is:

mV/A/m so. VD= (18x32x20)/1000 = 11.52
11.52x100/230 = 5% dead on

Am i right, or am i just being plian stupid somewhere and gone absolutly wrong....

 

[JUD]

hi guys

trying to get the right table in the BS7671 book and the On site guide, regarding wroking out my CCC and VD....

I worked out my design for my shed, its going in twin and earth flat, in round conduit. Looking in the BS7671 it looks to me that is a Method B.

There isnt a method b, on the flat cable in either book. I thinking its going to be multicore cable PVC.
so been looking at table 6D1 OSG & 6D2 But when working out VD on using 2.5mm T&E, using 32amp MCB, and is only 20m cable to be used.
When working out the VD im getting weird answers and getting 5% dead on volt drop.

my calculation for my ring is:

mV/A/m so. VD= (18x32x20)/1000 = 11.52
11.52x100/230 = 5% dead on

Am i right, or am i just being plian stupid somewhere and gone absolutly wrong....

Firstly, the CCC table you want in the OSG is 6E1 (page 131).

2.5mm² T&E Reference Method B is 23A. The CCC of a ring circuit is taken as 40A (20A per leg) so 23A is sufficient.

To work out the VD for a ring you use 26A (see page 54 OSG) as the design current so the calculation would be:

(18 x 26 x 20 ÷ 1000) ÷ 4 = 2.34V (1%)


It is also worth remembering that the maximum length of a ring circuit for voltage drop is 98m at ambient temperature of <30°C and 106m at ≥30°C

4 x (11.5 x 1000) ÷ 26 ÷ 18 = 98m (<30°C)

4 x (11.5 x 1000) ÷ 26 ÷ 18 ÷ 0.923 = 106m (≥30°C)


The 0.923 in the second calculation is the correction factor for the reduced operation temperature of the cable (Ct) when it's not carrying its full current capacity (see page 258 BS 7671) but is only applicable at an ambient temperature ≥30°C.

 

[JUD]

How can the CCC be 40A when a ring is normally protected by a 32A CPD?

I mean the CCC of the 2.5mm² that makes up the ring.

*EDIT* I guess you deleted your post

 

[danny124]

JUD, i got to say a big thanks.... Thats one hell of a reply

Thanks so much for the Info Sir

Danny

 

[pushrod]

Hi JUD if i am reading this correctly Danny is running a ring from his CU to his shed through a single conduit? ie both legs does this mean that there would be a derating of 0.8 for grouping involved - or would that not be relevant in this case? ( I appreciate it won't affect the overall design as there was a lot of slack , but was just wondering)

 

[JUD]

I never class the two legs of a ring as grouped as they are part of the same circuit.

 

[pushrod]

I never class the two legs of a ring as grouped as they are part of the same circuit.

but grouping is a heating thing so wouldn't it still count, a bit like you should unreel an extension cable if you want to use its full current carrying capacity.