Design Course More questions!!

[YoungBart]

Hi all, And sorry to be a pest with the questions.
Basically I'm on my design course, I've just found out I've passed my exam with a merit, But now I'm struggling with a few things in the project.

1) When Sizing a cable on a ring circuit for V/D what is the formula, I'm fairly sure you have to divide something by 4 but I'm not sure where in the formula it goes, ( I'm also aware that table 8A gives you sizes but I assume you need to show your formulas for the project)

2) I also have a number of 110v sockets (25 I think) Someone on here already suggested a 5KVA transformer, So would I feed all Sockets from that? (I have no idea on the KVA calcs, we only glanced at them In college)

Thanks Again
James

 

[Guest55]

1. Ring V.D. = cable mV/A/m x Ib x circuit length divide by 1000 then divide by 4.


2. i wouldnt spec just 1 transformer for that many sockets ??
division of supply and loads to reduce minimal distruption under fault maybe ??
try at least 4.


think about it for a while mr billy big boots i got a credit.
;-)
​

 

[YoungBart]

Thanks Biff, Just a quick question on the 5KVA transformer, Is the 5kva the 'loading' of the transformer (I.e I need to size the cable to 5KVA) or is it the load the transformer will take?
Thanks agan.

 

[infinity]

It's the load the transformer can supply

 

[infinity]

So 5KW pretty much

 

[alanl]

Why are you specifying ring final circuits?
They can help sometimes if you are struggling for capacity, but it is nearly always better to opt for a 4mm radial than a 2.5mm RFC in a commercial/industrial environment.
Easier to install, easier to test, and the cable always has overload protection if it is modified in future, which a RFC will not if someone spurs off and puts more than one socket outlet on it.

 

[ian.settle1]

Why are you specifying ring final circuits?
They can help sometimes if you are struggling for capacity, but it is nearly always better to opt for a 4mm radial than a 2.5mm RFC in a commercial/industrial environment.
Easier to install, easier to test, and the cable always has overload protection if it is modified in future, which a RFC will not if someone spurs off and puts more than one socket outlet on it.

The project may specify that ring circuits are to be used

 

[Guest55]

Why are you specifying ring final circuits?

distance , pure and simple.
although both methods will provide 32a of usable power , a ring using x2 2.5mm legs will give you a combined cpc of 3mm which will allow a much longer circuit than the 1.5 earth in a 4mm t&e cable.

although in all other respects i agree with your preference for radials , C&G design projects have this annoying habit of making the student use exceptionally long circuits for sockets & lights.

in fact on my project i had to spec a 4mm ring !! just to get round the brutal volt drop at terminal load of a circuit that had a socket 35mtrs from the DB.

basically , its all about the maths

 

[YoungBart]

I hadn't even considered radials to be honest, most of the sockets look like they will be used for computers, so I may end up sticking a few 4mm rings in anyway.
And a final question on the transformer note, do I size my cable (feeding the transformer) to 5kva or do I size it to the load of the 110v sockets it will supply?

 

[Silly Sausage]

in fact on my project i had to spec a 4mm ring !!
basically , its all about the maths

Haha...so did mine way back in 2400 days!

 

[wozz]

Rings can be handy for supplying computers.

As for the transformer I would find suitable ones and get manuf data sheets, use their figures and include the data sheets in your project. The transformers may have a fair bit of inrush.