Called calcs and volt drop confusion?

[threephase123]

Why do we protect ring final circuits with 32 amp mcb's if 2.5mm t&e can only take a maximum of 27 amps?

Also when calculating volt drop on a ring, length of cable is calculated from start of leg one of ring (outgoing) to end of leg 2 of ring (incoming)? Or is it the farthest point away from consumer unit?

 

[Sidewinder]

Because a single section in a full, sound, correctly installed and connected ring can never see 32A.

Neither!

 

[threephase123]

So how do we calculate length?

 

[Sidewinder]

End to end

 

[threephase123]

That's what I said..... From outgoing to incoming

 

[Sidewinder]

No, that's not what you said, as I read it.

The length of the ring is from end to end.

To calculate volt drop you have to know the current flow and it's distribution in the ring and the length of the individual legs of the ring.

 

[Manator]

It is possible to calculate the ring length, but you need the end to end measurements or the R1+R2 reading.

Lets say you have a R1+R2 reading of 0.48 for a 2.5mm twin and earth ring, r1 reading should be around 0.73 and r2 should be around 1.20. The approximate length of that ring would be 98.5 metres. I always remember that calculation because it is has close to the maximum I would ever like to get.

 

[PC Electrics]

Remember chaps, this is in the student area.

3Ph123,

In a ring final circit current flows from the ocpd (typically a 32A mcb) along both conductors to the point of utilisation. If the load were to be 32A and positioned at the centre of the ring, then 16A would flow down each leg. The actual current split down each leg is inversly proportional to the ratio of the leg resistances or lengths (provided the ring is made of the same cable all round).

So you can see that by splitting the current down two conductors, those conductors can be smaller than if only one conductor (a radial) were used.

The voltage drop is calculated by knowing the loads in use and the load distribution. It is calculated from the ocpd to the point of use. The maximum size of a standard rfc is quoted as 107m round in the OSG. This is calculated using an assumed load distribution of 16A at the ring centre and 8A evenly distributed around each leg (iirc). The voltage drop will be the same at all points on an rfc no matter what the load distribution. You could calculate the voltage drop for any load.

 

[Sidewinder]

PC,

I did remember it was the student area, however, I still won't give direct answers, I want the students to at least try to figure it out for themselves.

I don't even give direct answers when I am teaching!

I will always try to get my students to have a go first.

 

[threephase123]

For example. If I have a ring final circuit enclosed in conduit ran in singles PVC. With a run end to end of ring of 50m supplying 6 13 amp sockets used for portable equipment such as drills and hand grinders what size cable would be sufficient? Could someone take their time to show all calculations. I'd really appreciate help for project

 

[Zeespark]

Supply voltage = Volts

Ib - design current = Amps

Protective device type = BS

In - protective device rating = Amps

Length of run of cable = Metres

Power factor =

Cable Type :

Installation Method :

Ambient Temperature =

Number of circuits including this one =

Length of cable in thermal insulation =

Apply Correction factors:

From TABLE 4C1 : Cg =

From TABLE 4B1 : Ca =

From TABLE 52.2 : Ci =

Protective device factor : Cc =

It = tabulated current carrying capacity

It = In / (Cg x Ci x Ca x Cc)

It = / ( )

It = Amps

From TABLE Cable selected = mm

 

[Sidewinder]

three,

There is not a single definitive answer to this problem, you have 6 off 13A rated sockets, are these singles or doubles?

Where are the loads connected?

What project BTW?

---------- Post Auto-Merged at 21:34 ---------- Previous post was made at 21:33 ----------

Zee,

"Not quite" for an RFC mate as you have 2 current paths!

 

[Zeespark]

I apologise, just trying to help the OP learn what it takes to design a circuit fully is all.

AndyGuinness

 

[Sidewinder]

Zee,

I know, no need to apologise, for a radial spot on.

Because of the configuration of a ring, it's a bit more in depth to do the full analysis.

See Kirchoff for explanations! ; )

 

[threephase123]

My tutor has told me to design a circuit of choice and show all calculations. They would be double sockets

 

[Sidewinder]

Choose an easier circuit!!!

 

[threephase123]

Well here it goes.

Assuming I am using 2.5mm for ring.

18xibx50/1000/4 is this the right calc for a ring?

I din't know what ib to use for portable hand tools such as drills, grinders and hoovers? What ib should I use for portable hand tools?

 

[Sidewinder]

No it is "technically not"!

What is the load distribution around the ring?

What course are you studying?

 

[threephase123]

2330. The 6 sockets are going to be part of a Worksop ring above a bench. I want to use diversity as there won't be any fixed loads just portables.

 

[Sidewinder]

You don't need to worry about diversity.

Don't "do" a ring final or a socket radial, to do these "perfectly" is impossible in an "academic" sense.

Do another circuit with known fixed loads to make your life easy!!!

You can learn the more complex design rules later.