Cable confusion

[iandandav]

Supply
PME earthing
Earth loop imp < 0.8ohm
Capacity 15kva
Max prospective short circuit current 16kA
50 hertz frequency

Lounge and Dining Room
Ring circuit 32A
MCB protected
Circuit length – 42m
Floor area – 23m sq
10 double sockets on ring
1 double socket on short spur (20 cm through single layer terracotta block wall not shown)
RCD protected

The circuit

Key for diagram
A - Behind plasterboard on surface of insulation < 100mm
B - In PVC conduit laid in floor concrete
C - Attached to joist (insulation > 100mm)
D – Under PVC shield in wall rendering


My copy of onsite guide is giving me a cable size of 6mm sq, yet from my previous experience I would have thought 2.5mm sq. for a standard circuit like this. Am I interpreting the tables wrong ?

 

[Gees]

Could you POSSIBLY get three 6mm cables in one socket? 2 ring and a spur??

 

[HullMarc]

Yep. Keyword is 'ring'.

 

[LoveRocket]

You need an Iz of 20A in order to install a 32A ring.

You do know that when you're applying derating factors, you only apply the most significant one?

PS. Before you install this, do you really think a ring circuit is needed/sensible? Why not a radial (or two) - it will make future alterations/additions much much easier, and won't overload cables if there's unbalanced loadings or loose connections.

 

[Spark123]

TN-CS (PME) should have a Ze of less than 0.35 ohms

 

[plugwash]

You do know that when you're applying derating factors, you only apply the most significant one?

I was under the impression that you needed to work out the size of cable needed for the worst conditions the cable will pass through. So if two derating factors apply to the same part of the cable you would need to apply them both but if different factors apply to different parts of the cable you need to work things out for each part of the cable and then take the worst case.

 

[LoveRocket]

You do know that when you're applying derating factors, you only apply the most significant one?

I was under the impression that you needed to work out the size of cable needed for the worst conditions the cable will pass through. So if two derating factors apply to the same part of the cable you would need to apply them both but if different factors apply to different parts of the cable you need to work things out for each part of the cable and then take the worst case.

Yeah, that's what I meant, I just couldn't be bothered/find the words to explain it properly!

 

[electronicsuk]

Equally interesting is your diagram, which I assume is supposed to show the physical layout of the circuit. I can't see how some of those cables are going to be in the safe zones.

 

[seasickstevie]

Assuming the two legs in the concrete are >100mm deep, and the legs in the ceiling ('attached to joists') are >100mm above ceiling, I can't see any that aren't in safe zones.

 

[seasickstevie]

Although I've just noticed the drop at the end of 'A' that drops below the floor is actually out of SZ.

 

[electronicsuk]

Assuming the two legs in the concrete are >100mm deep, and the legs in the ceiling ('attached to joists') are >100mm above ceiling, I can't see any that aren't in safe zones.

I'm looking at 'A' on the near side of the drawing, where there appears to be a cable that drops down into the floor when it nears a door frame. As 'A' apparently signifies ' Behind plasterboard on surface of insulation < 100mm', I don't see how this would be compliant.

 

[seasickstevie]

Got there before you

 

[electronicsuk]

Got there before you

Yup. Also not sure about D in the rear right corner of the drawing - it seems there are two opposing walls with accessories attached, but the adjoining wall between them has a buried cable with no accessories to create a safe zone. Same in the near right corner, too.

 

[iandandav]

Equally interesting is your diagram, which I assume is supposed to show the physical layout of the circuit. I can't see how some of those cables are going to be in the safe zones.

Do you mean the socket (circles) on the very far right of the diagram as you look at it. If so Yes ive noticed this its just a mistake on the drawing. the circle should be drawn over the corner

 

[iandandav]

Assuming the two legs in the concrete are >100mm deep, and the legs in the ceiling ('attached to joists') are >100mm above ceiling, I can't see any that aren't in safe zones.

They are about 50mm havent found anything specific to this re the depth. do you have a reference?