What size SWA? & Xlpe vs pvc swa

[Avo]

Hi,

I finally succumb to better judgement and dared to pull out one of the 2.5 t&e cables to my workshop which ran 16 meters in 50mm conduit. I have another to pull out now but I want to use it to drag through some armoured instead. This way I can forget the need for an RCD at head end and not have to worry about discrim.

Also means I can up the power to the workshop.

Keeping in mind I have two bends to pull this stuff through, what swa size would be realistic for:

2 x 16A radials

1 x 3A lighting (Mainly LED lamps, but total wattage is only around 120W)

1 x 3A fire/intruder alarm

The two radials will be on RCBOs, possibly the lights also. The alarms unprotected. (Anyone RCBO/RCD alarms on dedicated circuits?)

Realistically max continuous load would never be over 1.5Kw on each radial, of course there might be a rare occasion a 1Kw fire gets put on but I have a balanced flue gas heater partly for that reason.

I was hoping to get away with 6mm2 / 3 core?

 

[sparkytim]

Your load requirement appears to be only around 3 KW allowing for diversity

A 2.5 mm protected at 20 Amps gives you 5Kwatt potential.

 

[Avo]

Really?

Forgot to mention the conduit is 2' underground, whether that makes much difference...

I thought I had to calculate the total mcb values and make sure they stay within a percentage of the total current carrying capacity of the cable once de-rated for said conditions?

 

[sparkytim]

No the total of MCB values is not an issue (Just look at a common domestic Consumer Unit installation and add up the MCB values, often way in excess of the service fuse value)

The issue is what is your DESIGN load and allowance for diversity.

 

[Avo]

Okay, that seems odd to me.

I appreciate in the real world it likely wouldn't happen, but if every circuit in a domestic cu was running at peak this would possibly push the cable beyond it's limits would it not?

Isn't that what all the regs are supposed to stop! I guess that is why they take diversity into account then...

 

[sparkytim]

The supply cable is protected from overload by its protective device. In the case of a domestic Consumer Unit the 60, 80 or 100 Amp service fuse.

In most situations your POTENTIAL load is not all on at the same time, i.e. you may have 40 Amp MCB for a shower circuit, 32 Amp Cooker cct, two 32 Amp Ring ccts, 20 Amp Immersion cct, three 6 Amp lighting ccts.

Just adding up the MCB values is not a true Load assessment.

 

[Avo]

The supply cable is protected from overload by its protective device. In the case of a domestic Consumer Unit the 60, 80 or 100 Amp service fuse.In most situations your POTENTIAL load is not all on at the same time, i.e. you may have 40 Amp MCB for a shower circuit, 32 Amp Cooker cct, two 32 Amp Ring ccts, 20 Amp Immersion cct, three 6 Amp lighting ccts.

Just adding up the MCB values is not a true Load assessment.

I see, thanks for taking time out to respond to my dumb questions sparkytim.

I'm going to go spend the night reading up on circuit design, you've got me intrigued now.

 

[Smilers]

You could prove it to yourself by putting every appliance and light on in your house and then slap a clamp meter on at your board to see what is getting used just for fun :/)

 

[Avo]

You could prove it to yourself by putting every appliance and light on in your house and then slap a clamp meter on at your board to see what is getting used just for fun :/)

Aye, but I'm not doubting it.

Just surprised a bit by the regs. People always talk about the regs in a way that makes you think they are completely stupid and pointless, whereas most I've read are pretty sensible and seem common sense to me.

I was surprised to read they allowed for diversity (common sense)

 

[Avo]

Looking on the screw fix website i see that they have listed both xlpe and pvc swa cable. Obviously the xlpe is better, but im a little confused.

Both the pvc listed and xlpe apparently both sport temperatures up to 90 degrees. So, do you think screwfix lists it incorrectly or am i missing something.

Furthermore, with xlpe is the outer insulation made of xlpe or is it just the inner insulation.

 

[Manator]

The xlpe is just the inner insulation, however you must get the correct glands, they are usually marked up as low smoke. I suspect as is usual the screwfix website has mixed up some information.

 

[Riggy]

The xlpe is just the inner insulation, however you must get the correct glands, they are usually marked up as low smoke. I suspect as is usual the screwfix website has mixed up some information.

How does the insulation material affect gland selection?

 

[Avo]

Low smoke glands??

Anyway, I've had another look at the situation and measured the removed t&e. It's actually longer than I thought and I'm going to need 25m.

Unless anyone has any thoughts otherwise I'm still running the above circuits but I'm going to install 4mm2. I'd prefer 6mm2, just not sure it'll thread through.

 

[Manator]

How does the insulation material affect gland selection?

I can only presume its to allow for compliance throughout the entire length and materials. The gland boots are sometimes marked low smoke.

 

[Sidewinder]

Also the diameter of pvc/swa/pvc is IIRC different in certain csa's to that of xlpe/swa/pvc.

However, you can't run xlpe at 90 deg C unless the devices that it is connected to are rated to 90 deg C.

The CCC of xlpe @ 70 deg C is the same as that of pvc @ 70 deg C.

 

[Sidewinder]

No it is NOT!

Think about it mate...

 

[apprentice87]

Hi All,

Paul is of course entirely right in every thing he says... The different cable types DO have different gland requirements and i have in the past had terrible struggles when trying to re-terminate old cables....

john...

---------- Post Auto-Merged at 20:46 ---------- Previous post was made at 20:41 ----------

Hi Again,

On the question of whether a particular MCB or whatever was rated for 90c or not, would it not depend on whether the CU as a WHOLE was rated to have accessories run at that temperature installed in it???

What about the effect of the temperature on ADJACENT accessories [de-rating perhaps???]

I have no idea!!! It is just something that occurred to me..

john...

 

[green-hornet]

Nice thinking, and you are indeed correct, any supply fitted to cope with higher temperatures should have this temperature factor throughout the installation, otherwise a weak link is inbuilt into the design.

Not very good design if one componant is not suitable for the total design temp.

 

[Sidewinder]

Wylex, Crabtree, Volex, all Siemens, but, probably NOT rated at 90 deg C! ]

 

[steptoe]

is the DNO HED and meter rated at 90 ? :|