Theory against real world

[androiduk2002]

So been at college tonight and we covered cable calcs and diversity , something I am sure you all know alot more than me about :Salute .

Heres the figures we was given I am interested as to what you would really do.

Customer phones , wants supply to garage , with the following information what size SWA would you run to supply this;

1 A3 Radial

4 lights

1 3kw heater

Cable run of 10 mtrs 5 mtrs buried underground, through back of CU outside of building till it goes underground.

Passes through 500 mm thermal insulation.

To be protected by a BS EN 60898 MCB.

 

[Evans Electric]

Without thinking about it ...a 3c x 2.5 SWA .

 

[NozSpark]

Whats the Ze?

Or do we assume TNS 0.8ohms?

 

[androiduk2002]

Assume TNCS 0.35 ohms

 

[NozSpark]

So a quick calc got my a design current of about 30a

That got me a 4mm 3core SWA before the insulation correction, which sedated the cable by 50%,,,, so I would have to guess at 10mm SWA (maybe?)

 

[androiduk2002]

Well going by the calc as have been taught we calculated to a 25mm SWA

 

[NozSpark]

Well that's all well and good, but what design current were you working to?

 

[Apache]

Well going by the calc as have been taught we calculated to a 25mm SWA

Wow

TLC's calculator gives 4mm, not allowing for derating. If cable just passes through insulation for a short distance do we need to derate?

 

[androiduk2002]

The calc as we have been taught :

20 A Radial no diversity

4 lights at 100 W after diversity applied 1.74 A

3Kw heater no diversity 13.04 A

Design Current 35 A

In 40 A MCB

Ci of 0.51 Cc of 0.91

It = 87.14 A

---------- Post Auto-Merged at 22:08 ---------- Previous post was made at 22:03 ----------

Passing through more than 400mm of thermal insulation derates a cable by a nats over 50%

 

[Evans Electric]

Good grief!!!

 

[NozSpark]

With that max demand I get 10mm before any de-rating using my simple calculator so 25mm sounds about right

 

[ADS]

The calc as we have been taught :20 A Radial no diversity

4 lights at 100 W after diversity applied 1.74 A

3Kw heater no diversity 13.04 A

Design Current 35 A

In 40 A MCB

Ci of 0.51 Cc of 0.91

It = 87.14 A

---------- Post Auto-Merged at 22:08 ---------- Previous post was made at 22:03 ----------

Passing through more than 400mm of thermal insulation derates a cable by a nats over 50%

So why not 16mm - or have I missed something?

94 Amps - 16.0mm 6943X 3 core SWA Steel Armoured Cable

 

[androiduk2002]

Table 4D4A pg 280 BS 7671 Current Carrying Capacity referance method D a 16mm has a rating of 78A to small

 

[ADS]

Table 4D4A pg 280 BS 7671 Current Carrying Capacity referance method D a 16mm has a rating of 78A to small

Not if it's XLPE - acccording to TLC ......94 Amps

As your thread says - theory against real world lol

 

[Evans Electric]

Dear God Almighty !!!.... is this a commercial garage or what ?

 

[androiduk2002]

I believe XLPE comes under table 4E4A 16mm is 91A so yes acceptable but would be even more costly ? and as it would be 3 core then the current carrying capacity is even further derated to 75A .

Please if I am incorrect then please correct me

 

[Sidewinder]

Not if it's XLPE - acccording to TLC ......94 AmpsAs your thread says - theory against real world lol

ADS only if your termination devices can handle 90 deg C!

 

[androiduk2002]

Dear God Almighty !!!.... is this a commercial garage or what ?

Nope just your average garage 5 mtrs from your house ! !

This is why I brought the topic up

 

[fletchflash]

I can honestly say if a domestic customer rang me for a quote to put some lights, sockets and a heater in their garage 5metres from their house its VERY unlikely I'd quote for a 25mm SWA 3core. Sure they wouldnt be too happy with it clipped to their external wall for the other 5 metres.

How much insulation does it pass through!? Also is unlikely radial will be pulling 20 amp's, maybe this could go on a 16amp mcb?

I would probably put it on a 4mm, 6 at most. If insulation was really that thick i'd take a different route.

Good post!

 

[Evans Electric]

Why will this "Average " garage be pulling 94 A ? Where are you getting that from ?

Heater ....12.5A

Lts........... 0.5A

1 Socket ... ?????? 4 A

17A ???