Discuss undersized SWA to garage code? in the Electrical Forum area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

Reaction score
38
Hi,
Got an undersized cable going to a garage not sure how to code it:
Cable is 2 core 2.5 SWA at least 40m long
Cable comes of a 15a 3036 fuse in the house and splits into DB3 (a 20a switch fuse coming of the live side of DB4 main switch) and DB4 an old wylex rcb main switch board with 2 16a and 1 6a plug in RCD.
Obviously voltage drop way above 5% but essentially the whole lot is protected by a 15a fuse in the house so more a nuisance than dangerous.
The earth is exported via the steel sheaf from the TN-S in the house. I know it's generally advised that anything over a 20m run should be spiked but I need a reg to back that up.

Any thoughts?

Steve
 
Can you provide a drawing of the arrangement? your post is a bit confusing.
 
I know it's generally advised that anything over a 20m run should be spiked but I need a reg to back that up.

I'll go along with post 2. Where did you hear the 20m rule?
 
I'll go along with post 2. Where did you hear the 20m rule?

The guy may come from an industrial background. Most specs and designs for large areas have a 20-30metre rule of thumb, to keep lengths of final circuits down to manageable length. Depending on circuit, I would always try and design final circuits under 30mts, by trying to fit sub boards.

Sometimes you can't such as hi bay lighting, or areas where siting another board is just not possible, other than in the electrical/technical room.

But the OP is a distribution circuit and so I would not thought any rule on distance would apply as such
 
Thanks for the responses.
Goes like this:
Origin > DB1
Origin > DB2

DB2 15a Fuse (3036) > 2.5 2 core SWA >Garage>DB3 and DB4 (sheaf used as earth)

DB3 is a 20a Switch fuse
DB4 is an old wylex board RCD main switch to 2 x 16a and 1 x 6a plug in MCBs

Also looking for R1 + R2 tables using the steel sheaf as a CPC..
Thanks
 
Cable to garage voltage drop safe to declare an exception?

Hi,
I've got a customer with a 2.5mm 2 core swa cable on a 15a 3036 fuse feeding a garage 40m away
Although the cable can easily handle the current it's more than 5% voltage drop.
The earth is via the armour.
I've recommended a board change for other reasons but i've suggested a 3core 6mm swa of a 20a mcb feeding a garage box with a 6a and 16a for lights and some sockets. 6mm gives me a big enough CPC.
He's not keen.
Given that we're talking about a voltage drop which won't make much difference to what he'll be using (lights, a drill etc) the only safety problem I can see is the size of the cpc (armour). Does he have the option of me installing an earth rod to get over the cpc size and then signing of the cable size on the cert as an exception? Not ideal but basically safe?
Its a TN-S BTW

Thoughts ?
 
Re: Cable to garage voltage drop safe to declare an exception?

Hi Steve,
What's the design cuttent of the garage circuit? From what you say , lights and a drill, it won't be much. I would of thought the vd would be fine.
The armour for cpc should be ok too, I'm assuming no extraneous conductive parts.
 
Re: Cable to garage voltage drop safe to declare an exception?

I would stick to my guns. Cable too small for garage usage. When he puts his fan heater on whilst he uses little drill ?.
 
Garage has a 20a switch fuse and a board with 2 x 16a and 1 x 6a mcb. So loads are more likely to pop the house fuse.
So I guess the voltage drop isnt really a safety issues so a C3.
Just need to get a value for the armour as a cpc. Guess I'l just measure it!!

Cheers
s
 
Garage has a 20a switch fuse and a board with 2 x 16a and 1 x 6a mcb. So loads are more likely to pop the house fuse.
So I guess the voltage drop isnt really a safety issues so a C3.
Just need to get a value for the armour as a cpc. Guess I'l just measure it!!

Cheers
s

or measure the loop impedance art a garage socket.
 
There's a lot more to correct cable sizing than just reading the tabulated ccc.

Agreed, so I did a quick calc! Based on a 15A load ( fuse rating at origin ), multi core 2.5mm swa, buried, 40m, ze 0.8ohms (TN-S), 5% VD. These are all assumed at worst case!

Correction factors:
Ci : 1
Cg : 1
Cc : 0.9
Ca : 1
Cf : 0.725
Total correction : 0.653

Adjusted cable rating : 22.97A
Calculated R1 + R2 : 0.71 ohms ( based on a core rather than armour ). Will be lower when calculated with armour.
Calculated Zdb : 1.51 ohms
Max Zdb at 0.4s : 2.43 ohms
VD : 4.696%

All still seems ok!
 
Last edited by a moderator:
Thanks for this TD , very useful.
Problems:
You say that:
<Resistance> Will be lower when calculated with armour.
But according to this manufacturers tables the resistance of the armour (8.8 ohms/km) is a higher than the 2.5 copper(7.41ohms/km). how does this affect your calcs?

Exported earth TN-S:
The armour is too small to for the earthing requirements at the garage end?

Any thoughts
many thanks for this.
Regads
Steve


best regards
Steve


Appendix 4 - Cable Data-Resistance, Impedance and []R1 + R2? Values - Guide to the Wiring Regulations - Locke - Wiley Online Library
 
Last edited:
To all contributors
I started another thread on this subject in General to discuss remedial works. Unfortunately it has been frozen. many thanks to the people who posted replies to that thread and my apologies for not replying. Apologies if this drifts into solutions as was intended in the new thread.
Regards
Steve
 

Reply to undersized SWA to garage code? in the Electrical Forum area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc

Electrical Forum

Welcome to the Electrical Forum at ElectriciansForums.net. The friendliest electrical forum online. General electrical questions and answers can be found in the electrical forum.
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock