SWA Maximum current question

[cr0ft]

Hi all, please bear with me, this is my first shot doing cable calculations for my dad's garage. I'd appreciate a more experienced pair of eyes please!

I'm thoroughly confused with current carrying capacities for SWA cable. Basically the SWA cable I will be using for my dad's garage wiring is already in place. It's 4mm 3-core with 1 of the cores being used as the CPC.

Here's a photo of it.

I'm assuming the correct table to use for this to calculate current-carrying capacity and volt drop is table 4E4A and 4E4B on pages 348-349 of the green book? The cable will be clipped direct for some of it's length and run underground for most of it. As it's effectively a 2-core cable as 1-core is the CPC am I ok to assume 43A as it's current carrying capacity before corrections or should I use the 36A for 3-core? If I can assume 43A then I will use a 40A MCB on the way that feeds it from the house consumer unit (it's on a dual RCD board by the way, or it will be). if not then I'll have to use 32A MCB but obviously this means it may trip if someone draws too much current in the garage.

Working on a 12 metre run supplying a ring main and lighting circuit (38A) I get a maximum volt drop down this cable of 0.012V/A/m * 38A * 12m = 5.472V . Am I right in assuming that this leaves me just 6.9V - 5.472V = 1.428V allowable drop on the lighting circuit in the garage? If so I am going to need to run the lighting circuit in 2.5mm to get the run length that I need without volt drop going over 3%.

I'm allowed 5% on the garage ring mains so that leaves me 11.5V - 5.472V to play with on the garage ring mains, right?

Replacing the cable with 6mm SWA isn't really an option as it's been lock-blocked over now.

 

[Technician]

Just a quick reply;

When doing cable calculations the tabulated values for the

conductors are in appendix 4. Use these to determine what

the conductor size should be.

Armouring CAN be used as the cpc but has to be assessed

separately. If the armouring is not used as a CPC which under

certain circumstances is the preferred option (buried conductors

being one) then the current carrying capacity is for the cores

and one of these will be the cpc.

Table F5 i is for non-armoured yes, but go to the regs appx 4

and you will see the tables for tabulated values on one side

with allowable volt drop on the other.

HTH

 

[cr0ft]

Thanks mate. Just stumbled across it in the regs book. So are you saying that if I'm using 1 of the 3 cores as a CPC I can assume it's a 2-core cable for current carrying capacity? I didn't quite follow your answer there.

 

[Technician]

Use the procedure page 139 of the OSG new version

for conductor sizing.

You will also need the table of R1 + R2 calculations.

It was on Page 166 of the old guide. This gives you

the calculated reads for R1/R2.

Yes, the armoured is a three core, two of them will be

L and N and the other the cpc. The cores will be identified

of course, do not forget to use the proper sleeving to identify

the conductor function.

That table I referred to is now on page 182. table I1.

 

[lkayelectrical]

Will you be glanding off the swa on the other end? Also do you have a bottom for that cu?

 

[Technician]

Good Point Ikaye; I had not looked at the pic.

 

[cr0ft]

Will you be glanding off the swa on the other end? Also do you have a bottom for that cu?

Hi Ikayelectrical. The install you see in the picture is coming out. That's how I found it when I went to view it yesterday. It was done by a 'semi-retired electrician'. The CU is discontinued so I've convinced my dad to get a new CU with dual RCD protection and we will remove the up-front RCD (which is under-rated by the way too, 63A rating on a 100A supply).

I will be glanding off the SWA at both ends to earth the armouring, yes.

Do my calculations seem right, particularly with regard to having to use 2.5mm on lighting in garage?

I think I'm going to be limited to a 13m radial circuit in 2.5mm to keep within the volt drop limits for the lighting circuit in the garage. R1+R2 calculations are well within the limits for maximum Zs for lighting and ring mains circuits, even adding in R1+R2 for the SWA run too (around 12m).

Just wish they had put in 6mm SWA then we wouldn't have to run lighting in 2.5mm .

 

[Technician]

I did read an NIC paper on the termination of armouring,

according to them only one end needs termination, preferably

closest to the supply if it is not used as a cpc.

 

[cr0ft]

Thanks for that. I'm guessing I will still use a cable gland on the garage board end then and just not earth it up to the board there?

 

[Doc Hudson]

Will you be glanding off the swa on the other end? Also do you have a bottom for that cu?

Good Point Ikaye; I had not looked at the pic.

This point has already been discussed on Crofts other thread here.

http://www.talk.electricianforum.co.uk/electrician-talk/18448-niciec-assessments-advice-needed-please.html

Doc H.

---------- Post Auto-Merged at 11:52 ---------- Previous post was made at 11:41 ----------

Thanks mate. Just stumbled across it in the regs book. So are you saying that if I'm using 1 of the 3 cores as a CPC I can assume it's a 2-core cable for current carrying capacity? I didn't quite follow your answer there.

The current capacity relates to the physical structure of the cable and its ability to dissipate heat. If you have a thee core cable you will use the figures for three core not two core. The tables you mention (4E4A and 4E4B on pages 348-349) will be correct if you have 900C Multicore Armoured, Thermosetting, if it is 700C Multicore Armoured, Thermoplastic it would be tables 4D4A & 4D4B a few pages back. Thats assuming they are copper conductors, if not the tables for Aluminum are further on in appendix 4.

Doc H.

 

[cr0ft]

Hi thanks. I take it the temperature rating of the cable will be marked on it somewhere? Given that it's buried underground already is there an obvious difference between the two? If it is indeed 70C cable then the maximum current is 30A so my only option would be to put in a lower MCB to protect the cable. I doubt he will ever pull more than 30A through the lights in the garage (2) and the small ring mains (6 double sockets). If I do it this way then the installation is safe if not ideal. I can't see any other way of doing it without increasing the cable size though, something I am sure he won't opt for.

Many thanks!

 

[NozSpark]

You've put the demand as 38A, I assume you have added together the 32A and 6A breakers?

In reality you should get away with a 20A radial (assuming only one person using general power tools and no welder) for the sockets without too much of a problem, as for the lights just add up the wattage of the fittings (assuming 100W for each normal lamp holder).

With the above in mind your actual max demand would be < 21A

 

[Sidewinder]

Also please remember that you cannot run a 90 deg C rated cable at 90 deg is the terminal accessories either end are not also 90 deg rated.

Most are only 70 deg rated, thus you have to rate the cable at 70 deg for its current carrying capacity.

 

[Technician]

Sidewinder, I am a bit thrown by your wording here.

It is possibly just one spelling mistake.

You appear to say that a 90 Cel rated cable cannot be run at

90 Cel if the terminal accessories are not also rated

at 90 Cel.

I follow that, so do I take it that the assessment is

carried out for the lowest rated accessory, termination

kit, cable, whatever. If that is what you mean, fine, and

it would explain the reg about designing specifically for 70

Cel unless otherwise stated.

 

[Sidewinder]

It is what I mean Technician, it is merely a reiteration of correct design practice.