Practical Real World Question

[steptoe]

OK,

in relation to an ongoing thread in another part of the forum I thought I'd put a question in here,

this is an actual real world situation, my own house, so, as you may have guessed I have already made the answer  up,

but, I would like you guys to tell me why, and how, and what you would do.

as an aside, the earthing arrangement is perfectly fine to export the TNCS on, if you need any more info regarding that particular aspect then I can provide that, but it is not relevant to the cable sizing , OCPD etc .

what size OCPD do I need to supply my shed?

what protection do i need to provide?

there will be a mini pvc CU fitted to the shed.

TNCS incomer

100A main switch

6mm T&E >50mm from the surface of any wall  [ 12m length ]

surface clipped not in contact with any insulation

changes to 10mm 4core SWA for outside run @ 12m length , 2 cores are redundancy so CANNOT  be used for anything else, but loading on them CAN be ignored for the purposes of this exercise.

the cable is already installed so is non change-able, you have to work out the max permissable loading and what protection I require.

I cant find a limiting factor other than It, although this leaves next to nothing VD wise for the final circuits.  

Obviously cant do shock and thermal constraints.  

any help?

is it that simple considering the resistance of the cable and the likely type of loading? not simply the max load.

 

[wozz]

no, not really, you have given a installation method and indicated Ci=1.  

shock and thermal constraints cant be calculated without Ze.

 

[steptoe]

well, how do you calc it on a new build then?

its not written in stone but all the books take it as being 0.35 ohms max.

 

[binky]

is your swa buried, run in conduit or dangling from washing line? or am I barking up wrong tree if you want to discuss thermal constraints?

 

[wozz]

havnt got time tonight but thermal constraints is to do with the adiabatic equation.

 

[steptoe]

is your swa buried, run in conduit or dangling from washing line? or am I barking up wrong tree if you want to discuss thermal constraints?

SWA is buried,

I think Wozz is wondering what to do on a new build where he has NO actual readings, no offence intended Wozz, that is what the guidance in the books are for.

 

[binky]

buried swa has different ratings to swa in air, can't be arsed to go look them up!

 

[steptoe]

that is really not going to be the defining issue,

can I ask a question,

why are people looking at issues that are NOT going to be the issue of the OCPD, or rating of.

 

[wozz]

SWA is buried,

I think Wozz is wondering what to do on a new build where he has NO actual readings, no offence intended Wozz, that is what the guidance in the books are for.

haha you think wrong.  

you can take the maximum if you want but you will run into trouble on larger installs.

I would run through my calcs so far but im busy tonight and i wont be copying them onto here.  

I will get round to it.  

that is really not going to be the defining issue,

can I ask a question,

why are people looking at issues that are NOT going to be the issue of the OCPD, or rating of.

come on then expand on how cable installation doesnt effect the max OCPD on a circuit. 

 

[steptoe]

haha you think wrong.  

you can take the maximum if you want but you will run into trouble on larger installs.

I would run through my calcs so far but im busy tonight and i wont be copying them onto here.  

I will get round to it.  

yes, you will,

its not a larger install though, is it,?

and Ze was stated as being good, so how would you normally work out your cable calcs for a new build semi with a TNCS at a stated 0.35 hms   ?

the basics of this is regarding the cable resistance and the type of loading,

all basic stuff that hasnt been asked, or accounted for,

oh, apart from the PMs I got that got it spot on with no major issues or further info from the OP.

 

[wozz]

Come on then as no one is biting, and so many are that good lets see some calcs? 

 

[sellers]

what brand CU?  we don't want to be mixing breakers!

 

[sprocketflup]

what colour is the carpet?

 

[SLIPSHOD & SLAPDASH]

Carpet ?  What colour wood flooring.

 

[binky]

that is really not going to be the defining issue,

can I ask a question,

why are people looking at issues that are NOT going to be the issue of the OCPD, or rating of.

becuase we are trying to think what obscure bit of electrical theory you are trying to highlight. And to annoy you :innocent

 

[steptoe]

Come on then as no one is biting, and so many are that good lets see some calcs? 

I havent done any calcs, :|

Its a standard install,

Its on a 40 type C ,

The only thing needed to be calc'd on this was the resistance of the cable to ensure Zs would be good, for a C ,

It was never going to get too high a load for VD to be an issue and still keep Zs within scope,

Ive said it before,

Stop over thinking some things.

 

[apprentice87]

Hi All,

I think everyone has gone a little off track here. I am a bit busy at the mo, but i will have a quick calc and post what i think later tonight if i have time.

In the meantime, what Steps asked is not difficult.... Just approach it as you would if you were doing an EICR...Remember, the cable is already installed... All you need to do it to make the decision "is the OCPD fitted appropriate for the circuit it is protecting" Not difficult...

john...

 

[steptoe]

Hi All,

I think everyone has gone a little off track here. I am a bit busy at the mo, but i will have a quick calc and post what i think later tonight if i have time.

In the meantime, what Steps asked is not difficult.... Just approach it as you would if you were doing an EICR...Remember, the cable is already installed... All you need to do it to make the decision "is the OCPD fitted appropriate for the circuit it is protecting" Not difficult...

john...

well put John,

as I said, sometimes more often than not folks over complicate stuff like this,

I didnt actually say OCPD size at the start as I thought it very easy to work out what size needed,

in fact a couple of folks PM'd me with some very basic calcs and the OCPD they would fit as a max,

without having a firm Ze or Zs it is very easy to calc the max allowed current,

hence why I said VD or loading wasnt an issue, the max load is what the worst case cable could carry,

VD on such a short run was always going to be inside the CCC of the cables.

now, at a max Ze of 0.35 for TNCS which is what all the tables are based on, its easy to work out the max resistance of the cable to find what size and type of MCB will comply.

that is why I said final/shed circuits were irrelevant, they would be at least twice permitted Zs of the OCPD at source, and it wasnt that BIG of a shed.

one of the PM's I got,

Using the parameters youve given, BRB table 4D2A tells me the 6mm t+ewill take 46A, and table 4D4A says the 10 SWA will take 67A.

Volt drop on the t+e works out at 4.4mV/A/m x 12m x 46A = 2.428V and on

the SWA 4.4mV/A/m x 12m x 67A = 3.537V so total volt drop = 5.965 or

2.59% of 230v so within tolerance for that.
 
So bearing in mind that the most vulnerable cable will take 46A id use a45A MCB, the cables dont necessarily need RCD protection, so Id run it

from an unprotected part of the board, rod the shed and have an RCD in

the shed CU.

what was so hard about working that out?

simple , and straight to the point,

 

[Sharpend]

Makes me wonder why you couldn't answer it yourself and needed to put it too the forum as a student question? Still I'm glad you found the replies helpful!

 

[apprentice87]

Hi Steps, well said!!! There are some more very interesting points that i have to add about this scenario, but i have to pop out again in a mo, so i will try to do it later!!

john..