Outbuilding

[Traineeboy]

Yes thanks that makes sense 

 

[Traineeboy]

As I won’t be using the steel armour as my CPC  I guess it doesn’t need to Gland it ? 

Or do I need to at the supply end ? 
 

I’ve been told different things in the passed 

yes you have to at both ends ( even if you aren’t  using steel as CPC) 

yes but just the supply end. 
 

could you please clarify. 
 

 

[Murdoch]

As I won’t be using the steel armour as my CPC  I guess it doesn’t need to Gland it ? 

Or do I need to at the supply end ? 
 

I’ve been told different things in the passed 

yes you have to at both ends ( even if you aren’t  using steel as CPC) 

yes but just the supply end. 
 

could you please clarify. 
 

SWA should ALWAYS be glanded , at either end (BS 7671 doesn’t stipulate) but I always do both ends unless the outbuilding is a TT

ive seen an EICR which gave a C2 for SWA glanded only at the load end - and that’s wrong

 

[Traineeboy]

SWA should ALWAYS be glanded , at either end (BS 7671 doesn’t stipulate) but I always do both ends unless the outbuilding is a TT

ive seen an EICR which gave a C2 for SWA glanded only at the load end - and that’s wrong

That’s good to know. When my boss does flats and each flat has its own board he brings the SWA rear entry and just puts tape around it where the steel wire ends. I guess that’s a wrong way to do it then. 

 

[Andy™]

That’s good to know. When my boss does flats and each flat has its own board he brings the SWA rear entry and just puts tape around it where the steel wire ends. I guess that’s a wrong way to do it then. 

yep, thats the wrong way

 

[Traineeboy]

On the same topic as armoured cables and outbuildings etc... 

I once saw an armoured cable come in the house into an adaptable box then flexed our to a plug. It powered an IP rated sock in the garden. 

although that’s not the way you would do it I presume there was nothing wrong with That. Providing the steel wire had been earthed and the correct cable size had been used. The plug had a 13a fuse so the cable was protected. I know if I did it I would have used an FCU rather than a plug but presume there are no regs to say you can’t use a plug. 

 

[ProDave]

On the same topic as armoured cables and outbuildings etc... 

I once saw an armoured cable come in the house into an adaptable box then flexed our to a plug. It powered an IP rated sock in the garden. 

although that’s not the way you would do it I presume there was nothing wrong with That. Providing the steel wire had been earthed and the correct cable size had been used. The plug had a 13a fuse so the cable was protected. I know if I did it I would have used an FCU rather than a plug but presume there are no regs to say you can’t use a plug. 

That will be someone thinking it is not "fixed wiring" so is outwith the wiring regs and phart pee etc.

 

[apprentice87]

Traineeboy..

Cable calculation is very involved, but i will try to give you a bit of an idea..

First thing, you need to do is to decide the design load.. Let us say 30A

Next you need to decide upon an OCPD to protect the cable.. That is what the OCPD is for.. 1, protecting the cable, AND 2, providing ADS, so that in the event of an earth fault [that may cause "exposed conductive parts" like say the the metal casing of a piece of equipment] to become "live" in the event of a fault, the supply is cut off in the required time to prevent cable damage and reduce duration of electric shock!!. [look up "disconnection times]

Yes, the current carrying capacity of the cable is important and you need to check this, and there are various calculations involved based on "installation method" etc.. BUT and it is a very big BUT, usually the deciding factor is one of two things.. either volt drop, or EFLI.

From this, you can see that the CABLE has to be selected to suit the OCPD, NOT the load at all..

Never mind what the actual incoming voltage is, you just take it that at the origin of the installation, [where the DNO supply cable ends at the cutout meter etc] that the voltage is 230v..

You know what the load current is, and you can look up from tables what the volt drop in millivolts per amp per metre is, so knowing the 3 or 5% bit, you can select a suitable cable from the point of view of volt drop..

Next thing, you need to look up what the maximum Zs is for the OCPD, or work it out for yourself.. I always work it out myself..... Lets say we had a 32a one, a "B" curve one...

Now to make this trip off in the required time, [both 5 seconds AND 0.4 seconds is the same current required] we need a fault current to flow of FIVE times the 32A.. This equals 160A..

Now, the calculated Zs for our circuit [so that in the event of a fault enough current will flow to trip the OCPD] must be no higher than 80% of the ACTUAL Zs would be [like if we went out and measured it] The idea is, that if there is a fault, the cable would obviously get hot, and this increases the resistance of the copper, [so that less current would flow] so we have to have an allowance orf 20% to allow for this..

I do it like this though.. The reciprocal of 0.8 is 1.25...

So, remember the 160A??

Multiply this by 1.25, you will end up with 200A

Now, in the rules, they now decide that you have to pretend that the supply voltage is only 218v [i think it is] [to allow for volt drop caused by the fault current]

So divide 218v by the 200A and you will end up with 1.09 ohms..

This is the MAXIMUM earth loop impedance  that we can have measured at the END of our circuit.. [the earth loop is the route the "fault current" would take if you were to short the earth and "live" together, so from the cutout all through our "live" cable, across the fault, and all the way back down the earth cable to the earth terminal]

You will be thinking.. "what about the resistance of the cables going back to the power station then??" and this would be correct.. That is what "Ze" is..

Now, you can either measure Ze, or ask the DNO, but the USUAL maximum figure for TNS is taken as 0.8 of an ohm and for TNCS, as 0.35 of an ohm and the DNO try [but do not guarantee] to make sure that these figures sre not exceeded]

Even though we can measure Ze and find it is generaly lower than this, it is good practice to assume these figures [the ones above] are correct so that even if the supply from the electric board gets worse our circuit design is still ok!!

So, we have worked out for our 32A "B" curve OCPD that we need a maximum Zs of 1.09 ohms. Take from this the Ze, [say we are on TNS, so 0.8 ohms] to leave 0.29 ohms..

We now need to go back to the cable makers charts to find a cable where the figure for R1 + R2 [R1 being the "live" leg of our cable and R2 the earth leg, remembering that the CPC can be smaller than the "live" [think twin and earth] or of higher resistance material [steel armour instead of copper cable] is LESS than the 0.29 ohms for the length of cable we need, and we are good to go!!!

There is more to it than this, but the above will give you the basics..

You will find that for any longish lenght of cable, that even though you might THINK that you only need a cable that can carry 32A, you might ACTUALLY need one that can carry, say 80 odd amps if not much more], just so that volt drop and EFLI [Zs to you!!] at the END of the circuit are low enough...

Hope this helps....

john..

 

[Traineeboy]

Traineeboy..

Cable calculation is very involved, but i will try to give you a bit of an idea..

First thing, you need to do is to decide the design load.. Let us say 30A

Next you need to decide upon an OCPD to protect the cable.. That is what the OCPD is for.. 1, protecting the cable, AND 2, providing ADS, so that in the event of an earth fault [that may cause "exposed conductive parts" like say the the metal casing of a piece of equipment] to become "live" in the event of a fault, the supply is cut off in the required time to prevent cable damage and reduce duration of electric shock!!. [look up "disconnection times]

Yes, the current carrying capacity of the cable is important and you need to check this, and there are various calculations involved based on "installation method" etc.. BUT and it is a very big BUT, usually the deciding factor is one of two things.. either volt drop, or EFLI.

From this, you can see that the CABLE has to be selected to suit the OCPD, NOT the load at all..

Never mind what the actual incoming voltage is, you just take it that at the origin of the installation, [where the DNO supply cable ends at the cutout meter etc] that the voltage is 230v..

You know what the load current is, and you can look up from tables what the volt drop in millivolts per amp per metre is, so knowing the 3 or 5% bit, you can select a suitable cable from the point of view of volt drop..

Next thing, you need to look up what the maximum Zs is for the OCPD, or work it out for yourself.. I always work it out myself..... Lets say we had a 32a one, a "B" curve one...

Now to make this trip off in the required time, [both 5 seconds AND 0.4 seconds is the same current required] we need a fault current to flow of FIVE times the 32A.. This equals 160A..

Now, the calculated Zs for our circuit [so that in the event of a fault enough current will flow to trip the OCPD] must be no higher than 80% of the ACTUAL Zs would be [like if we went out and measured it] The idea is, that if there is a fault, the cable would obviously get hot, and this increases the resistance of the copper, [so that less current would flow] so we have to have an allowance orf 20% to allow for this..

I do it like this though.. The reciprocal of 0.8 is 1.25...

So, remember the 160A??

Multiply this by 1.25, you will end up with 200A

Now, in the rules, they now decide that you have to pretend that the supply voltage is only 218v [i think it is] [to allow for volt drop caused by the fault current]

So divide 218v by the 200A and you will end up with 1.09 ohms..

This is the MAXIMUM earth loop impedance  that we can have measured at the END of our circuit.. [the earth loop is the route the "fault current" would take if you were to short the earth and "live" together, so from the cutout all through our "live" cable, across the fault, and all the way back down the earth cable to the earth terminal]

You will be thinking.. "what about the resistance of the cables going back to the power station then??" and this would be correct.. That is what "Ze" is..

Now, you can either measure Ze, or ask the DNO, but the USUAL maximum figure for TNS is taken as 0.8 of an ohm and for TNCS, as 0.35 of an ohm and the DNO try [but do not guarantee] to make sure that these figures sre not exceeded]

Even though we can measure Ze and find it is generaly lower than this, it is good practice to assume these figures [the ones above] are correct so that even if the supply from the electric board gets worse our circuit design is still ok!!

So, we have worked out for our 32A "B" curve OCPD that we need a maximum Zs of 1.09 ohms. Take from this the Ze, [say we are on TNS, so 0.8 ohms] to leave 0.29 ohms..

We now need to go back to the cable makers charts to find a cable where the figure for R1 + R2 [R1 being the "live" leg of our cable and R2 the earth leg, remembering that the CPC can be smaller than the "live" [think twin and earth] or of higher resistance material [steel armour instead of copper cable] is LESS than the 0.29 ohms for the length of cable we need, and we are good to go!!!

There is more to it than this, but the above will give you the basics..

You will find that for any longish lenght of cable, that even though you might THINK that you only need a cable that can carry 32A, you might ACTUALLY need one that can carry, say 80 odd amps if not much more], just so that volt drop and EFLI [Zs to you!!] at the END of the circuit are low enough...

Hope this helps....

john..

Thanks John very helpful  

 

[binky]

or you could just use the Doncaster cables calculation tool  :^O . 

 

[Traineeboy]

What I’m trying to understand is if my house breakers are the following 

20,16,32,32,32,40,6,6

Totalling 184 

I fully understand that all circuits are not all used at once or to their full potential but how is the main fuse it Calculated. My main house fuse is 60 a and never had an issue. 

So..... 

If this outbuilding will have 

32

32

32

32

6

Totalling 142 

Am I basing the cable on being able to take 142a and what about it’s protective device ? 

Hope that makes sense. Sorry if I seem slow but I am dyslexic so reading takes me a bit to grasp. 

 

[Andy™]

What I’m trying to understand is if my house breakers are the following 

20,16,32,32,32,40,6,6

Totalling 184 

I fully understand that all circuits are not all used at once or to their full potential but how is the main fuse it Calculated. My main house fuse is 60 a and never had an issue. 

So..... 

If this outbuilding will have 

32

32

32

32

6

Totalling 142 

Am I basing the cable on being able to take 142a and what about it’s protective device ? 

Hope that makes sense. Sorry if I seem slow but I am dyslexic so reading takes me a bit to grasp. 

adding breaker sizes doesnt work very well and doesnt give a realistic value

i.e you have an old house with a 5a for lights and a 30a for sockets. total 35a.

you rewire it with

6a up lights

6a down lights

32a down sockets

32a up sockets

40a cooker outlet

6a smoke detectors

16a garage sockets

you now have a total of 138a, over 100a more than before, ye the actual power being used hasnt changed from before

 

[Traineeboy]

So really then I need to know Exactly what the customer wants to power. 
 

i.e

immersion heater

kettle 

treadmill

stereo

TV

computer 

lamp

 heater 

etc 

 

[Murdoch]

What I’m trying to understand is if my house breakers are the following 

20,16,32,32,32,40,6,6

Totalling 184 

I fully understand that all circuits are not all used at once or to their full potential but how is the main fuse it Calculated. My main house fuse is 60 a and never had an issue. 

So..... 

If this outbuilding will have 

32

32

32

32

6

Totalling 142 

Am I basing the cable on being able to take 142a and what about it’s protective device ? 

Hope that makes sense. Sorry if I seem slow but I am dyslexic so reading takes me a bit to grasp. 

If the 400 foot away building is intended to have the breakers above this raises some more very important questions about the heating, bonding, gas, and then you can look at diversity ...

Look at page 122 in the OSG and see how it applies to your own home and the building you outline above - let us know your thoughts and we'll give you some pointers 

So really then I need to know Exactly what the customer wants to power. 
 

i.e

immersion heater

kettle 

treadmill

stereo

TV

computer 

lamp

 heater 

etc 

To an extent but surely your boss knows this

Immersion heater? water? see post above

Either way when a quote or estimate is provided I recommend that the maximum design load is stated clearly - it will avoid awkward questions later and replacing the supply cable. 

 

[Traineeboy]

Thanks this week I will do all the calculations etc and see if I get it. Thanks for your pointers 

 

[Traineeboy]

If the 400 foot away building is intended to have the breakers above this raises some more very important questions about the heating, bonding, gas, and then you can look at diversity ...

Look at page 122 in the OSG and see how it applies to your own home and the building you outline above - let us know your thoughts and we'll give you some pointers 

To an extent but surely your boss knows this

Immersion heater? water? see post above

Either way when a quote or estimate is provided I recommend that the maximum design load is stated clearly - it will avoid awkward questions later and replacing the supply cable. 

Here is an example ... 

———————————

Ring est power needed 7KW

7000w Divide by 230v = 30.43A 

No diversity allowed for 1st ring 

———————————————-

Lights est power needed 1KW 

1000w divide by 230 = 4.34A 

4.34A - 33% = 2.91A 

————————————————

Immersion heater est power needed 3KW 

3000w divide by 230 = 13.04A 

No diversity allowed 

——————————————————-

Heater est power needed 3KW 

3000w divide by 230 = 13.04A 

10A + 50% of 3.04 = 11.52 A 

————————————————————

Totalling 57.9 A 

I know I need to apply volt drop to the cable and also work out tabulated current of which Ca and Cf apply. 

Then to double check size or CPC I would need to do the  adiabatic equation. 

Am I on the right lines ?

 

[Murdoch]

So I would do as @binky above suggested next and next try the calculator on line and see what that gives you - bear in mind as you will have a lighting circuit put 3% in as your volt drop then see what cable size it suggests (if there is one)

 

[Traineeboy]

For SWA it’s saying 70mm2 - Flip that is massive. Don’t think I’ve ever worked with that size before lol 

 

[poni]

i.e

immersion heater

kettle 

treadmill

stereo

TV

computer 

lamp

 heater 

etc 

its the heating that you need to know Kettle, water heater, room heater, not all will be no at the same time (kettle 3mins ,water heater depends on how much water you need, room heater on depends on time of year heat loss of room and the length of time its occupied) the rest are low current and he's  unlikely to have the kittle on when he's on the treadmill 

 

[binky]

when it comes to diversity, I prefer the Western Power calc, total load * 0.4  which suggests 25mm as suitable?