Cable Calculations

[Puthor]

Hello i need some help with the cable calcuations.

The question is as followd

Lighting Circuit (12 lamps)

Domestic House

Type B Circuit Breaker

Clipped Directly to wooden joist

-(above plasterboard ceiling coverd by thermal insulation exceding 100mm thikness)

Flat twin & earth cable

30 meters lenght of circuit

TNC supply

Ib=

In=

It=

Iz=

Vd=

R1&R2=

Zs=

If=

s=

When i try to solve it i get.

Lighting Circuit (12 lamps) = 1200

Domestic House = 230v

Type B Circuit Breaker

Ib=5.2

In=6a (But i dont know witch one. Closest is 6a but in BS7671 there is 4 diferent circit brakers. It cant be the C and D so is it the "Type B Circuit-Braker to BS EN 60898" or is it "Fuses to BS 88-2.2 and BS 88-6"

Clipped Directly to wooden joist

-(above plasterboard ceiling coverd by thermal insulation exceding 100mm thikness)

Flat twin & earth cable

It= In/Cc*Ci*Cg*Ca*Cr (But i dont need to use the Cc or Cr right?) It say nothing about the "Ambient Temperature" do that mean is 1? and same with "Grouping" 1? The "Thermal Insulation" is that 10.5a 1mm cable?(page 282 BS7671)

In/Ci*Cg*Ca

6/1*1*?(10.5? or 1mm) I think its 10.5 but that be to high so 1?

=6?

(im prity sure somthing is wrong here)

Iz=1mm

30 meters lenght of circuit

44*5.2*30/1000

Vd=6.864

TNCS supply

R1&R2= 1.129

36.20*1.04*30/1000

Zs=7.67

If=29.98

(v(if^2*t))/k=s

(v(29.98^2*0.1))/115=0.082

s=0.082

It would help me very much if somone could tell me what im doing wrong. And how i can correct it. Thanks allot in advance.

 

[Doc Hudson]

Welcome to the forum Puthor, it is good to see you have had a go at answering some parts of your question yourself. Keep checking back here during the course of the day and I am sure someone will offer some help. Some of our members are out at work and may not get back till later so make sure you check back later in the evening or tomorrow morning for more answers. Why not go to our introduce yourself section and tell us a bit about who you are while you are waiting.

Doc H.

 

[Zeespark]

Could it be the rule of thumb of 100 watts per light fitting Canoeboy?

AndyGuinness

 

[Puthor]

I think it is the rule of thumbs of 100 watts yes.

 

[Zeespark]

I don't disagree with you Canoeboy, but could that rule of thumb be getting the boot with all of the different varieties of lamp that can now be gotten a hold of?

AndyGuinness

 

[Puthor]

Aha. so "Ci" is 0.78

so it should be 6.6?

1. Ib 5.2

2. In 6

3. It 6.6

-------------

4. Iz 10.5 (Not sure im right here?)

-------------

5. Vd 6.864

6. R1&R2 1.129

7. Zs 7.67

8. If 29.98

9. s 0.082

 

[meerkat]

Hi Phutor,

It is difficult to calculate without full details required, is this a :-

TN-C-S Max Zs = 0.35Ω

or a TN-S Max Zs = 0.8Ω?

Ib = P/V = 1200 (12 x 100w) / 230 = 5.2Amps

In = MCB type B = 6Amps

It = In/ Correction factors, see page 255 (brb) section 3 no correction factors reqd, so It = 6Amps

Iz = a Flat cable with CPC 70˚C with a CCC > 6 Amps, use table 4D5, Ref method is 101 above plasterboard with insulation exceeding 100mm, column 2, a 1mm cable CCC = 10.5Amps, look at voltage drop from same table column 7 = 44 Mv/A/M

Fault protection formula = Ib ≤ In ≤ Iz = 5.2A

 

[Doc Hudson]

I don't disagree with you Canoeboy, but could that rule of thumb be getting the boot with all of the different varieties of lamp that can now be gotten a hold of?AndyGuinness

I guess they are still using the 'allow 100watts per point rule', but as you say Zeespark, with retail outlets not selling 100watt lamps anymore it does seam a bit silly to say the least. Are 60watts also obsolete yet?

Doc H.

 

[Zeespark]

I think they are - a very unwise decision IMHO.

AndyGuinness

 

[Puthor]

Thanks a lot!

That help a ton.

Yes sorry i ment a TNCS system

I tried another question after that but got stuck again.

But this one is to do with Iz (cable size)

12kW cooker with socket-outlet

Domestic House

Bs 3036 fuse

Enclosed in cnduit in an insulated wall

Flat twin & earth cable

18 meters length of circuit

TNCS supply

The cable for "Enclosed in conduit in an insulated wall"( Method A ) for flat twin & earth is 57 at max but it needs to be 60 amps at least?

Am i not looking for the right cable?

1. Ib 52.17

2. In 60

3. It 60

4. Iz ?

 

[Doc Hudson]

Thanks a lot!That help a ton.

Yes sorry i ment a TNCS system

I tried another question after that but got stuck again.

But this one is to do with Iz (cable size)

12kW cooker with socket-outlet

Domestic House

Bs 3036 fuse

Enclosed in cnduit in an insulated wall

Flat twin & earth cable

18 meters length of circuit

TNCS supply

The cable for "Enclosed in conduit in an insulated wall"( Method A ) for flat twin & earth is 57 at max but it needs to be 60 amps at least?

Am i not looking for the right cable?

1. Ib 52.17

2. In 60

3. It 60

4. Iz ?

You will need to take Diversity into account with a cooker. The appliance rating is the value of all hob rings, grills, oven elements etc, but in the real world nobody has everything on at the same time. The On-site-guide has the recommended diversity values for cooking appliances. If you cant find it someone will probably add a table or page number later. Remember most domestic cookers work fine on a 30amp or 32amp fuse.

Doc H.

 

[Puthor]

Ah yes thats how it is

Thanks Doc!

 

[a1]

Are 60watts also obsolete yet?Doc H.

No Doc,

I have boxes and boxes of them in (my) stock. ]

 

[Doc Hudson]

No Doc,I have boxes and boxes of them in (my) stock. ]

Keep them locked up for another 5 or 10 years then you can sell them @

 

[Puthor]

Thanks again that helped me alot. I hope i'm right now

So these 2 are correct?

12kW cooker with socket-outlet

Domestic House

Bs 3036 fuse

Enclosed in cnduit in an insulated wall

Flat twin & earth cable

18 meters length of circuit

TNCS supply

1. Ib 27.65

2. In 30

3. It 30

4. Iz 6mm

5. Vd 3.63

6. R1&R2 0.22

7. Zs 0.57

8. If 403.5

9. s 1.569

----------------------------------------

Lighting circuit (12 lamps)

Domestic house

Type B circuit breaker

Clipped direct

Run through loft (45

 

[meerkat]

Hi Puthor,

I am not sure about your adiabatic equation answer for scenario one,

show your calculation, so we can see how you get 1.569mm

everything else checks out fine,

(MEERKAT)

 

[Puthor]

Hello sorry for replaying so late.

I did (√(If

 

[Puthor]

BRB page 245?

The fault current for this instalation is ony 403.5 and to trip a 30a in 0.1 sec it needs 450?

or am i doing somthing wrong?

 

[skydog]

diversity ?

its a good idea, but in the real REAL world, i know there ARE people out there that in the middle of winter, when the gas has run out, will put on just about every

electrical socket outlet and circuit that will generate heat, so thats diversity out of the window.

i'm taliking about nans and students lol.

then, and only then can they benefit from the underfloor heating i.e. the conducting twin and earth

 

[meerkat]

Hi Puthor

BRB page 245?The fault current for this instalation is ony 403.5 and to trip a 30a in 0.1 sec it needs 450?

or am i doing somthing wrong?

the scenario fault current is 403.5 Amps

450 Amps will trip fuse in 0.1 Sec's

300 Amps will trip fuse in 0.2 Sec's

therefore 403 Amps falls somewhere in between, ie 0.15 Sec's

I hope this helps,

(MEERKAT)