Lighting ccts/ total power demand.

[Tom123]

Hi,

I'm at the final stage of my coursework and there is a few things which I'm trying to clarify i.e.

100m2 is the total area of the rural-premises.

I've used a 1-phase TN-C-S (TT was recommended but the choice of earthing system is not that important).

My main lighting circuit has 160m total length, and the volt drop using following formula for 1.5mm2 conductor size is:

Vdrop = (Ib x mv/A/m x L)/1000

Vdrop = (5.6 x 29 x 160)/1000 = 26 or 11%

According to Appendix 12 table 12A the maximum allowable vdrop on lighting ccts is 3% (6.9V) therefore do I have to use 4mm2 conductor size in order to meet the regulations or my formula is wrong ?

The second part is diversity:

12kW cooker, Ib = 61A.

Ib after diversity = 37A (60%) In = 40A <- this values has been used in Ipsc calculations.

As I look at the "Electrical installation design guide" it tells me to use diversity on cooker cct at the design stage so I can use lower rating of MCB.

So I'm just wondering, should I not use the worst case scenario i.e. 61A or is 37A legally acceptable ?

The final Ib has to be =< 100A (suppliers limit), mu current Ib currently is 180A(including Ib for cooker after diversity) so I can use any diversity factors to fit within the limit or could you recommend some real-life formula for it ?

Thanks in advance,

Tom

 

[Zeespark]

Earthing is the MOST important issue (I would TT the installation), and install 2 or even 3 circuits instead of 1 hugely loaded one. 37 Amps is legally acceptable. Diversity calcs can be found in the OSG.

AndyGuinness

 

[NozSpark]

With your volt drop calculation you have assumed that the full load is on the end of the cable and not distributed along it's length.

 

[Tom123]

Thank you, that's the answer I was looking for.

About the demand I will simpy apply it at the end of the report with my diveres cooker cct value.

 

[SLIPSHOD & SLAPDASH]

Earthing is the MOST important issue (I would TT the installation), and install 2 or even 3 circuits instead of 1 hugely loaded one. 37 Amps is legally acceptable. Diversity calcs can be found in the OSG.AndyGuinness

The supply type has nothing to do with this question. I agree with splitting the circuits but we dont know if this is a warehouse or a cottage. What does 37A is legally acceptable mean ? Your just complicating things.

 

[Zeespark]

I was referring to the 37A being the diversified cooker load Slips.

Andy

 

[SLIPSHOD & SLAPDASH]

Lets hope the cooker police aren't watching this or we will all be in the cells Wet Fish

 

[Zeespark]

Right to Tom and Slipshod, I apologise. I have just redone My numbers by the OSG (Red)

Table 1B page 97:

10A + 30% full load + 5A if a socket outlet is built into the cooker switch.

12000/230 = 52A

10A + (30% of 52A) = 25.6 + 5A = 31.6 A

A 32 Amp MCB would suffice for the job Tom.

Its been a while (2 years +) since doing a cable calculation, so I apologise of I have screwed the pooch so to speak.

AndyGuinness

 

[johninlondon1]

cooker = 12kW/230 = 52.17 amps, Diversity = first 10 amps = 10.00 amps

30% of remainder ie (52-10) x 30/100 = 12.65 amps

Socket outlet incorporated 5.00 amps

Total Cooker Circuit after applying diversity = 27.65 amps

Most people make the common mistake of taking 30% of full load instead of full load deduct the 10 amps then the remainder multiplied by 30%.

 

[Tom123]

how about a lighting cct, the total length for a 5.6A with 10A mcb (10A due to inrush currents) is 200m.

I've used a radial calculation formula: max. cable length = (6.9 x 1000)/(average current x mV/A/m x Ct)

6.9 is the 3% volt drop from 230V supply

Average current = 2.8A

mV/A/m is 29 for 1.5mm2 conductor

I've said the Ct factor is 0.41 to make that cable length 207m to satisfy requirements. What should I do in order to legally meet those requiremets without putting a random numbers in ?

 

[johninlondon1]

Ct is the adjustment for the temperature of the conductors at operating temp, normally 70 celcius for pvc conductors. You would use 1.2 as the correction factor.

 

[SPECIAL LOCATION]

cooker = 12kW/230 = 52.17 amps, Diversity = first 10 amps = 10.00 amps30% of remainder ie (52-10) x 30/100 = 12.65 amps

Socket outlet incorporated 5.00 amps

Total Cooker Circuit after applying diversity = 27.65 amps

Most people make the common mistake of taking 30% of full load instead of full load deduct the 10 amps then the remainder multiplied by 30%.

:Salute

Quite so John-Down-The-Big-City!!!! :^O

Or for the lazy sods.....

which most electricians are

Just read last but one paragraph of page 176 Green On-Site-Guide..

(or page 160 old money red book)...

Part of Appendix 8

"STANDARD CIRCUIT ARRANGEMENTS"

Cooker circuits in household & similar premises

A 30A or 32A circuit is usually appropriate for a household or similar cookers of rating up to 15KW!

So just remember the Up to 15kW rule-of-thumb for your bog standard cooker on 32A MCB...

jobs a goodun!

Guinness

 

[Sidewinder]

One last point, the legally bit, not really an issue, you can design as you please as long as your cables & protective devices are co-ordinated with the load.

BS7671 is not law by the way.

Even if you get nuisance tripping of your mcb under full load as long as the mcb rating is below the final cable rating you have not broken the law.

 

[steptoe]

how about a lighting cct, the total length for a 5.6A with 10A mcb (10A due to inrush currents) is 200m.I've used a radial calculation formula: max. cable length = (6.9 x 1000)/(average current x mV/A/m x Ct)

6.9 is the 3% volt drop from 230V supply

Average current = 2.8A

mV/A/m is 29 for 1.5mm2 conductor

I've said the Ct factor is 0.41 to make that cable length 207m to satisfy requirements. What should I do in order to legally meet those requiremets without putting a random numbers in ?

why?

why dont you use a different curved OCPD?

uprating the protective device isnt the answer to inrush currents, you will soon find that out once you start using larger devices,

why does everyone think this is the answer to their poorly designed lighting circuits? just bang a 10A in, she'll be right.!!!