Pop Quiz - Trainees Cooker Question

[Nigel]

Ok thanks

Burnsy had detailed exactly what we are looking for. It is very easy to get caught up in just saying things without really knowing where to back it up in the Regs book. That is why I have specifically mentioned Regs.

 

[Jay 1987]

yes i understand, thank you, will try again !

 

[Nigel]

Ok thanks

Burnsy had detailed exactly what we are looking for. It is very easy to get caught up in just saying things without really knowing where to back it up in the Regs book. That is why I have specifically mentioned Regs.

yes i understand, thank you, will try again !

My advice would be to go deeper into calculations on cable size. Think correction factors and how they will influence your design.

 

[Jay 1987]

Ok

 

[Nigel]

Ok. Forum Mentors. Can we vote on a winner please???

Use the ‘Winner’ to vote.

 

[Jay 1987]

Ok. Forum Mentors. Can we vote on a winner please???

Use the ‘Winner’ to vote.

Is there no more probationary period ?????

Don’t want to leave it till Xmas day lol

 

[gazdkw82]

It would be nice to get an outcome with answers

 

[Nigel]

It would be nice to get an outcome with answers

I will rustle something up over the weekend and then await to be picked to pieces by others. Haha

 

[telectrix]

I will rustle something up over the weekend and then await to be picked to pieces by others. Haha

hopefully there won't be the wrong kind of snow on the tracks.

 

[MDJ]

and then await to be picked to pieces by others. Haha

I cannot wait

 

[gazdkw82]

Still waiting ;-)

 

[Nigel]

@gazdkw82 - can you explain where you got the Ci rating factor of 0.88 from please?

PS - I know the answer ;-)

 

[Nigel]

Ok so here goes:

Cable Calculations:
Rating factors we need to consider are as follows:
Ca = ambient temperature
Cf = OCPD
Ci = Insulation

So, we won't realistically exceed 25 degrees in a dwelling so the rating factor here is 1.03 (Table 4B1 70 deg thermoplastic).

As the OCPD is a BS 3036 we need to apply a RF to this also as the breaking accuracy is rubbish. For this we apply 0.725 (App 4 Section 4).

Then we have insulation which is already covered when we decide a cable and installation method. So, for T&E cable, Table 4D5 has already applied the RFs and then printed the max CCC of the different size cables. Our reference method will be 100 for T&E above plasterboard in less than 100mm thermal insulation (Table 4A2).

So now we need to look at our design current (lb). The cooker’s maximum demand is 49.6 amps. Now, we are allowed to apply diversity to this load as it is unlikely that the cooker will every run at full load (311.1). The ONG gives us some calculations here:

Remove 10 of 49.6 (leaves 39.6)
Take 30% of that figure away (leaves 11.88)
Put back the 10 we took away earlier = 21.9A

So; lb = 21.9A

If we then looked at Table 4D5, which is what we have already agreed is our table for CCC for T&E cable and reference method 100 then for a cable to safely hold 21.9 amps we would need a cable size of 4mm.

However, we have not applied our rating factors yet; so:
lb
(Ca x Cf)
-----------------------------------
21.9
(1.03 x 0.725)
-----------------------------------
21.9 / 0.75
New lb = 29.2A

So, we now look again at Table 4D5 and we can now see we need a 6mm to take 29.2A and an OCPD of 30A.

We check the voltage drop which we are told is not to exceed 5% on a non-lighting circuit (Table 4Ab) so our maximum permitted VD is 6.9v (we use Nominal Voltage for VD not measured voltage. Nominal is always 230v)

So, the calculation for VD is:

(mV/A/m) x lb x Length
1000

We get our mV/A/m figure from Table 4D5 which for a 6mm T&E is 7.3.

7.3 x 29.2 x 25
1000
----------------------------
5329 / 1000 = 5.33

VD = 5.33v (which is less than our maximum permitted of 6.9v) so within limits.

So, we would need to install 6mm T&E for this cooker based on what we know so far.

In terms of no isolating switch Regulation 463.1.3 & 464.1 clearly stipulate isolation is required. Also, it could be argued that Regulation 465 also applies.

Now we know the customer has to have an isolator we have two choices. One with a socket or one without. Diversity states we need to add 5 amps if the isolator has a socket. If we add 5A to our lb, that becomes 34.2A. Checking Table 4D5 again and now we need to install a 10mm cable and an OCPD of 45A. This changes the job completely and an option with costs would need to be presented to the customer to make an informed decision.

Now to look at the CU. There is currently no RCD protection and Regulation 522.6.202 clearly states that we must have RCD protection for our new circuit. If we were to install a cooker isolator with a socket Regulation 411.3.3 will also stipulate RCD protection for the socket outlet regardless of the installation method.

As BS 3036 do not do RCDs we would need to offer the customer either a CU change or install a standalone RCD and enclosure to protect our circuit. The preference being the CU change.

Bonding – Table 54.8 states that in a PME system the minimum cross-sectional area of the main bonding conductor where the tails are 35mm or less is 10mm. So, the customer would need to upgrade the bonding conductors to 10mm also.

In conclusion:

The customer needs a CU upgrade to include RCD protection, main bonding increased to 10mm and either a 6mm or 10mm T&E depending on if they want a socket with the isolator.

 

[gazdkw82]

Brilliant breakdown.

I'm going to go through it in more detail later when I can get some books out.

I messed up on a few things so id like to review.

Didn't realise about the dersting factor for BS3036.

 

[Nigel]

Brilliant breakdown.

I'm going to go through it in more detail later when I can get some books out.

I messed up on a few things so id like to review.

Didn't realise about the dersting factor for BS3036.

You referenced 0.725 in your calcs????

I know you got your Ci Rating Factor from Table 52.2 but you need to read 523.9 carefully to understand why this is incorrect.