Cable size and load calculations

[Cazmac1]

Hi all, I need some help with working out the correct cable size for some work I'm doing at the moment.

I am currently working on testing Mobile units carrying out PIR and remedial works.

I have already worked out my answer which I will keep to myself as not to invite negative comments, the problem I have is that there seems to be some gray areas and I am in disagreement with a colleague at work.
I have also used many online calculator's and they give widely differing answers.

I have an existing 2.5mm thermoplastic/thermosetting 70 degree radial circuit, this is feeding an above sink 2KW water heater. The customer wishes to add a zip boiler above the sink that is rated at 2.2KW.
My question is can I just extent the exsiting circuit and by adding another 13A switched fuse spur or do I need to run another circuit?
Here are all the detail's...
Cable as stated above
Ambient air temp 30 degree
Installation method C (installed in surface mounted plastic Trunking)
Grouping derating factors. There would be a maximum of 3 other cables installed in the trunking at anytime but these vary. I suppose it would be best to use a worse case scenario.
Lighting circuit 1.5
Ring circuit 2.5
another radial 2.5
Cable run not more then 10 meters.

Could you also provide the answer as a single circuit for a type C installation.

PS: as both boilers will be constantly switching on and off at different times as the water in each will heat and cool at different times Can diversity be applied???

Thanks in advance.

 

[Cazmac1]

ok I've made some mistakes lol
believe me I did work it out right,
page 334 4D2a
method B = 23 as per the original post. got my C and B mixed up back to school for me.

 

[telectrix]

enclosed in conduit on a wall....or trunking is method B...

go look in a book...

so why is ref. method B not quoted in tables for T/E?

 

[Cazmac1]

so why is ref. method B not quoted in tables for T/E?

This is what I find confusing?

 

[telectrix]

it's because the IET, in their infinite wisdom, consider T/E to be only either surface clipped or buried in insulation.

 

[GLENNSPARK]

it's because the IET, in their infinite wisdom, consider T/E to be only either surface clipped or buried in insulation.

well that to me is just stupid...

i mean if its in trunking....then its in bloody trunking..isn`t it...

 

[telectrix]

yeah. table 4D5 in bgb. method B is non-existant. so it's a choice between A and C. 20A or 27A. split the diff. and call it 24A . everyone's happy, 1 radial will suffice. next job , please.

 

[Cazmac1]

I like your reasoning but still does not answer the question in full. I think I will just continue to use the one radial and let the other guys run a new circuit. Unless I can prove my argument with sound reasoning from the regs I'm on a loser. Maybe I'll contact the IET myself.

 

[snowhead]

If your job depends on the decision, just run 2 circuits and you're safe.

I doubt you will get decisive answer for this, it'll be 50 / 50.