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.

 

[Pete999]

If you are intending to extend the existing radial that has a 2kw water heater connected to it, to add a further 2.2 kw will be pushing it. My advice would be to add a separate radial to feed the Zip boiler. Or are you doing away with the existing water heater? by the way what is a type C installation?

 

[telectrix]

18A total load with intermittent switching of both appliances. OCPD of 20A, what's the fuss?

 

[Cazmac1]

If you are intending to extend the existing radial that has a 2kw water heater connected to it, to add a further 2.2 kw will be pushing it. My advice would be to add a separate radial to feed the Zip boiler. Or are you doing away with the existing water heater? by the way what is a type C installation?

Sorry my mistake should be installation method C

 

[Cazmac1]

18A total load with intermittent switching of both appliances. OCPD of 20A, what's the fuss?

I agree with you, If I was installing two water heaters in a house I would run two circuits, But we are talking about temporary mobile accommodation that requires test every six months or when re-sited.
The fuss is that if I choose to put both boilers on the same circuit and can't back it up via the regs I could lose my job.

What does OCPD mean???

 

[telectrix]

over current protection device. i.e fuse/mcb.

 

[Cazmac1]

Thanks
Installation method C says that a 2.5 mm cable at 30 degrees can carry 23 amps. but when you add the de-rating grouping factors it takes it above 23 amps.

18.2 divided by 0.7 = 26 amps


This means I can't run both boilers at full load on a 20 amp CB.

The gray area I was talking about is the de-rating for the grouping factors. The only information I could find in the regs seems to be related to PVC singles grouped together. Is the same de-rating applied to T+E as they are sheathed.

 

[Pete999]

Can't you just install another radial? or are there other problems you haven't mentioned

 

[Cazmac1]

Just want to add to my previous post... what I should have saids is yes I can run both boilers on an 20 CB, but the cable will become very warm?

 

[GLENNSPARK]

whats this `method C` all about?

you have said its going to be installed in a trunking.....so try method B for a start..

 

[Cazmac1]

Can't you just install another radial? or are there other problems you haven't mentioned

There are no problems installing another radial. the problem is that we are under pressure to turn out these units and any time saving is a must. The other problem is it has to be in plastic trunking. Due the the design of the units (they have a raised plastic boarder on the top of the wall) it is difficult to install the trunking and make it look good.

 

[Cazmac1]

whats this `method C` all about?

you have said its going to be installed in a trunking.....so try method B for a start..

Has to be C, no other option.

 

[Cazmac1]

were getting of the track here.
What I want is someone to work out the the correct size of conductor and CB using the info in my original post so I can confirm that I have worked it out right as per the regs.

 

[Cazmac1]

Just checked the regs and can see why i've been questioned on the installation method lol.
seems B is the correct method for trunking I will recheck my calculations.

 

[GLENNSPARK]

Just checked the regs and can see no way i've been questioned on the installation method lol.
seems be is the correct method for trunking I will recheck my calculations.

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

go look in a book...