7.5Kw Single Phase Aircon Unit , Have I Calculated Correctly ?

[wsoppitt]

Hello fellas , gotta quote for connection of domestic 7.5 kw aircon unit ( my first one , so want to get it right )

from consumer unit to external unit is approx 10m and client wants it run in trunking , run will go from understairs cupboard through cloakroom , through conservatory where i will utilise ducting put in by aircon company which will take me to outside unit .

Plan to run in 6mm t+e internally then jointed to 6mm sy multiflex for external run . 

40amp type c mcb at consumer unit and 40 amp ip65 rotary isolator externally next to unit .

am i missing anything ?

also what would be a competitive price for this ,  got job through " my builder " so will need to be competitive .

Trunking runs are simple , straight runs of approx 2m then through wall , 2m then through wall and then 3m until utilise aircon ducting.

If im missing something or there is something that i should know please let me know , dont want to mess up .

thanks fella .

wayne

 

[ProDave]

Which bit of the UK is warm enough to need a 7.5KW Air con unit?

 

[steptoe]

what is your Ze,?

you need to be really careful of your Zs on a 40a type C with only a dinky little 2.5mm cpc .

 

[wsoppitt]

Which bit of the UK is warm enough to need a 7.5KW Air con unit?

Confirmed tonight what the power rating of the unit was and they confirmed it was a 7.5kw unit , already installed , so my question is still relevant .

 

[NozSpark]

Without checking I'd be surprised that 6.0 t&e would be OK for 10m on a C40

 

[SPECIAL LOCATION]

10m worth of 6.0mm T&E will have an R1+R2 of 0.1ohm

Max Zs for 40A typeC is 0.57ohm..

80% rule of thumb is Max Zs 0.456ohms...

Minus our R1+R1 leaves  0.356 ish worth of Ze to play with..???

Hope its not a TNS with up to 0.8ohms???

Competitive price??

£50 is competative....

Dunno about feasible though??

 

[wsoppitt]

what is your Ze,?

you need to be really careful of your Zs on a 40a type C with only a dinky little 2.5mm cpc .

Dont know what ze is , just been asked to quote for initial connection of unit .

10m worth of 6.0mm T&E will have an R1+R2 of 0.1ohm

Max Zs for 40A typeC is 0.57ohm..

80% rule of thumb is Max Zs 0.456ohms...

Minus our R1+R1 leaves  0.356 ish worth of Ze to play with..???

Hope its not a TNS with up to 0.8ohms???

Competitive price??

£50 is competative....

Dunno about feasible though??

PME supply .

 

[Steve3948]

But for your design you need to know.

Unless you allow for worst case and bump your price up, is that competitive - No.

 

[wsoppitt]

Without checking I'd be surprised that 6.0 t&e would be OK for 10m on a C40

You are right that 6mm is not quite up to the job with the bgb stating that max current carry capacity of 6mm in conduit/trunking being 38 amp or 46 amp clipped direct .

10mm is obviously the correct answer at 52 amp ccc but that isnt gonna get me the work when competing against people who have already quoted for 6mm t+e .

To be thorough i should bond the pipework , but that hasnt been included in the competitions quote so why would the client believe it is neccessary when all the others dont include ? 

 

[NozSpark]

All you can do is quote for doing the job correctly,,, maybe you need to give your quote face to face so that you can explain why you're quoting for a "thicker" cable

 

[SLIPSHOD & SLAPDASH]

Are you sure the 7.5kw is  the cooling/heat load & not the electrical ?

Edit

Got that the wrong way round

Are you sure the  7.5kw is the electrical  load & not the cooling/heat ?

ie, my boiler at home is rated at about 30kw, but runs on a 3A fuse.

An example here   http://www.aircon247.com/p/849152/airconditioningcentre-kfr75lw-75kw-toshiba-powered-easy-install-low-wall-air-conditioning-system-.html

7.5 kw heating/cooling but runs on a 13A supply

 

[kme]

Agreed.

I`ve got a 4KW unit in my house - its on a fused spur, with a 5A fuse in it!

The electrical load bears no relation to the heating / cooling rating.

And, even if you ARE talking about the electrical load, and want someone to justify cutting the cable size to minimum, just `cos someone else has......better go unhitch your horse, and find your spurs! Here`s the problem we all face regularly:

1. You want the job.

2. You think that means you have to beat every other quote.

3. Some muppet has drawn a size out of a hat, and decided to use it, even though it doesn`t comply - so he`s cheaper on materials.

4. As pointed out above, if he`s chosen to use the heat/cooling capacity to work on - you can come out looking good.

5. You don`t want to do a cowboy job.

Look at 1-5 above, and decide which apply to you. 

n.b. Ensure you don`t need to supply the interconnect cable to the indoor unit; or cabling for a wired remote.

HTH

KME

 

[SPECIAL LOCATION]

Is it just me or does anyone else get the impression that someone here hasn't got a clue how to design a new circuit...

Understand the essential details needed to calculate correct cable sizes and protective devices etc..

Verify the design complies with regs...

and

Cost the materials & labour for installing and testing that new circuit....????

  :shakehead

You CANNOT base a quote on what someone else has quoted.. 

You have to be clear yourself of whats needed and quote your time & materials to do a proper job.. 

Following like sheep cuz someone else says they need cable "xyz" when you haven't verified your own calcs is just asking for trouble!!

We used to go through loads of design calcs and questions at college for various OCPD's

maybe they don't do this anymore??

 

[steptoe]

Ive just got a job I quoted on at more than double the other person,

on walking round the job I was asking questions etc, and looked at the CU,

'why do you need to see our fuse box and water pipes and gas meter'  was the clients question, 'none of the other guys needed to look at them' ,

explained the RCD and earthing to them and they were happy, CU was OK dual RCD and all bonding in place,

I got the job as ,  'you just seemed to be a lot more thorough and knew what you were actually going to price for with no hidden extras unless something weird crops up'

 

[wozz]

6mm twin on a 40 always worries me, a assessor once told me it was the most common problem of that nature he comes across.  

 

[Steve3948]

But if design is correct then its fine, many factors need to be considered.

 

[wozz]

Yes, but i think he was getting at the fact many seem to think there is plenty of leeway with a 6mm twin on a 40A when in fact there isnt much.   Which i believe is probably true.  

 

[Zeespark]

WSoppitt, do you have the manufacturers name and model number for the A. C. unit at all? I could be wrong here ( I haven't seen the spec of the unit, hence why I have asked) but could a type D breaker be needed due to the initial in rush current of the A. C. units?

I could, and probably am, well wrong with My thoughts. :innocent

Andy Guinness

 

[SLIPSHOD & SLAPDASH]

Most of these size  units are designed to run off  a 13A plug or sometimes a 16A B type breaker is specified .

 

[Marvo]

Are you sure the  7.5kw is the electrical  load & not the cooling/heat ?

This was my first thought. A 7.5kW single phase motor is pretty unusual, your starting current could be as high as 200Amps.Your specifications don't make sense to me, as already mentioned you need the make and model number of the air con before you go any further with this.