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Topic Title: Cable sizing for a 3 phase 19Kw steam cooker could some check my calculations
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Created On: 09 February 2017 09:48 AM
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 09 February 2017 09:48 AM
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colliemcc

Posts: 5
Joined: 08 February 2017

Doing a project, in which I am installing a 3 phase 19kw steam cooker.

My Calculations are as follows, just not sure which (A) I use when picking the cable Ib or It

400v, 50Hz, 19Kw, power factor 1, cable run 20m, Ambient temp 50^C SWA run on its own on tray.

Ib= kwx1000/(?3xVl)
Ib= 19000/(?3x400)
Ib=27.42A

Pick the size of the MCB at 32A
Then check this from the Regs for size of cable for 27.42Amps?

or

Do I use It from below.

or is it

It = 32xCaxCg Ca=1/.79 and Cg is 1
It= 32x1/.71x1
It = 45A


Any help much appreciated
Thanks is advance

Calum
 09 February 2017 10:57 AM
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mapj1

Posts: 12039
Joined: 22 July 2004

There are two approaches, that should both come to the same answer.
Either treat it as 3 single phase loads of 1/3 the power each.

(19kW /3 is 6.3kW per phase = 27 amps or so.)

or
as a between phases load of 19kW from 400V, (47 amps) and divide by square root of 3,

47 amps /1.7 = 27 amps.

so, 2.5mm cable will be pushing it very hard, probably too hard if it is anything other than in open air, and 4mm will run cool.

Rule of thumb, when whistling between teeth and saying, 'hmm' maximum current numbers for 2.5mm, 4mm 6mm and 10mm to carry around when in open runs are 27A, 37, 47A and 67A. Not quite right being a mish mash of various cable types, but an expectation setter - if your answer when done properly is more than double or half this, then perhaps double check it - it may be right, but it is then a corner case, like lots of grouping, or of course volt drop limited rather than current capacity.

If it is resistive heating, and I'd expect a steamer to be, then there wont be much inrush , and a 32A breaker will run warm but happy.

-------------------------
regards Mike


Edited: 09 February 2017 at 11:03 AM by mapj1
 09 February 2017 12:05 PM
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colliemcc

Posts: 5
Joined: 08 February 2017

Thanks for the reply.

What is the It calculation for ?, as I thought it was the one you used to select the cable as it is the one to include factors such as temp and grouping factor etc. ie cable to be select must be able to carry 45amps from my calculation

Or is It the max current that the cable will have to with stand in the event of a fault.
 09 February 2017 12:53 PM
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AJJewsbury

Posts: 17795
Joined: 13 August 2003

Ib= kwx1000/(?3xVl)
Ib= 19000/(?3x400)
Ib=27.42A

Pick the size of the MCB at 32A
Then check this from the Regs for size of cable for 27.42Amps?

or

Do I use It from below.

or is it

It = 32xCaxCg Ca=1/.79 and Cg is 1
It= 32x1/.71x1
It = 45A

Whether you use Ib or In as the basis of your calculation depends on whether you need to provide overload protection or not. If not then it's safe to presume that the current the cable will be carrying for a significant length of time won't be higher than Ib that it's safe to select a cable based on that (with correction factors), if however overload is possible and you're relying on the protective device (In) to prevent higher currents flowing then you need to base the cable rating on that value.

- Andy.
 09 February 2017 01:37 PM
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colliemcc

Posts: 5
Joined: 08 February 2017

Thanks Andy for the reply, What is the It calculation for?
 09 February 2017 01:52 PM
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AJJewsbury

Posts: 17795
Joined: 13 August 2003

What is the It calculation for?

It is the "tabulated" current carrying capacity for a cable.

The "obvious" way to select a cable might be to apply all your correction factors to the tabulated ratings and then select the size you need from those - e.g. 2.5mm2 might start off with a rating of 27A but after applying your factors for grouping and ambient would end up being able to carry just 21.33A safely, likewise 4.0mm might de-rate from 37A to 29.23 and 6.0mm from 47A to 37.13A. So if you needed overload protection and a 32A MCB you can see you'd have to pick 6.0mm2.

Obviously the problem with that approach is that you have to do the calculation for every likely cable size - which is a bit tedious.

So people usually do it the other way around - apply the factors 'upside-down' to the required current rating - which gives you a higher value that you can compare with the tabulated values directly. So if you started off with 32A MCB, you'd need a cable with 32/0.79 = 40.5A - which leads you to 6mm2 directly.

- Andy.
 09 February 2017 02:06 PM
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broadgage

Posts: 3170
Joined: 07 August 2007

All the above presumes a balanced load, which is not always the case.
The maker of the equipment may keep in stock a limited number of standard size elements and use a suitable number to achieve the desired loading.

Also in my view a reasonable allowance should be made for manufacturing tolerances and mains voltage variations.

For a continuous long hour load I would allow A margin of about 10% and would therefore calculate on the basis of a nominal 21KW load not 19KW.

I would be rather concerned about a cable rated at 27 amps being used for a calculated load of 27 amps that is in fact 30 amps because firstly the current was calculated at 230 volts but the actual supply is 242 volts, and secondly because manufacturing tolerances mean that the "19KW" elements are in fact 19.8KW.

If the load was intermittent, then it should be fine.
 09 February 2017 02:12 PM
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colliemcc

Posts: 5
Joined: 08 February 2017

The Rating on the Cooker

18.2 KW
26.2 A

I put down 19 to allow a little extra.

Thanks

Colm
 09 February 2017 02:15 PM
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AJJewsbury

Posts: 17795
Joined: 13 August 2003

Also in my view a reasonable allowance should be made for manufacturing tolerances and mains voltage variations.

There's already a bit of tolerance in the system - cable ratings match MCB ratings (close protection as it used to be called) and MCBs are guaranteed not to trip with anything up to a 13% overload - so presumably there's a similar margin built into the cable tables.
- Andy.
 09 February 2017 02:33 PM
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OMS

Posts: 22864
Joined: 23 March 2004

LoL - round it up to the next number divisible by 3 - so 21kW

Divide by 3 = 7kW

We know a kW is 4A in old money - so 7 x 4 = 28A

So that's a 32A MCB

Then we need a cable that can carry 32A with a potential number of factors - in most cases 0.8 is fine and it allows the cable to not run at full temperature

so, 32/0.8 = 40A

Assume it's clipped direct

Quick look at 4D4A and we need a 6.0mm2 4Core good for 42A

Most electricians will be familiar with a 32A supply on a 6.0mm2 circuit in my experience

Crack on, trouser the cash, job jobbed, move on

OMS

-------------------------
Let the wind blow you, across a big floor.
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