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Topic Title: 3 phase immersion heater
Topic Summary: cable size and breaker
Created On: 04 September 2007 11:15 AM
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 04 September 2007 11:15 AM
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caboom

Posts: 100
Joined: 13 April 2006

what size cable is needed for a 6kw 3 phase immersion heater and what size breaker? cable run less than 5 metres.
 04 September 2007 01:37 PM
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rocknroll

Posts: 8878
Joined: 03 October 2005

Hope this helps, or do you want us to do it for you?

Cable selection procedure;

1. Determine the design current (Ib) of the circuit.

2. Select an appropriate size circuit protective device (In).

3. Apply correction factors (C) to the circuit as indicated.

4. Determine the effective current carrying capacity (Iz) and from the regulations find (It)

5. Ascertain the maximum permissible voltage drop and check the circuit voltage drop is in accordance with the formula: vd = mV/A/m x Ib x length ÷ 1000

6. Check the disconnection time of the circuit such that Zs(act) is equal to or less than Zs(Max).

7. Check the thermal constraints using the adiabatic equation.

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 04 September 2007 02:29 PM
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Theapprentice

Posts: 54
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This is a trick question, isn't it??
 04 September 2007 02:34 PM
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davebarman

Posts: 285
Joined: 27 February 2006

I assume that you are wiring it across 3phases and not linking it out and putting it across a single phase supply?

regards

dave

-------------------------
Never knock on Death's door. Ring the doorbell and run like hell, he hates that!
 04 September 2007 04:22 PM
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caboom

Posts: 100
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no. you guys do it for me. see what you all come up with.
 04 September 2007 04:23 PM
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caboom

Posts: 100
Joined: 13 April 2006

and yes wiring across 3 phases
 04 September 2007 04:42 PM
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rocknroll

Posts: 8878
Joined: 03 October 2005

Lets have your results first

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 04 September 2007 04:51 PM
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caboom

Posts: 100
Joined: 13 April 2006

i have no results thats why im asking for your help guys! come on you experts work it out and show your results....
 04 September 2007 04:57 PM
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Ricicle

Posts: 843
Joined: 23 October 2006

If you give me a share of your payment for the job I'll work it out for you.

-------------------------
Empty barrels make the most noise.
 04 September 2007 05:10 PM
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rocknroll

Posts: 8878
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i have no results thats why im asking for your help guys! come on you experts work it out and show your results....


Why dont you give it go first, then we will help you

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 04 September 2007 08:21 PM
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fredman

Posts: 221
Joined: 26 July 2007

Me thinks that you are not going to get a reply, tell you what tell us how much you got in for the job and we can then add the consultancy charge and you can pay us before we help.

xxxboys
Naughty I shouldnt say that

Edited: 04 September 2007 at 08:24 PM by fredman
 05 September 2007 02:57 PM
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caboom

Posts: 100
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nope cant do it guys. so any serious takers out there? thought this forum was about helping each other out especially those still learning?
 05 September 2007 03:06 PM
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rocknroll

Posts: 8878
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What do you mean you cant do it, have you worked out your design current yet? Do you knoe how to do it?

A clue;
Ib(design current) = kW ÷ √3Uo

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------

Edited: 05 September 2007 at 03:09 PM by rocknroll
 05 September 2007 03:07 PM
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caboom

Posts: 100
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no and no
 05 September 2007 03:11 PM
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Theapprentice

Posts: 54
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Originally posted by: caboom

nope cant do it guys. so any serious takers out there? thought this forum was about helping each other out especially those still learning?


I'm learning but really this is rudimentary stuff

3 elements @ 2KW apiece how hard can it be

Go on have a guess
 05 September 2007 03:23 PM
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caboom

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so theapprentice what size breaker and cable then?!
 05 September 2007 03:59 PM
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rocknroll

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Design current (Ib)

6000 ÷ √3 x 400

(Ib) = 8.7 Amps

Select circuit protective device, Type B 10A MCB

Select cable, 1mm²

Assuming no correction factors

Voltage drop, 5 x 8.7 x 44 ÷ 1000
= 1.9 volts

Assume TN-C-S of 0.35 Ze

R1 + R2 from OSG page 158

5 x 36.20 ÷ 1000
= 0.18 ohms

Zs = Ze + (R1 + R2)
Zs = 0.35 + 0.18
= 0.53

If = Uoc / Zs
If = 240 / 0.53
= 452 Amps

Max Zs for 10 Amp MCB = 4.80 ohms


Check for thermal constraints;

Now who will check a 1mm² is okay, just the job for AJ

Job done.

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------

Edited: 05 September 2007 at 06:53 PM by rocknroll
 05 September 2007 04:18 PM
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markLEDS

Posts: 121
Joined: 09 August 2007

rocknroll,

Would Ib not be (6000/3)/230 = 8.7a?

3 phase immersion heaters are commonly just a number of single phase elements, rather than 3 elements each with 400v across them.

It makes hardly any difference, so Im splitting hairs really. Guess the answer should be to get the loads from the manufacturer.

Mark
 05 September 2007 04:36 PM
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rocknroll

Posts: 8878
Joined: 03 October 2005


Ok, what if this is three phase and earth only no neutral, there are two fault conditions, phase to phase and phase to earth, is the 1mm² still okay.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 05 September 2007 04:56 PM
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caboom

Posts: 100
Joined: 13 April 2006

ok. we got there in the end!. so why is an niceic registered sparky saying to me that it should be 16A breaker and 2.5mm cable!!!!!!!!!!!!!
IET » Wiring and the regulations » 3 phase immersion heater

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