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Topic Title: Sizing CPCs in parallel circuits
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Created On: 03 December 2008 03:08 pm
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 03 December 2008 03:08 pm
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kalookicedogs

Posts: 181
Joined: 23 October 2008

Advice please,

I am currently designing a circuit which has lead me, for volt drop purposes to use 2 x 400mm 4c in parallel. This is something I dont like doing but that is another topic. Current wise each cable is capable of carrying the circuit design current in it's own right. How should I size the cpc. Should it be 240mm (half of one of the cables in parallel which can take the full load current), or should it be half of the cross sectional area of each line conductor, which effectively would be 800mm/2 = 400mm.

Any thoughts gratefully received.

Regards, Mitch

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 03 December 2008 05:05 pm
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OMS

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That must be some volt drop if the design current is such that a single 400mm can carry all the load including derating for grouping - or you have a hell of a run to overcome.

Alternatively are you severely limiting the design by imposing an unreasonable volt drop to this distributor circuit.

To answer your question, personally I would evaluate the CPC required by means of an adiabatic expression rather than trying to adopt a "deemed to comply" approach which is unlikley to be economic - particularly if I can get back to a single 400mm2 cable.

If you want to use to use this approach you would need S/2 if copper (as you say a 400mm conductor) or K1/K2 x S/2 if you want to use the armour (say 622mm2).

You may want to check my figures but I think a 400mm 2 4 core will have an armour CSA of 452mm2 so 2 in parallel will have an armour CSA of over 900mm2 and as such this exceeds the minimum of 622mm2 which gives you compliance by default.

Perhaps if you provide further details of the design load and distance a more economic design may be achievable - if you want to continue with a parallel pair of 400mm2 then you can claim the CPC is adeuate on a deemed to comply basis - although at this sort of level, I would always calculate based on knowledge of the protective device and prevailing fault level

Regards

OMS

EDIT - I based the info on the use of thermosetting XLPE (ie a 90C cable)

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Edited: 03 December 2008 at 05:07 pm by OMS
 03 December 2008 05:19 pm
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rocknroll

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Obviously a bit of guesswork on your part;

Your load will be equally shared along the parallel conductors so a slightly different equation comes into play.

In >= Ib
It >= In / (n) x Cg
(n) number of conductors per phase

Example;
A circuit with an Ib of 500A protected by a MCCB of 630A(In)
Cable grouping 2 cables per phase 0.80

In >= Ib
MCCB 630A >= 500A

It >= In / (n) x Cg
It >= 630A / (2 x 0.80) - 393A

Select a cable from the regs which has a current carrying capacity of >= 393A:

Remember you need to factor 2 into your vd calculations as well;

Ib x L x (mV/a/m) / 2 x 1000

Then you can go on to work out fault protection and sizing of the cpc using the good ole adiabatic.

Job done ; happy

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
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"Oh! The drama of it all."
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"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: 03 December 2008 at 10:45 pm by rocknroll
 03 December 2008 08:02 pm
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Jaymack

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Originally posted by: kalookicedogs
I am currently designing a circuit which has lead me, for volt drop purposes to use 2 x 400mm 4c in parallel.

Who's the unlucky contractor who's doing the cable laying? he'll need some clydesdales for that job. Have you considered single cores or for shorter runs - busbars?

Regards
 04 December 2008 08:37 am
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kalookicedogs

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Thanx all for info so far. Yes, this is all based on multicore XLPE 90 oC cable. The run is 320m burried direct into the ground. Protection is provided by a 400A ACB set to 315A. The customer specifies a volt drop not exceeding 3%. 400mm cable seems excessive but I selected it on the basis that should one cable fail then the other is still capable of carrying the design current. I looked at the possibility of running single core XLPE/pvc/awa/pvc but could find no data for reference method D for this type of cable???? Does any one know why this is?

Thanx, Mitch

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 04 December 2008 09:23 am
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Jaymack

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I don't know why you would consider 100% redundancy for this cable supply unless it's for some strategic importance, is there another option of having the local electricity supplier lay on a local supply?
Have you considered a supply to a local transformer at a higher voltage?
Have you considered a group of smaller cable in parallel - say 150mm² both for cable handling and a measure of redundancy?

Regards

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 04 December 2008 09:32 am
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OMS

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Protection is provided by a 400A ACB set to 315A. The customer specifies a volt drop not exceeding 3%.


I would go back and argue the 3% allowance and explain to your client the grossly disproportionate design required to achieve it.

Unless there is some voltage critical element at the load end (and if there is how will the ESQCR tolerance impact on this) then I would suggest that the distributor could conservatively run at say 7% giving at least a further 5% in the final circuits. (I have assumed that the client has some control over the distribution transformer).

Personally, I would set a voltage required at the load and work back from there towards the distribution TX. In addition, is the load really approaching 315A - and have you allowed a temperature correction based on the comparison of Ib to It.

I would be really suprised if this actually needs a totally redundant parallel conductor just to achieve a volt drop criteria (unless of course there is a real insistence on a 3% limit which I would robustly challenge)

Regards

OMS

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 04 December 2008 09:35 am
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mstaple

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Mitch, I hope you are not competitively tendering for this job! If the client doesn't ask for 100% redundancy, why do it?

Usually we run parralel conductors to REDUCE the csa of the cables to make it easier to install. Not add another one of equal size.

I think Jaymack and OMS have some good ideas for you there.

Edited: 04 December 2008 at 09:37 am by mstaple
 04 December 2008 09:42 am
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Jaymack

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Originally posted by: OMS
I would go back and argue the 3% allowance and explain to your client the grossly disproportionate design required to achieve it.

Not unreasonable for a distribution circuit. I'm more accustomed to 2%

So many questions, so little time.
 04 December 2008 09:51 am
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OMS

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Not unreasonable for a distribution circuit. I'm more accustomed to 2%


Well I suppose it depends on how you define distribution circuit - perhaps what I should have said is that limiting a large installation to a 5% criterion between the supply offloading point and the final circuit load is rarely achievable with any form of realistic and economic design.

If you are talking about a few sub mains within a small commercial building then 2% might be reasonable.

I think its important not to get too carried away with Appendix 12 "deemed to comply" scenarios without considering the tolerances in the ESQCR as well in order to achieve a sensible solution


Regards

OMS

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 04 December 2008 10:15 am
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kalookicedogs

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I have calculated that I could use 2 x 240mm which would obviously be more manageable, but in the event of one cable failing the acb would not be suitably protecting the cable. This could obviously be monitored via voltage drop detection but is it best practice. What is deemed as best practice for protection of circuits in parallel. Should each circuit be protected individually of should the conductors be rated to allow for redundancy, as I have initially done here.

Your thoughts??


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 04 December 2008 10:25 am
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Jaymack

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Originally posted by: OMS
Not unreasonable for a distribution circuit. I'm more accustomed to 2%

Well I suppose it depends on how you define distribution circuit - perhaps what I should have said is that limiting a large installation to a 5% criterion between the supply offloading point and the final circuit load is rarely achievable with any form of realistic and economic design.
If you are talking about a few sub mains within a small commercial building then 2% might be reasonable.
I think its important not to get too carried away with Appendix 12 "deemed to comply" scenarios without considering the tolerances in the ESQCR as well in order to achieve a sensible solution

I suppose you have to attempt to justify your claim to be a consultant, I'm not convinced.

Regards

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 04 December 2008 10:30 am
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Jaymack

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Originally posted by: kalookicedogs
I have calculated that I could use 2 x 240mm which would obviously be more manageable, but in the event of one cable failing the acb would not be suitably protecting the cable. This could obviously be monitored via voltage drop detection but is it best practice. What is deemed as best practice for protection of circuits in parallel. Should each circuit be protected individually of should the conductors be rated to allow for redundancy, as I have initially done here. Your thoughts??


I think you should stay in the shallow end of the pool or hire someone who kens whit they're doin.

Regards
 04 December 2008 10:44 am
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OMS

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I suppose you have to attempt to justify your claim to be a consultant, I'm not convinced.


I don't justify anything - my role is to borrow the clients watch and then charge them a fee for telling them the time

I don't know why your not convinced, unless of couse you can defy the laws of physiscs within a reasonable budget.

It takes two things to be a consultant - grey hair and hemorrhoids. The grey hair makes you look distinguished and the hemorrhoids make you look concerned.


Well all I can say is my hair is receding faster than it can turn grey and I don't sit down long enough for piles - I do have the supercilious attitude and complete lack of ethics however - does that count

Regards

OMS

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 04 December 2008 10:55 am
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OMS

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but in the event of one cable failing the acb would not be suitably protecting the cable


Protecting it from what ?


May I suggest you go back to basics here - define the load, you have the distance and can set a sensible volt drop limit along with an installation method - ie buried direct or in ducts

Correct your Volt Drop calculation for power factor and temperature (temperature correction applies to the resistive component only) - and see what cable size results.

Determine if this is physically the most sensible solution bearing in mind any redundancy issues.

Evaluate the cable size for thermal withstand in earth fault and short circuit conditions. Determine if you need overload protection.

Derive the set points of the MCCB/ACB for communication to the commissioning contractor.

Regards

OMS

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Let the wind blow you, across a big floor.
 04 December 2008 10:57 am
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kalookicedogs

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Originally posted by: Jaymack

Originally posted by: kalookicedogs

I have calculated that I could use 2 x 240mm which would obviously be more manageable, but in the event of one cable failing the acb would not be suitably protecting the cable. This could obviously be monitored via voltage drop detection but is it best practice. What is deemed as best practice for protection of circuits in parallel. Should each circuit be protected individually of should the conductors be rated to allow for redundancy, as I have initially done here. Your thoughts??




I think you should stay in the shallow end of the pool or hire someone who kens whit they're doin.



Regards


Jaymack,

Maybe you should hire yourself a secretary who would then be able to type out your responses in legible queen english.


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 04 December 2008 11:03 am
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OMS

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Joined: 23 March 2004

Maybe you should hire yourself a secretary who would then be able to type out your responses in legible queen english.


Physician heal thyself

Pay no attention to Jaymack - its a Caledonian thing, try reading it with a wheezing Glasgi accent.

That said, if you are in any doubt as to what is going on with this distributor try to get some independant advice

Regards

OMS

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Let the wind blow you, across a big floor.
 04 December 2008 11:13 am
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kalookicedogs

Posts: 181
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Thanx OMS. To quote Rock and Roll on a similar thread

"There are problems associated with parallel cables that need to be considered;

1. You cannot guarantee the load will be equally shared between the two conductors.
2. Should a conductor fail the remaining conductor needs to be capable of carrying the full fault current, especially if you go down the route of an MCB for each cable.
3. As was mentioned the length must be the same, but this also related to the cable type and csa, if you are going down the route of different size cables in parallel then the calcs get a bit more involved. "

As with most topics, people do things in different ways, some right and some wrong which is why I asked the question on best practice opinions.

Thanx

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 04 December 2008 11:27 am
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OMS

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Yes I know all about the problems of parallel circuits - what I was trying to get you to think about is " do you need to complicate a simple design".

As I pointed out above - first sort out the criteria you are trying to design for - post them on here if you like - and then go from there - only introduce parallel conductors if there is no alternative - think along the lines of simple first and then use more complex methods if simple doesn't work.

To be fair, if you are running 300+ meters then this is going to be a machine pull anyway so over consideration of the conductor size in relation to manual handling etc is not actually your higest priority.

Apologies if I'm teaching you to suck egges here but go back and revise the volt drop calculation again and take temperature into account - it is highly unlikely that the design current matches the tabulated current carrying capacity of the cable and remember that a direct buried conductor benefits from both a lower ambient temperature and a significant thermal mass surrounding it which allows good heat dissapation - ie the cable rating will increase when you bury it - it runs cooler so the resistance is not as high as the tabulated volt drop values which are usually presented at 70C or 90C.

Regards

OMS

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Let the wind blow you, across a big floor.
 04 December 2008 11:33 am
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kalookicedogs

Posts: 181
Joined: 23 October 2008

Thanx OMS for your helpful comments. Will look again at all points mentioned.

Regards, Mitch


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My ex wife was an amazing house keeper. Problem is I never got the damn thing back.
IET » Wiring and the regulations » Sizing CPCs in parallel circuits

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