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Size of Cable

02/09/2021 9:34 AM

Hello All,

what size of cable for 2000A phase current for three phase system. (XLPE insulation armored cable)

the 2000A comes from 22 inverter, all inverters current are connected to (Solar) main switch board.

the (Solar) main switch board will be connected to main switch board of the building side. (distance only 20meter)

cable will be laid underground in duct.

Questions:

1: what size of cable do I need?

2: Do I need to consider the grouping of conductor correction factor for the 2000A?

3: I calculated the cable size to be 4 runx3c cable (515A) is it correct?

4: do i need neutral cable for the system

I'm doing my degree project please help me

Please someone guide me

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#1

Re: Size of Cable

02/09/2021 10:23 AM

That depends on what you wish to survive when 2000A of current flows and how long this current will be sustained.

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#2
In reply to #1

Re: Size of Cable

02/09/2021 10:34 AM

Will the current will be generated from the solar system (inverters) during the day.

the current will flow from the main switch board of the solar system to the building main switch board. the voltage is 415v, 50HZ.

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#3

Re: Size of Cable

02/09/2021 1:36 PM
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#4

Re: Size of Cable

02/09/2021 5:09 PM

1. The cable manufacturer will sometimes provide ampere ratings for their cable at different ambient temperatures and bundling factors.

2. Yes. The grouping of the cable is a term in the industry sometimes called 'bundling' when one needs to de-rate a cable based on the number of separate conductors bundled together in the cable or the number of cables bundled together in a duct. Unless the cable manufacturer specifically rates a 'built-up' cable of multiple conductors. Refer to IEEE Std 835-1994 for their recommendations.

You can also look up MIL-HDBK-299 on https://quicksearch.dla.mil and look for a cross-linked polyethylene (XLPE) if you want to find something similar. (The LSFNWA cable is a four conductor cable with XLPE. The LSMDY cable type is another one that may be specified with multiple conductors. )

3. Depends on what the 3c cable is. I think that just means 3 conductor. If the cable is rated for 515 amps when placed in a duct with three other similar cables, then the answer is yes. If 515 amps is a single cable alone in buried duct, then the answer is probably no. Do you have a data sheet for the cable? A part number of the cable?

4. Will you have un-balanced phases? If the answer to my question is yes, then your answer is yes.

Best wishes with your (schooling) quest.

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#13
In reply to #4

Re: Size of Cable

02/10/2021 8:56 PM

Hi Mr. Robin,

Thank you for you comment.

My concern is regarding the grouping correction factor when I'm sizing the cable of 2000A do I need to consider that each inverter is one circuit (since each inverter will not generate the same current at the same time) ?

I would size anything based on the cable rating and submit my project, but I want to understand more about this issue.

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#5

Re: Size of Cable

02/09/2021 5:10 PM

You should include a note that when using parallel conductor runs it is critical that all conductors be the same length to ensure equal resistance - otherwise the shortest, lowest resistance conductor will carry the bulk of the load, will heat up, increasing its resistance, then the current will shift to another conductor, then it will heat up, and so on - the current oscillating from one conductor to another until catastrophic failure. An electrical inspector here said that a sign that the conductors were not the same length was if the installation was neat - if the conductors go to their respective lugs in a tidy and orderly fashion it usually means that they have been cut to different lengths to mate cleanly with both close and distant lugs - looks good, but is strong evidence that the installation is wrong.

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#6

Re: Size of Cable

02/10/2021 12:41 AM

Here is an Australian manufacturers handbook which can be down loaded from the link.

If you refer to page 35 you will get relevant information to assist you but it would appear you will need a minimum two 630 Sqmm cables in parallel to handle the current depending on the configuration of the cables.

https://www.olex.com.au/eservice/Australia-en_AU/fileLibrary/Download_540225217/Australia/files/OLC15984%20Handbook%20July%202016%20FA%20V3%20LR.pdf

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#7

Re: Size of Cable

02/10/2021 2:31 AM

British Standard 7671. Again.

If in doubt, consult a qualified Electrical Engineer.

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#9
In reply to #7

Re: Size of Cable

02/10/2021 10:35 AM

Two things to consider:

  1. Will the circuit protective device open if a short circuit is placed on the cable, before the cable melts?
  2. Will there be an acceptable voltage at the far end of the cable at the circuit protective device's full load current given the volt drop on the cable?
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#8

Re: Size of Cable

02/10/2021 7:57 AM

The current rating of any cable/busbar system is determined by how much heat the load generates, which must be less than how much heat the system can dissipate. There will be a maximum running temperature that is permitted which will be found in the contract specification directly or indirectly if the spec refers to tables in an approved standard. How many watts of heat are generated per meter is just the square of the current multiplied by the resistance of the cable per meter. How much heat is dissipated per meter depends on the cable insulation, the ambient temperature of the soil, the maximum temperature of the soil, the size of the duct, the material the duct is made of, if the duct ends are sealed, if there are any other cables in the same duct, if the route slopes, how deep the duct is buried, what the surrounding material is made of. There are tables which average out most of these factors and tell you how much to de-rate any cable run in an underground duct. There is a second table that tells you how much more to de-rate the cable for every degree that your site ambient is higher than the ambient used to formulate the tables.

Cables only get made in standard sizes so using the manufacturer's data, choose a cable that is rated just above 2000amps, multiply the stated cable rating by the various de-rating factors and if the result is still above 2000amps use that cable. If the result is below 2000amps choose the next higher rated cable and repeat the de-rating calculations until you get a result above 2000amps.

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#10

Re: Size of Cable

02/10/2021 10:37 AM

<...4: ...need neutral cable for the system...>

A glance at the intended schematic drawing would say, and it can't be seen from here.

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#11

Re: Size of Cable

02/10/2021 12:13 PM

You have 22 three phase inverters all synchronized, or 22 single phase inverters, also synchronized?

Question 3 implies you have the answer for Question 1, except the dimension of the conductor is omitted? A set of 4 parallel conductors to conduct 2000 amperes implies 500 MCM conductor section, before derating, for example.

You might find that when connecting 22 devices to a single conductor, that a bus bar might be mechanically more suited, in which case you might consider making your entire run of bus duct, rated for your unspecified voltage.

Question 4 requires a knowledge of the anticipated loads or downstream equipment. If you have no line to neutral loads, then you do not need a neutral. If you are connecting to a step up transformer, then no neutral is required.

Do you need to consider the heating effect of the harmonics generated by the inverters when sizing your conductors? This is a function of the inverter design, it often requires some consideration.

A more efficient design would be 1 inverter, also easier to synchronize and maintain...

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#12
In reply to #11

Re: Size of Cable

02/10/2021 8:51 PM

Dear Mr. Rwilliams,

Thank you for you reply.

based on my understanding the inverters are 3 phase synchronized with the grid.

the THD is also between the range of grid code.

my question is when designing the cable for 2000A do we need to consider the grouping correction factor (each inverter of the 22 is one circuit)

I have attached the cable manufacturing size rating..

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#14
In reply to #12

Re: Size of Cable

02/11/2021 1:29 AM

You will have only 4 parallel circuits, the 22 circuits are combined at your solar distribution panel. Those 4 circuits are the ones that need to be derated if they operate close to one another.

The cable is sized to carry the maximum output of all 22 inverters together.

The need for group conductor derating depends on your exact and specific method to interconnect the inverter switchboard to the main distribution bus.

The cables between the inverter board and the switchgear must run in 3 phase groups, but it is not really practical to stuff 6 current carrying conductors of that size in a single conduit, so now you just need to consider duct bank derating, i.e. 4 conduits, where you specify the spacing between conduits, the conduit material, and what they are buried in.

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#15
In reply to #14

Re: Size of Cable

02/11/2021 1:56 AM

Here is a sample duct configuration similar to your needs, showing the information you need.

https://www.calcware.com/?page_id=73

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#16
In reply to #14

Re: Size of Cable

02/11/2021 2:04 AM

Dear Mr. Rwilliams,

thank you for your valuable information.

I have attached my Single line diagram for your reference.

now I understand more about the system.

I would put anything in my degree project but I'm very interested to learn new things.

the busbars will be connected to the MCCB 4p

so now i need to consider 4 circuits depending on the gap between each other am i right?

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#17
In reply to #16

Re: Size of Cable

02/11/2021 2:23 AM

The use of a 4 pole circuit breaker assumes you need to provide a neutral conductor, which appears to be unlikely, but may be required by local codes.

The 4 circuits you need to consider derating are the 4 parallel cable runs you need to get your peak rating. or perhaps 5, all in 3 phase groups, perhaps with a neutral, if you anticipate single phase loads directly on your distribution system. Just like the diagram, which shows 6 x 500MCM cables, which could be a link between 2 switchboards.

The derating assumes that you have 3 current carrying conductors in each group, so the derating applies only to the number of adjacent groups of conductors.

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