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Cable Size For 3 Phase Motors


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#1 chuahtc

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Posted 04 June 2010 - 08:07 AM

Hi all,

I'm trying to commonise the cables used for few sizes of 3-phase motors used in my work.

Here's a summary of what we have

1. 2hp & 3hp with DOL start (FLA 3.5A & 5A)

2. 7.5hp, 10hp & 15hp coupled with centrifugal pump with DOL start (FLA 11A, 14A &21A)

3. 15hp & 20hp coupled with radiator fan with Wye-Delta start (FLA 21A & 28A)

4. 50hp air compressor with Wye-Delta start (FLA 66A)

5. 75hp motor coupled with a centrifugal pump with Solid State soft start (FLA 100A)

And the cable size we are currently using for the above

1. 4 x 4mm2 (36A cable current rating)

2. 4 x 10mm2 (64A cable current rating)

3. 4 x 10mm2 (64A cable current rating)

4. 4 x 25mm2 (114A cable current rating)

5. 4 x 50mm2 (170A cable current rating)

From what I read in this forum, we can safely size the cable to 125% of the motor FLA assuming the cable is not too long to effect voltage drop.

And the new size of cable to use as below,

For group 1,2 & 3, use the same cable (for more durability)

For group 4, use 4x16mm2 cable with 86A current rating.

For group 5, use 4 x 35mm2 cable with 140A current rating.

These motors needs to be mobilised every now and then, therefore using the smallest size is most preferable for ease of moving from site to site.

The maximum cable length that we use is 30m the most.

Are these cable sized correctly?

Is there any effect between different type of starting and application on these selection?

Thanks in advance.



#2 AB2005

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Posted 07 June 2010 - 11:41 AM

QUOTE (chuahtc @ Jun 4 2010, 01:07 PM) <{POST_SNAPBACK}>
1. 4 x 4mm2 (36A cable current rating)

2. 4 x 10mm2 (64A cable current rating)

3. 4 x 10mm2 (64A cable current rating)

4. 4 x 25mm2 (114A cable current rating)

5. 4 x 50mm2 (170A cable current rating)


Hi,

The current rating quoted by you is the max caring current of cables. There are many factors which must be taken into account during cable sizing not only the LONG LENGTH.

1) What is your envirnmental temparature? (Temp factor)
2) Are these cables single lay or other cables pass near with them. (Grouping factor)
3) Generally, we must select 125% > rated cable due to high starting torque of load for a long life usage (Ageing factor)
"Don't assume any thing, always check/ask and clear yourself".

#3 chuahtc

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Posted 08 June 2010 - 01:36 AM

QUOTE (AB2005 @ Jun 7 2010, 07:41 PM) <{POST_SNAPBACK}>
Hi,

The current rating quoted by you is the max caring current of cables. There are many factors which must be taken into account during cable sizing not only the LONG LENGTH.

1) What is your envirnmental temparature? (Temp factor)
2) Are these cables single lay or other cables pass near with them. (Grouping factor)
3) Generally, we must select 125% > rated cable due to high starting torque of load for a long life usage (Ageing factor)


Hi AB2005,

1. The environmental temperature is around 50degC. How much are the effects for this variable?
2. The cables are single lay

Can you advise? Thanks.

#4 AB2005

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Posted 08 June 2010 - 08:21 AM

QUOTE (chuahtc @ Jun 8 2010, 06:36 AM) <{POST_SNAPBACK}>
Hi AB2005,

1. The environmental temperature is around 50degC. How much are the effects for this variable?
2. The cables are single lay

Can you advise? Thanks.


OK. The de-rating factor for 50C is 0.7. For example, in your application # 4 where the FLA is 66,

66 X 1.25 (Safety/Ageing factor) = 83A

The current caring capacity of cable must be 83A after applying other factors. If you use 86A cable with 30M length and single lay application then,

86 X 0.7 = 60A

It means this cable is not adequate for this application. If you use cable with 114A current caring capacity, then,

114 X 0.7 = 80A

80A is closed with our requirement 83A. Apply this rule and calculate the others. Neglecting this rule can shorter the life of cable.


"Don't assume any thing, always check/ask and clear yourself".

#5 chuahtc

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Posted 11 June 2010 - 01:08 AM

QUOTE (AB2005 @ Jun 8 2010, 04:21 PM) <{POST_SNAPBACK}>
OK. The de-rating factor for 50C is 0.7. For example, in your application # 4 where the FLA is 66,

66 X 1.25 (Safety/Ageing factor) = 83A

The current caring capacity of cable must be 83A after applying other factors. If you use 86A cable with 30M length and single lay application then,

86 X 0.7 = 60A

It means this cable is not adequate for this application. If you use cable with 114A current caring capacity, then,

114 X 0.7 = 80A

80A is closed with our requirement 83A. Apply this rule and calculate the others. Neglecting this rule can shorter the life of cable.


Hi AB2005,

Thanks for the example there. Really make things easier to understand. smile.gif




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