Cable Sizing for Electric Motors

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cshaopin

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Hi I'm new here. I'm involved in Offshor Crane Design field and I would like to know how to size the incoming cable for 2 units of 185kW Electric Motor as prime mover for our crane.

As per clients spec, they provide us incoming power feeder of 3runs x 4core-185mmsq. Power supply @ 415VAC,50Hz-3phs.

They also mentioned that the motor has inrush current up to 7 times the rated current.

Therefore I would like to know how to calculate that the 3runs x 4C-185mmsq is sufficient to accomodate the high inrush current. More preferably i would like to know how to size cable for high amp motors.

The starting method is Star-Delta.
 
This is a little larger than anything I have experienced..............

185000/415x1.732=257a ?!.

Then I found this that led me to believe you have 350kcmil conductors which seem as if they will handle your load unless they are AL, I do not know what 4 core means?

I am under the impression that inrush doesn't really matter when sizing conductors as it is only momentary.
 
What you are describing is clearly an international standards type project. In IEC world motors are rated in KW and can have extremely high LRA or inrush. I've seen 22x FLA before.

To size the conductors, you need to know first the installation method since that is a factor, and armored or unarmored, etc. You can look on the internet for cable manufacturers to find ampacities for IEC cable by types and install method. One example, 4 core in conduit or trunking is good for 340A continuous. at 30Deg C ambient. The sizing will be based on FLA and whether this is a continuous or intermittent load. Being a crane I would say non-continuous. but at that, derating for a 40Deg C ambient (91% of Rated) is now 309A. Even at 50Deg C your probably still OK. But know you need to look at short circuit capability as IEC dictates that cable must be able to withstand the SC current for 3 seconds, so you have to find the capability of the cable. This often causes the cable to get over sized. Get a copy of the IEE Wiring Regulations, I believe the latest Edition is 18th.
 
Kingpb is cool.

If you get something smaller and covered by the NEC. Look under article 430 . In the code book. It's interesting and my favorite chapter .
 
barbeer said:
I do not know what 4 core means?
Click to expand...
I believe it means four conductors.

"3runs x 4core-185mmsq" = 3 conduits, each containing four 185 sq. mm conductors.
 
Barbeer:

Our standard is using mmsq. 4 core means inside 1 cable we have 4 conductors. Each conductor's cross section area is 185mmsq. The thing that concerns me is that this incoming power feeder is used by 2 motors. Provided that the motors should not be started together, at normal operation, both motors should be drawing 2 x 257A = 514A under full load condition.

Kingpb

Yes its IEC standard. The cables will be armoured cables. 185mmsq conductor has a ampere rating of 245A @ 40 Celsius. The derating factor as follows:

Ambient air temperature (?C) Derating factor
15 1.26
20 1.20
25 1.15
30 1.10
35 1.05
40 1.00
45 0.94
50 0.88
55 0.81
60 0.73
65 0.65
70 0.57
75 0.47
80 0.34
85 0.19

Being a crane its non-continuous load. Because of armoured cable's physical stiffness, its hard for us to do the cable routing inside a small confined space. And this crane is located in ambient temp of 60 deg celcius.

Therefore I plan to design 2 runs for each phase. 2x150mmsq for each phase.

LarryFine:

Yes you are correct.
 
4 core is IEC terminology for 4-conductor. So, there are 4 separately insulated conductors bundled together w/ ground conductor, with a common sheath. They can be armored or unarmored. Conduit does not really come into play as large size multi-conductor cable usually is installed either in cable tray (armored or unarmored) or run without any raceway (armored). Installation practices outside US often don't rely on conduit systems. They make special cable clamps whereby you simply strap it to the wall. Conduit sleeves will be used where physical protection may be required. This is not abosolute, my description is intended as general information.
 
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