AC CABLE SIZING 1

[orlyboy]

How to size the cable for the data given below:

Considered Cable = 70 mm2
Route Length = 110 m
Load Current = 57.27A
Voltage = 110 VDC

 

[ScottyUK]

Cable type?
Protective device?
Disconnection time?
Acceptable volt-drop?


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If we learn from our mistakes I'm getting a great education!

 

[jghrist]

Thread title says AC Cable Sizing. OP says Voltage = 110 VDC

 

[waross]

Step one: from tables select a suitable cable based on ampacity and temperature rating.
Step 2: from tables check voltage drop and increase the cable size if necessary.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[orlyboy]

My apology to All,

Title suppose to be DC CABLE SIZING, I am suppose to edit the title but I don't see a tab to edit the said title.

Type of Cable = XLPE
No. of Core = 2 cores
Acceptable VD = 3%
Type of Installation = Installed in Air (ladder)

Now, is 70mm2 cable is sufficient to handle a load with a current given (57.27A)?

Thanks

 

[juleselec]

1) Current rating (underated) of a 70mm2 XLPE cable on ladder is about 300A. Don't know what your ambient temperature is, or the bunching of the cables, but even derated, the cable ampacity should be more than your full load current. You better confirm that however.

2) Voltage drop - Assuming stranded, plain copper, the max dc resistance of a 70mm2 conductor is approx. 0.268 Ohms/km. Therefore, the voltage drop over 110m is Vd = 2 x 57.27 x 0.268 x 110 / 1000 = 3.377Vdc, which is a voltage drop of 3.07%. So it fails the voltage drop criteria.

3) Short circuit temperature rise - probably won't be a problem unless there's a high dc fault capability. In any case, you need to know the protective device type, rating, tripping characteristics / disconnect time, fault let-through energy, etc and the maximum dc fault current.

 

[SparkyLarks]

If you are approving the use of the cable. You should be able to do this calculation yourself. Or you need someone to review it, peer, supervisor, or outside consltant.

You also need to look at the protection for the circuit to ensure that it is suitable first. The cable size is then based on the protective device and not the load.

Is it 2 core 70mm2 or 2 single cores. I haven't ever come across 2 core 70mm2 cable before and I would think it is expensive.

All this information regarding current carrying capacity voltage drop should be on the cable datasheet.

Anixter have a great cable handbook if you ask your local rep for it.

Here is the 600/1000V XLPE single core informatin from it,

http://onlinecatalog.anixter.com/common/objects/pd313673.pdf

Assuming that it is 2 single cores installed on cable tray.
Current carrying capacity is 310A, though you wil neet to derate for , ambient conditions, no of conductors in the vicinity , Installation method etc.

You then need to compare that to your protective device. If the upstream device will let 300A flow in the cable then it unlikely to be sufficent. I souwl suggest that something inthe 63A range might be a good place to start

Volt drop is .68V/Am so for you is 111m x 58A x .68V/Am = V

I'll let you do that bit yourself

 

[undervoltage]

Concentrate on Voltage Drop while you are sizing the low voltage cables, when it comes to MV cables the key parameter to be kept in mind is the cable's short circuit withstand capability.

In your case, calculate the total derating factor for the cable. It depends upon the method of installation and the enviroment.