Cable sizes

[nek]

How to calculate the correct cable size by taking into consideration distance,impedance,motor size,voltage drop,starting current.
Here is an example:Motor size:160 kw motor (4 pole)
:distance:90 meters

 

[hekimoglu]

There are so many formulas that we had studied in education life but i could not exactly remember them! but the best way of finding the cable size is using standard cable tables!

 

[tawakol1]

Nek, Cable sizing can be done by using the free voltage drop program which is available in the following site:

http://www.monachos.gr/en/calculators/voltage_drop.asp

You have to use also any catalogue of any cable manufacturer to get the cable size which can carry the load and meet the required voltage drop.

I hope that to be useful

Tawakol

 

[dpc]

I don't know your location, but keep in mind that in most countries, the sizing is dictated by local codes not strictly the laws of physics.

In the US, a motor feeder must be sized at 125% of the full-load current, based on tables in the National Electrical Code. Voltage drop concerns can cause the size to be increased, but it can never be smaller.

 

[Hertz]

Don't forget the rules for derating cables for ambient temperature, and also for the number of current carrying conductors in raceways.

 

[peebee]

I'd tend to recommend AGAINST tables and programs for voltage drop calcs. I've seen several cable sizes get screwed up by people putting too much blind faith in those things -- and I know one guy who got fired after screwing up once too often by relying on such short cuts.

The calcs are not that hard. It all pretty much comes down to V=IZ. Z can be determined from tables in the NEC or from manufacturer's data.

Suggestion: if you don't know how to size cables, well, get an engineer, that's what we're here for.

 

[jbartos]

Suggestion to nek (Electrical) Aug 19, 2003 marked ///\\How to calculate the correct cable size by taking into consideration distance,impedance,motor size,voltage drop,starting current.
Here is an example:Motor size:160 kw motor (4 pole)
:distance:90 meters
///Additional information is needed:
1. Copper conductor or aluminum conductor
2. Rated or specified minimum motor transient starting voltage, e.g. 85% of motor nameplate rated voltage.
3. Cable sizes and their ampacity ratings at certain ambient temperature
4. Cable insulation temperature rating\\\

 

[alehman]

Voltage drop is highly dependent on the power factor of the load. Motors have very low power factor when starting. Accurate formulas are available in most electrical design guides and some manufacturer's catalogs.

 

[uno]

Hope you are not using a medium voltage motor. Else it may be required to start with a minimum cable size based on the short circuit level of the system.This is to cope with the heat energy without damage to the cable (insulation and conductor) .Cable will then have to be sized based on the interrupting capability of the circuit breakers/fuses that protect them. Generally, low voltage motor feeders are not sized for short circuit withstand current. On medium voltage systems, cable impedences have little effect on reducing short circuits , however in low voltage systems or when the loads are far from the source the cable impedances have a major effect in reducing the fault levels at the load end.

Also check if the cable are not laid underground with heated pipes or other high current cables. This might require a software for ampacity calculation .

 

[jghrist]

I also tend to agree with peebee on the benefits of calculating voltage drops instead of using tables or programs. At least learn enough of the basics to make a simplified calculation as a reality check. When dealing with motor starting voltage drops, though, you have the problem of figuring out what Z to use and what I to use.

 

[prk]

would it not be advisable to size the cable on full load current and then check if voltage drop at starting current do not cause a voltage drop at motor end such that it is less than 80% or so. starting is a transient phenomenon and short can very well be met with the short time rating of the cable which is sized on the full load current.
in case cable is being sized for a MCC feeding , say 10 motors, would the starting current of 10 motors will be summed up to size the incommer of MCC.

 

[Rarichan]

As detailed above many, the main points for the cable sizing can be summarised as below.
1. Ampacity of the cable to meet the load current (Remember to derate the cable for the installation conditions and grouping factors)
2. If it is a medium voltage cable you would also have to check the minimum size to be used for the short circuit point of view.
The simple formula is S = I x sqrt(t)/k.where S is the minimum cross section of the conductor, I is the short circuit current , 't'is the operating time of the disconnecting device(you can consider 100ms)and k is the constant depending on the type of cable (conductor, insulation with lead sheath or not etc)which can be found from any cable catalouge.
3. Now you have to look for the voltage drops during starting and normal running with the formula VD=sqrt(3)xI(R x cos(phi)+X x sin(phi))xL where R and X are the resistance and reactance of the cable and L is the total length of the cable in km.
The size satisfyng all the three conditions to be selected.

If you want I have an excell sheet to find the size for cables satisfying the above conditions.

Cheers
vrj

 

[prk]

rarichan - i will be thankful if you can send a copy of your excell sheet to me. my email is pradeepkumar@ongc.net .
thanks in advance.

 

[boomsystems]

Just go by any cable manufacureres user guide. Do not get confused by above replies.

 

[jsummerfield]

Refer to the National Electrical Code tables in article 310. Consider the calculation notes found in Annex B Application Information for Ampacity Calculation.

"These ampacities were calculated as detailed in the basic ampacity paper, AIEE Paper 57-660, The Calculation of the Temperature Rise and Load Capability of Cable Systems, by J. H. Neher and M. H. McGrath. For additional information concerning the application of these ampacities, see IEEE/ICEA Standard S-135/P-46-426, Power Cable Ampacities, and IEEE Standard 835-1994, Standard Power Cable Ampacity Tables."


John

 

[alehman]

suggestion to boomsystems: Manufacturer's tables are fine as long as you are careful to apply them correctly and understand which tables to use for a given situation. You must understand the exact conditions for which the tables were calculated (e.g. power factor used for voltage drop calculation). Commonly real world conditions don't match pre-calculated tables, therefore use of calculation for some quantities may be necessary.