cable cross section calculation

While calculating the size of the cable the de rating factors due to temperature rise depending on various locations and conditions of cable laying ( ambient temperature,ground temperature,variation in temperature due to thermal resistivity of of soil and depth of laying etc) are available and to be multiplied by the de rating factor depending on the site conditions. The other factor is permissible rise of temperature of cable depending on the type of cable ( XLPE,PVC, PILC ) These calculations vary depend on each case and depending on ground conditions and mode of laying and type of cable used.There is no standard formula.

What core?

Are you talking about the conductor or transformer core?

However, for temperature calculations you have to consider:

1. Ambient temp. The ref will always be ISA, (International std. temp) i.e 15°C at sea level..Will be different for free air, vs. duct etc.
2. Conductor temp. is a function of the cable insulating material, and is given by the cable manufacturer.
3. Core temp. (in the case of power transformers), depends on many variables, such as oils, seals core saturation temp.

Any calculations done without it, are incomplete.

Wangito.

Hey Wangito,

Many thanks for explanation.

What I am asking for is the "real" cable core temperature.

Means not depend on insulation but on effective load.

Example: Motor 160kW, direct online, 400V, Ia 8x, cos 0,85, start cos 0,35, cable length 350m delta Ustart max: 10%.

Because of cable length the laying factor is not so important (let say normal condition), but voltage drop is critical item. Do you agree? OK :-)

My Question is why we have to take the max. Rw of the cable core for our calculation? Example XLPE= R90= R20(1+a*delta temp.).

The core resistance should be according to load and not max. temperature. Why nobody consider this?

And the next item what is not clear to me is :

For what physical reason we have to use for starting phase the starting cos? We have 8x In for starting, and the cable have to bring this current to the Motor.

Cheers

Wormnhaque

Wormnhaque,

I ran some calculations and got a temperature rise for the cable core, at starting current =8 times running current, for both copper and aluminum conductors, single set and two sets in parallel.

Temperature rise per second of inrush, degC:

copper parallel, 0.22; copper single, 0.3; aluminum parallel, 0.35; aluminum single 0.46.

You ask: The core resistance should be according to load and not max. temperature. Why nobody consider this?

For a worst case starting period of 10-seconds, the cable core temperature will not rise enough to require calculating the resistance at the new temperature of the metal.

You ask: For what physical reason we have to use for starting phase the starting cos?

It helps explain the high inrush current of the motor, but I do not use it when calculating starting currents. I don't know when it would be used, except for analysis of an entire distribution system.

You ask: My Question is why we have to take the max. Rw of the cable core for our calculation.... The core resistance should be according to load and not max. temperature. Why nobody consider this?

Codes and standards are a minimum design guideline, with some built-in cushion for safety. They have evolved from experience--mostly failures, fires, and injuries. In the USA, the National Electrical Code works with R_w at 75-degC, and motor conductors are supposed to be sized for 125% of the full-load current for the motor. This gives you some safety margin before you are likely to exceed the permissible or tested temperature limit for the conductor's insulation. Derating tables in the code tell you by how much to increase the conductor size for higher ambient temperatures. This carries your safety margin along with you. The tables are checked against the calculated temperature rise of conductors which are being cooled by the specified ambient conditions (cables in free air, or in conduits, or directly buried, etc.). This allows me to confidently select a particular cable for a job without having to be a physicist or engineer and without having to carry a calculator.

You say: ...but voltage drop is critical item. Do you agree?

Yes, I agree. I also calculated the voltage drop for this application, 350m length and approximately 390-amps (running current). For one set of conductors it was about 3% and two sets in parallel, it was around 4.5%. These are probably OK for running the motor, although high (suggested design is 5% drop total). However, the inrush voltage drop is substantial and could cause you problems. Therefore, you would have to increase the conductor size to reduce the voltage drop over the distance you mention. When motors are run on low voltage, they typically will draw more current. During starting this will increase the voltage drop further. If I were to design the supply to the motor you mentioned, I would increase my conductor size to limit the voltage drop to no more than 2.5% of the supply voltage, and I would also select a suitable type of reduced voltage starter (star-delta, part winding, soft start or other). Any other problems you may encounter will depend on the type of load the motor is handling and the starting inertia of the load.

Regards--JMM

Dear Sir

I faced with voltage drop on starting of a 300 KW. MV 6.6 Kv motor.

Then I checked and found there were in 3 points cable on incoming 6.6 Kv crossing itself , by placing some wood between crossing points the as mentioned drop vanished.

I seek for some teoritical explanation for this cross effect.

Regards

Farzin