Is this voltage drop calculation correct?

Dear friend,

Your calculation seems o.k if it is for COPPER cable.For Alluminium cable either two runs of 50 mm2 or to be on safer side 70mm2 cable runs are o.k

D.D.DESAI

Actually the voltage drop calculation is not an algebraic equation rather its a vector equation. To simplify the calculation we shade extension of the equation, thus use the one you have referred.Though its a general practice, but many of us are not aware about vector equation.

Thank u guys.

But appreciate if you can also address my doubts mentioned below the calculation in bold.

If we were to use star-delta starters for the same 55kW in my example, how would things change as i have 6 runs of cable now. Appreciate a detailed reply.

Thanks again.

In Star-delta starting- the motor starts in star then changes to Delta therefore the initial starting current would be same but duration would be less.

As regards the normal voltage drop, the current would be phase current instead of line current, therefore you have to take phase current for calculation i.e line current /1.732.

other calculations will remain same.

Thank you for your reply Mr. Ram. But I am not too clear. So is this the way I should do it. Pls see the following.

Cable Selected: 3 Core × 35 mm²

Cable Data:

Cable Resistance @ 20°C = 0.6270 Ω/km

Cable Reactance @ 50 Hz = 0.0860 Ω/km

Cable De-rated Current = 83.1 A

Cable Length = 0.200 km

Motor Full Load = 55 kW

Voltage = 440 V

Power Factor = 0.85

Efficiency = 0.89

Full Load Current = 1000 × kW / √3 × V × p.f. × effy.

= 1000 × 55 / (1.73 × 440 × 0.85 × 0.89 x 1.73)= (95.4 A old value in brackets) 55.1 A

Minimum no. of cable runs required = (1.1) 0.66 ie (2) 1 runs

Volt Drop = √ 3 × IFL × L × ( R CosÆ + X SinÆ ) × 100 / n × V

= 1.73 × (95.4 ) 55.1 × 0.2 ( 0.627 × 0.85 + 0.086 × 0.527 ) × 100 / 2 × 440

= (2.17) 0.43 % Maximum Allowable Volt Drop 2.5% Feeder to Motor (Running)

Maximum Starting Current (Multiple of I FL) = 6

Starting Power Factor = 0.3

Locked Rotor Starting Current Ist= 6 x Full Load Current (Direct On Line)

= 6 x 95.4

= 572.4 A

Volt Drop (Starting) = √ 3 × IST × L × ( R CosÆST + X SinÆST ) × 100 / n × V

= 1.73 × 572.4 × 0.2 ( 0.627 × 0.3 + 0.086 × 0.954 ) × 100 / 2 × 440

= 6.09 %

Maximum Allowable Volt Drop 15% Feeder to Motor (Starting)

Therefore Recommended number of cables (2) 1 Runs of 3C × 35 mm²

So am I right to conclude that as my calculations now only show 1 run now and I require two runs, I must try with a reduced size cable instead of 35 mm². Pls confirm.

Star-delta starter

Motor

Previously 2 run of 35mm² but now ???

Use the cable data for 1 size higher. Hopefully the size cable box of motor can accommodate, as well as its technical suitability.

Hello to everyone

Can anyone help me to solve this problem to find voltage drop

Load(costumer) = 140 kW

Distance from transformer is 250 m

Does it work with NYY cable 4x95 mm2

If somebody can please solve it step by step , because i want to see it.

Thank you

Dear V Srini....

your voltage drop calculation for star delta feeder is correct but this volatage drops is needs to mulplied with 2 for overall volatge drop calculation because you need to lay two cable for one motor windings .and these cable will be in series of the motor winding and form delta at switchgear end .So its better to multiply by 2 to your voltage drop calculation .i.e voltage drop will be 0.86% instead of 0.43 %.

Mr Ram : please provide your input in this regard also.

Have you consider the following for cable sizing.

MINIMUM CABLE SIZING SELECTION BASED ON THE FOLLOWING POINTS.

1. SHORT CIRCUIT LEVEL OF THE SYSTEM

2.CONTN. CURRENT CARRYING CAPACITY OF CABLE.

3.VOLTAGE DROP OF CABLE DURING STARTING AND RUNNING.

IF THE ABOVE REQUIREMENT IS NOT MET.THEN U HAVE TO GO FOR HIGHER CABLE SIZE.

A.IN STAR-DELTA STARTER DURING STARTING VOLTAGE DROP WILL BE LESS.BUT IF IT IS COMING WITH IN LIMIT WHICH EVER SIZE U SELECTED IS OK.IF IT IS EXCEED THE MAX LIMITED VALUE THAN U SHOULD SELECT NEXT HIGHER SIZE.

B.FOR VOLTAGE DROP CALCULATION U HAVE TO CONSIDER THE NUMBER OF RUNS WHICH U CALCULATED DURING THE CABLE SIZING.

R=(Resistance x Route Length/No of Runs)/m.

X=(Reactance x Route Length/No of Runs)/m.

Determination of 'SHORT CIRCUIT LEVEL OF THE SYSTEM' suitability is not required for LV cable. Guess why.

Hi Prasanth,

Thanks for contributing to this topic. I feel that my calculations took care of other items other than the s.c analysis. So do you see anything else which I should take care of?

Thanks

Just one small comment:

You say × effy.

Should be ÷effy (divided by efficiency)

In your formula mentioned you also used the power factor.

Considering that you indicated the power in kW (which is usually the mechanical power at the shaft (output power)) you should not use any power factor.

If you indicate the input power (kVA, apparent power) drawn from the power grid, you should use the power factor.

tomad

The formula is correct.Please do not confuse!

This is a classic example of cable calculation. Rolling up your sleeves and getting stuck into discussions about vector analyses, power factor involvement and interesting formulae has dragged attention away from the basics. What has happened here is that you have jumped straight into detailed calculations, which might then prove irrelevant.

Your cable is sized firstly according to the protection device. Look up manufacturer's data for the MCCB you are intending to use. 125amp is required for SD, or 160amp for DOL. Instantly, you can see 35mm2 is not big enough.

As you intend to use SD, you need a neutral. 3-core cable is not appropriate. 4-core cable when paralleled also gives you 2 neutrals, which is not required either. So either use 1 larger cable, or use parelled single-core cables with 1 neutral.

Copper is massively expensive now. There have been many instances of copper cable being stolen from site, which of course plays havoc with the programme of the works, causes insurance issues, jointing problems etc etc. Consider using aluminium.

Think of how this cable is going to be delivered and stored on site. It's easier to protect 1 drum of large cable from theft, than several drums of smaller size.

Use XLPE insulation to maximise the current capacity of the cable. Although in this instance volt drop will define your cable size.

The above leads me to advise considering PVC insulated 4-core 70mm2 Cu, or 4-core 95mm2 Al. This avoids the unnecessary (in this instance) complication of paralleling two cables.

To answer your questions, no - using SD you calculate volt drop on running current.

Your other question is not clear. You could elaborate.

Hi Baby Guinness,

Appreciate your reply looking at things from a different perspective. But pls understand that when I size this cable, due to some constraints, I will not get the MCCB data, etc. I am only given the motor kw and the distance from the switchboard. In cases where it is a packaged equipment I am given the approximate kVA or kW consumed by the packaged equipment.

I however appreciate your remark, where we have to look at the practical issues also. But I have to submit these calculations to the client first and then only look at consolidating the cable sizes I can use to reduce the number in variety. Also my calculations are to be submitted for about 500 such feeders to packaged equipment/motors. So my approach would be to calculate everything first and then look at the practical issue of reducing the variety.

As to my question, I was using this calculation (shown above) assuming DOL. So I wanted to know whether there was any change required if I use S-D. Because if you see the above calculations, which uses DOL, I can get the number of runs based on the size I select. So if I happen to get, say 3 runs, I have a problem with S-D. So I was not too sure what was the right way to go about it. Ram Vinod had given an explanation, which I thought was logical.

I would appreciate if you too can put in the suggested calculation, which you think I should use.

Thank u for your time and also all the other members.

I hope that we can continue on such practical design issues raised by many more members.

Sorry V Srini I may have misunderstood.

A DOL starter has 3-core supply, and 3-core from the starter to the motor. However the supply cable has to be rated to the MCCB feeding it. For 55kW this is 160amp.

A S-D starter has 4-core supply (neutral can be 50% sized), and 6 cores between the starter and the motor. Thus, a S-D starter must be mounted next to the motor. The assisted start however lowers the MCCB size to 125amps.

Are you sizing the supply cable, or the cable between the starter and the motor?

If you have 500 units as you say, I would use a VSD and talk directly to a manufacturer about bulk discount. The VSD can absorb excess volt drop (down to 380v at least) allowing you to make drastic savings on cable. Copper prices increase every day, but as installers switch to aluminium, that too is rising in price.

Hi Baby guiness,

Apologise for not being more clear in my description.

By 500 units I meant about 500 feeders in this project totally. They are from different MCCs to either motors of varying kW or different packaged equipment like air compressors system (mounted on a skid), etc.

The power supply is 440V, 50Hz, 3 ph.

In this case, I have some feeders (from starters provided) in the MCC to the motor loads or distribution feeders from the MCC (!!) panel to the packaged loads.

In my calculations, I have taken all loads as DOL. In that context, I presume my calculations are right (!!!).

But my question is how should I go about the calculations if I know for sure the some motors are S-D starting type or VSD type or softstarter type. Also how should I size the feeder differently if it was a pure distribution feeder to a packaged equipment. (note that a packaged equipment may contain its own skid-mounted starter panels and that the motors may be again of either DOL or S-D starters).

Hope in my attempt to make myself more clear, I have not ended up confusing you more. I hope to receive a complete suggestion from all experts.

Thanks

I agree with you

C.S. Pant

HI

This is sivakumar from chennai. The above calculation is wrong. I think you made some arithmatic mistake in voltage drop. Please check.

Siva,

Pls put the correct one as i do not know what is the mistake

please check starting current of SD motor from the motor data sheets and then recalculate . Rest of the things remains unchanged Mukund sundarkar Sunil Hitech Engineers Ltd. Mumbai