Derive and Size Cables - 3 Phase Induction Motors

Power = √3*V*I*CosΦ is correct. And you need to consider efficiency also.

1. Line = 131 Amps. (What is the formula for this?) Assuming 415V and (CosΦXη)=0.8, you will get this value

2. Phase = 75.6 Amps (What is the formula for this?) Line current = 131A, so Phase current = I_line/√3 ≈75 A

c. Direct On-Line Starter

1. Overload Relay range = 90-135 Amps (what is the formula for this?). Line current = 131A, so you need a realy whose range covers that.

2. Maximum backup fuse = 200 Amps (what is the formula for this?). Recommended fuse you need to get from manufacturer, based on some guidelines, and lots of tests No easy formula.

3. Recommended cable for Al. = 95 sq.mm (Based on which current?) Ampacity tables available from reputed manufacturers available, no easy formula.

4. Recommended cable for Cu. = 70 sq.mm (Based on which current?) As above

d. Star-Delta Starter

1. Overload Relay range = 50-90 Amps (what is the formula for this?). Relay is now WITHIN the Delta. So sees only phase current 75A. See diagram

2. Maximum backup fuse = 160 Amps (what is the formula for this?). As earlier

3. Recommended cable for supply side

3.1. For Al. conductor = 95 sq.mm (Based on which current?) Line current

3.2. For Cu conductor = 70 sq.mm (Based on which current?) Line current

4. Recommended cable for Motor side

4.1. For Al. conductor = 50 sq.mm (Based on which current?) Phase current

4.2. For Cu conductor = 35 sq.mm (Based on which current?) Phase current

Hope this helps.

Good Answer.

I assume in part C (2), I = 131Amps. For Fuse or MCCB sizing in this case, it seems 30-40% larger sized. And the result amps from (1.4 X 131) would come close to the next breaker size of 200A by the manufacturer :)

Hope that would help too...

OK, my mistake, let me detail a little bit. For a rating of 100 HP or 75KW motor, the regulation is that max. permissable current = 1.5 X Full Load Current. I.e. 1.5 X 131 = 196.5A. Since the manufacturer might be having the closest MCCB/Fuse rating of 200A, then that would be the selection :)

i suppose the NEC and other codes in USA and Canada specify the backup fuse/CB rating, but i am not familiar with them. My ignorance should have been no excuse for assuming that the OP is in an IEC area. Thanks for correcting.

The Type 1 and Type 2 coordination as per IEC60947-4 is established by good manufacturers by doing a large number of tests....that is why i mentioned that the OP has to refer to their recommendations.

Hi! Thanks. Using the three-phase formula, we found out the selection of the fuse or MCCB to be of 200A. [Note:That would be for any form of starter.]

Similraly the cable could be sized accordingly. Assuming the length to be of 20 meters or so, the following cable size be used for the said breaker or fuse:

4C X 95 mm^2 XLPE/SWA/PVC/CU

+ 1C X 50 mm^2 PVC/CU/ECC

I could put an equation but would need the % voltage drop etc. Also, there is a chart by the manufacturer when to select the size of the cable after finding out the voltage drop?