Load Calculations

Both formulas are correct. If you have single phase motors (with single phase supply, obviously) the total supply current is 54 amp as you say.

If they're 3 phase and supplied at 220*√3 = 380 volt, that reduces the amps by √3. But you now have 3 supply lines, which gives another √3 factor, so amps in each line is down by 3.

If the motors are 3 phase and wired for 380 volt star/220 volt delta, (and you have all 6 terminals) you could connect in delta at 220 volt, and line current is then 54/√3 = 31 amp.

You also need to allow for efficiency as well as power factor to calculate current.

Thanks!

One more doubt, isn't balanced 3 phase load(I mean 3 single phase loads) same as 3 phase load(single unit)?

Also how should I calculate the current of my Panel Board? The total load is 56 KVA. So should I use single or 3 phase formula?

One more doubt, isn't balanced 3 phase load (I mean 3 single phase loads) same as 3 phase load(single unit)? If you have 3 motors 4kW each supplied at 220 volt, current to each (ignoring PF and eff) is 18 amp as in your original post. So total current = 54 amp. (Same as for one 12 kW motor). 220volt*3*18amp/1000 = 12 kW.

If the motors are supplied one from each phase of a 3-phase supply, 380 volt to give same 220 volt phase-neutral to the motors, amps to each is still 18. Total power is still 12 kW, as 380*√3*18/1000 = 380/√3*3*18/1000 = 220*3*18/1000 = 12 kW. Same as for one 12 kW 3-phase motor, 18 amp in each line.

Also how should I calculate the current of my Panel Board? The total load is 56 KVA. So should I use single or 3 phase formula? It depends on whether the motors are single or 3-phase (assuming you have a 3-phase supply) and the voltage (see #1). But the total KVA of the 3 motors is still 12kW/PF/efficiency.

Thanks bro, really appreciate that!

No problem, I had to think about it a bit, which was good.

Bro, need your help again!

I have a lighting panel board. Obviously all loads are single phase (230 V P-N). Load on phase A is 5 KVA, on B is 4 KVA and on C is 4 KVA.

Total KVA is 13 KVA.Supply volgate is 230/400 V, 60 Hz.

Now, for Cable selection and Protective device for the above panel, I do the calculations as below.

13000/230/0.95=59.5 A

So I would select cable and Protective device keeping in view the above continuous current(i.e. 60 A)

But what I got from the consultant is different. He calculated the current to be:

13000/380/1.732/.96=19.75 A

He also chose the 3 phase cable and protective device accordingly(i.e. for 20 amps).

Why did he use 3 phase formula for single phase loads? What formula should be used in this case both for the cable and protective device??

Thanks

This is already covered in earlier posts, (with lights in place of 1-phase motors). If all the lights are fed from one single-phase 220V supply, current in each bank is ~ 20A, total ~ 60 A as you say. But if each bank is fed between one phase of a 380V 3-phase supply and neutral, the current to each bank is still ~ 20 A and this is the current in each phase. So the consultant is right.

It might be worth considering the situation when 1 bank is switched off, maybe current in the common neutral is > 20A. I'll try to work it out when I have a minute, but perhaps the consultant knows the answer!

This is what confusing me. All lights are single phase and are fed between phase and a neutral(230 V). Total amps are 60. Then how on earch is he selecting MCCB(3phase) with 30 AT/100AF rating. Does 30 AT mean it will trip if current in any phase exceeds 30A or it means that it will trip when total current exceeds 30 Amps?? Also same for the cable.

As I said in #1, if you had just 1 phase, 220 V, and all loads fed from that, total current is 60 A. But 3-phase 380 V is different, the higher voltage means lower amps (by a factor of 3) and because the currents are out of phase there's another 3, so current is back to 20 A. This is current in each phase of the 3-phase system.

I assume 30 AT/100AF means it is rated for 30 A. I don't know what the 100AF means (I'm not a full-blown electrical engineer!) but it's probably to do with the current needed to trip in a specified time, maybe 0.1 sec. 30 A rating does not mean it will trip at anything over 30 A - see attached link. 30 A is its normal operating current. For fuse protection the link says if the fuse is marked '30 amps', it will actually stand 40 amps for over an hour. That sounds low to me, I always thought fuses blew at ~ 100% overload i.e. 2 x rating, but the principle is the same - the fuse rating is the operating figure, not the rupture current.

http://electrical-engineering-portal.com/what-is-the-difference-between-mcb-mccb-elcb-and-rccb

Mistake!

In 1st para I missed out √ in a couple of places. I drafted it in Word as it's been known to get lost from CR4 (probably my fault but still irritating if any length) and forgot to edit after copying in. Corrected below.

As I said in #1, if you had just 1 phase, 220 V, and all loads fed from that, total current is 60 A. But 3-phase 380 V is different, the higher voltage means lower amps (by a factor of √3) and because the currents are out of phase there's another √3, so current is back to 20 A. This is current in each phase of the 3-phase system.

I assume 30 AT/100AF means it is rated for 30 A. I don't know what the 100AF means (I'm not a full-blown electrical engineer!) but it's probably to do with the current needed to trip in a specified time, maybe 0.1 sec. 30 A rating does not mean it will trip at anything over 30 A - see attached link. 30 A is its normal operating current. For fuse protection the link says if the fuse is marked '30 amps', it will actually stand 40 amps for over an hour. That sounds low to me, I always thought fuses blew at ~ 100% overload i.e. 2 x rating, but the principle is the same - the fuse rating is the operating figure, not the rupture current.

How the hell is 3 single phase the same as a 3 phase?

Eh?