Volts drop - 3 phase delta pump supply

[leolec]

Came across a supply for a 3 phase delta water pump at a friends farm the other day, the cable appeared too small at 2.5mm swa given the distance (1/4 mile) so far as I was concerned. The 25A 3 phase 30ma RCD had been fried but the cable looked to be ok. Not being a 3 phase expert I pointed my friend at the faulty component which allowed him to call the right supplier.

But it's left me thinking - do the normal volts drop rules apply to delta configs?

 

[johnboy6083]

whats the FLC on the pump mate?

 

[johnboy6083]

as far as im aware normal volt drop applies, and using cablemate, i just did a cable calc, guessing 12A FLC, 25A type D, 5.5kw pump with power factor .8
came in at 10mm2 because of volt drop.

Sounds undersized for me, but ofcourse will need all details for a correct answer

 

[Engineer54]

Well it must be a dammed small motor to be wired in 2.5mm over a quarter of a mile!!

Motors generally require being wired with a cable that is sized, that will not drop voltage during it's starting/run-up time, when a higher current is flowing than it's normal FLC...

 

[johnboy6083]

thats why i was taught that you size the cable for 150% FLC, unless on star delta, or VSD,

 

[IQ Electrical]

Came across a supply for a 3 phase delta water pump at a friends farm the other day, the cable appeared too small at 2.5mm swa given the distance (1/4 mile) so far as I was concerned. The 25A 3 phase 30ma RCD had been fried but the cable looked to be ok. Not being a 3 phase expert I pointed my friend at the faulty component which allowed him to call the right supplier.

But it's left me thinking - do the normal volts drop rules apply to delta configs?

Is this a pressurisation unit as opposed to a genuine water pump?

 

[Engineer54]

Yep ... that sounds about right!! Starting currents can be around 10x plus of FLC depending on motor configuration etc....

 

[leolec]

Thanks very much for the responses - I didn't go into the problem in too much detail as it was well out of my comfort zone, it is a pump, the head must be over 100ft so it must be fairly powerful. I assumed the installer knew what they were doing as it's all connected to a uv filtration unit and looks the part. Granted its not giving a massive flow or a high pressure, but it took out the 3 pole RCD and the 3 pole 16A MCB. I've been reading one or two threads about harmonics but ....? Anyway the thought at the mo is to fit a non-return valve to the intake and move the pump next to the UV filter. Stands a chance as long as the system can be primed.

 

[raithrovers1]

Is the pump supplied by an inverter drive?

 

[leolec]

Is the pump supplied by an inverter drive?

I don't know - didn't get that far with it - what should I be looking for - I assumed delta as only L1 L2 & L3 went to the control switch, the chasis was earthed and the neutral from the rcd went to the met, would this be the same in an inverter drive?

 

[Chr!s]

The volt drop needs to be such that it doesn't affect the operation of the connected equipment. If its the case that the pump will work safely at lower voltages then this way allow a larger volt drop to be utilized.

 

[leolec]

as far as im aware normal volt drop applies, and using cablemate, i just did a cable calc, guessing 12A FLC, 25A type D, 5.5kw pump with power factor .8
came in at 10mm2 because of volt drop.

Sounds undersized for me, but ofcourse will need all details for a correct answer

So there are other acceptable volt drop tables for 415V?

 

[IQ Electrical]

So there are other acceptable volt drop tables for 415V?

Normal voltage drop percentages do apply but as Chr!s has stated, the manufacturers tolerances are the limiting factor in these cases.

 

[leolec]

right - so if it were to be an inverter drive then harmonics may have distroyed the MCB & RCD as they could be overheating and the installer possibly should have installed cartridge fuses depending on manufacturers recommendations

 

[Engineer54]

Purely going on what the OP has told us,... ie cable size is 2.5mm, length of run around a quarter of a mile, a pump capable of rising water around a 100ft and through a UV filtration unit, and then onto wherever.

Now, i don't know about anyone else idea's here, but to my mind that cable is undersized for the size of motor it's supplying, and to say that motor has been struggling, ...is an understatement!!!

 

[raithrovers1]

We really need the data off the pump.
As mentioned before, the cable seems to be underated. Johnboy has done the calculation and it came out at 10mm.
As to why the MCB & RCD were destroyed?- could be harmonics if a drive is fitted although I don't see why a drive is required for a pump that does not need the speed changed.
Could be loose connections.
2.5mm cable is rated for about 20A. After this it heats up. If the pump is drawing more than 20A then this will cause the terminals on the MCB and RCD to heat up and could cause failure after a prolonged period of time.

 

[leolec]

With you there have sent a text to my brother to find out what the model is should have that info in the next couple of days will post as soon as

 

[Engineer54]

Out of interest, ...How long has this pump been installed, or been in service??

 

[leolec]

I think it was about 12 months ago

 

[malcolmsanford]

Chr!s quite rightly was refering to regulation 525.4 concerning the possibility that the motor would work without undue affect on start up with a larger % VD, but that is just on the start up, and it also as Chr!s states you need to refer to the makers technical data.

If though once the motor was runing and it was still getting an above accepted volt drop, which would impair the performance of the pump, then the pump would labour and take high amps, as the 2.5mm is such a long run I would be amazed if it was not producing high amps due to volt drop, but that does not explain why if it was labouring the MCB did not activate on overload, or indeed the pumps overload protection be it external or internal.

I can only assume that the volt drop was just keeping the pump running at the MCB tripping tolerance and therfore the protection device was at full or just above maximum demand for that 12 months, and with the conductor heating up on the load enough to take out both RCD/MCB.

I would definately be contacting the installers and getting them involved, it maybe over a warrenty period but with what you have said so far I think you would have a good case for them to come out and rectify this problem FOC.

 

[Knobhead]

To be honest it looks like your brother will be getting the cheque book out and the spade.
You’ll have to brush up on your cable jointing skills. ¼ mile of 10mm 4C is one hell of a cable drum if you haven’t got the gear to handle it. I’d put it in sections and through joint.