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Cable sizing for a 3 phase Chiller
1 Posts
I have a question regarding the supply to a 3 phase no neutral Chiller.

The details stamped on the side of the chiller are - 400v / 50hz / 3ph / 386 max amps.

Am I right in thinking as 386amps is the max load shared across the 3 phases then the load per phase is 386\1.732 (square root of 3) which makes it 223amps per phase so I have sized the SWA feeding this machine at 70mm 3 core with separate 35mm CPC.

Hope to get a unbiased  opinion from other knowledgeable electricians.

4 Replies
What's the kilowatt rating of the chiller? You can use that to confirm the load.

70mm should be okay if the max load is 223a/phase. What about the length to see if volt drop will be an issue  etc? 
610 Posts
I would expect that "386 amps" means 386 amps PER PHASE at full load.
Therefore a cable with a rating of at least 386 amps is required.
Confirm with the supplier if in doubt, but without wishing to sound patronising it think that it means what it says, without any need for furthur calculations, corrections, or assumptions.

I would be very cautious with cable sizing for large chillers for several reasons.
Firstly, most use is expected in hot weather, and cables might need to be de rated for UK summer ambient temperatures. If overseas in a hotter climate than the UK, then de ratting is almost certainly required.

Secondly, chillers tend to be very sensitive to voltage drop. A voltage below the nominal or expected voltage will tend to increase the current drawn, this will either trip the internal overload protection, or might result in the current exceeding the namplate by say 5%
In areas with a lot of air conditioning, utility voltage tends to drop on hot days.

And finally, chillers tend to be a long hour load in hot weather, and voltage drop represents money wasted in cable losses. This can be significant on such a large load.

Finally, double check if this chiller requires any other auxillary supply for controls. Some types need an additional single phase 230 volt supply for controls. If providing same is not easy, then the makers can generally supply a transformer in the control panel that derives the 230 volt control supply from the 400 volt main power circuit.
Be aware that any such transformer may be an extra cost, and that the input current to this control transformer will be a slight addition to the stated current, perhaps 5 or 10 amps.
John Peckham
688 Posts
I agree the supply needs a proper cable calculation. 
3413 Posts
Unless it is a very unusual piece of kit, that current will be per line, so a 400A supply (think of that as  3 fuses, each of 400A)
How big is it, and does it have electronic (inverter) controls. If it is star-delta start, then actually the suply will need quite a bit more in hand to get it going, at least twice that, maybe rather more.
That is a substantial load, so you will need to find out about the rating of the substation transformer you intend to connect it to, and the length of the route - as others have alluded, voltage drops will get you once you are more than some tens of metres from the substation, and apparently oversized cable may be needed.


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