DC Motors - Cable sizing

Do realize that the guidance in BS7671 is only that, helpful collection of cable ratings in some common combinations. It is not reasonable to expect the committee to have considered all possible situations, and quite often the results are that the next size up is recommended  just to be sure, especially with large grouping factors or extreme sizes. And cable maker's data need not align with it, and may well be more accurate, for their specific cables.

Has the existing cable type ever failed due to overheating - why is it being replaced ?  If it is still in use Is it practical to put temperature disclosing stickers on the existing cables to see how hot they get in practice ? For DC you do not need to worry about the inductance, or skin effects, so larger cables will be slightly lower impedance than they would on 50Hz, and spacing can be increased.


How different are the ERA figures for DC ?

Just seems strange that for every other cable they provided data for cables buried but not in tables 4D3A & 4E3A. When speaking with a cable manufacturer even they thought this was strange.


No the cable hasn't failed but has been damaged due to other works going on and now needs to be replaced.


Well i can't say how different the figures are because there is no data for them in BS7671 but for a 300mm2 single-core 2 cables DC within a duct the ERA is saying the CCC is 734A this is at 90C.

Working on a project where we are replacing an existing cable serving a DC motor. The motor is 690V and has a current of 1440A. The client has advised that the existing cable is 2 x 2c 300mm2 armoured buried below ground for 130m. Now when looking at the tables in BS7671 the maximum load that a 300mm2 can take is 446A (90C) or 379A (70C) so with the parallel cables this would only provide a current-carrying capacity of 892A ( 758A @70c). We are now concerned that the existing cables installed cira 20 years ago are undersized for the full load current of the motor.

Seems odd that a cable had been run at close to twice(ish) its rating for 20 years and presumably isn't showing any signs of ever overheating - so I wonder if there's a bit more to this that meets the eye. How often is the motor used and how long does it run for? If it's only occasionally or only for short durations or isn't heavily loaded mechanically then it might be that the thermal equivalent current is a lot less than the rating plate. That's presuming the 1440A is correct - that sort of power would be good for hauling a decent sized train.


 

Just seems strange that for every other cable they provided data for cables buried but not in tables 4D3A & 4E3A.

There are quite a few combinations that the tables don't provide for - e.g. the one I stumbled over was no method B in table 4D5 (where you'd expect T&E to have a higher rating than allowed for in table 4D2A)


Looking at table 4A2 (which I suspect is of IEC origin rather than a BS 7671 original) - I get the impression that someone expects single core cables below ground to be in some kind of trough rather than being directly buried, but I might be wrong.


   - Andy.

This is one megawatt approx, and the train motor analogy is about right  -. 690VDC is close to  the sort of voltage used on the London Underground. (nominal 630V but newer units 750 volt ready for upgrades to come.) Along train has a number of motor units, each capable of a hundred kW or so during acceleration.

As has already been said the current cable might be sufficient IF the motor is only lightly loaded and drawing a lot less than the nameplate  rating.

Or if the motor sees only limited use with the cable not being loaded for long enough to reach the steady state temperature suggested by the nameplate.

Or might the existing cable be larger than believed ? Perhaps someone had last minute doubts about 300mm and installed 400mm, which would still be overloaded but not as badly. Older cables do not always have the conductor size marked on the outer sheath.


Unless you can be reasonably certain that the motor is significantly under loaded, or that use is only short term, and WILL REMAIN SO, then in my view any replacement cables should be fully loaded in accordance with published figures.


Remember that the existing cables MIGHT be about to fail. A common failure mode is softening of the insulation which then creeps away from the conductors, usually at bends. Not apparent from external inspection.


Consider also the energy lost in the cables and the cost thereof. Reducing the losses from say 6% to 4% would save about 20Kw or about £3 an hour. for a long hour load that would be in the region of £25,000 a year. A quarter of a million pounds wasted in ten years puts into perspective the cost of larger cables.


Presumably the 690 volt DC supply is obtained on site from a transformer and rectifier unit. Reducing cable losses will also reduce the load to be supplied by the transformer/rectifier and the losses therein.

Such losses might be about 3% of full load. So reducing the load by say 20Kw will reduce the losses in the transformer/rectifier by 3% of 20Kw. About 600 watts saved, or about 10 pence an hour, or about £800 a year for long hour operation.