DC Motors - Cable sizing

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.

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.