Tri rated cable current carrying capacity

[Jako13!]

I have a 37KW motor and need to replace some burnt out 35mm2 tri rated cable(loose connection). I’ve looked at 25mm2 Tri rated cable the data sheet says it’s current carrying capacity is 136amps. The lengths will be 0.5m.Is there any other consideration I should take into consideration? It will be in trunking with the control circuit. Do I need to apply any de rating factors?

Thanks guys

 

[plugsandsparks]

You will need the Full Load Current off the motor, sometimes labelled FLC or read the Amps related to how it is connected and the voltage. Once you have your current then you can get the cables rating off the manufacturer for the containment you are using, how full it is with other stuff and the ambient temp. Please note though if the trunking has other cables in it you will need the rating for the lowest temp cable in the trunking. In other words dont run at 90 degrees if its sat on a 70 degree cable,,, lol eek

 

[Jako13!]

You will need the Full Load Current off the motor, sometimes labelled FLC or read the Amps related to how it is connected and the voltage. Once you have your current then you can get the cables rating off the manufacturer for the containment you are using, how full it is with other stuff and the ambient temp. Please note though if the trunking has other cables in it you will need the rating for the lowest temp cable in the trunking. In other words dont run at 90 degrees if its sat on a 70 degree cable,,, lol

Thank you plugsandsparks. The other cable is 1mm tri rated so, am I correct in thinking max operating temp is 85degrees in 45degrees ambient air temp? The plastic trunking is full with control wiring. So if the cable is rated at 136 is there a grouping factor that should be applied as well. The overloads set to 70amps which is FLA.

 

[plugsandsparks]

Couple of things. This is generally guess work as your particular set up will have different needs than what Bs7671 can make out. So sometimes its test and verify, in other words you may have to run it to see but not guess, hope for the best and never return... lol

Your main issues as i see them are temperature and what other cables will be impacted by running these in the same plastic trunking, again the trunking has to be considered if it is up to the job.

So all the cables in that trunking need to be considered, like your 1mm control cable to make sure they do not expire due to temps.

I can easily get to 70 amps de-rating the 25sqmm Tri-rated using the above factors alone. However your motor may have a duty cycle which reduces the average temp rise, the control cable maybe a few mA - so this type of engineering can easily go from massive over-engineered to running at the ragged edge.

You may wonder why it was run in 35sqmm in the first place.... either way its a short cable, either just run at 35sqmm as (apart from the loose connection) you have some history for that application which hopefully as not caused a problem or run your 25sqmm and keep an eye on the temps.

 

[Jako13!]

Yeah that definitely my problem supervisor told me to run it in 25 but my question to him was why it was 35 in the first place. His response was that the cable can take 136amps so it’s all good. But with derating as you’ve stated it could possibly not be protected against overload. I guess this isn’t a question of BS7671 and more 60204. However I’m still not clear on these calculations.

 

[westward10]

136A is for a single conductor in free air after that you apply correction factors. For three grouped conductors this is 0.69.

 

[plugsandsparks]

Sadly its not a great answer from your supervisor. Your motor really sets the max demand, if an overload or short cct condition arises either the motor overload will deal with it or the fuse or breaker, providing there is some appropriate upstream cct protection. You can by all means get out BS7671 and start to plod your way through de-ratings for ambient temp, bundled cables, trunking etc and then get onto the other cables and what they would run at but i think you will loose the will to live. The de-ratings are a guide and do tend to err on the side of over engineering from my experience but they can at least give you a worst case scenario.
As this is such a short run, if it were me i would simply replace like for like, its a no brainer