Warm Cables

[Jadele]

Hi guys I am looking for a bit of advice. As shown in the attached pic I have a 3 phase DB joined to a smaller enclosure which houses 10 x 20 amp contactors.

These are fed via C16 and C10 breakers from various phases in the main DB via 2.5 stripped T&E. All the cables pass to the other enclosure via a 50mm coupler.

The outputs of the contactors are connected to various banks of GU10 LED lighting via 1.5 and 2.5 T&E depending on maximum current and distance.

The current draw of 4 of the outputs are around 8-11 amps each and the rest between 3 and 5 amps of continuous load.

From this I have 2 questions.

1 - The 2.5 cables between the breakers and the contactors seems to get quite hot. They are only warm from the breaker to about 3 inches before the coupler but after that they get much warmer. It appears to only be the cables with the 8+ amp loads on that are doing this and warming the others. That said the outputs are not getting anywhere near the same temperature.

When I say Much Warmer you can still comfortably hold your hand on them it would never burn you. I would say they get to about the temperature of a warm bath. I rarely deal with cables that are getting warm except the odd shower cable and wondered what peoples thoughts are on it? I am just concerned that once the door is all closed if they were left on for a very long time they may get a runaway temperature. Just to note that the 2.5mm cables were 1.5 and were doing the same thing so I increased them to 2.5 to see if this eliminated the problem. While better it still exists. Could this just be a grouping issue and normal and nothing to worry about?

Also the highest demand output that runs at 11amps trips even a c20 breaker occasionally at startup due I guess to in-rush current. Is there anything I can do apart from breaking down the demand.



thanks

 

[John D v2.0]

That warmth in itself isn't a problem, however you'd be wise to check the calculations for cable sizing if you're concerned. If you have 10 circuits (with some load >0.3x) going through bunched together, you're going to have to derate them a fair amount.

 

[Jadele]

Hi John that's what I thought which is kind of why I went up to 2.5mm from the 1.5mm that was there. The thought being that even at .5 derating the 2.5 would be fine and reduce the heat. Like I say the outputs are barely warm its just that bit shown in the diagram in red where the coupler is. I see loads of installations where cables are ram packed and they don't seem to have an issue although I wouldn't necessarily of known the loading at the time.

 

[ban-all-sheds]

Hi John that's what I thought which is kind of why I went up to 2.5mm from the 1.5mm that was there. The thought being that even at .5 derating the 2.5 would be fine and reduce the heat.

But even at .5 derating, 1.5mm² is OK for 10A.

 

[Jadele]

But even at .5 derating, 1.5mm² is OK for 10A.

Well yeah but I thought it would run cooler as the 1.5 would have been in the top end of its spec.

So you think it's all ok then and normal. All connections are tight. No undue resistance on the cables according to the fluke. Zs is great from the output side back.

 

[John D v2.0]

nothing you've said is inherently a problem, but assuming you're the designer and you're happy the calculations you've done haven't missed anything and your testing is good, and you've used good quality approved materials, I can't think what else you can do other than sign it off!

 

[John D v2.0]

although an interesting one is the ambient temperature one, because if your db has a high ambient temperature, would that be covered by the grouping factor or would you use a higher temperature for derating them?

 

[Jadele]

I am not the designer. They fell out with the designer I was just looking at the c 20 tripping and found it to be warm etc. I have clamp metered it and nothing is beyond the limits of the cables and now even derated the 2.5 should be ample. Just would of thought that 2.5 should of stayed cool at 10amps ish.

 

[Jadele]

Ambient temp defeating may be an issue too but I need a thermometer to stick I. The cabinet and the. Close it to see what it gets too.

 

[EFLImpudence]

How 'hot' is 'warm'?

There is very little difference between a bath which feels too cold and and one which is too hot to get in.

 

[Jadele]

Need to get a thermometer from somewhere but like I say no where near to hot to keep your hand on. Can't think of a way to explain it. Maybe similar or a touch warmer than a laptop charger that has been. Charging for a while.

 

[securespark]

They can get pretty hot!

 

[John D v2.0]

those cheap IR thermometers are really good for checking temperatures quickly. I was surprised how accurate the one I got is, I forget the price but it was under 20 quid.
The only thing is struggles with is shiny surfaces so you have to stick something on copper pipes.
I reckon it's within a couple of degrees of actual at the most.