Multicore cable current capacity

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I am looking at a multicore cable for remote switching of a machine. The supply to the machine is fused at 5A (240v).

The cable to the control panel 16 cores, each of 0.25mm², going to various devices in an adjacent room. The cable is max 5m long.

Switching should be intermittent, across various pairs, but I have to consider that in the event of a fault, it might carry current for an extended period. It is improbable that any core would carry more than about 2A, but I am considering that in the event of a fault it might carry 5A (being the capacity of the fuse for the whole machine) on a pair of cores for an extended period.

How do I verify that the cable is adequate for the current? Bunching does not really apply because if one pair is carrying 5A, none of the others will carry anything.
 
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In real terms it works in reverse to what the regulations say. Working with motor vehicles it is common to find a completely burnt out cable going into a harness but in the harness when opened the cable is good. The heat transfer cable to cable is better than cable to air. Of course when it happens we have to renew the multi-core or open the harness as we can't be sure. But normally in the multi-core or harness it is OK.

So in real terms it's down to what a single 0.25mm² can take in free air.
 
At 30C, a bit over 6A

unless someone has a better table
 

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Since this is part of the electrical equipment of a machine, it must meet the requirements of BS EN 60204-1. It doesn't give a derating factor for 16 core multicore, but for 7 pairs the derating factor is 0,34, for 10 pairs 0,29. Current-carrying capacity of 0,2 mm2 is 4.3 or 4.4 A depending on installation method, so the allowable maximum current in your cable would be around 1.5A.
 
do you mean 1.5A per pair, or per whole cable?
 
If you could be certain that only one pair would be energised at any time, even under fault conditions, then it would be the same for the whole cable.
What is the machine?
 
It's a boiler and its ancillaries.

I find it hard to grasp how to derate, when the total load, spread over one or more pairs, is fused at 5A.
 
I was particularly interested in how downrating works for multicore, not getting into discussions about the things on the ends of it.

The cable we have was left over from a remote control panel for something else.
 
Bunching does not really apply because if one pair is carrying 5A, none of the others will carry anything.
It could if the 5 amps is going back and forth over several pairs. That would not be a good design but it is not an impossible situation.
 
I was particularly interested in how downrating works for multicore, not getting into discussions about the things on the ends of it.

The cable we have was left over from a remote control panel for something else.
If the thing on the end of it is a machine, then it needs to comply with the harmonised standard for electrical equipment of machines. The 30 or so international experts responsible for that standard over the last few decades have decided on the derating factor. Committees who are responsible for other application standards might use different factors.
 
Common sense needs to be applied as well as regulations when designing systems and selecting control cables for the system.

Regulations for the derating of bunched cables for general installation work, such as domestic installations, has to allow for the worse case of all cables carrying current to the maximum rating of that cable. ( or the ratings of the over current protection devices supplying the cables in the bunch ).

When designing cabling for a specific installation the applicable derating factors need only suit the application and do not need to comply with ratings in guide-lines / regulations intended to cover all foreseeable installations.
 
Bernard, for an installation, you might be right, provided that the deviation is justified and documented. For machines, many Notified Bodies will not accept deviations on things like CCC of conductors.
 

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