OMS:

fiftyhertz:

Hi all,


Got another one for you all, do you use Ib or In for cable calculations. I must admit that i have always used design current however after looking at recent threads on overload protection it appears i could be wrong. The way i read the attached extract is that, In should be used if there is potential for simulatenous overload, however Ib can be used if this is not the case.

Correct.


The immediate example that comes to mind are cleaners rings which are likely to never have over 2A connected load, however if we start putting the design current to 28A (OSG), they can only be grouped with 4 other cables before a 4mm is required. Do we not need to apply common sense to these calculations otherwise we will just get extremely oversized cables? 

Would each of the grouped final circuits be liable to simultaneous overload - that's up to you as a designer to ascertain. As an example, would the cleaners rings be populated with a lot of electric fan heaters if the main heating were to fail. If that's credible, then yes, you'll end up with "oversized" circuit arrangements - if not, then you can use Ib rather than In for your assessment of grouping


The same goes for lighting circuits, if we know the connected load will be 2A , it seems counter-intuitive to put 6A as the design current, if alterations occur is it not up to the next designer to asses the situation before sticking new luminaires on the circuit?

Lighting circuits are far less likely to be subject to simultaneous overload as people tend not to plug things in - so you could safely use Ib as the starting point for circuit sizing and grouping analysis.


 

 

Regards


OMS 

 

OMS:

fiftyhertz:

Hi all,


Got another one for you all, do you use Ib or In for cable calculations. I must admit that i have always used design current however after looking at recent threads on overload protection it appears i could be wrong. The way i read the attached extract is that, In should be used if there is potential for simulatenous overload, however Ib can be used if this is not the case.

Correct.


The immediate example that comes to mind are cleaners rings which are likely to never have over 2A connected load, however if we start putting the design current to 28A (OSG), they can only be grouped with 4 other cables before a 4mm is required. Do we not need to apply common sense to these calculations otherwise we will just get extremely oversized cables? 

Would each of the grouped final circuits be liable to simultaneous overload - that's up to you as a designer to ascertain. As an example, would the cleaners rings be populated with a lot of electric fan heaters if the main heating were to fail. If that's credible, then yes, you'll end up with "oversized" circuit arrangements - if not, then you can use Ib rather than In for your assessment of grouping


The same goes for lighting circuits, if we know the connected load will be 2A , it seems counter-intuitive to put 6A as the design current, if alterations occur is it not up to the next designer to asses the situation before sticking new luminaires on the circuit?

Lighting circuits are far less likely to be subject to simultaneous overload as people tend not to plug things in - so you could safely use Ib as the starting point for circuit sizing and grouping analysis.


 

 

Regards


OMS