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Ib or In for cable calculations

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. 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?

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?

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0 broadgage over 4 years ago

In the example given, I see several answers. If the submain fuse is 100 amps, then there is clearly the potential for simultaneous full loading of three out of four ring final circuits. Unlikely with only a few PCs of course, but future use of the room might alter.

Options include downrating the circuits to 20 amps, splitting the circuits over 2 runs of conduit or trunking, useful if the consumer unit is in the area to be served.

Or simply use 4mm cable, not as extravagant as it sounds remembering that "a minimum of 4mm for small power circuits" is often a client specification for non domestic installations.

Or consider protecting the relevant submain with a 60 amp fuse, that will limit the long term load to only two ring finals nearly fully loaded. 60 amps is still an awful lot for one room !

Yet another option, if the wiring system permits, is to omit RCD protection at the origin of the sub circuit. Two ring finals will be more than ample for the expected load, with about 10 PCs on each ring. The socket outlets should of course be RCD types in this case. This gives the option of greater shock protection by use of sockets with 10ma RCD protection, and little risk of nuisance tripping.
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0 broadgage over 4 years ago

In the example given, I see several answers. If the submain fuse is 100 amps, then there is clearly the potential for simultaneous full loading of three out of four ring final circuits. Unlikely with only a few PCs of course, but future use of the room might alter.

Options include downrating the circuits to 20 amps, splitting the circuits over 2 runs of conduit or trunking, useful if the consumer unit is in the area to be served.

Or simply use 4mm cable, not as extravagant as it sounds remembering that "a minimum of 4mm for small power circuits" is often a client specification for non domestic installations.

Or consider protecting the relevant submain with a 60 amp fuse, that will limit the long term load to only two ring finals nearly fully loaded. 60 amps is still an awful lot for one room !

Yet another option, if the wiring system permits, is to omit RCD protection at the origin of the sub circuit. Two ring finals will be more than ample for the expected load, with about 10 PCs on each ring. The socket outlets should of course be RCD types in this case. This gives the option of greater shock protection by use of sockets with 10ma RCD protection, and little risk of nuisance tripping.
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Ib or In for cable calculations

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