Overload protection

MrJack96: 
 

If you have a 32 amp feeder feeding 4x 6 amps circuits feed in parallel is there something in the regs that states the cable between the 32amp fuses dosmt have to be rated for 32 amps as long as It meets certain condition? my thinking was as its limited to 6 amps upstream it’s unlikely to get overloaded.

I can not visualise the design. Does 433.3.3 apply?

Z.

I suspect that the O/P is referring to a sub main from a 32 amp OCPD that feeds a consumer unit with only four circuits, each with a 6 amp OCPD. Or electrically similar arrangements.

In that case I believe that the arrangement is acceptable, but rather poor practice. I perceive a risk of adding extra sub circuits or up-rating the existing circuits to say 10 amps each.

It would in my view be better to either size the sub-main cable to allow a for a full 32 amp loading, or alternatively to downsize the upstream OCPD to 25 amps.

MrJack96: 
Sorry I wasn’t clear so the 32 amp cable feeding a MCC then off of that the 6amp motor circuits

In that case I believe that the arrangement is acceptable but arguably not best practice. 

What size is the feeder cable protected by a 32 Amp protective device?

Z.

Does 433.3.3 apply?

More likely 433.2.2 I think - we're not omitting overload protection, just positioning it downstream of the conductor it's protecting.

The regs are worded in terms of a single protective device, but I reckon 4off 6A devices are pretty equivalent to one 24A device as far as overload is concerned.

Note that this approach only applies to overload protection - other overcurrent protection (faults) need to have protective devices placed upstream - but they of course may have significantly higher rating than the conductors and still be effective (the magnitude of the fault current and disconnection times being suitable co-ordinated with the device and conductor).

   - Andy.

IF the total load cannot exceed the 4 lots of 6A at once, that is to say 24 A, it is OK to use a cable rated for that, indeed the 32A breaker could probably be bigger so long as it still provides short circuit cover for that cable. But woe betide someone just adding another load to that second board without thinking too hard.

 So I'm with Broadgage - it is OK, and to regs it is perfectly acceptable, but not the sort of thing many people want to see or do. if they can avoid it.

Mike

MrJack96: 
Am I right in saying this is the same for if you was tapping of a bus bar the overload protection may be downstream so the conductors should be sized to the downstream circuit as long as Fault protection ADS upstream of the cable is in place and the ZS of the cable is met? 

Not quite the same in my view.

Small conductors tapped off large busbars MIGHT not even be protected against short circuit, this is permissible for SHORT conductors that are physically protected against damage.

For example, a SHORT length of say 6mm that connects a 32 amp switchfuse to a 315 amp busbar.  In such a case my preference would be for a larger capacity switchfuse, perhaps 100 amps, with terminals that accept say 50mm cable. 50mm is probably protected against short circuit by the upstream 315 amp fuses.

Also acceptable for SHORT conductors that connect batteries, transformers or generators to control panels or other equipment that incorporates OCPD. 

What the O/P is proposing is a cable that IS protected against short circuit by the upstream device, but is NOT protected against overload by any upstream device. Reliance being placed upon the downstream devices for overload protection. Acceptable but poor practice in my view.

The most extreme thought experiment example would be a wire that feeds the light switch in the substation - the lights probably draw single figure amps, and yet the at the origin PSSC is as high as it is ever going to be. 

So does the wire to that  HRC fuse get done in 240mm2 or something - even if it is only 5A ?

Generally, no  - we may prefer to accept that if disaster struck in the hop before the fuse, that whole (short ) length would be heat damaged (maybe even blown open circuit)  and have to be replaced.  It won't be as thin as 1mm2, though from a current rating consideration perhaps it could be, but that is more about sensible mechanical robustness, and lug sizes available. It does need to be routed and restrained so that it cannot fail in a way that causes any damage though.

Mike