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.
And regarding the specific case of the lighting circuit in a substation, the last one in which I was involved had a 32 amp fuse holder, with 32 amp fuse, mounted very close to the busbar with only a couple of inches of wire between the busbar and fuse holder. This rendered a fault almost impossible as no earthed metal or other phases were within reach of the very short wire.
A length of double insulated wire then went to a 10 amp fuse for the lights and a 20 amp fuse for the power socket. These fuses could be easily changed if needed. The 32 amp fuse was uncomfortably close to the live busbar, but was likely to last the life of the substation.
In the case of a single load that is very unlikely to produce an overload, then it is permitted to size the cable according to the load, provided that the cable is still protected against short circuit.
Examples of such loads include electric motors with a thermal overload trip, or simple resistance heaters.
In the case of multiple loads that are unlikely to draw excess current, then in my view this is permitted but poor practice. Four loads each limited to 6 amps are in my view permitted on a cable rated at 24 amps. Poor practice in my view because the load might be more liable to increase than with a single load. (Yea mate, the supply is 32 amps so no problem in adding a 5th motor)
In most cases i consider it simpler, safer and better practice to either upsize the cable or downsixe the OCPD.
In such a case I dont consider the length to be directly relevant.
I’m trying to get my head straight on this read something about a maximum length of 3 meters and installed in a way to reduce the risk of a fault.
That's the condition for omitting a fault protective device. Confusingly it's duplicated in the section on positioning overload devices downstream, but it's really just saying you still need to consider fault protection upstream (or omit it according to the rules for fault protection).
- Andy.
am I right in saying tapping off a bus bar in a metal panel with lengths less than 3 meters we can size cables according to the down stream overload protection. just as with a motor circuit we can have the main fuses only used for fault protection and can size the cable for the downstrmean overload protection?
yes, and you can have up to 3m even if the upstream protection would provide neither overload nor fault protection for the cable, and as much length as you like if the upstream does fault protection and overload is done downstream.
If you do forgo the short circuit protection, do not do so willy nilly - the wire that will be acting as its own fuse needs to be routed / protected to that the chances of it doing so are small, and the consequential damage that would occur if it did, is acceptable.
Mike.
Although it is PERMITTED to omit short circuit circuit protection as described above, it is my view very poor practice unless unavoidable.
Example, cables less than 3m in length that connect a 110 volt battery to a control panel. I would if reasonably practical provide short circuit protection WITHIN the battery by fitting a suitable fuse between cells near the middle of the string.
Example, cables less than 3m in length that connect the secondary side of a transformer to a fuse board or other equipment. I would if reasonably practical specify cables of sufficient size that they are protected against short circuit by the HV fuses or circuit breaker on the primary of the transformer.
Example, cables within switchgear, connecting for example a relatively small switch fuse to a relatively large busbar. I would if reasonably practical select cables large enough that they are protected against short circuit by whatever protects the supply into the busbar.
Just because under certain specified conditions you CAN omit short circuit protection, does NOT mean that so doing is a wise design choice, unless there is no realistic alternative.
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