Cooker hood wiring, 3 metre rule

[gazdkw82]

I need to install a cooker hood. Only close supply is the cooker switch. My plan was to install a FCU next to the switch and then terminate hood, fused appropriately.

My concern was, if the cooker supply is 6mm > I think I'm going to struggle terminating 2x6mm2 cables from the supply side of the switch to the FCU.

I have just heard that there is a 3metre rule that enables you to use a smaller CSA to supply the FCU.

Have you heard of this? Could someone point me in the right direction within the regs?

If this is true then the next conundrum would be obtaining a secure connection with 2 different size conductors into the terminal...... ferrules on the hood flex will help

 

[telectrix]

you can use 2.5mm or 1.5mm from switch to FCU as it is protected from overload by the 5A/3A fuse in the FCU. double the end over and connect to the load side of the switch (or outlet plate). hood will only be required when cooker is on anyway.

 

[gazdkw82]

you can use 2.5mm or 1.5mm from switch to FCU as it is protected from overload by the 5A/3A fuse in the FCU. double the end over and connect to the load side of the switch (or outlet plate). hood will only be required when cooker is on anyway.

I was always under the impression that the fuse protects the load only?

 

[pc1966]

I was always under the impression that the fuse protects the load only?

It protects the whole circuit from an end-point fault, but the cable before the fuse is not protected from any faults to that section.

That is the basic idea behind the '3m rule' where there is a change of cable rating and the regulations say it is acceptable to be protected by a down-stream fuse/MCB/etc, provided that the cable is short and otherwise well protected so unlikely to suffer damage or accidents.

For example for stuff like cables coming off a busbar to switched fuses, etc, all within the busbar chamber but without having to use 800A wires to your 60A switch-fuse, etc.

Reg numbers 433.2.2 and 434.2.1

The requirement 434.2.1 (ii) and (iii) to minimise the risk of faults, and consequences of them, has to be considered. But if you have the FCU immediately next to the cooker switch and the wires between them, maybe even short section of conduit or just a coupler and two male bushes, then you have that extra protection for the short length of small cross-section wire.

 

[telectrix]

I was always under the impression that the fuse protects the load only?

post#4 goes into deeper detail.

 

[gazdkw82]

It protects the whole circuit from an end-point fault, but the cable before the fuse is not protected from any faults to that section.

That is the basic idea behind the '3m rule' where there is a change of cable rating and the regulations say it is acceptable to be protected by a down-stream fuse/MCB/etc, provided that the cable is short and otherwise well protected so unlikely to suffer damage or accidents.

For example for stuff like cables coming off a busbar to switched fuses, etc, all within the busbar chamber but without having to use 800A wires to your 60A switch-fuse, etc.

Reg numbers 433.2.2 and 434.2.1

The requirement 434.2.1 (ii) and (iii) to minimise the risk of faults, and consequences of them, has to be considered. But if you have the FCU immediately next to the cooker switch and the wires between them, maybe even short section of conduit or just a coupler and two male bushes, then you have that extra protection for the short length of small cross-section wire.

Thankyou for that info.

If I was to terminate say a short run of 1.5mm2 from load side of the switch to hood, in normal working conditions, if a piece of current using equipment (in this case an oven) is using an amount of current that exceeds the maximum allowed for that short section, would there be adverse effects?

Is there conditions for an overload? Or does the current using equipments load not impact that small section between switch and FCU?

 

[streamer]

I wouldn't normally take a spur from a cooker circuit but, that said, if you take a spur from a ring final circuit then it's usually a 2.5 protected by a 32 amp mcb so it's not really any different.

 

[telectrix]

the current drawn by the cooking appliance will have no effect on the cable to the FCU, as said current don't go down that path.