Flex size for motor/downstream protction

[inflammable_tom]

Hello everyone,
I have a question about flex size and circuit protection

The triphase circuit in my workshop is fed from a 20A d curve mcb ( there is also a 30mA rcd before).

It then passes through 7m of 2.5mm wire in conduit to feed an isolator/two 16A sockets.

These will go to a 7.5hp motor and a 4hp motor for two wood machines. Only one will be in use at a time.

My question is this: is it ok to use 1.5mm2 flexes (h07-rn, 3m long)?

Do the downstream 10A and 6.5A motor overloads protect this smaller size of flex, or should I go for 2.5mm2?

I ask because i have the thinner stuff here already... but will buy thicker if needed...
thanks!

 

[sako243]

Do the downstream 10A and 6.5A motor overloads protect this smaller size of flex, or should I go for 2.5mm2?

It is not the motor overloads that will protect against cable faults. Think about it - on a 13A plug, for example, where is the fuse located - in the plug (usually) to "protect" the operator from a fault in the cable, it blows and disconnects the cable from the mains therefore making it safe.

Note they're not there to protect the cable per se but shut off power if a fault develops in the cable or equipment and stop it causing a fire or harm to people etc.

If the only protection device you have upstream (mains side) of the socket is 20A then ideally all cable should be sized to handle at least 20A continuous (rule of thumb). There's rules and regs for all of it but I'm not the person to dispense those just trying to explain the common sense aspect.

 

[dobbslc]

The current rating for a 1.5mm 5 core HO7 cable is only 15 / 16 amps (depending where you look) so it's too small for the 20 amp MCB, also a 7.0hp (5.5kw) motor sounds very juicy for that size cable.
Welcome to the forum by the way.

 

[hotponyshoes]

Presuming you are in the uk using 400v?
Even so, a 10a overload trip sounds low for a 7.5hp motor.
Might be worth checking that 10a trip covers the whole machine rather than just a control circuit/coolant pump/something.
If it does then the 1.5mm cable will be fine for a 10a load.
But, as said, you will want suitable upstream protection for the cable itself.
You could buy a 16a mcb and fit in place of the 20a but you could just buy the h07 in 2.5mm for £30 and do a better job all round.
The 2.5mm will also be physically bigger and have thicker insulation so last longer and be less likely to suffer damage in the first place.

 

[inflammable_tom]

Welcome to the forum by the way

Thanks!

 

[inflammable_tom]

Presuming you are in the uk using 400v? Even so, a 10a overload trip sounds low for a 7.5hp motor. Might be worth checking that 10a trip covers the whole machine rather than just a control circuit/coolant pump/something. If it does then the 1.5mm cable will be fine for a 10a load. But, as said, you will want suitable upstream protection for the cable itself. You could buy a 16a mcb and fit in place of the 20a but you could just buy the h07 in 2.5mm for £30 and do a better job all round. The 2.5mm will also be physically bigger and have thicker insulation so last longer and be less likely to suffer damage in the first place.

Hello everyone, I have a question about flex size and circuit protection The triphase circuit in my workshop is fed from a 20A d curve mcb ( there is also a 30mA rcd before). It then passes through 7m of 2.5mm wire in conduit to feed an isolator/two 16A sockets. These will go to a 7.5hp motor and a 4hp motor for two wood machines. Only one will be in use at a time. My question is this: is it ok to use 1.5mm2 flexes (h07-rn, 3m long)? Do the downstream 10A and 6.5A motor overloads protect this smaller size of flex, or should I go for 2.5mm2? I ask because i have the thinner stuff here already... but will buy thicker if needed... thanks!

Presuming you are in the uk using 400v? Even so, a 10a overload trip sounds low for a 7.5hp motor. Might be worth checking that 10a trip covers the whole machine rather than just a control circuit/coolant pump/something. If it does then the 1.5mm cable will be fine for a 10a load. But, as said, you will want suitable upstream protection for the cable itself. You could buy a 16a mcb and fit in place of the 20a but you could just buy the h07 in 2.5mm for £30 and do a better job all round. The 2.5mm will also be physically bigger and have thicker insulation so last longer and be less likely to suffer damage in the first place.

Presuming you are in the uk using 400v? Even so, a 10a overload trip sounds low for a 7.5hp motor. Might be worth checking that 10a trip covers the whole machine rather than just a control circuit/coolant pump/something. If it does then the 1.5mm cable will be fine for a 10a load. But, as said, you will want suitable upstream protection for the cable itself. You could buy a 16a mcb and fit in place of the 20a but you could just buy the h07 in 2.5mm for £30 and do a better job all round. The 2.5mm will also be physically bigger and have thicker insulation so last longer and be less

Im in France... so same voltages as uk. The amps are correct- I rechecked. Three phase draws less than single for same power, so might have seemed low

What got me curious is here they dont use plug with fuses, and its quite normal to have ( in single phase) a 2.5mm radial circuit of multiple 16A plugs from a 20A mcb- so a 1.5mm flex plugged in is only protected by this mcb.

I will just buy a new cable though!
Good point about 2.5mm being more robust.

Thanks for the replies.

Edited because my phone is playing up...

 

[hotponyshoes]

We don't have any fuses in our 16a plugs either. Or any others for that matter.
It's only the 13a 'household' plug that has a fuse in it.

Same setup here as well.
I have multiple 16a socket outlets on a single 20a breaker.
That just means you can have sockets at convenient places or have multiple things plugged in at once. If you turn too many on at the same time the breaker trips due to overload.
But if someone does something stupid like puts a trolly jack on a cable and lifts a car up crushing before going home for the night, having the correct size cable will cause the breaker to trip out. If the cable is undersized for the upstream protection it will melt and set fire to the workshop before the breaker trips.
If the downstream load is over the rating of the cable as well it could even catch fire just from using it.
Even if you are not worried about the fire risk, At the least, it will be running warmer than it needs to be which will result in conductor and insulation degrading and going brittle so it will need replacing more often.
SY cable might also be suitable for your application and its cheaper than h07 (although harder to fit the plugs onto)

 

[inflammable_tom]

Sorry my phone was playing up amd posted before i wrote the full response above

 

[hotponyshoes]

I'm sure most of us have seen under-rated flex plugged into high amp sockets!

I quite commonly see people wiring up (homemade) armchair lamps with 0.5mm flex (because it's only a 25w led bulb) and leave the original 13a fuse in the plug they fit..
Never actually seen the charred remains of a grandmother yet but I can imagine one moving the chair to get a better view of the telly and putting a chair foot on the cable then sitting down for a nap...

I've also got my own box of home-made adaptors for various applications.
If I plugged enough of those together I could go from a 3-phase 400v 64a outlet to a 110v 16a socket with no protection between!
But a bit of common-sense has kept me alive so far..