Choosing the right size cable

[contender1]

Hope someone can clear something up for me.
I've been selecting cable recently to wire up industrial machinery for test purposes only.

I usually purchase H07RN-F or H07BN4-F.

My boss has informed me that the cable needs to have a current carrying capacity to match the supply it's being being connected to. For example if the equipment is being connected to a 415v 32a supply the cable in the case of the H07RN-F needs to be 6mm irrespective of the load.
Say the load is known to only draw 3A I have been selecting lower CSA cable based on the load requirements of the equipment but he says that if I do this the cable should be protected by a suitable breaker.

I do a fair bit of pat testing and often see equipment with 2.5mm h07 cable connected to equipment which has a 32a plug fitted.

Is he getting mixed up with installation cable selection as opposed to selecting suitable cable for equipment.

If I'm making an extension lead I always use 6mm cable as It should be capable of carrying the full 32a as I don't know what will be connected to it.

Are there any books I can read on equipment wiring.

Sorry for the long question just really would like to clear it up.

 

[telectrix]

you select your cable according to the load current. then select your MCB/fuse according to Ib<In<Iz. pointles installing a 32A cable for a 5A load, but then the MCB rating must be below the max. ccc of the selected cable. e.g. if the cable will take 20A max, then the MCB In must be < 20A

 

[Strima]

You can have a lower cable CCC than OCPD size if there is no risk of overload.

 

[Wilko]

On the other hand, if you do what the boss says then you've avoided 2 risks .

 

[contender1]

Haha thanks guys.
The problem is I can't change the size of the MCB in the distribution board which will be rated at the full 32A as the sockets are used by all and sundry on the shop floor for all sorts of applications tool use etc.
Thats what he says, the cable should be capable of carrying the full 32A under fault conditions.

But this doesnt seem practical to me I know I have a water chiller in my workshop that only draws a few amps and is fitted with 2.5mm cable and a 32A plug are you saying that this should be protected with a 20 MCB in the panel ?

 

[Bobster]

Does the water chiller have inbuilt OL and short circuit protection. Maybe a 20mm fuse somewhere?

 

[Strima]

Thats what he says, the cable should be capable of carrying the full 32A under fault conditions.

Then I would hazard a guess that he doesn't know a lot about fault currents and circuit design as a fault current would be a lot higher than 32 amps (TT system excepted).

 

[contender1]

You can have a lower cable CCC than OCPD size if there is no risk of overload.

How do you know there will be no risk of overload?
I can understand a motor may be worked too hard and draw more than the FLC but this wouldn't apply to everthing

 

[Wilko]

Your water cooler example - might be better that it have a BS1363 plug (and fuse) or be internally fused as per @Spoon.

 

[contender1]

Then I would hazard a guess that he doesn't know a lot about fault currents and circuit design as a fault current would be a lot higher than 32 amps (TT system excepted).

He's a very experienced electrician who I have alot of respect for I guess what he's getting at is that providing the cable is big enough to carry 32A under fault conditions the breaker would trip before the cable melted

 

[contender1]

Your water cooler example - might be better that it have a BS1363 plug (and fuse) or be internally fused as per @Spoon.

Its a 3 phase piece of equipment

 

[contender1]

Does the water chiller have inbuilt OL and short circuit protection. Maybe a 20mm fuse somewhere?

It may do I'm not sure

 

[Strima]

How do you know there will be no risk of overload?
I can understand a motor may be worked too hard and draw more than the FLC but this wouldn't apply to everthing

Under normal conditions many appliances with a fixed load will not draw a higher load than their rating.

For example (Extreme example for those lurkers that may decide to google lighting circuit designs and actually think something on the internet is true...) a 100 watt lamp will not draw more than 100 watts so in theory you could supply the lamp using 1mm cable fed from a 32 OCPD. When you start to add more lamps then you need to consider the cable rating in relation to the OCPD.

 

[Wilko]

How do you know there will be no risk of overload?
I can understand a motor may be worked too hard and draw more than the FLC but this wouldn't apply to everthing

This is one of those risks I hinted at - risk of not knowing everything. If the load was a heating element say, then it really can't do anything other than open circuit, normal load current or fault current (limited by Zs).

 

[contender1]

Ok thanks for your advice.

On a side not it's much easier selecting the cables from our control panels to the motors and the other equipment they control as the devices do then have suitable breakers and adjustable overloads.

Ah well I'm learning all the time by the time I retire I may be getting somewhere