Selecting MCB

[tark]

There is a 6mm2 aluminium cable connected with a 4mm2 copper cable in a radial configuration. They are both buried in masonry, their total length is ~10m. I cannot think of any correction factors (no thermal insulation, no increased ambient temp etc.).

1. Can I use a 32A MCB to power that cable? From what I understand, 32A is slightly more than it should be, but is it safe?

2. Will those cables get extremely hot (like 70C) under full load of 32A?

3. Will those cables overheat and possibly catch fire if overload occurs? For example, at 32A x 1.1 = 35A MCB will not trip, at 32A x 1.45 = 46A it will take up to 1 hour to trip. Can those cables handle 46A for 1 hour?

4. And one not related to the topic question: Am I right to think that 2.5mm2 copper cable that hangs in the free air can handle 27A * 1.45 = 39A for up to 1 hour without insulation melting given there are no correction factors? It looks like quite a lot of power for such small sized cable. Can Wago 221 connectors that are rated at 32A also withstand 46A for up to 1 hour?

 

[Evans Electric]

Thats not an opening post ............more like a naval bombardment !  

 

[tark]

I am sorry, but what did I do wrong? Did I ask too many questions?

 

[Doc Hudson]

I assume you are referring to PVC Twin & Earth, not SWA or single sheathed or MICC, as you only mention the conductor material.  What tables are you referring to in your "from what I understand" aspect. As that may give us a better idea of what cable types you are talking about. Who will be signing the electrical certificate for the design aspect of this work? As they will be the ones carrying the can if it all goes pear-shaped, not an internet advisor, who may or may not fully understand any installation limitations.

Doc H. 

 

[tark]

I am sorry I made a mistake. By "cable" I was referring to 3 separate single core wires. To my excuse, I was reading quite a lot of technical documentation lately with regards to this matter and all the examples were talking about the PVC Twin & Earth.

In my case there will be no one signing anything.

 

[Andy™]

min CSA size of aluminium is 16mm², so your 6mm is non compliant

 

[binky]

are you sure it's Aluminium and not tinned copper stranded (old stuff, usually dark grey sheath and balck / red) .

 

[Tony S]

There is a 6mm2 aluminium cable connected with a 4mm2 copper cable in a radial configuration. They are both buried in masonry, their total length is ~10m. I cannot think of any correction factors (no thermal insulation, no increased ambient temp etc.).

1. Can I use a 32A MCB to power that cable? From what I understand, 32A is slightly more than it should be, but is it safe?

2. Will those cables get extremely hot (like 70C) under full load of 32A?

3. Will those cables overheat and possibly catch fire if overload occurs? For example, at 32A x 1.1 = 35A MCB will not trip, at 32A x 1.45 = 46A it will take up to 1 hour to trip. Can those cables handle 46A for 1 hour?

4. And one not related to the topic question: Am I right to think that 2.5mm2 copper cable that hangs in the free air can handle 27A * 1.45 = 39A for up to 1 hour without insulation

melting given there are no correction factors? It looks like quite a lot of power for such small sized cable. Can Wago 221 connectors that are rated at 32A also withstand 46A for up to 1 hour?

This is what confuses me.

Up to the 60’s 7/044 (6mm²) aluminium was available, I ran foul of it in a MCC panel.

 

[tark]

are you sure it's Aluminium and not tinned copper stranded (old stuff, usually dark grey sheath and balck / red) .

Yes, it is an old object.

This is what confuses me.

I am not sure what exactly confuses you.

By the way, what about my question #4? Can anyone please clarify if those calculations are correct?

 

[Ardet R]

I think you may be approaching the situation form the wrong direction.  You are looking at how dangerous is this whereas you should be looking at how safe is this?

You should be looking at a cable and checking what current it is permissable for it to handle and then work from that current and go lower in total demand to ensure the cable is safe to install.

The design current of a cable is the current it can carry on a continuous basis without causing direct damage to the cable or seriously reducing its life by heat damage.

The current ratings take into account the characteristics of the protective device and how it operates, however they are not intended to state that a significant overload would be carried on a regular basis only that it could withstand that overload which should only occur due to a fault which should be an unlikely occurrence.

Joining copper and aluminium conductors directly can lead to corrosion problems, so you would need to ensure the connection was sound; as mentioned above 6mm² aluminium is not permitted to be used by the wiring regulations.

A 4mm² copper cable can carry 32A if clipped direct without problems so long as it is connected to an MCB for overload and fault protection.  The cable will warm up under full load but should not exceed 70°C conductor temperature.

A single core unsheathed copper cable should not be hanging in free air as it is not a safe installation method. However the cable should not suffer significant damage if it is inadvertently overloaded so long as the protective device is working correctly.

No idea how the ratings for connectors are worked out, if you follow the manufacturers instructions and you do not overload them they should be fine.

 

[Tony S]

Yes, it is an old object.

I am not sure what exactly confuses you.

By the way, what about my question #4? Can anyone please clarify if those calculations are correct?

If the cable isn’t in free air it’s in another medium, so where is it?

Tinned stranded copper cable is so easy to mistake for aluminium stranded. The only way I could say for certain would be by cutting it with sheer cutters. Aluminium has a lower CCC than copper so jointing has to take in to account the lower fault current handling.