Bunched cables, derating.

[bernardgreen]

Eight, possibly ten, 1mm ² twin and earths from a switch panel to eight, possiby ten, lamps 50 metres away. The switch panel is fed from a fused switched outlet so the total current in the bunched cables is limited to 6 amps. ( accepting there will be short duration overload in fault conditions till the fuse blows ).

Guides and cable size selection require the cables to be 1.5 mm²

Common sense appears to suggest that 1 mm² is adequate. If only one cable is taking the full 6 amps then the others will have no current and therefor not contributing anything to the derating equation. That situation will not happen as largest lamp will be 100 watts

This is going to come up between a jobsworth electrician and a money conscious client.

 

[ban-all-sheds]

An FCU with a 6A fuse?

What CCC do you end up with for 1mm² ?

 

[JohnW2]

Eight, possibly ten, 1mm ² twin and earths from a switch panel to eight, possiby ten, lamps 50 metres away. The switch panel is fed from a fused switched outlet so the total current in the bunched cables is limited to 6 amps. ( accepting there will be short duration overload in fault conditions till the fuse blows ). Guides and cable size selection require the cables to be 1.5 mm² Common sense appears to suggest that 1 mm² is adequate. If only one cable is taking the full 6 amps then the others will have no current and therefor not contributing anything to the derating equation. That situation will not happen as largest lamp will be 100 watts.

I think yiou already know the answer. I totally agree with your common sense view, but I can't think of anywhere where the regs encompass any such common sense.

However, as you know, there is no complusion to comply with BS7671. If, as I imagine is the case, you can present a good argument that the proposed arrangement is electrically satisfactory and safe, that should theoretically be OK.

Kind Regards, John

 

[JohnW2]

What CCC do you end up with for 1mm² ?

We need to know the installation method, since some would give an answer above, and some below, 6A.

Kind Regards, John

 

[ban-all-sheds]

I'm assuming that Bernard has already worked it out.

Let's not forget that with a 5A FCU, his Ib for any of the cables can't be higher than that.

 

[JohnW2]

I'm assuming that Bernard has already worked it out. Let's not forget that with a 5A FCU, his Ib for any of the cables can't be higher than that.

One assumes from what he said that he had and that (for the installation method concerned, and together with any other derating factors), the answer for 1mm² cable must have been below 6A (he said 6A, twice, rather than 5A, although that seems rather odd).

Kind Regards, John

 

[ban-all-sheds]

Yes, odd.

I wonder if an option would be to split the cables into two groups and have 2 x 3A FCUs?

 

[JohnW2]

Yes, odd. I wonder if an option would be to split the cables into two groups and have 2 x 3A FCUs?

If that were possible, it would presumably solve the problem. Indeed, it's far from impossible that having them all on one 3A FCU could be an option. However, as I wrote to Bernard, I personally think that the 'common sense' approach (which I feel could easily be justified in engineering terms) would be perfectly OK - after all, BS7671 does not really address the issue of grouped cables in a situation in which the total current in all of the cables is seriously limited.

Kind Regards, John

 

[ban-all-sheds]

OTOH, all 3 of us could bother to read Note 9 in 4C1.

 

[JohnW2]

OTOH, all 3 of us could bother to read Note 9 in 4C1.

Ah!

However, I find it all a bit confusing and/or ambiguous. It says that if one has N cables, of which M will be carrying currents <30% of their grouped rating, then the 'other' (N-M) cables may be sized as if they were just a group of N-M. That's all reasonable, but what about the first M cables - as written, this note seems to imply that those first M cables have to be derated 'naturally', and I find it hard to believe that such is what they mean (I think I would expect them to be saying that they did not need to be derated at all).

What do you think?

Kind Regards, John

 

[bernardgreen]

It is a 5 amp fuse in the FCU which is supplied from a 6 amp RCBO. Given that a five amp fuse will carry 6 amps for a very long the supply is effectively "fused" at 6 amps by the RCBO.

Splitting into two groups of cables is out of the question as there is only one cable duct

 

[JohnW2]

It is a 5 amp fuse in the FCU which is supplied from a 6 amp RCBO. Given that a five amp fuse will carry 6 amps for a very long the supply is effectively "fused" at 6 amps by the RCBO.

As far as the regs are concerned, that doesn't matter. If the cable is protected by a 5A fuse, then it only needs to have a CCC (after derating) of 5A.

Splitting into two groups of cables is out of the question as there is only one cable duct

I don't think BAS was talking about physically separating the cables, merely splitting them onto two FCUs so that 3A fuses could be used (hence required CCC after derating only 3A). However, as I asked, might it not even be possible to have them all running off a single 3A FCU? What potential total load are you talking about?

Kind Regards, John

 

[bernardgreen]

I am given to believe the maximum lamp rating will be 100 watts so 1kW if all are on at the same time.

 

[JohnW2]

I am given to believe the maximum lamp rating will be 100 watts so 1kW if all are on at the same time.

OK - so you probably do need at least a 5A fuse. So, per BAS's suggestion, is there any possibility that they could be fed from two 3A FCUs (5 lamps per FCU)?

I forgot to ask - with the current single FCU proposal, why on earth is there to be a 5A FCU after a 6A MCB?

Kind Regards, John

 

[ban-all-sheds]

What do you think?

N = M?