6mm cable for a hifi?

[HappyHippyDad]

Evening all.

A customer has asked for a dedicated radial circuit to feed 4 2g sockets.

He has asked for the cable to be rated at 45amps as he said at times the hifi equipment may reach this current. He is very specific that he wants the cable rated at this as a minimum.

He has asked for the 6mm to be terminated into an enclosure by the hifi which then feeds out in 2.5mm 'individually' to each of the sockets which are right by the enclosure (like an old spider circuit for lighting).

My plan was to use a 45A MCB for the circuit as the 2.5mm cable supplying the sockets will not need overload (O/L) protection and fault protection will be offered by the 30mA RCD (TT). Reference method will be 'C'.

I'd be interested in any opinions on this as it is a different style of circuit to 'normal' and those shown in appendix 15, but as stated fault protection is in place, O/L protection in place for the 6mm cable and protection against O/L can be omitted for the 2.5mm cable feeding the sockets (433.3.1 (ii)).

Cheers.

 

[Simonslimline]

The CU is inside (in a nicely decorated downstairs loo), it is plastic and I would not be able to terminate the SWA into it. I guess I could use 10mm for the whole run in conduit but I dont see a problem with a well terminated joint.

I am using table 4E4A for the XLPE SWA as it is 90c thermosetting clipped direct.

You have asked 2 questions in this post, I have answered, if you feel the need to pursue them further then I'll try and accomadate for perhaps another post or 2 but If you feel able to offer your thoughts on which cable you would use... T&E/conduit, SWA etc etc based on the the facts above (tidy bathroom etc) that would be appreciated?

You should be using table 4D4A, Unless the accessories are also rated at 90c.

 

[Andy78]

The CU is inside (in a nicely decorated downstairs loo), it is plastic and I would not be able to terminate the SWA into it. I guess I could use 10mm for the whole run in conduit but I dont see a problem with a well terminated joint.

I am using table 4E4A for the XLPE SWA as it is 90c thermosetting clipped direct. I am using column 2, 2 core single phase as only 2 of the conductors will be carrying current and the 3rd used as the earth. I feel this is an ambiguous area, do you feel that it should be column 3 (3 core 3 phase, where you will have 3 live conductors each contributing to heat)?

You have asked 2 questions in this post, I have answered, if you feel the need to pursue them further then I'll try and accomadate for perhaps another post or 2 but If you feel able to offer your thoughts on which cable you would use... T&E/conduit, SWA etc etc based on the the facts above (tidy bathroom etc) that would be appreciated?

Of course you can, with a bit of thought and appropriate methods.

 

[Geordie Spark]

For the first 20 milliseconds of inrush this might be true, but microwave ovens are the same and they don't need special circuits. If his system requires 45A during operation, his speakers will be on fire, or if it's transients he is worried about, he needs to get the reservoir capacitors in his amplifier changed.

Most HiFi woo collapses under engineering scrutiny. Where would all that power go? That's 10kW of heat it would be chucking out. A normal domestic system idles at maybe 20-50W input (listening audio power typically between 0.1 and 10W) and peaks at maybe a few hundred. OK, the power factor is typically low with a rectified load running well below maximum rating, so maybe that 200W requires 2A or so. Class A amplifiers consume their maximum power input at all times, wasting all that is not required for audio as heat. I use one of those, it requires 1.5A.

You can convincingly fill a large stadium on 45A. FWIW I used to be a live audio engineer and I've done it.

One of my dubious claims to fame is that I've actually done this!

It was a JBL 4560 as part of a 5 kW concert rig which was being used at a Young Farmers' gig in a bloody big barn and I was driving it hard.

It lasted as far as the third or fourth number from the end of the gig when there was a loud CRACK & flames appeared from the front of the cab.

The rig kept working with reduced sound and the band finished the set while the roadies rushed to chuck pints of beer on the fire - unbeleivable - roadies chucking beer onto a fire without drinking it first!!

The driver was a total loss but the cab was saved & back in use the following night with a new driver in it.

 

[Lucien Nunes]

He is more worried about 'transients' rather than the total audio power.

I've no wish to interfere or cast aspersions on your customer's equipment or method, but in fact the power to deliver audio transients does not flow through the mains cable at the time the transient occurs, it is provided by the energy already stored in the reservoir capacitors in the amplifier. Otherwise, when the mains is at a voltage-zero, the amplifier would have no power available and the audio would cut out 100 times a second (every half wave) and he would hear an ear-splitting buzz or maybe nothing at all.

Between the supply cable and the reservoir caps is the mains transformer. The primary is in series with your radial circuit and in his case is probably wound with dozens of feet of wire no more than 1mm², all bunched together and pre-heated by the iron losses of the transformer. Any current that goes through your circuit on its way to replenish the capacitor charge, also has to make it through the transformer winding. Thus, there is no point aiming for a supremely low impedance or high CCC because the transformer winding doesn't have these ideal characteristics itself.

Clearly a very high circuit impedance is bad, as the voltage will sag at high volume. But if you've ever run a big PA off a small genny, you'll know how far you can go with that before it has any tangible effect.

 

[DPG]

I've no wish to interfere or cast aspersions on your customer's equipment or method, but in fact the power to deliver audio transients does not flow through the mains cable at the time the transient occurs, it is provided by the energy already stored in the reservoir capacitors in the amplifier. Otherwise, when the mains is at a voltage-zero, the amplifier would have no power available and the audio would cut out 100 times a second (every half wave) and he would hear an ear-splitting buzz or maybe nothing at all.

Between the supply cable and the reservoir caps is the mains transformer. The primary is in series with your radial circuit and in his case is probably wound with dozens of feet of wire no more than 1mm², all bunched together and pre-heated by the iron losses of the transformer. Any current that goes through your circuit on its way to replenish the capacitor charge, also has to make it through the transformer winding. Thus, there is no point aiming for a supremely low impedance or high CCC because the transformer winding doesn't have these ideal characteristics itself.

Clearly a very high circuit impedance is bad, as the voltage will sag at high volume. But if you've ever run a big PA off a small genny, you'll know how far you can go with that before it has any tangible effect.

Tell that to the likes of PWB electronics! Daz

 

[HandySparks]

The CU is inside (in a nicely decorated downstairs loo), it is plastic and I would not be able to terminate the SWA into it. I guess I could use 10mm for the whole run in conduit but I dont see a problem with a well terminated joint.

I am using table 4E4A for the XLPE SWA as it is 90c thermosetting clipped direct. I am using column 2, 2 core single phase as only 2 of the conductors will be carrying current and the 3rd used as the earth. I feel this is an ambiguous area, do you feel that it should be column 3 (3 core 3 phase, where you will have 3 live conductors each contributing to heat)?

You have asked 2 questions in this post, I have answered, if you feel the need to pursue them further then I'll try and accomadate for perhaps another post or 2 but If you feel able to offer your thoughts on which cable you would use... T&E/conduit, SWA etc etc based on the the facts above (tidy bathroom etc) that would be appreciated?

I would consider terminating the armour at an external box and then running just the insulated cores through the wall and conduit to the CU. Much easier to bend than the complete SWA cable and no extra connections required. Do have to run a wire for the armour earth though.

 

[Lucien Nunes]

I've just taken a well desrved coffee-break and done a little test. To represent domestic conditions I put a laboratory grade dynamometer wattmeter in the supply to the audio system at my workbench: Kef Concertos driven by a Quad 520 and a sub shelved at 30Hz driven by a P2500. Speaker and amp efficiency varies, but these aren't much above average. Depending on what accessories are on, it idles around 60-100W. The highest I managed to get without risking damage to the speakers was when playing Gary Moore unpleasantly loud, drowning out the noise of the machine tools. Average power: 225W, peak power: 410W. Average current: 1.5A, peak current: 2.3A. This is at volume levels that would be classed as a legal nuisance in a domestic situation.

 

[davesparks]

The CU is inside (in a nicely decorated downstairs loo), it is plastic and I would not be able to terminate the SWA into it. I guess I could use 10mm for the whole run in conduit but I dont see a problem with a well terminated joint.

I am using table 4E4A for the XLPE SWA as it is 90c thermosetting clipped direct. I am using column 2, 2 core single phase as only 2 of the conductors will be carrying current and the 3rd used as the earth. I feel this is an ambiguous area, do you feel that it should be column 3 (3 core 3 phase, where you will have 3 live conductors each contributing to heat)?

You have asked 2 questions in this post, I have answered, if you feel the need to pursue them further then I'll try and accomadate for perhaps another post or 2 but If you feel able to offer your thoughts on which cable you would use... T&E/conduit, SWA etc etc based on the the facts above (tidy bathroom etc) that would be appreciated?

If you can install T&E in trunking then why can't you run the conductors and bedding of the SWA in trunking having glanded it to a skeleton box on the outside? No matter how good the joint an unbroken conductor is better.

As for the ccc you are using two current carrying conductors therefore treat it as 2 core cable, there is nothing ambiguous about this.
What is incorrect, and potentially very dangerous, is to install a cable to operate at 90degrees when it will be terminated into accessories designed for a maximum operating temperature of 70degrees. There is even a note at the bottom of the table which states that the 70degree rating needs to be used when terminations are only rated to 70degrees.

What would I do? After I'd finished laughing at the nonsense idea of a 45A supply to a hifi I would discuss the electrical science of the situation and help the customer to understand why what they are asking for is a waste of their money.
If they insist on having this supply installed then I would look at routing the cable internally as I don't agree with running internal circuits on the outside of buildings where not necessary for a good reason (I don't think convenience is a good reason) and if it had to go on the outside then, unless it's being buried at some point or otherwise requires armouring, I would not consider SWA as my first choice. I would use an insulated and sheathed UV resistant cable which doesnt require any sort of external connection or glanding and can be drilled through the wall and run in bodgers trunking as necessary.

 

[davesparks]

But if you've ever run a big PA off a small genny, you'll know how far you can go with that before it has any tangible effect.

Every time the drummer hits the big drum the generator slows down, great fun!
And if the generator has come from speedy then you have to count yourself lucky that you've got any volts at all!

 

[NDG Elecs]

Not at all constructive HHD but I would tell them it will make little to no perceptible difference to the audio quality that reaches their wax filled lug holes!

However, it could be a nice earner so play along I suppose!

 

[Lucien Nunes]

I am actually doing some work, honest. But while I work, I'm listening to the War of the Worlds soundtrack at comfortable volume - note power reading (64W). Earlier I was playing the Clash loud enough to hear across the car park, through the OCPD shown... I promise you that is in circuit, a 0.315A quick-blow fuse. It won't stand the inrush though, I bypassed it while switching on.

 

[davesparks]

I still think if they are an audiophool that it's worth selling them a supply wired in bare, highly polished, 2H6 MICC with some technical bull****e

 

[DPG]

I am actually doing some work, honest. But while I work, I'm listening to the War of the Worlds soundtrack at comfortable volume - note power reading (64W). Earlier I was playing the Clash loud enough to hear across the car park, through the OCPD shown... I promise you that is in circuit, a 0.315A quick-blow fuse. It won't stand the inrush though, I bypassed it while switching on.
View attachment 31623View attachment 31624

Great experiment. And a nice old watt meter as well. Daz

 

[jackhammerJIM]

Get him to email the specs for the amplifier . It will have a 13 amp plug with a 5 amp fuse inside .

Then ask him why a 45 amp supply is needed for a piece of equipment protected by a 5 amp fuse .

 

[Marvo]

I'd just keep quiet, get a written spec from him and just install the circuit as long as you can make it regs compliant. Call it 'bespoke audio compliant electrical work' on the invoice and charge accordingly.