Which size SWA for a 40 metre run into music room?

[Deluks]

I'll soon be building a music rehearsal room in my garage, and wanted to know a few things about SWA.
I've read on here that 2.5mm is suitable for garage use, but as I'll be running a couple of guitar amps, a PC and lighting, will I need more? Will 4mm or even 6mm be more appropriate?

I've already got a 2 way consumer unit with rcd, and with a 32a and 6a MCB's. Will this be suitable for my requirements? and will using 12v downlights give me more 'headroom' for my power requirements, or will it use the same power as 'normal' lighting?

Hope someone can advise me on what to add to my 'shopping list'

cheers

 

[plugwash]

40m is quite a long run, i'd advise going for a 40A cuircuit in 10mm on the grounds that you only want to dig that trench once and usage of the outbuilding can change.

 

[dw3101]

Also technically speaking low voltage lights may take a tiny bit more power as the transformers arent 100% efficient.

Id personally use mains ones, as I hate the thought if the transformers failing.

 

[ban-all-sheds]

OTOH, 12V halogens put out a slightly brighter light which probably balances things out.

But anyway:

while this forum is generally quite good much of the advice on the rest of this site is pretty sh1tty.

What's all that about??

 

[AdamW]

I wouldn't use 12V lighting: guitars and amps are very prone to any EM interference (by the very nature of pick-ups, I guess). It is possible that you will pick up some hum from the transformer.

 

[Deluks]

I wouldn't use 12V lighting: guitars and amps are very prone to any EM interference (by the very nature of pick-ups, I guess). It is possible that you will pick up some hum from the transformer.

Thanks for the tip. I was under the impression that it was the other way round, and that 240v lights caused interference?

So as an alternative lighting source? How about a pair of these double 60w spotlight units, bearing in mind that whatever I use will have to go on the wall, as the ceiling is only 7ft tall and that's before a couple layers of 15mm p/board.

One of these on each wall??
http://www.tlc-direct.co.uk/Products/GLQB10BK.html


I am on a tight budget here though, and I've noticed that 10mm swa costs nearly twice as much as 4mm, and as there are no plans to add anything else to the room so would I get away with 4mm? and would it be able to handle the following two applications......

Lighting needs:
What I basically need is two of the aforementioned spotlights for the music room (10ft x 12ft) plus a 4ft (single or double) fluorescent strip for the rest of the garage (about the same size), which will be used for storage/workshop purposes. Would the 6a MCB be able to cope with this?

Power needs:
I'll need 3 or 4 double wall sockets in the music room, plus another 1 or 2 doubles in the rest of the garage for plugging in tools/lawnmower etc. Would the 32a MCB be able to handle this (the music room and garage area would never be used at the same time, it'd be one or the other)

I could easily put up with 3 double sockets in music room and 1 double in the rest of garage if it will mean cheaper outlay on cable (I.Y.K.W.I.M)


Thanks again for any advice, I did notice that all the bright sparks hang out in the electric forum The other forums are a bit hit 'n miss.

 

[Beanzy]

If you want to push the cable size this much you'll be into some serious maths. Appendix 4 of BS7671 section 7 gives the details. Below I've just roughed out an outline approach, you'd need to be on site to check other factors out.

As a rough guide the absolute maximum you can get from your buried 2core 4mm SWA would be 0.9 of 38A giving 34.2 (and that's without taking the power-factor of the guitar amplifiers into account). Remember they're a very dynamic load in which demand which with variations in the music dynamics. If you use a 3 core SWA cable you'll be using 0.93 of 33A so you're down to 30.69 A.

If we de-rate the loads as if they were Fluorescents for convenience sakes you'll be treating the demand of your sockets as 57.6A during peaks. If you're expecting to deliver a 32A ring then you're just about ok with buried 10mm 3core SWA for this, however we haven't added the lighting (avoid anything with transformers and avoid fluorescents like the plague in your music environment. In terms of your given situation you'll be well in with 2 core which will enable 50A (derated .75) . Remember SWA needs proper terminating to secure the armour glands. You should only drop about 2.85V from the origin to the Distribution board on 2 core 10mm.

You may want to consider having an earth pit installed with an RCD to give you a TT set-up here. The risk of circulating earth currents at this distance is great, and you need a clean earthing system if you're going to avoid ground loop hum etc.

Suggestion. Either drop the demand to a 20A radial circuit with about 3A for lighting if you want to do 6mm. or just bite the bullet and get the 10mm in. In a situation like this it would be well worth getting an electrician in to do the measurements and calculations so you get the best possible solution for your given situation. You'll need to ensure the integrity of the earthing and the disconnection times of the system. Remember people will be touching live amplified instruments in a system well outside your main equipotential zone, so they'll be particularly vulnerable in the event of a fault. Also look at bonding of any incoming services as there could be a sizeable voltage between these and the earthing of your main system on the SWA, not to mention the difference between any incoming metalwork and your earth rod.

 

[Lectrician]

Along comes the day when it's it's freezing cold.......out come the heaters!

 

[plugwash]

note that tlc are an expensive place to buy swa

http://www.electricalshop.net/acatalog/SWA.html is considerablly cheaper.

local wholesalers may be cheaper still but if they think you are wasting thier time they may crank the price up to something insane.

you could use 4mm on a 20A breaker but don't be surprised if you have problems when you start plugging in more than one heater or similar.

 

[AdamW]

you need a clean earthing system if you're going to avoid ground loop hum etc.

Agreed, ground hum can be a nightmare with any guitar amp. People have been known to disconnect the earth terminal of the flex, but that is TOTALLY a bad thing to do!

 

[Big_Spark]

The type of installation your talking about will NOT NEED a clean Earth. Come on fella's, we are talking Guitar amps, not Critical server rooms for christ's sake.


In order to comply with the regulations, assuming a maximum demand of 40A on the circuit, and protected by a 40A RCD, then 6mm SWA is sufficient for the task at hand.

With regards to lighting, High Frequency Control Gear Fluorescent luminaires will be the answer, they will give you all the light you need, and avoid the problems of any potential hum in the systems you use. Further, they will keep maintenance costs to a minimum.

If you choose fittings that use compact lamps, such as TL-C or TL-D types, you get a large light output for very little input.

Personally I would suggest that this is not a DIY job, bear in mind it would all need to be certified as safe to use, especially if your not the only person who will use it. Should you ever decide to use it is some pseudo or full commercial way, then it gets even more "interesting".

 

[Beanzy]

The type of installation your talking about will NOT NEED a clean Earth. Come on fella's, we are talking Guitar amps, not Critical server rooms for christ's sake.

Folks always end up trying to record in places like these. Sound engineering was where I started out and ground hum and mains induced transients are a real pig. Hiss hum clicks and pops can ruin a whole series of takes, chucking hours down the drain, and somehow you never hear them until after you listen back or try to combine two takes. I was going to say to get a filter across the incomer until I saw he was on a budget. And introducing fluos in there is asking for trouble, HF just gives HF noise less audible perhaps but why stick it in there in the first place?

If they're never going to record and don't really mind the background hum while rehearsing then it's not an issue.

Would the 40A set-up cope with the transient loads and lightng after derating for burial? I'd not recommend going for C types if he's not got someone in who can assess the situation on the ground.

 

[Big_Spark]

Beanzy..You lost me on the type C comment..for protection it should have a 100mA RCD not a MCB of any type.

Regarding the Fluorescents, TL-C type fittings with HF electronic ballasts are designed for use in critical environments, they will not cause the kind of interference being talked about here.

 

[AdamW]

Guitar amps (and their cousins, the bass amp and the PA amp) are very prone to hum from the earth. Not sure why, but I would hazard a guess that it is due to the increased sensitivity of the pre-amp to cope with the very low input signal voltage from the guitar.

Also I have read that guitar amps are often badly designed from the outset.

When I have played in professionally-designed and installed studios then the hum is virtually non-existent, takes a keen ear to pick up on it. But when I have plugged my amp in at home (3 different homes over the last few years) I get hum of varying degrees. Same amp, just different plug sockets!

It would seem the studios have either very-well designed wiring lay-outs (doubtful as they invariably share a building with other uses and therefore can't be built "perfectly"), or they have some form of of filter or conditioner on the power wiring. Maybe they even build the room as a Farady cage and ground everything to that

Strangely, I've never tried to rectify earth hum in any way. Dunno why, seems a bit daft now I think about it.

 

[Beanzy]

Beanzy..You lost me on the type C comment..for protection it should have a 100mA RCD not a MCB of any type.

Now you've dropped me here. RCD only? what protects the circuit/cabling?
I feel like we're on the same road but different lanes here somehow
Reading back with all the talk of earth etc it may not read right, but the type C comment was not about earth leakage.

Type C : was talking about coping with the overall load, combined with the effects of the dynamics of the amps, they're like having something continually in start-up mode, every time the guitarist goes from John Williams mode to Johnny Axeman the surge is very large, hence my de-rating the cable at 0.75. This also covers the burial of the cable if inst. method 3, I can see it may be 1, 2 or 16 depending on how he does it. Would you count it as ref method 1, 3 or 4?

Without an on-site scout about it's a bit difficult to see how he can push the cable size if he keeps the load up at 32A + 6A especially when the 32A will be inductive. Maybe I'm just too cautious? I know I drive like a granny on the Isle Of Wight, so probably.

This one isn't a studio but a rehearsal room though, so the earth issues I mentioned before may not be so critical. The only sound studio I've installed from scratch (ie no building to start with) we had to have two 8x2 trenches dug and stick a pair of earth plates in about 6ft apart. (not my design, I was only a snotty nosed sprat then). Then compact the earth once filled in so we could get on with the rest of the job.

BTW If anyone has problems with cable interference in a set-up; rather than just drop the screen on an XLR, try putting a resistor in first, up the value from 50 Ohm until you get the result you're looking for (prob about 200Ohm). If you do have to drop a screen then do it at the equipment end or in the Krone Bay if it's a larger set up.