Wiring garage sockets with steel conduit, how do I do this?

[Space cat]

I mean the current that flows both ways round the ring, from the source meeting at the appliance socket can presumably be double what it would be on a spur.

That's only true if the socket is at the mid-point of the ring. In practice the current will not split equally between the two cables so, while a 2.5mm 'spur' direct from the CU (better known as a radial) is typically rated at 20 amps, a ring of the same cable is only good for 32 amps - and you could still overload it by putting the full load close to one end.

It's got two routes, therefore twice the area of copper --

Since you're obviously interested in the subject, you should know that doubling the area of copper will not give you twice the capacity if the doubled up copper is all in a bigger cable. I'll let you think about that one; it's a thermal problem.

-- or it joins to the CU in two places.

A ring starts and ends at the fuse/breaker in the CU. Having said that, I inherited an upstairs ring which started and ended at a junction box under the floor. This in turn was on the end of a 4mm radial from the CU and, just for good measure, that radial stopped off at a single socket on its way to the junction box. I can only imagine that whoever put it in had a lump of 4mm cable going spare!

 

[shagster]

I mean the current that flows both ways round the ring, from the source meeting at the appliance socket can presumably be double what it would be on a spur.

That's only true if the socket is at the mid-point of the ring. In practice the current will not split equally between the two cables so, while a 2.5mm 'spur' direct from the CU (better known as a radial) is typically rated at 20 amps, a ring of the same cable is only good for 32 amps - and you could still overload it by putting the full load close to one end.

Rings sound like they have a few gotchas! So are rings harder to test than radials, and are they more or less likely to fail? Copper cost not being an issue would one wire a garage with four double sockets as a ring with smaller cable or a radial with the larger? I don't plan to use the garage as a commercial workshop, the most current will be for the pressure washer, 2.4KW, a couple of other 1500W drills and 600-800W appliances might also be plugged in, unlikely to be on at the same time.

Since you're obviously interested in the subject, you should know that doubling the area of copper will not give you twice the capacity if the doubled up copper is all in a bigger cable. I'll let you think about that one; it's a thermal problem.

That figures, it would be derated with temperature.

A ring starts and ends at the fuse/breaker in the CU. Having said that, I inherited an upstairs ring which started and ended at a junction box under the floor. This in turn was on the end of a 4mm radial from the CU and, just for good measure, that radial stopped off at a single socket on its way to the junction box. I can only imagine that whoever put it in had a lump of 4mm cable going spare!

Just to check my understanding, even though 4mm temperature derated current capacity is way less than 2.5mm x 2, this arrangement is perfectly safe for the reason that the breaker has to deal with the loads at the start and end of the ring, so for most houses, the ring at 2.5mm is equivalent to the 4mm radial. I guess a safer ring system would be breakers on each end of the ring - isn't there a safety issue when the ring breaks, or does the CU breaker deal with that too?

I guess there is some house size at which 2.5mm ring/4mm radial no longer applies and you would switch to 4mm ring, 6mm radial, which would go by number of sockets or length or ring or maybe both? I'm only curious how far inside this limit the average domestic property is.

thanks,
S.

 

[riveralt]

Rings sound like they have a few gotchas! So are rings harder to test than radials

Yes but since you said that you were not going to buy/hire test equipment that shouldn't be an issue.

and are they more or less likely to fail?

No evidence to justify that but with a ring final circuit you are less likely to know - think about why.

Copper cost not being an issue would one wire a garage with four double sockets as a ring with smaller cable or a radial with the larger?

It will depend on the size of your garage since there are suggested limits to the size of ring and radial final circuits. Why do you need a radial with a larger cable. Nothing to stop you have one 2.5mm T&E 16Amp MCB radial for each socket if the needs be. On a recent rewire I did, a new additional eight way board allowed each appliance socket to to be individually protected by its own circuit and RCBO.

I don't plan to use the garage as a commercial workshop, the most current will be for the pressure washer, 2.4KW, a couple of other 1500W drills and 600-800W appliances might also be plugged in, unlikely to be on at the same time.

So at last you are starting to think about what you want to use the garage for. This will help the electrician design the correct circuit(s) for your garage.

Just to check my understanding, even though 4mm temperature derated current capacity is way less than 2.5mm x 2, this arrangement is perfectly safe for the reason that the breaker has to deal with the loads at the start and end of the ring, so for most houses, the ring at 2.5mm is equivalent to the 4mm radial.

You appear to be off on another tangent again here. Derating occurs when temperature rises and or voltage drops - you should research the answer why. But since most of the BS7671 current carrying capacity cables are set at 70c you are going to have really increase the temperature to cause the cables to be derated when clipped direct. It is only when you use conduit etc, bury the cable in insulation or have long runs is derating going to be an issue.

I guess a safer ring system would be breakers on each end of the ring - isn't there a safety issue when the ring breaks, or does the CU breaker deal with that too?

Think about what you are saying here and the consequences of one of the MCB's tripping.

I guess there is some house size at which 2.5mm ring/4mm radial no longer applies and you would switch to 4mm ring, 6mm radial, which would go by number of sockets or length or ring or maybe both? I'm only curious how far inside this limit the average domestic property is.

I would like to see you fit 4mm and 6mm cables in the todays sockets - They can be made to fit but its not for the faint hearted. House size has nothing to do with it - It is the circuit length and voltage drop that causes the problem. With large houses you can break down the circuits to manageable means and 'normal' size cables by introducing sub mains.

But all this is circuit design - that's what you pay an expert to do.

 

[shagster]

Yes but since you said that you were not going to buy/hire test equipment that shouldn't be an issue.

But everything that's done on my house could potentially be reflected in the ongoing maintenance costs and or circuit capability, so I'd like to know the ins and outs.

So at last you are starting to think about what you want to use the garage for. This will help the electrician design the correct circuit(s) for your garage.

But when I sell the house I won't be giving a list of things the garage sockets can and can't be used for. Surely there is some standard way of doing this job, and then if the buyer comes along and uses a mig welder he finds that the breaker isn't up to the job that's his problem. My question relates to how this would be done for new build, where the designer has no idea of the final use for the garage. I'm looking for "If you wire it with XXX then you will be able to use no more than A, B and C at the same time", rather than specifiying all my requirements up front.

I guess a safer ring system would be breakers on each end of the ring - isn't there a safety issue when the ring breaks, or does the CU breaker deal with that too?

Think about what you are saying here and the consequences of one of the MCB's tripping.

OK, so the breakers must be connected together in some way such that both break when one does. How about that?

But all this is circuit design - that's what you pay an expert to do.

I'm not aware of anyone who does circuit design when they're not doing the final job, but perhaps they do exist, or maybe anyone would do it, but they just don't advertise. I would normally pay an SE to get the specification for a building job, and I would have some comeback if the work was completed to the spec and found to be inadequate. Are you telling me I can do a similar thing for the wiring, if so how does that work?

regards,
S.

 

[Aragorn84]

But when I sell the house I won't be giving a list of things the garage sockets can and can't be used for. Surely there is some standard way of doing this job, and then if the buyer comes along and uses a mig welder he finds that the breaker isn't up to the job that's his problem. My question relates to how this would be done for new build, where the designer has no idea of the final use for the garage. I'm looking for "If you wire it with XXX then you will be able to use no more than A, B and C at the same time", rather than specifiying all my requirements up front.

Surely you can work this out for yourself. What has selling the place got to do with anything?

I would imagine that the garage supply in most new builds would consist of the bare minimum, probably a 16A socket supply and a light.

And this is going to cover what the majority of folk will do out there.

I'm sure your capable of adding up the wattage of the items that are most likely to be used out there together and decide for yourself if this is adequate or needs to be made bigger.

I'd imagine most people might use the odd power tool, and have a light or two.

Moving up the chain you might have heavier tools, or a tumble dryer, but its upto you to decide the likelyhood of your missus filling the tumble dryer while your out there with your 2KW table saw.

At the extreme you might have a 2.5HP compressor, 2kw oil filled heater, a couple of 1kw panel heaters, a plasma cutter and 150A mig welder. Now clearly its unlikely your going to use all of that simultaneously, but its possible you'll have the panel heaters on, and the compressor running for some painting, or perhaps the compressor and plasma cutter running interspersed with the Mig welder and the heater on in the background the whole time while you weld up your rusty 1950's VW Beetle in the middle of winter.

Only you can know what the proposed loading is likely to be.

If you want a generalisation, a 20A radial is probably enough for most peoples needs.

 

[EFLImpudence]

OK, so the breakers must be connected together in some way such that both break when one does. How about that?

Would it not be just as well to amalgamate these two breakers into one of, say, a higher current rating and connect both conductors to it?

Then the circuit would be capable of supplying more current.

Or, just have one conductor that could handle this current.

Oh! Wait a minute...

 

[shagster]

Surely you can work this out for yourself.

Well, obviously not, or I wouldn't asking.

What has selling the place got to do with anything?

As for selling, I was only using that as an example of why I believe there must be a recognised way of giving the circuit 'reasonable' capacity, without needing to specify all the appliances that will go in every socket. I've made do with single sockets in garages in previous places I've lived. I only want the extra sockets for convenience, not because I plan to populate them with stuff that will be permanently on. I don't have a mig, or a bar heater that I run in the garage. But if you are going to stick loads of double sockets in a garage perhaps it might make sense to handle some reasonable level of loading on all of them, to avoid people (buyers? visitors? family members?) misunderstanding what they are for, and the capacity of the circuit as a whole. I don't really know what makes best sense in this regard. If I have a double socket in four rooms in a 2-bed house (with 30A circuit), maybe I should have 30A circuit in my garage as well.

I would imagine that the garage supply in most new builds would consist of the bare minimum, probably a 16A socket supply and a light.

I suspect this will do, but it would be annoying if the circuit couldn't support, say, freezer plus pressure washer, or maybe the relocation of the gas boiler into the garage at some future point in time. Neither of these things are things I want to do now, but who knows? As for the light I've not got to that yet, but I was assuming it would be connected to the house lighting circuit (garage is connected to the house).

I'm sure your capable of adding up the wattage of the items that are most likely to be used out there together and decide for yourself if this is adequate or needs to be made bigger.

I'm led to believe that the provision of the garage circuit is more complicated than that, based on other responses in this thread, but OK.

If you want a generalisation, a 20A radial is probably enough for most peoples needs.

OK, thanks.

 

[shagster]

Would it not be just as well to amalgamate these two breakers into one of, say, a higher current rating and connect both conductors to it?

But the single, higher current breaker won't trip when the ring gets broken, which is what you want isn't it?

Ah, never mind, it doesn't really matter, I guess you just assume the ring won't be broken, because you have your wiring checked ever few years

 

[EFLImpudence]

Would it not be just as well to amalgamate these two breakers into one of, say, a higher current rating and connect both conductors to it?

But the single, higher current breaker won't trip when the ring gets broken, which is what you want isn't it?

No, but neither would either one of your two.

Nor, for that matter, would any breaker - that's not what they do.
They (MCBs, fuses etc (NOT RCCBs)) react to over-load and short-circuit currents.

 

[ban-all-sheds]

Rings sound like they have a few gotchas! So are rings harder to test than radials, and are they more or less likely to fail?

I am assuming a ring is preferable for this kind of application, based on what I read here:
http://en.wikipedia.org/wiki/Ring_circuit

Just to check my understanding, even though 4mm temperature derated current capacity is way less than 2.5mm x 2, this arrangement is perfectly safe for the reason that the breaker has to deal with the loads at the start and end of the ring, so for most houses, the ring at 2.5mm is equivalent to the 4mm radial.

A ring is never equivalent to a radial.

With radials the entire length of the cable is adequate for the current which could flow in it. With rings it isn't.

I guess a safer ring system would be breakers on each end of the ring - isn't there a safety issue when the ring breaks, or does the CU breaker deal with that too?

How would that be safer? What if there's a break in the neutral but not line?

I guess there is some house size at which 2.5mm ring/4mm radial no longer applies and you would switch to 4mm ring, 6mm radial, which would go by number of sockets or length or ring or maybe both? I'm only curious how far inside this limit the average domestic property is.

The only reasons to use 4mm² for a ring would be length or de-rating factors.

 

[SimonH2]

Would it not be just as well to amalgamate these two breakers into one of, say, a higher current rating and connect both conductors to it?

But the single, higher current breaker won't trip when the ring gets broken, which is what you want isn't it?

Correct

Ah, never mind, it doesn't really matter, I guess you just assume the ring won't be broken, because you have your wiring checked ever few years

And there is one of the reason quite a few people would like to see rings (or RFC, Ring Final Circuits to use the current terminology). They are one of the few situations where it's allowed to use cable which has a lower current rating than the upstream protection (fuse or circuit breaker). It relies on the two paths sharing the current to an extent that neither branch is overloaded - which will be the case unless the ring is broken or you put a lot of load close to one end of the ring.

The problem is, of course, that if the ring gets broken, few users would notice as everything would keep working - but it's now easy to overload one of the cables.


Without wanting to sound condescending, you clearly lack much electrical knowledge. It's OK to say you want the drops done in conduit (I'd suggest plastic is quite adequate) and the fittings to be metalclad - but leave the rest to your electrician who will already know all this.

 

[shagster]

Without wanting to sound condescending, you clearly lack much electrical knowledge. It's OK to say you want the drops done in conduit (I'd suggest plastic is quite adequate) and the fittings to be metalclad - but leave the rest to your electrician who will already know all this.

Nothing condescending there, I don't deny it.

I think I'm going to just specify 16A and a separate breaker for each double socket. The CU is right next to the garage, so it's not going to break the bank in terms of copper, and as you probably guessed I'm not on a budget . I'll leave the spark to work out if I'm peppering the joists above the CU with too many holes or if the bunch of cables coming from the CU are going to get derated for some reason.

cheers,
S.

 

[Aragorn84]

So now your suggesting that using up 4 ways in your consumer unit to provide 64A worth of power to your garage is the best solution?!

 

[shagster]

So now your suggesting that using up 4 ways in your consumer unit to provide 64A worth of power to your garage is the best solution?!

Remember I don't know anything (much) about electrics. It seems like the most flexible solution. If you want to tell me why I can't have that, then great! In fact it's for this reason that I want to ask these kinds of questions on a forum instead asking 2-3 sparks for quotes.

[EDIT] I mean, seriously, can you imagine any spark getting back to me if I had this many questions for him, he'd be long gone by now, but you're still here

 

[ban-all-sheds]

You can have that.

It's barking mad, but you can have it if you want.