spur length

I'm trying to find out if there is a maximum distance you can install a spur socket from it's 'donor' ring main socket. Any ideas?

How long do you want?

There are 2 limiting factors:-

1) Volt drop across the cables. You are allowed 4% from the point of origin. (at 230V thats 9V)
2) The fault loop impedance must be low enough for the circuit protection to operate within 0.4s for TN instalations

Its hard to arrive at an exact figure without knowing more about your requirments.

A basic guide however would be:-

A straight 2.5mm cable from the CU would be limited to about 16-20m for a double socket at 26A. Allowing for 10m effectively used in the ring (100m^2 -> 40m cable /4) I'd say that leaves 5-10 meters for the spur. Id say anything over 4m however needs a real calculation done. You can increase the length by using a larger spur cable.

These figures take no enviromental, instalation method or grouping factors into account so are not absolutes just guestimates.

brown-nought said:

A straight 2.5mm cable from the CU would be limited to about 16-20m for a double socket at 26A.

Click to expand...

you run a double socket at 26A for any significant time and you will cook it

plus you can't just add the length of the spur to your "effective" length of the ring because a ring is a 32A cuircuit

brown-nought said:

100m^2 -> 40m cable

Click to expand...

Where did you get that figure?

well it would be if the area was absoloutely square and the CU was on one side of it and everything was level and all cables run horizontally

having said that most rings don't serve anywhere near 100 square meters of floor area

Yep 40m came from a max size perfectly square ring - which must be the shortest length a maximum area ring can be.

I agree 26A isn't good for a twin socket, but it is the theoretical max that it could draw - and hence gives a Vd figure. The ring Vd calculation would need to assume 32A if protected by a 32Mcb.

As I said the figures were a guestimate.

The easiest way to get a real value for the length would be to measure the rings approx length using its ring conductor resistance value. Then calculate the Vd in the ring using 1/2 the length. From the point of spuring measure the ring Zs (or use the rings max Zs value.) Then calculate the max spur possible which still achieves 0.4s and under a 4% drop.

brown-nought said:

Yep 40m came from a max size perfectly square ring - which must be the shortest length a maximum area ring can be.

Click to expand...

No - the shortest is 35.45m.

I agree 26A isn't good for a twin socket, but it is the theoretical max that it could draw - and hence gives a Vd figure.

Click to expand...

You'd better hope for terrible VD then - might keep the current down to a level where your socket does not catch fire...

The easiest way to get a real value for the length would be to measure the rings approx length using its ring conductor resistance value. Then calculate the Vd in the ring using 1/2 the length. From the point of spuring measure the ring Zs (or use the rings max Zs value.) Then calculate the max spur possible which still achieves 0.4s and under a 4% drop.

Click to expand...

Splendid plan, but try using ¼ of the ring length, not ½....

ban-all-sheds said:

No - the shortest is 35.45m.

Click to expand...

- Of course a circle is less

ban-all-sheds said:

Splendid plan, but try using ¼ of the ring length, not ½....

Click to expand...

Yes you are right I forgot that the current is split, the max value will be based on quarter the ring length. The value will become less as you move towards one end of the ring.

At the middle each leg will carry ½ the current ½ the length. Thus Vd will be L/2 * I /2.

Totally pointless talking about voltage drop limiting spur length. No one is reasonably going to install a spur on the assumption it will be required to provide so much current as you suggest. Voltage drop is based upon design current, not maximum possible current.

Difficult to decide what the actual design current might be, but on the assumption that any 13A socket, double or single, is rated at 13A, then this might be a rather better number. Or assume that average curent per socket when in use is maybe 5A?

You might find that disconnection time becomes the limiting factor.

Howie - is this a hypothetical Q, or do you actually need to install a long spur?

Damocles said:

Totally pointless talking about voltage drop limiting spur length. No one is reasonably going to install a spur on the assumption it will be required to provide so much current as you suggest.

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Respectfully, I know its unlikely, but home owners should be consider electrically unskilled operatives and who knows what a spur will be used for in the future. I'm sure none of us would install a spur socket intended to supply a 26A load but there is nothing physically to stop someone later trying to use it that way. I'm not even suggesting they are drawing a constant 26A it could be a short term load.

How did I decide what figure to use? Volt drop should be calculated from the load not the rating current. So what is the load current of a twin 13A Socket? Using the maximum demand tables this says it should be the rated current of the socket - here 2x13. Thats why I used 26A. I cannot see what other figure I should use without getting into grey diversity allowances.

If you can find other regs or guidelines that say I could do something else I'm happy to be corrected and informed. However I'd say if I used 26A nobody could later come back and say I had under allowed.

The simplest solution to use a lower figure would be to spur from the load side of an FCU. That would double the V.d. limit distance.

Anyway this is getting a bit academic without knowing what howie was planning.

brown-nought said:

So what is the load current of a twin 13A Socket? Using the maximum demand tables this says it should be the rated current of the socket - here 2x13.

Click to expand...

No - the rated current of a socket outlet, single, double, triple, whatever, is 13A, not n x 13A.

I think the important limit is that of keeping the cable resistance low enough to ensure disconnection times are met. This is a safety issue. If voltage drop occures, that might conceivably be a safety issue but is much more likely only to be a nuisance issue.

The regulation sensibly states that a circuit should be designed to provide the current demanded, not over-engineered for something extra. This might mean that you are planning to add a spur which you know is only going to be used for a radio and table lamp. Yes, someone may come along in the future and use a heater. But the necessary thing is to ensure the system is safe. If you need to cheat a bit and assume 10A maximum current to stretch the voltage drop cable length a little, then I think that would be considered acceptable if it makes the installation doable. It is not against the spirit of the regulations.

Would you concede the quite likely scenario that it is possible to wire a house with just one 32A socket ring, and that it is also quite likely that this would never trip. I feel it is much better practice if considering the installation as a whole to split the sockets into up/down/kitch as appropriate for reasons of discrimination rather than total load. This means the rings are very likely to be under-loaded. This makes assuming a low overall demand more reasonable.

I am not saying anyone should ignore voltage drop. But I think it is better to keep advice simple as much as possible. The important thing is disconnection time. Anyone reading advice here should get that point before starting to worry about voltage drop.

Which unfortunately has still not answered the original post. The actual answer is that it would depend upon the existing length of the ring and at what point you spurred off. Advice has to be. keep it short.

Guys,

I'll shut up about V.d. after this - promise! - Agreed disconnection is more important from a safety perspective.

B-a-s are you sure about 13A for doubles?? (Trips are always limited by a 13a fuse and singles by design but MK specs rate their doubles at 13a per outlet. i.e. 26a)

I've racked my brains to justify V.d. concerns and the only case that comes to mind might be under volted fan heaters getting too hot!

At the end of the day I'm a pragmatist too and would try hard to make the calculation fit the result needed. (but we ain't got one howie!!)

Damocles Do you really fit more than one ring?? I've always seen it as an energy saving feature.

Well sadly it depends whose paying. Yes, hard to say exactly how much electricity might be saved by reducing voltage drop, which just means power wasted heating cables. But probably a power station or two by good design. Has to be balanced against the cost of that extra copper. More rings likely means they are each shorter.