# spur

system TN-C-S 0.35

32 amp circuit breaker type b

Zs Max 1.44

amps 160

2.5 mm2 + 1.5 mm2

r1 + r2 resistance per metre is 19.51

you use 13 mtr 2.5 mm2 wire to spur from upstairs room socket to the loft.

volt drop

18 mV x amp x length / 1000

what will be the first task?

do we need to find out the resistance at the point where we will spur off from?

I HAVE "guide to the wiring regulations" in fromt of me , but i am still unable to figure out how to calculate the amount of wire required for a spur.
mrsonic
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Step one was to make a lead to test the Line / Neutral impedance. One came with meter for Line / Earth but that is not really any good when we are using RCD as it is the Line / Neutral readings which are the limiting factor.

I have argued the fact many times as to if the tripping current for magnetic part of MCB/RBCO or the volt drop should be limiting factor. However both are very close and since the impedance to trip the magnetic part is slightly lower than impedance to keep volt drop within 5% I personally go for magnetic part of MCB. i.e. with direct short it will disconnect within 0.1 seconds.

You of course want it to trip at the end of spur so with B type MCB at 32A the impedance must be lower than 1.44 measured. Assuming the house is in use then using the correction formula is not required and I would not want to be sailing so close to wind to need to use it for spur. So 1.44 – 3 x 0.018 = 1.38 ohms so with a impedance of 1.38 ohms you can install the maximum of 3 meters with a spur in accordance with 433.2.2 which limits spur length. After which you need to use a fuse connection unit and then the name changes and it is no longer a spur but a radial.

But you are not permitted 13 meters for a spur so in your case you need a fused connection unit and then it is the 13A fuse and 2.42 ohms impedance so unlikely a problem.

As to on site guide when I took my BS7671:2008 the guide had not been printed so I have never bought one. I will admit the "(ii) Its length does not exceed 3 m, it is installed in such a manner as to reduce the risk of fault to a minimum, and it is installed in such a manner as to reduce to a minimum the risk of fire or danger to persons" seems to be missed out of the earlier copies of the on site guide but the device protecting a conductor against overload is the fuse in the plug at end of the spur so it must be within 3 meters. Also the position of the protective device has neither branch circuits nor outlets for connection of current-using equipment is of course the reason why only a single device is permitted at the end of the spur, be that a single or double socket or a FCU.
ericmark
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greetings

can explain "So 1.44 – 3 x 0.018 = 1.38 ohms "

1.44 is the MAZ Zs for 32 amps type b

do we add r1 + r2 results obtained from continuity tests to Ze to find the Zs for the ring circuit? then subtract from 1.44 ?
mrsonic
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i meant Rn
mrsonic
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To measure R1 + Rn you need cold cables i.e. an installation not yet commissioned. Where one does not own an earth loop impedance tester one could use the R1 + Rn method but it is a lot of work to do when one can simply plug in the earth loop impedance meter.

So what I was saying is before you add a spur. If on a B32 MCB supply the socket you intend to take the supply from reads 1.38 ohms or less, then you can add the max of 3 meters for a spur, without when completed exceeding the total 1.44 ohms limit.

However the job you are looking at from your description will exceed the 3 meter limit so can't be done as a spur and will need a fuse connection unit and a radial from a fused connection unit with 13A fuse will allow a R1 + Rn of 2.42 ohms which is unlikely to be exceeded.
ericmark
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what is 0.018?

table 4.1 says that 2.5 mm2 at 20 degress is 7.41
per meter

rn value ,i think, is the same.

you say cold cable.

are you taking a percentage off 7.41 to bring it to .018 ?

20 degress is equal to what %?
mrsonic
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so when the temperature increases the resistance increases.

the increase in resistance is added to the test you did when the wires were cold?
mrsonic
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If you look in the appendix of the 17th Edition it shows on table 4D5 with the max current ratings a column with Voltage Drop (per ampere per meter) well of course volts per amp can also be expressed as ohms. It is from here I get the 0.018 ohms per meter. Now on page 258 it gives the formula to correct this figure and I have put this into excel. One of the advantages with excel is one can enter known figures and see if it all works correct before going to the unknown. I used the published 106 meters now permitted for a ring main to check all working as it should.

However I do not agree with the 106 meters figure. It works on the idea that we no longer are worried about earth loop impedance as the RCD now takes care of that and all we worry about is volt drop. However to get 106 meters the line / neutral impedance is 1.5 ohms so a short circuit line to neutral would not trip the MCB/RBCO with the magnetic part of the trip. Since with 1.44 ohms one can still have 101 meters it seems odd to want to squeeze the extra 5 meters.

Design current is another point and it is accepted that a ring mains design current is 26A even though it has a 32A MCB. The thinking is 20A at centre and 12A even spread which give average of 26A.

In spite of Ali-tube being recommended by manufactures for partition walls we still have the tables giving us the temperature for a 70º cable rather than the 90º cable now recommended.

The list goes on. So to me trying to squeeze the last bit of length from ring, radial or spur is not the way forward and I would always aim for under the 1.44 ohm impedance.

The maths is not too easy
ericmark
Rank: Project Manager
Posts: 1914
Joined: Fri Oct 02, 2009 8:49 pm
Location: Mold, North Wales.

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