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Topic Title: Measuring Circuit Lengths (Radial/Rings)
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Created On: 11 July 2018 02:15 PM
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 11 July 2018 02:15 PM
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FiftyHertz

Posts: 20
Joined: 13 April 2018

Hi,

Just to clarify, when measuring the length of the circuit run in meters:

For a radial, it will only be the furthest point that will be measured too and not every point, for example in a corridor with cleaners sockets - the length would only be to the last socket then a 2.5m additional length for the drop. (No other drops to sockets would be accounted for)

For a ring, every socket drop and rise (2.5m + 2.5m) will be accounted for and a point to point measurement is used instead of only measuring to the final point.

Looking forward to hearing your comments.

Thanks
 11 July 2018 02:37 PM
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broadgage

Posts: 2976
Joined: 07 August 2007

I would say that the total length of cable should be counted, presuming the common arrangement whereby the cable drops to each socket and then another length rises and continues to the next socket.
Why would this cable NOT count, the current passes through it and volt drop is thereby caused, regardless as to whether it be in horizontal or vertical runs.
Simply measure the total amount of cable used to install a circuit, and that is the length for a standard radial circuit.

If junction boxes are used, then the position is different and the effective length is then the total length from origin to most distant point, and not the total amount of cable used. Spurs or tees to intermediate sockets are then of no consequence since the current to the most distant outlet does not pass along them.
 11 July 2018 02:43 PM
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mapj1

Posts: 11436
Joined: 22 July 2004

Depends how you wired it - if each socket is on a JB in the manner of a spur from the radial backbone, then you are right. If it is looped through, then electrically it is as good as linear, and mentally you could pull it out straight without any disconnections, you need the total length.

I'd do it the other way, and fit a shorting link at the origin, and walk along plugging a meter in. If you know the cable size, the length will follow from the resistance, and as normally it is Zs we want or voltage drop, this is a more useful quantity.

-------------------------
regards Mike
 11 July 2018 03:14 PM
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FiftyHertz

Posts: 20
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Thankyou very much for that, makes complete sense now. One more thing, is looping in and out for a radial exclusive for sockets, and would a junction box generally be used for wiring multiple fused spurs/Isolators on a radial circuit.

Thanks.
 11 July 2018 03:26 PM
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broadgage

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Multiple fused spurs may be connected via junction boxes, or by looping the supply cable in and out.

If the circuit cables are larger than 2.5mm, then junction boxes might be better. It can be a challenge to get into a standard back box, a pair of 4mm twin with earth and the outgoing flex or cable. A single 2.5mm is rather easier to handle.
Larger sizes are doable with extra deep back boxes, but a challenge with ordinary boxes.
Pvc singles in conduit are more manageable in back boxes.

Similar arguments apply to SINGLE 13 amp sockets, doubles are easier due to the greater space in the box.
 11 July 2018 04:54 PM
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sparkingchip

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Avoid junction boxes like the plague, if they are really required then go down the maintenance free road, even if you think they will always be accessible.

Andy B.
 11 July 2018 05:01 PM
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sparkingchip

Posts: 11373
Joined: 18 January 2003

Originally posted by: FiftyHertz

Hi,

Just to clarify, when measuring the length of the circuit run in meters.

Looking forward to hearing your comments.

Thanks


Being pedantic, a comment you may not be expecting is length is measured in metres and you measure things with a meter.

You can actually measure the length of a cable in metres with a cable length meter.

Andy B
 11 July 2018 05:14 PM
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mapj1

Posts: 11436
Joined: 22 July 2004

Also, not all radials are a simple daisy chain or a spine with ribs like they get drawn in black boards (or white boards nowadays). A radial circuit can be anything that does not have loops, so may have branches like a Christmas tree, may be fed anywhere part way along, not just from an end ...

So, there may be more than one far point. From the point of voltage drop verification only one is the 'furthest far point' Arguably testing of R2 or Zs should be performed at each "end" as an OK from an end only verifies one path, there may be breaks in other branches, so there is more testing to do with the spine and ribs or tree approaches.

-------------------------
regards Mike
 11 July 2018 05:22 PM
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sparkingchip

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If you are doing calculations for a 4.0 mm radial with 2.5 mm spurs, do one calculation for the entire length as 2.5 mm and if it passes all is well, without getting bogged down in three calculations for the total of the two cable sizes.

Andy B.
 12 July 2018 04:23 PM
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FiftyHertz

Posts: 20
Joined: 13 April 2018

What about measuring lighting radials, is there any best practice guidelines. For instance, is it measured to the furthest PIR/Lighting control module/switch where this is used to distribute to the luminaires and measure the furthest luminaire.

Thanks

Edited: 12 July 2018 at 04:46 PM by FiftyHertz
 16 July 2018 11:47 AM
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AJJewsbury

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Joined: 13 August 2003

What about measuring lighting radials

Again, for most purposes you're interested in the length of cable the current needs to flow along - so say for a light and switch on a junction box arrangement you'd measure from the CU to the light and add on the length of the switch drop.
- Andy.
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