Spur supporting spur

[biblio]

In a ring circuit can the length of spurs be additional to the 50m maximum for the ring, or do all the spur lengths have to be counted as part of the 50m maximum?

In a ring circuit, in order to determine the safe length for any one spur cable does the voltage drop have to be calculated individually for each separate spur (taking the resistance of the ring itself into account, but not other spurs)?

Is the resistance of all the spurs in a ring circuit discounted when calculating the voltage drop in the ring itself; but the resistance of the ring (but not of any of the other spurs) taken into account in determining the voltage drop at the end of each single spur in turn?

In an existing ring circuit can extra spurs still be added by inserting (accessible) junction boxes, instead of inserting a socket in the ring? My 1908 house was re-wired with junction boxes serving a spur supporting a second spur, which I want to amend.

 

[breezer]

what 50M, do you mean the 100m floor space area?

jb's accesible or not are not always a good idea, so if you can remove / change it do so

 

[biblio]

breezer, 50 metres maximum cable length for an MCB, was 60 metres for cartridge fuse protection. I know about the 100 metre square maximum floor area. Thanks.

 

[securespark]

Ring circuits are generally limited by floor area, not cable length.

 

[AdamW]

50m maximum, on a ring... you would find it hard to keep cable runs down to 50m on any moderately large house. Where did you read/hear of this limit? The only reason I could think behind a 50m limit would be if 50m of 2.5mm cable would give a significant volt-drop at the rating of the MCB... but it wouldn't.

The main rule with spurs is that you wire from the ring to the new accessory in the same size cable as the rest of the ring is run in, unless you are running from an FCU in which case you can usually get away with 1.5mm.

You can use accessible junction boxes to wire a spur, following the above rule. Some might prefer the use of an FCU mounted on the wall, but a random FCU in the middle of nowhere can look a bit odd. And if the JB is accessible and obvious it should be more than safe to use one. Perhaps you could put a label inside the spurred accessory detailing where to find the JB: a spark in the future may want to love you, twice, for such helpful details.

 

[ban-all-sheds]

breezer, 50 metres maximum cable length for an MCB, was 60 metres for cartridge fuse protection. I know about the 100 metre square maximum floor area. Thanks.

Even if those were the limits for cable lengths (which they are not) they couldn't possibly be that way round, since ones for fuse protection are lower than MCBs (it's all about disconnection times).

I don't know where you're getting your ideas, but they are strange...

 

[biblio]

AdamW, ban-all-sheds; I have come across the 50metre maximum cable length in a few places, here is a site eg
http://www.diydata.com/planning/ring_main/ring_main.htm

Please bear with me all of you (don't leave me in the middle of the river) please please please look at my other three questions ...

 

[biblio]

ban; this site gives the 50metres for MCB, and 60metres for cartridge

http://www.diydoctor.org.uk/projects/ringmain.htm

RSVP (please look at my other questions too) biblio

 

[AdamW]

Please bear with me all of you (don't leave me in the middle of the river) please please please look at my other three questions ...

I answered your last question re: JBs.

Regarding taking into account the resistance of cable and voltage drop for your spur: provided the existing ring is wired in 2.5mm, and that your spur isn't incredibly long and is also wired in 2.5mm (as it should be if it is not fused at the ring), then it is unlikely you will have a problem. If it was, the only way around it is to either rewire the ring in heftier cable, but with that amount of bother you may as well just run the spur as a new radial circuit from the CU and leave the ring alone.

How long do you anticipate this spur being? Or is it just a theoretical question?

 

[biblio]

AdamW
Thanks. If it is the case that there is not a 50/60metre length of cable limit in a ring circuit (but see the two links, above eg) then my first three questions are probably theoretical, yes. That is why I asked them all, as everything becomes more critical when restricted by this limit of cable length. Can you confirm there is no such limit? Spurs can save cable length in my house.

 

[AdamW]

I can't confirm if there is or isn't, but hopefully one of the sparks on here will check their little maroon book and let us know.

 

[securespark]

This from OSG:QUOTE

The number of non-fused spurs does not exceed the total number of socket outlets and items of stationary equipment connected directly to the circuit.

UNQUOTE

BUT: I wouldn't bother with a single spur if I had the opportunity to wire from scratch - all my sockets would be on the ring, and no JB's would be allowed.

A 32A ring can serve 100m2. But the OSG recommends separate circuit for kitchen.

So, if you have 1 ring for First floor, one for Ground and one for kitchen, then you can't go far wrong.

There is nothing in the OSG about a limit of cable length.

 

[ban-all-sheds]

AdamW, ban-all-sheds; I have come across the 50metre maximum cable length in a few places, here is a site eg
http://www.diydata.com/planning/ring_main/ring_main.htm[/QUOTE]
I've asked them to justify this - I will let you know if they reply.

Please bear with me all of you (don't leave me in the middle of the river) please please please look at my other three questions ...

Patience, Glasshopper..

 

[ban-all-sheds]

ban; this site gives the 50metres for MCB, and 60metres for cartridge

http://www.diydoctor.org.uk/projects/ringmain.htm[/QUOTE]
I've asked them to justify this - I will let you know if they reply.

RSVP (please look at my other questions too) biblio

Patience, Glasshopper..

 

[ban-all-sheds]

There is nothing in the OSG about a limit of cable length.

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