adding sockets to a radial circuit

[asterisk]

The ISBN for the brown cover version is 0863413749. An Internet search for that number returns a number of online book shops. It seems to cost £15 from wherever you buy it but P&P charges vary.

I could be wrong, but IEE don't seem to charge for postage in the UK.
http://www.iee.org/Shop

Delivery seems to vary from 3-5 days to 3-5 weeks.

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[snowflake]

Does'nt the actual length of the cable and the choice of MCB or Fuse restrict the number of sockets in a radial circuit.

It's the other way round - the choice of MCB or fuse restricts the length of the cable.

You should get a copy of the On Site Guide.

To cut along story short, what is the maximum length of 4mm cable I can use on a 32amp MCB (B) as opposed to a 30amp Cartridge Fuse.
Many Thanks (in anticipation)

 

[ban-all-sheds]

That's a Jolly Good Question. Having mentioned the existence of a 30/32A 4mm² radial, I find that the OSG does not list that as a valid circuit.

Whether that is because it has too many restrictions on installation method and grouping factors, or whether it gets hard to meet disconnection times with the assumed Ze values, I don't know - some advice from someone with proper cable calc s/w would be handy.

In terms of voltage drop you'd be OK up to 25m, and provided the cable is either on the surface, or just embedded in plaster you'd be OK for capacity. But if the cable is in conduit or trunking, or in insulation, or bunched with other cables then you cannot use a 32A breaker on it - 25A would be the max.

 

[dingbat]

That's a Jolly Good Question. Having mentioned the existence of a 30/32A 4mm² radial, I find that the OSG does not list that as a valid circuit.

Page 151, Table 8A - Final circuits using 1363 socket-outlets and connection units - Circuit A2 = radial, 30/32A 4/2.5 sq mm, 75 sq m.

If the design load is 32A then, at a tabulated (Table 6F in OSG, p.126) volt drop of 11 mV/A/m and an allowable maximum of 9.2V you can use up to 26m before correcting for temperature. At this length the (R1 + R2) of that circuit would be 0.31 ohms (Table 9A, p.158 and assuming good connections). For a 32A, Type B, BS60898 MCB, the maximum (temperature-corrected) Zs to meet disconnection times is 1.2 ohms, so, as long as your Ze is less than 0.9, you'd be sorted.

But I have to point out that the idea of working out how long a cable you can use for a particular MCB/fuse is a*se about face. You calculate your design load first. Then you select a protective device. You next calculate the required current-carrying capacity of the cable depending on conditions and select your cable to suit. Then you check that the voltage drop is within tolerance (4%) and if not, you select a higher rated cable. Then you calculate Zs and if it's acceptable you finally check that the chosen circuit protective conductor is capable of carrying the prospective fault current.

Easy, really!

 

[ban-all-sheds]

That's a Jolly Good Question. Having mentioned the existence of a 30/32A 4mm² radial, I find that the OSG does not list that as a valid circuit.

Page 151, Table 8A - Final circuits using 1363 socket-outlets and connection units - Circuit A2 = radial, 30/32A 4/2.5 sq mm, 75 sq m.

pp42-46, Table 7.1 - Conventional Circuits - 30/32A radial with 4mm not listed....

If the design load is 32A then, at a tabulated (Table 6F in OSG, p.126) volt drop of 11 mV/A/m and an allowable maximum of 9.2V you can use up to 26m before correcting for temperature. At this length the (R1 + R2) of that circuit would be 0.31 ohms (Table 9A, p.158 and assuming good connections). For a 32A, Type B, BS60898 MCB, the maximum (temperature-corrected) Zs to meet disconnection times is 1.2 ohms, so, as long as your Ze is less than 0.9, you'd be sorted.

But I have to point out that the idea of working out how long a cable you can use for a particular MCB/fuse is a*se about face. You calculate your design load first. Then you select a protective device. You next calculate the required current-carrying capacity of the cable depending on conditions and select your cable to suit. Then you check that the voltage drop is within tolerance (4%) and if not, you select a higher rated cable. Then you calculate Zs and if it's acceptable you finally check that the chosen circuit protective conductor is capable of carrying the prospective fault current.

Easy, really!

Yup - and that's how the maxima in Table 7.1 are arrived at, including the assumption of a reasonable grouping factor. And I reckon that if you did the calculation yourself, when you got to the part about "calculate the required current-carrying capacity of the cable depending on conditions", you'd find that any derating would mean that 4mm was not suitable for a 32A load.

 

[dingbat]

I don't disagree with you on potential derating factors, but Table 8A is intended to summarise acceptable circuits for 'household and similar installations', where cable embedded in building materials can be taken as method 1 and grouping is not normally a serious issue.

 

[snowflake]

Does'nt the actual length of the cable and the choice of MCB or Fuse restrict the number of sockets in a radial circuit.

It's the other way round - the choice of MCB or fuse restricts the length of the cable.

You should get a copy of the On Site Guide.

Many thanks for the extremely speedy response. Where would a copy of the On Site Guide be available from ?

I have read in Diy Manuals that the maximum length of 4mm cable I can use is 15 metres on a 32a MCB, and 38 metres on a 30a cartridge fuse.Problem is I want to extend the existing circuit over 15 meters but the consumer unit is a Crabtree Starbreaker with plug-in CB,s but plug in Cartridges are not available. Is there an easy solution.

 

[plugwash]

for 4mm on a type b 32A the limit is 27 meters and volt drop is the limiting factor

for 4mm on a 30A cartridge the limit depends on your Ze

with a worst case TN-S the limit is 18 meters with disconnect time being the limitng factor

with a worst case TN-C-S the limit is 29 meters and volt drop is the limiting factor

if you need a cable longer than this you have a few options
1: downrate to a 20A breaker (this may cause neusense tripping though)
2: add your new sockets on a seperate cuircuit
3: turn the cuircuit into a ring (you have to be carefull about ring spur rules if you do this though)

 

[ChrisSnow]

Hi all.

I'm wanting to add 3 sockets to a recently added conservatory. I have what I reckon to be a radial cct to my lounge with 2.5mm cable. Strangely it has a 30 amp cct breaker on it in the breaker box! I expected to find a 20amp.

Here's the plan. I'll extend the radial cct into the conservatory via an air brick with 2.5mm cable and wire three double sockets in, in parallel.

The new total area served by the cct will be approx. 30 square meters.

Would this be OK?

Also, can I continue to use the 30amp breaker? I would guess not an have replaced it with a spare 15amp with a view to bunging a new 20amp in.

Thanks in advance for any advice!

Chris.

 

[ban-all-sheds]

30m² is fine.

You must replace the breaker asap.

Can you not turn the circuit into a ring?

 

[ChrisSnow]

Making it a ring had crossed my mind but running a second cable to the box to complete the ring is not a trivial matter in my house!

Many thanks. Chris.

 

[plugwash]

you said you had put it on a spare 15A
personally i would just leave it on that unless you have problems with neusence tripping