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This is my first post so please be gentle!
I've been asked to extend an RFC into an extension at the back of an 8 year old property.
It is a TN-C-S supply with 2 RFC's, one for the Kitchen and one for the rest of the house.
I've done some testing on the main house RFC as follows:
r1=0.84
rn=0.84
r2=1.53
R1+R2 = 0.59
Zs= 0.78
All figures are pretty close to their theoretical values so happy so far.
My issue is when calculate the length of cable I come out with 0.84/0.00741 = 113 metres

According to table 7.1(i) in the onsite guide the maximum length of cable for a standard circuit of 2.5/1.5 protected by type B MCB is 106 metres so this circuit is already outside of the limit.

I have the current volt drop calculation as Vd = L x Ib x (mV/A/M)/ 4 x 1000.
So VD = 113 x 32 x 18 /4000 = 16v
5% of 230V = 11.5, so its already over the allowable 5%

This is very disappointing as this is a relatively new build and there are spares in the board. I am surprised that the designer did not install two separate RFC's, one for up and one for down.

The extension is relatively small, 4m x 4m so extending the ring will add a maximum of 16 metres which would cost the customer very little.

The alternative is a new radial circuit from the CU at the front of the property and on the opposite side of the house which means all floor boards up and a very costly job.
There are no fixed loads on the ring and just the usual T.V's, games consoles lamps etc so loading is not an issue and I cant see volt drop being a problem and Zs is good.

What would you do? I think I've answered my own question with my calculations!

Thanks for your help.
 
Last edited:
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ipf

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Is your cable length the measurement of the RFC in total?
If so, you can half that....2 x 2.5's to farthest point on ring.
It seems like it's probably OK to me, too.
 

Megawatt

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Arms
This is my first post so please be gentle!
I've been asked to extend an RFC into an extension at the back of an 8 year old property.
It is a TN-C-S supply with 2 RFC's, one for the Kitchen and one for the rest of the house.
I've done some testing on the main house RFC as follows:
r1=0.84
rn=0.84
r2=1.53
R1+R2 = 0.59
Zs= 0.78
All figures are pretty close to their theoretical values so happy so far.
My issue is when calculate the length of cable I come out with 0.84/0.00741 = 113 metres

According to table 7.1(i) in the onsite guide the maximum length of cable for a standard circuit of 2.5/1.5 protected by type B MCB is 106 metres so this circuit is already outside of the limit.

I have the current volt drop calculation as Vd = L x Ib x (mV/A/M)/ 4 x 1000.
So VD = 113 x 32 x 18 /4000 = 16v
5% of 230V = 11.5, so its already over the allowable 5%

This is very disappointing as this is a relatively new build and there are spares in the board. I am surprised that the designer did not install two separate RFC's, one for up and one for down.

The extension is relatively small, 4m x 4m so extending the ring will add a maximum of 16 metres which would cost the customer very little.

The alternative is a new radial circuit from the CU at the front of the property and on the opposite side of the house which means all floor boards up and a very costly job.
There are no fixed loads on the ring and just the usual T.V's, games consoles lamps etc so loading is not an issue and I cant see volt drop being a problem and Zs is good.

What would you do? I think I've answered my own question with my calculations!

Thanks for your help.
I’m impressed with your calculations good job
 
The regs say that as long as a limit of 100m^2 is not reached, the number of sockets On ring is unlimited.
The main thing would be to maintain the ring in full.
100^2m is a fair size but might be worth checking the total area including the proposed extra space. Apart from that, as long as you are getting good readings, it should be fine
 
Also, when calculating your length, it’s a ring so the maximum length would actually be half your figure (the mid point) which is why the preferred method is to use the area rather than the length...
 

Risteard

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The regs say that as long as a limit of 100m^2 is not reached, the number of sockets On ring is unlimited.
The main thing would be to maintain the ring in full.
100^2m is a fair size but might be worth checking the total area including the proposed extra space. Apart from that, as long as you are getting good readings, it should be fine
100m^2 is noted as a historical requirement, i.e. it is no longer a requirement. It hasn't been a Regulation since the 16th Edition was withdrawn.
 
100m^2 is noted as a historical requirement, i.e. it is no longer a requirement. It hasn't been a Regulation since the 16th Edition was withdrawn.
It’s in the 18th edition under Appendix 15 - this gives the guidelines and still mentions the “historical limit of 100m^2.
I guess the main points here, taking in regulation 433.1.204 as well is that as long as there are no immersion heaters,
Or other heavy current-carrying accessories over 2kw on the ring, it should be fine as most accessories would only use a fraction of the cables Iz
 

Risteard

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My point was precisely that it's noted as a historic requirement. Historic means that it isn't now a requirement - just that it used to be one.
 

Midwest

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Arms
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The historical limit or guidance is maximum floor area, is it not :)
Would the new extension this to be exceeded?

I'd be more concerned over the design of one RFC for the rest of the house and inconvenience etc, in the event of a fault. Whats the extension for?
 

Pete999

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Arms
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The historical limit or guidance is maximum floor area, is it not :)
Would the new extension this to be exceeded?

I'd be more concerned over the design of one RFC for the rest of the house and inconvenience etc, in the event of a fault. Whats the extension for?
WD, if this issue is really causing you concern, might you consider wiring the extension sockets as a Radial? you mentioned this as une of the options open to you, personally I would opt for this option, but that's just my thought, good luck at whatever option you decide to take.
 
  • Thread Starter Thread Starter
  • #12
Is your cable length the measurement of the RFC in total?
If so, you can half that....2 x 2.5's to farthest point on ring.
It seems like it's probably OK to me, too.
The cable length calculation I used was

Vd = L x Ib x (mV/A/M)/ 4 x 1000.
So VD = 113 x 32 x 18 /4000 = 16v

I’ve already divided the end to end value by 4. This is how I was taught to calculate volt drop at the mid point on a RFC.

Does this look correct to you?
 
  • Thread Starter Thread Starter
  • #13
The regs say that as long as a limit of 100m^2 is not reached, the number of sockets On ring is unlimited.
The main thing would be to maintain the ring in full.
100^2m is a fair size but might be worth checking the total area including the proposed extra space. Apart from that, as long as you are getting good readings, it should be fine
The floor area including the extension will be just below 100m2.
It’s going to be used as a play room.
The customer wanted an oil radiator installed but that will not be possible off the existing ring so a new radial may be the only option.

It’s very frustrating for a new build by one of the countries leading developers!
 

Midwest

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Arms
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The floor area including the extension will be just below 100m2.
It’s going to be used as a play room.
The customer wanted an oil radiator installed but that will not be possible off the existing ring so a new radial may be the only option.

It’s very frustrating for a new build by one of the countries leading developers!
What flooring will you have to remove?
 

Pete999

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Arms
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The floor area including the extension will be just below 100m2.
It’s going to be used as a play room.
The customer wanted an oil radiator installed but that will not be possible off the existing ring so a new radial may be the only option.

It’s very frustrating for a new build by one of the countries leading developers!
Most builders aren't interested in future proofing WD, bash it in quickest way, unless of course, anything off piste is discussed in the planning stages, yeah go for the Radial.
 
  • Thread Starter Thread Starter
  • #16
What flooring will you have to remove?
It’s the large sheets of tongue and grove chipboard. I’ll have to cut a channel out across the whole width of the house which spans two bedrooms to be able to drill the joists. They are kids bedrooms too packed full of stuff.
I am considering running it in 4mm SWA around the perimeter of the house as there is a gravel soak away so no digging required.
More expensive material but much less hassle.
 

Midwest

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It’s the large sheets of tongue and grove chipboard. I’ll have to cut a channel out across the whole width of the house which spans two bedrooms to be able to drill the joists. They are kids bedrooms too packed full of stuff.
I am considering running it in 4mm SWA around the perimeter of the house as there is a gravel soak away so no digging required.
More expensive material but much less hassle.
Then you’d cause more damage to the structural floor, than an inconvenience to continuity of service.

If the figures hold out, I’d alter the RFC. Would the final Zs be ok, if you did so?
 

Pete999

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Arms
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It’s the large sheets of tongue and grove chipboard. I’ll have to cut a channel out across the whole width of the house which spans two bedrooms to be able to drill the joists. They are kids bedrooms too packed full of stuff.
I am considering running it in 4mm SWA around the perimeter of the house as there is a gravel soak away so no digging required.
More expensive material but much less hassle.
Not an ideal method, is surface wiring a no no? PVC trunking.
 
I have the current volt drop calculation as Vd = L x Ib x (mV/A/M)/ 4 x 1000.
So VD = 113 x 32 x 18 /4000 = 16v
5% of 230V = 11.5, so its already over the allowable 5%
The max length for RFCs of 106m uses a design current of 26A (there's a table earlier in the chapter in the OSG that should state this), distributed evenly around the ring. I presume multiple calculations were used to arrive at the 106m figure.
 

davesparks

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I'd extend the ring personally as long as the Zs is going to be within limits.
The onsite guide with its pictures and tables is just a guide, not a requirement. The point of the
The volt drop won't be a problem as the actual load is going to be less than the 32A you've used to calculate it.
 

Wilko

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Hi - as per Dave.
If the extension has heating that doesn’t rely on this circuit likely the biggest new load will be a vacuum cleaner :) .
 

ruston

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I agree too as as long as your ZS is within the the limits of the installation methods .
If I remember correctly 100mtr square was the limit of an RFC without calculation.
 

Risteard

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Arms
Esteemed
The cable length calculation I used was

Vd = L x Ib x (mV/A/M)/ 4 x 1000.
So VD = 113 x 32 x 18 /4000 = 16v

I’ve already divided the end to end value by 4. This is how I was taught to calculate volt drop at the mid point on a RFC.

Does this look correct to you?
I wouldn't use 32A as the design current.
 

stevethesparks

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Arms
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if your r1 & rn are 0.84 I would hope to see the r2 at around 1.4
 
De-rate it to 20A RFC if possible, VD would be less than 5% if using the full 20A load but as the main usage would be the kitchen RFC I doubt the remainder would use 20A (unless electric heating!). Higher max Zs value too!
 
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