reference method explained

[Edd]

This is a page I wrote to help the people at work understand the reference method.
Hope its of some help to others


Reference methods for domestic installations
The reference method box on the EICR is there to indicate how the particular circuit is installed.
This then allows us to verify that the cable used is capable of carrying the current for that circuit.
A basic house with floor boards would have different reference methods for different circuits and also different methods for the same circuit.
A Twin and Earth may be run up a wall clipped in a cupboard from the CCU, then laid under the floor and down the wall in metal conduit in the wall to a cooker panel. This is 3 different types of run so looking in the regs we need to pick the one that has the greatest impact on the current carrying capacity of the cable.

The reference methods used specifically for flat twin and earth are these.

1. 100, under thermal insulation less than 100mm
2. 101, under thermal insulation greater than 100mm
3. 102, in a stud wall with cable at the edge’s and full of insulation
4. 103, in a stud wall with cable running through the middle of the insulation.

In our above example the cable runs clipped up the wall in a cupboard. This is reference Method C.
It then runs under the floor, this is Method, B, 100 or 101 depending on the presence of thermal insulation.
It then runs down in the conduit buried in the wall to the CCP this is Method B
I would use reference method 100 or 101 if there’s insulation as this is a worst case scenario. If there’s no insulation then I would use B.

Common Installation methods for domestic installations.
T&E installations see above

Method A

• Singles or multi core cables run in a conduit buried in insulation

Method B

• Surface mini Trunking.
• Run under floor boards if NO insulation.
• Running down in or on a brick wall in conduit.
• Running down in cavity wall with no insulation. With or without conduit

Method C

• Clipped to a wall or ceiling.
• Buried direct in plaster.
• SWA running down wall.

Method D

• SWA buried in the ground.

Effects on Current carrying capacity with method
Method 103 reduces the current carrying capacity greater than A
101 and A are close to the same, But 101 is for T and E, Method A and 101 reduces the current carrying capacity greater than B.
Method B reduces the current carrying capacity greater than C.

 

[marshr02]

My question is if cable clipped to wall in a cupboard is 'C', how is this different to cable clipped to a joist (no ins) - quoted as 'B'? Cheers

 

[SirKit Breaker]

My question is if cable clipped to wall in a cupboard is 'C', how is this different to cable clipped to a joist (no ins) - quoted as 'B'? Cheers

Joist is wood, flammable, wall is usually brick, breeze etc non flammable.
Cheers..........Howard

 

[SirKit Breaker]

This is a page I wrote to help the people at work understand the reference method.
Hope its of some help to others


Reference methods for domestic installations
The reference method box on the EICR is there to indicate how the particular circuit is installed.
This then allows us to verify that the cable used is capable of carrying the current for that circuit.
A basic house with floor boards would have different reference methods for different circuits and also different methods for the same circuit.
A Twin and Earth may be run up a wall clipped in a cupboard from the CCU, then laid under the floor and down the wall in metal conduit in the wall to a cooker panel. This is 3 different types of run so looking in the regs we need to pick the one that has the greatest impact on the current carrying capacity of the cable.

The reference methods used specifically for flat twin and earth are these.

1. 100, under thermal insulation less than 100mm
2. 101, under thermal insulation greater than 100mm
3. 102, in a stud wall with cable at the edge’s and full of insulation
4. 103, in a stud wall with cable running through the middle of the insulation.

In our above example the cable runs clipped up the wall in a cupboard. This is reference Method C.
It then runs under the floor, this is Method, B, 100 or 101 depending on the presence of thermal insulation.
It then runs down in the conduit buried in the wall to the CCP this is Method B
I would use reference method 100 or 101 if there’s insulation as this is a worst case scenario. If there’s no insulation then I would use B.

Common Installation methods for domestic installations.
T&E installations see above

Method A

• Singles or multi core cables run in a conduit buried in insulation

Method B

• Surface mini Trunking.
• Run under floor boards if NO insulation.
• Running down in or on a brick wall in conduit.
• Running down in cavity wall with no insulation. With or without conduit

Method C

• Clipped to a wall or ceiling.
• Buried direct in plaster.
• SWA running down wall.

Method D

• SWA buried in the ground.

Effects on Current carrying capacity with method
Method 103 reduces the current carrying capacity greater than A
101 and A are close to the same, But 101 is for T and E, Method A and 101 reduces the current carrying capacity greater than B.
Method B reduces the current carrying capacity greater than C.

What a cracking post. I have spent ages trying to explain this in simple terms on the 17th edition course, you hit the nail on the head.

Cheers............Howard

 

[marshr02]

Joist is wood, flammable, wall is usually brick, breeze etc non flammable.
Cheers..........Howard

Thanks for reply.....With respect, that doesn't make sense to me. The reference methods are designed to prevent the cable breaching 70 degrees, and flammability problems happen above this. The thermal conductivity of the joist versus the brick wall - that makes sense though......although the reference methods make no mention of different thermal conductivities - only clipped to something or touching the p/brd ceiling etc...

 

[SirKit Breaker]

Thanks for reply.....With respect, that doesn't make sense to me. The reference methods are designed to prevent the cable breaching 70 degrees, and flammability problems happen above this. The thermal conductivity of the joist versus the brick wall - that makes sense though......although the reference methods make no mention of different thermal conductivities - only clipped to something or touching the p/brd ceiling etc...

I see where you are coming from, and we are all probably looking too deep. The only difference i can see, is that one is clipped to wood, and the other to masonary of some sort or other, would it make any difference it you recorded one or the other? i doubt it. The IET works in wonderful ways, and i think that some of what they write is only done so we can argue the ---- about things, but it keeps a smile on my face.

Cheers.........Howard

 

[marshr02]

Taking this summary, then it's virtually impossible to wire a house without using reference method B. 4mm cable, ref B is rated at 30A - YET the OSG details a 32A, 4mm radial circuit as an approved circuit (A2 I think). Therefore it would seem safe to assume that the CCC of 30A is okay for use with a 32A breaker?

 

[marshr02]

Cheers Howard. What I don't get is why the IET doesn't just do some real world tests, and issue reference methods with real world examples. My particular issue is 32A, radial, 4mm circuit ... yet ref B shows a CCC of only 30A - can't get my head round that. Cheers...

 

[telectrix]

well, according to osg, there is no method B for T/E.therefore, it's either method C or 100,101,etc.

 

[Englishman]

We never fill in the reference method column, partly due to the fact that they won't give us copy of the bloody regs, never been moaned at yet.

 

[marshr02]

well, according to osg, there is no method B for T/E.therefore, it's either method C or 100,101,etc.

OSG page 52, table 7.2.

 

[marshr02]

We never fill in the reference method column, partly due to the fact that they won't give us copy of the bloody regs, never been moaned at yet.

Yes, but I presume you actually derate cables based on installation i.e. going through insulation etc (which is now the norm in new builds?) I'm coming from the point of doing the job right, the paperwork is a secondary issue...cheers

 

[Richard Burns]

My question is if cable clipped to wall in a cupboard is 'C', how is this different to cable clipped to a joist (no ins) - quoted as 'B'? Cheers

The reference method B referred to is cables under floors, in building voids ,etc., i.e. that cable is in a restricted space and may therefore heat up more than if it were in the open.

Taking this summary, then it's virtually impossible to wire a house without using reference method B. 4mm cable, ref B is rated at 30A - YET the OSG details a 32A, 4mm radial circuit as an approved circuit (A2 I think). Therefore it would seem safe to assume that the CCC of 30A is okay for use with a 32A breaker?

The standard circuit A2 does mention that the csa of the cables has to be assessed for CCC based on the installation method.

Because this is such a common circuit, it may well be installed incorrectly fairly often, but one can consider that domestic circuits almost never trip on overload and the likely current being carried will be far less than 30A so there is no significant danger, although as you say it will not meet the requirements of the regulations in many cases.

 

[Sintra]

Very informative Edd. Thread now stuck.

 

[marshr02]

Yes thanks Richard. I also stumbled across the answer in BS7671 - cables installed in a building void. For the benefit of others on this thread - If cables under floors are ref B --> nearly ALL domestic installations have ref B (or worse) --> Standard Cct A2, recommended for domestic installations must implicitly condone ref B for 4mm cable (Iz=30A) on a 32A CB.