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Hi guys n girls,

Building an extension and considering extending the downstairs lighting circuit which is 1.5mm t+e.

It is on a 5 amp breaker and I do not know the current length so am unsure if I can extend to the osg maximum of 105m? Also not sure of the osg maximum for bs3871 type mcb.....

If i measure the resistance at the furthest point of the line conductor and divide by the value for 1.5 (12.10 milli ohms) in table i1 of the osg is this likely to give me an accurate figure for existing length (assuming all connections are perfect).

According to my calcs the highest allowable figure should be 3.17ohms for R1+R2 and 1.27ohms for line on its own.

Am I best just running a new lighting circuit instead on a dedicated breaker? Help or advice appreciated.
 
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Off top of my head the Zs for a type 2 5A is between 6 and 6.5 ohm. Does the existing circuit have additional rcd protection.
 
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Off top of my head the Zs for a type 2 5A is between 6 and 6.5 ohm. Does the existing circuit have additional rcd protection.
Hi there, thanks for the reply. In answer to your question no the two lighting circuits in the house are currently not the protected but all others are.
 
As you are extending the circuit it is now a requirement for all domestic lighting circuits to have additional rcd protection. Are you an electrician.
 
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As you are extending the circuit it is now a requirement for all domestic lighting circuits to have additional rcd protection. Are you an electrician.
I new it was a requirement for new installations but wasn't sure if extending an old circuit with an installation done under the old regs whether it would need to comply with the new regs. The electrician question, current level of qualification would be Electrical Trainee but working towards nvq2 and 3.
 

davesparks

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It's a Hager cu mate. TLC do a Hager compact RCBO I will probably pick one of those up.
It being a 5A mcb as opposed to 6A suggests that this is quite an old CU so you need to check that a new RCBO will fit correctly and approved by the manufacturer for use in that board.
 
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It being a 5A mcb as opposed to 6A suggests that this is quite an old CU so you need to check that a new RCBO will fit correctly and approved by the manufacturer for use in that board.
Ok Dave thanks. Would it be better to replace the cu at this point for an all RCBO unit rather than faffing about with an older unit trying to get new parts to fit etc?
 
If you have a spare way a compact rcbo is likely to fit. Email Hager technical with a picture of your consumer unit.
 
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