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# length of circuit calculation - head scratching stuff!

Discuss length of circuit calculation - head scratching stuff! in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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#### sivoodoo

Welcome to ElectriciansForums.net - The American Electrical Advice Forum

Hi all,

How do I work out the length of a circuit?

I have these figures

R1+R2 = 0.40

The circuit is wired in 2.5mm/1.5mm T&E

I take it I need to know the resistance of the T&E (where do I find this?)

Could someone give me the formula or point me in the right direction

Thank you.

A

#### AlanDavies

I got the data from here:Untitled

1.5mm = 12.10 mOhms/m
2.5mm = 7.41 mOhms/m

12.10 + 7.41 = 19.51

19.51 mOhms/1000 = 0.01951Ohms/metre

0.4 / 0.01951 = 20.5m

I think, can someone else check my maths.

Last edited by a moderator:
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#### sivoodoo

Brilliant!!!
Thank you so much.

S

#### Spudnik

This info is in the BRB.
Somewhere.

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#### sivoodoo

thanks, ive been looking in the big red one and i cant find it yet?

A

#### AlanDavies

On Site Guide - appendix 9 - page 166

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#### Spudnik

On Site Guide - appendix 9 - page 166
Sorry, yes, the little red book!

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#### sivoodoo

Thank you all

How accurate is this measurment, some of the results I have seen dont add up?

Last edited by a moderator:

#### Des 56

##### -
Arms
Esteemed
Thank you all

How accurate is this measurment, some of the results I have seen dont add up?

Very accurate indeed

Varying readings could be a number of factors
Parallel earth paths when recording the R1+R2
Loose or bad connections at outlets
If its a ring
Spurs giving an higher reading because of the extra lenghth
Be sure that the Rn and R1 of the ring are exactly equal
Interconnections on the ring

T

#### trebor

I got the data from here:Untitled

1.5mm = 12.10 mOhms/m
2.5mm = 7.41 mOhms/m

12.10 + 7.41 = 19.51

19.51 mOhms/1000 = 0.01951Ohms/metre

0.4 / 0.01951 = 20.5m

I think, can someone else check my maths.

exacto mondo 0.4 x 1000/ 19.51 = 20.5 metres

all the figures for resitance of conductors are in the OSG or GN3

B

#### bigal

Forgotten how good this site is. Thanks for the reminder.
bigal

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