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# the maximum length of cable permissible

Discuss the maximum length of cable permissible in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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#### Banburyman

Welcome to ElectriciansForums.net - The American Electrical Advice Forum

[FONT=&quot]I found this question in "practical giude to inspection, testing and certification" book

A lighting circuit is to be wired in l mm2 twin and earth thermoplastic [/FONT]
[FONT=&quot]cable, the circuit is protected by a 5 A BS 3036 fuse. What would be the [/FONT][FONT=&quot]maximum length of cable permissible to comply with the earth fault loop impedance requirements (Zs)? (Ze, is 0.45.)[/FONT]

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Untitled

Chris

B

#### Banburyman

it is how I did it
Zsmax for 5A 3036 is 9.58
[FONT=&quot]Ω
rule of thumb 9.58 * 0.75 = 7,17[/FONT]
[FONT=&quot]Ω
[/FONT]
[FONT=&quot]Zsmax - [/FONT][FONT=&quot]Ze[/FONT][FONT=&quot] =
7,17[/FONT]
[FONT=&quot]Ω[/FONT] - 0.45 [FONT=&quot]Ω[/FONT] = 6.72 [FONT=&quot]Ω[/FONT]
[FONT=&quot]R1+R2 = 36,20 m[/FONT]
[FONT=&quot]Ω/m

Length * (R1+R2)/1000 = 6,72
[/FONT] [FONT=&quot]Ω[/FONT][FONT=&quot]
lenght = 185 meter

I try to prepare myself to C&G2391 exam but it is very difficult as I have to use a book with so many errors.
According to Practical Guide to Inspection Testing and Certification the answer is 498meters (this is one of many errors in this book)
thanks a lot I just needed somebody to confirm my calculation
[/FONT]

#### old dog

##### -
Arms
the rule of thumb is now 0.8 in the 17th edition

##### -
looks like they used the max zs value of 100% of 17.7 ohms for the 5 second disconnection time .
remember the .8 rule is only to give us a safety margin, not the max zs allowed.