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Posted to uk.d-i-y
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Trailing socket 'rules' ?
In article . com,
writes:
From CRC Handbook of Chemistry and Physics, 77th edition, 1996-1997,
2.588 mm^2 copper wire is 0.00319 ohms per metre. 25m x 2 x 0.00319 =
0.152 ohms. So theres no way that 25m of 2.5mm^2 would have any
noticeable effect on fuse or mcb action, ditto for 25m of 1.5mm^2.
Your figures are miles off those in the wiring regs (Table 4H3B):
0.019 ohms per metre for 2.5mm˛ @ 60C
0.032 ohms per metre for 1.5mm˛ @ 60C
0.037 ohms per metre for 1.25mm˛ @ 60C
25m x 2 x 0.019 = 0.95 ohms.
Max Zs for 13A BS1362 fuse is 2.53 ohms for 0.4 seconds disconnect
(Table 41B1), so that extension cord alone is using up 38% of the
allowable Zs. (Strictly, Table 41B1 only applies to fixed wiring,
but the principle is the same regardless.)
Also, checking Table 41C, max allowable CPC impedance with a 13A
BS1362 fuse is 0.75 ohms. 25m of 1.5mm˛ would be 0.8 ohm alone,
without adding on the resistance of the other parts of the CPC.
(Again, Table 41C only applies to fixed wiring, but the principle
is the same.)
0.152 ohm drops 1.99v at 13A, so v drop is still ok with wire half that
size.
25m x 2 x 0.019 x 13 = 12.35V drop.
This could be on top of max allowed drop in the wiring installation.
--
Andrew Gabriel
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