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Posted to uk.d-i-y
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Doubling up underground cable.
In article m,
writes:
I am considering installing a cylinder and boiler in a garage. I may
use two 3 kilowatt immersions. I need to take some cable from the
Consumer Unit to the garage underground. Supplying the boiler,
immersion, lights and sockets in there is more than what a direct
burial 2.5mm cable can provide. Doubling up two of these cables is cost
effective as heavy direct burial cable is expensive. Is it within regs
to have these cables on one 42A mcb at the CU and take it to a garage
CU in the garage and then take all the circuits off the Garage CU?
No.
Or is it best to have each cable on an mcb at the CU and no garage CU
with just two isolators as the cable enters the garage?
Any views?
You have to conform to the "conductors in parallel" part of
the regs. In this case, you could probably do it by using
a pair of ganged breakers at the CU (so if one trips, it
switches off the other too). For more than two cables, you
would normally need a set of ganged breakers at both ends
of the parallel run. However, you will have to sit down and
work out all possible fault scenarios, and that the protective
devices are effective. The harder ones to meet are a short in
the cable near the CU, where the fault current is being back-
fed via the other conductor.
Another factor is that you will have to derate the cables
if you lay two together (derating factor) as they'll heat
each other up.
As someone else already said, you really don't want to go
this route...
--
Andrew Gabriel
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