Zoomup:
A B20 at the origin and a B16 M.C.B. at the outbuilding garage unit should just about discriminate on overload, but not on a dead short or high earth current fault where they may operate together. Although if the house M.C.B. is warm and the outbuilding M.C.B is colder they may operate in a similar way under overload conditions, and trip off together.
Z.
will need 10.0 or 16.0mm2 for Volt drop limitation at 20 Amps over 65 metres.
No it won't - 16mmsq is more than you need to supply a small house or flat !!
It's not a lighting-only circuit, so the larger volt drop figure applies. You may need 6mmsq, not more.
Selectivity between 20A and 16A is not guaranteed, and wont be for fuses either, though if you accept the overload case being limited by the breakers at the load end, then the source end protection only needs to handle short circuits in the cable, and disconnect in 5 seconds or less for a short at the far end just before the load end breakers, so the source end MCB could be 32A.
Chris Pearson:
Many suggestions above, but what is the cheapest compliant solution? That's what the customer appears to want.
As for selectivity - how often are you going to get a fault in such a position that it would trip two protective devices; and if it did, would it bother the customer?
Chris Pearson:
Many suggestions above, but what is the cheapest compliant solution? That's what the customer appears to want.
As for selectivity - how often are you going to get a fault in such a position that it would trip two protective devices; and if it did, would it bother the customer?
mapj1:
will need 10.0 or 16.0mm2 for Volt drop limitation at 20 Amps over 65 metres.
No it won't - 16mmsq is more than you need to supply a small house or flat !!
It's not a lighting-only circuit, so the larger volt drop figure applies. You may need 6mmsq, not more.
..........................................................................................................................................................................................................................................6.0mm2 as an absolute minimum, 10.0mm2 preferably.
Well, a house does not normally have a 65 metre run for its internal main supply cable.
The outbuilding will have lighting so the 3 percent max. Volt drop will apply in my opinion. Also if 10.0mm2 or 16mm2 is installed for a 65 metre run rated at 20 Amps. it limits Voltage drop and it allows for future extra loads.
With a 20 Amp load and a 65 metre run using 6.0mm2 cable I calculate that the Volt drop will be just short of 10 Volts. Doesn't that create a loss of just under 200 Watts?
Z.
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