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- 8 Feb 2008
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George765
Well I expect that you thought that you were asking a simple question – can I use 16mm2 PVCTWE for a 50A distribution circuit to my garage – but
1) You have not given sufficient information;
2) I expect that you now know that the problem is far more complex than you ever imagined.
Information
1) Is the garage attached or detached?
2) Is this cable required to provide any bonding connections?
3) What is the length of this proposed circuit?
4) Details of the 50A mcb - such as BS number, type, energy limiting class, fault rating and make would be nice.
5) Details of the supply – earthing system, phases, line to earth and line to neutral impedances, prospective fault currents due to short circuits and earth faults and so on.
So how can an answer be given without all of this – well we could make assumption using worst case parameters and see what that gives us.
Assumptions
1) There are no issues concerning bonding – this circuit just needs a connection to earth that can carry fault current safely.
2) With no mcb details we could forget it and just assume that the 100A BS 1361 cut-out fuse will provide fault current protection.
3) We will assume that the cable can carry the full 50A without causing excessive voltage drop.
4) We will assume that the earthing system is TN-C-S.
Now the answer revolves around the cables ability to carry fault current without damage.
If we know the conductor sizes, the insulating material and the circuit format we could get a figure for the ‘withstand’ of the cable. We should do this for all conductors involved in any type of zero impedance fault (shorts or earth faults). However, in this case we could just check the 6 mm2 circuit protective conductor (earth wire).
The cable withstand can be estimated from S2K2 where S is the size and K is magic.
K can be taken from table 54.3 and is 115.
So we have 6 * 6 * 115 * 115 = 476100 As.
We don’t know the fault current so let us assume first that it is very high – so high that the fuse operates within the first half cycle of the supply wave – 0.01s.
We were given a let through energy for a 100A Busman fuse of 57300 As. We will double it and then take it up to 150000 As for good measure.
We need I2t <= S2K2
And we have: 150000 < 476100 As
So we have passed the first test with flying colours.
Now we will assume that the fault current is low but that the circuit can meet the 5 second disconnection time required by BS 7671 (i.e. we are assuming that the length of run is not too long).
For the 100A fuse this means that the fault current must be 550A or more.
We need S = root(I2t)/K to be <= 6 mm2
S = root(550 * 550 * 5)/115 = 10.7 mm2
Oh dear we have failed.
Now will your circuit fail – I don’t know - provide full details and I might be able to tell you.
Well I expect that you thought that you were asking a simple question – can I use 16mm2 PVCTWE for a 50A distribution circuit to my garage – but
1) You have not given sufficient information;
2) I expect that you now know that the problem is far more complex than you ever imagined
.Information
1) Is the garage attached or detached?
2) Is this cable required to provide any bonding connections?
3) What is the length of this proposed circuit?
4) Details of the 50A mcb - such as BS number, type, energy limiting class, fault rating and make would be nice.
5) Details of the supply – earthing system, phases, line to earth and line to neutral impedances, prospective fault currents due to short circuits and earth faults and so on.
So how can an answer be given without all of this – well we could make assumption using worst case parameters and see what that gives us.
Assumptions
1) There are no issues concerning bonding – this circuit just needs a connection to earth that can carry fault current safely.
2) With no mcb details we could forget it and just assume that the 100A BS 1361 cut-out fuse will provide fault current protection.
3) We will assume that the cable can carry the full 50A without causing excessive voltage drop.
4) We will assume that the earthing system is TN-C-S.
Now the answer revolves around the cables ability to carry fault current without damage.
If we know the conductor sizes, the insulating material and the circuit format we could get a figure for the ‘withstand’ of the cable. We should do this for all conductors involved in any type of zero impedance fault (shorts or earth faults). However, in this case we could just check the 6 mm2 circuit protective conductor (earth wire).
The cable withstand can be estimated from S2K2 where S is the size and K is magic
.K can be taken from table 54.3 and is 115.
So we have 6 * 6 * 115 * 115 = 476100 As.
We don’t know the fault current so let us assume first that it is very high – so high that the fuse operates within the first half cycle of the supply wave – 0.01s.
We were given a let through energy for a 100A Busman fuse of 57300 As. We will double it and then take it up to 150000 As for good measure.
We need I2t <= S2K2
And we have: 150000 < 476100 As
So we have passed the first test with flying colours
.Now we will assume that the fault current is low but that the circuit can meet the 5 second disconnection time required by BS 7671 (i.e. we are assuming that the length of run is not too long).
For the 100A fuse this means that the fault current must be 550A or more.
We need S = root(I2t)/K to be <= 6 mm2
S = root(550 * 550 * 5)/115 = 10.7 mm2
Oh dear we have failed
.Now will your circuit fail – I don’t know - provide full details and I might be able to tell you.
