bakey1959 said:

I cannot see a problem with using the lower value, but would appreciate confirmation and guidance from others.

There absolutely is a key issue with using the lower value, because that does not represent the actual prospective fault current at either end of the cable.

However, potentially, also the 328 A value if that is calculated at the furthest point in the circuit is also a problem, because a fault may well occur at the "near end" of the cable.

This is why designers ought to use an "adiabatic plot" over the characteristics of the protective device ... discussed in detail in Section 8 of the IET Electrical Installation Design Guide (EIDG).

BUT

There is another issue with the fact you're using circuit-breakers... as discussed in section 8.5 of the EIDG, if it's in the "instantaneous" portion of the curve, as 328 A or more would be, you need to use either:

(a) The manufacturer's stated value of let-through energy I²t; or

(b) The let-through energy I²t from the circuit-breaker product standard.

You plug this let-through energy value in place of the product I²t, in the adiabatic formulas in 543.1.3 and 434.5.2, rather than calculating I and looking up t. THe let-through energy does depend on the actual fault current, so you need to know the range of prospective fault currents (near end and far end of the cable) and look up the let-through energy for both, and the largest calculated S is the one to use.

The reason for this, is that, for a cpc, as well as 541.1,3, you also need to meet Regulation 434.5.2 (which relates to the selection of the protective device in relation to fault currents ... fault currents includes prospective fault currents for all of L-L, L-N and L-PE faults.

If you want to use values from the product standards BS EN 60898 and BS EN 61009, these lead to the following minimum csa (copper conductors) for Type B and C circuit-breakers (see Table 8.6 of the EIDG) ... the use of manufacturer's data often leads to lower csa requirement:

bakey1959 said:

I cannot see a problem with using the lower value, but would appreciate confirmation and guidance from others.

There absolutely is a key issue with using the lower value, because that does not represent the actual prospective fault current at either end of the cable.

However, potentially, also the 328 A value if that is calculated at the furthest point in the circuit is also a problem, because a fault may well occur at the "near end" of the cable.

This is why designers ought to use an "adiabatic plot" over the characteristics of the protective device ... discussed in detail in Section 8 of the IET Electrical Installation Design Guide (EIDG).

BUT

There is another issue with the fact you're using circuit-breakers... as discussed in section 8.5 of the EIDG, if it's in the "instantaneous" portion of the curve, as 328 A or more would be, you need to use either:

(a) The manufacturer's stated value of let-through energy I²t; or

(b) The let-through energy I²t from the circuit-breaker product standard.

You plug this let-through energy value in place of the product I²t, in the adiabatic formulas in 543.1.3 and 434.5.2, rather than calculating I and looking up t. THe let-through energy does depend on the actual fault current, so you need to know the range of prospective fault currents (near end and far end of the cable) and look up the let-through energy for both, and the largest calculated S is the one to use.

The reason for this, is that, for a cpc, as well as 541.1,3, you also need to meet Regulation 434.5.2 (which relates to the selection of the protective device in relation to fault currents ... fault currents includes prospective fault currents for all of L-L, L-N and L-PE faults.

If you want to use values from the product standards BS EN 60898 and BS EN 61009, these lead to the following minimum csa (copper conductors) for Type B and C circuit-breakers (see Table 8.6 of the EIDG) ... the use of manufacturer's data often leads to lower csa requirement: