If as 541.1 allows, I select the size of the CPC rather than use the calculation method, I would be able to select a 1mm² conductor based on the fact that the corresponding line conductor is also 1mm².

This then correlates with the calculation method outcome when the 120A generic value for a D6 circuit breaker (taken from Appendix 3) is used.

My assumption, maybe naively, has been that the calculation method would nearly always produce a smaller CPC than the selection method?

bakey1959 said:

I would be able to select a 1mm² conductor based on the fact that the corresponding line conductor is also 1mm².

How? The combination of overcurrent protective device and line and neutral conductors also need to meet Regulation 434.5.2?

So, if 434.5.2 is leading to 1.5 sq mm, this applies to L and N too. 434.5.2 applies in addition to

Usually, though, with prospective fault currents under 1.5 kA, the energy let-through I²t taken from manufacturer data (for 6 A Type B and C circuit-breakers at least) would permit selection of 1.0 sq mm. It's probably true for many D6 breakers too.

What is the manufacturer and part number/range of the D6 circuit-breaker you are using? I will see if I can get hold of the data to illustrate what  I'm saying.

bakey1959 said:

My assumption, maybe naively, has been that the calculation method would nearly always produce a smaller CPC than the selection method?

Yes, it can but not always.

It's worth noting that whilst our usual selection case of I_Z ≥ I_n ≥ I_b (Reg 433.1.1) relates to protection against overload current, wheras Reg Group 434.5 relates to protection against fault current. Selection of a conductor size S has to satisfy both conditions (overload and fault) and therefore the highest S calculated using each of the Regulations (and of course, not forgetting volt-drop) is the minimum size to be selected.

The circuit breaker is a Hager NDN106A (6A 10kA SP Type D).

It's worth noting that whilst our usual selection case of I_Z ≥ I_n ≥ I_b (Reg 433.1.1) relates to protection against overload current, wheras Reg Group 434.5 relates to protection against fault current. Selection of a conductor size S has to satisfy both conditions (overload and fault) and therefore the highest S calculated using each of the Regulations (and of course, not forgetting volt-drop) is the minimum size to be selected.

Is the suggestion then, that for normal MCBs, the 2nd paragraph of 435.1 doesn't apply? (i.e. where a suitable overload protective device has suitable breaking capacity, it may be assumed that requirements for fault protection are met).

  - Andy.

Thank you,

Just as an example, from the 2007 data sheet data sheet I quickly found on the internet for this device, I believe the let-through energy for the D6 would be as follows:

• For I_pf = 3 kA, I²t = 5,900 A²s
• For I_pf = 6 kA, I²t = 10,500 A²s
• For I_pf = 10 kA, I²t = 15,000 A²s

Rearranging the formula k²S² ≥ I²t from Regulation 434.5.2, we have S ≥ {√(I²t)}/k ... which spookily looks just like the formula in 543.1.3

k = 115 for copper conductors, so using the values above,

• For I_pf = 3 kA, S ≥ {√(5900)}/115}, or S ≥ 0.67 sq mm, so minimum S = 0.75 sq mm (flex from connection unit etc) or 1.0 sq mm (fixed wiring)
• For I_pf = 6 kA, I²t = 10,500 A²s, or S ≥ 0.89 sq mm, so minimum S = 1.0 sq mm
• For I_pf = 10 kA, I²t = 15,000 A²s, or S ≥ 1.1 sq mm, so minimum S = 1.25 sq mm (flex from connection unit etc) or 1.5 sq mm (fixed wiring)

So, if you know the prospective fault current at the DB end of the circuit is 6 kA or less, then 1.0 sq mm is OK for the D6 NDN106A, but if it's closer to 10 kA, you will need 1.5 sq mm. We already know it's lower than 3 kA at the remote end ...

This applies to cpc and live conductors equally.

For another manufacturer's product, or if Hager have provided a more up-to-date data sheet, the let-through energy values will be different, leading to potentially different results.

AJJewsbury said:

Is the suggestion then, that for normal MCBs, the 2nd paragraph of 435.1 doesn't apply? (i.e. where a suitable overload protective device has suitable breaking capacity, it may be assumed that requirements for fault protection are met).

The first paragraph of 435.1 says that the requirements of Sections 433 and 434 apply to the device.

The the third paragraph says the second paragraph is an assumption and you might need to check for circuit-breakers, and the calculation I just posted proves that there are limitations (even with this current-limiting circuit-breaker).  The assumption is, however, generally good, and it would be unusual to have > 6 kA on a B6 in a final dis-board, but it could happen.

AJJewsbury said:

Is the suggestion then, that for normal MCBs, the 2nd paragraph of 435.1 doesn't apply? (i.e. where a suitable overload protective device has suitable breaking capacity, it may be assumed that requirements for fault protection are met).

The first paragraph of 435.1 says that the requirements of Sections 433 and 434 apply to the device.

The the third paragraph says the second paragraph is an assumption and you might need to check for circuit-breakers, and the calculation I just posted proves that there are limitations (even with this current-limiting circuit-breaker).  The assumption is, however, generally good, and it would be unusual to have > 6 kA on a B6 in a final dis-board, but it could happen.

It doesn't seem like a particularly useful assumption, if it needs to be checked even for common MCBs. (using generic BS EN 60898 data there are whole swathes where energy let-though data would suggest larger conductors than Iz ≥ In would give (even ignoring the dubious maths of 1 ≥ 1.06 etc). If it's really only a safe assumption for fuses, would it be better just to say so?

   - Andy.