Earthing Conductor Size- Good?

[Cookie]

Alright, so what do you think of these CPC sizes in relation to the phase conductors?


90*C insulation, TN-C-S supply


MCB / Live / CPC / Multiplier

15 - 2.08mm2 - 2.08mm2 - X 1

20 - 3.31mm2 - 3.31mm2 - X 1

60 - 13.30mm2 - 5.261mm2 - X 0.3955

100 - 26.67mm2 - 8.367mm2 - X 0.3137

200 - 85.01mm2 - 13.30mm2 - X 0.1564

300 - 177mm2 - 21.15mm2 - X 0.1195

400 - 304mm2 - 26.67mm2 - X 0.0877

500 - 456mm2 - 33.62mm2 - X 0.0737

600 - 760mm2 - 42.41mm2 - X 0.0558


0.3, 0.1 and 0.05 multiplier difference ever encountered in the UK?

 

[Cookie]

To me that the the main issue - that proper design to meet shock and overload safety is not always applied. Then fancy electronics is added to try and fix the results of that...

I'm elated to know that you can see it. Not everyone can. AFCIs and GFCIs are being applied to fix a problem that the NFPA will not admit to. I'd be banned for saying it on a US forum, but the NFPA should not be writing electrical codes. 60 years of ignorance regarding basic electrical theory has been on perpetual display every code cycle.

Yes, the CPC table you started with looks awfully small for the larger breaker sizes (100A and above).

They are usable, but only just when the disconnection let-through is carefully controlled. To me that is not good enough for "rule of thumb" as that ought to be safe for all common combinations, and going to smaller CPC ought to be part of detailed analysis to justify it.

I think the same. But I'd like to prove it. I'm convinced that some NEC installations are a fire hazard despite being code compliant.

 

[Cookie]

A quick search pulls this up with a lot of details:

myCableEngineering.com > The adiabatic equation

Electrical cable sizing software. Current capacity to BS 7671, ERA 69-30 and IEC 60502. Impedance and voltage drop to IEC 60909 and CENELEC CLC/TR 50480. Cloud based - any device, anywhere.

mycableengineering.com

For copper wire and 70C PVC (common case) then k=115

Seems to easy vs the NEC's equation. I'm still confused though, where are they getting 143 as K?

 

[pc1966]

Seems to easy vs the NEC's equation. I'm still confused though, where are they getting 143 as K?

That k=143 is for "Thermosetting, 90°C (XLPE, EDR)" cable
The k=115 is for "Thermoplastic 70°C (PVC)" (and also CSA below 300mm)

 

[pc1966]

The k value is a combination of the material (same copper in both cases) but also the insulation temperature limits (max working and max survival) which differs by cable type and use.

 

[Cookie]

The k value is a combination of the material (same copper in both cases) but also the insulation temperature limits (max working and max survival) which differs by cable type and use.

Right, but how do you turn these variables into a number?

 

[pc1966]

Right, but how do you turn these variables into a number?

Covered here:

myCableEngineering.com > Calculation of k Factor

Electrical cable sizing software. Current capacity to BS 7671, ERA 69-30 and IEC 60502. Impedance and voltage drop to IEC 60909 and CENELEC CLC/TR 50480. Cloud based - any device, anywhere.

mycableengineering.com

The temperature effect is not quite as simple/linear as described earlier due to the conductor resistance increasing with heating (so a positive feedback situation here), hence the formulae including 'beta' for that coefficient and its non-linear solution with logs and square root.

But equally that is why fuses go so quickly when you get past a certain point and limit energy so effectively!

 

[Cookie]

Covered here:

myCableEngineering.com > Calculation of k Factor

Electrical cable sizing software. Current capacity to BS 7671, ERA 69-30 and IEC 60502. Impedance and voltage drop to IEC 60909 and CENELEC CLC/TR 50480. Cloud based - any device, anywhere.

mycableengineering.com

The temperature effect is not quite as simple/linear as described earlier due to the conductor resistance increasing with heating (so a positive feedback situation here), hence the formulae including 'beta' for that coefficient and its non-linear solution with logs and square root.

But equally that is why fuses go so quickly when you get past a certain point and limit energy so effectively!

That is unless anyone knows the 150*C impedance of the circuit?

Is using a k of 226 realistic for 90*C PVC insulation?

 

[pc1966]

k is not 226, it is 226 times a factor based on start/stop temperatures.

So if you plug in some values for your cable it ought to give to something closer to the k=100-150 range normally seen.

 

[pc1966]

Here is a simple spread sheet co compute the k values for copper, aluminium and iron based on that web page. Gives numbers very close to tabulated.

OK, so I can't upload a spreadsheet!

 

[Cookie]

k is not 226, it is 226 times a factor based on start/stop temperatures.

So if you plug in some values for your cable it ought to give to something closer to the k=100-150 range normally seen.

Ok, now I'm confused.

 

[pc1966]

If you paste these in to spreadsheet cells it should give you the k-values for copper, aluminium and iron/steel:

=226*SQRT(LN(1+(C6-C5)/(234.5+C5)))

=148*SQRT(LN(1+(C6-C5)/(228+C5)))

=78*SQRT(LN(1+(C6-C5)/(202+C5)))

Assuming your start temperature is located in cell C5 and your stop temperature in cell C6 (both deg C)

 

[pc1966]

For example, typical PVC has start 70 and stop 160 (deg C) for the adiabatic change. The above equations (if set for 1 significant digit) should give you:

• 115.0
• 76.0
• 41.7

For the k-values.

 

[Cookie]

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