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]

Sorry for the way the forum compresses the table. That wasn't my intent.

 

[DefyG]

Where have the figures come from as obviously not standard UK cable sizes?

 

[Cookie]

NFPA 70, National Electrical Code. I can link the tables if it will help.

 

[DefyG]

NFPA 70, National Electrical Code. I can link the tables if it will help.

Thanks but don't think it will as I/we work to standard UK cable sizes and BS 7671.
Typically, a 100A supply would be 25mm and earth size determined by tables /calc from BS 7671 and/or adiabatic equation!

 

[Cookie]

Right, but the physics are still the same. I'm wondering if these CPCs are ok or not from a theory perspective.

 

[DefyG]

If they are tabulated in the NFPA document I guess someone somewhere in the US has previously done the calculations to prove they are?

 

[Cookie]

If they are tabulated in the NFPA document I guess someone somewhere in the US has previously done the calculations to prove they are?

Well, that would be an assumption...

 

[DefyG]

Well, that would be an assumption...

It would just as we in the UK assume the tabulated values in BS 7671 have been calculated and vetted by others before going to print. We don't go out of our way to prove them wrong but might need to verify a conductor by use of adiabatic calc!
Not really sure what your point is, if non standard sizes seems meaningless!

 

[streamer]

It would just as we in the UK assume the tabulated values in BS 7671 have been calculated and vetted by others before going to print. We don't go out of our way to prove them wrong but might need to verify a conductor by use of adiabatic calc!
Not really sure what your point is, if non standard sizes seems meaningless!

@DefyG I've just googled the sizes and it seems they are the metric equivalent of AWG (American wire gauge) system so it seems they are standard American sizes. Every day's a school day!

 

[Cookie]

It would just as we in the UK assume the tabulated values in BS 7671 have been calculated and vetted by others before going to print. We don't go out of our way to prove them wrong but might need to verify a conductor by use of adiabatic calc!
Not really sure what your point is, if non standard sizes seems meaningless!

These are standard sizes for North and South America.

The ratio between live and earth is huge, so much so my point revolves around the uncertainty that that the CPC will survive a fault.

 

[DefyG]

Non standard to the UK so meaningless to me and I'm sure other UK electricians as I am not upto speed with the US regulations on conductor sizes or calculation using a US adiabatic formula!
Have you tried calculating yourself to prove the sizes are ok or not?
Need more information in any case.
Perhaps the US regulations do not allow for protection of the cable in the event of a fault?

 

[Cookie]

Non standard to the UK so meaningless to me and I'm sure other UK electricians as I am not upto speed with the US regulations on conductor sizes or calculation using a US adiabatic formula!
Have you tried calculating yourself to prove the sizes are ok or not?
Need more information in any case.

Truth be I'm not sure how to go about calculating them as there is nothing explicit in the NEC about it. Closest thing is the formula for tap conductors. Which I guess could work in theory.

Perhaps the US regulations do not allow for protection of the cable in the event of a fault?

That has gone through my mind, and thats what I'm trying to figure out to be honest.

 

[Cookie]

This applies to tap conductors, but nothing in regards to the CPC.

 

[pc1966]

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

Not as "rule of thumb" design, typically ours is 0.5 or a few smaller ones in the case of T&E which are normally limited to the 6-50A region.

Those cable sizes are mostly OK provided your breaker operates fast (i.e. magnetic trip) and your PFC is moderated for the smaller sizes. Running adiabatic calculations for k=100 (typical copper 90C thermoplastic) I get:

MCB​	Live​	CPC​	Multiplier​	I2t limit​
15​	2.08​	2.08​	1​	4.33E+04​
20​	3.31​	3.31​	1​	1.10E+05​
60​	13.3​	5.261​	0.3955​	2.77E+05​
100​	26.67​	8.367​	0.3137​	7.00E+05​
200​	85.01​	13.3​	0.1564​	1.77E+06​
300​	177​	21.15​	0.1195​	4.47E+06​
400​	304​	26.67​	0.0877​	7.11E+06​
500​	456​	33.62​	0.0737​	1.13E+07​
600​	760​	42.41​	0.0558​	1.80E+07​

Looking at some let-through curves for Hager 100A-250A MCCB (just to hand) it is showing about 5E5 at 5kA PFC, and 4E6 at 40kA PFC. Hence short runs on a 40kA board for your CPC at 200A and below are at risk, even for a high but not unheard of "domestic" fault of 5kA, your 60A and below is at risk.

Of course, depending on the length and so on, it might be OK. But to me it is really marginal as if you have Zs and/or fault impedance that fails to hit the magnetic trip region then those MCCB let-through values are in the 6E6 to 6E7 region and a risk of burning them out even at the highest size in that table.