Cable sizing Query for Harmonic Filter

Hi Mike

Thanks for your response and apologies for my delay in responding (it was the last thing I looked at before the Christmas break and just getting back to it now!).

OK so, some more info; yes the ADF is connected in parallel to the load via the dedicated ACB on the main switchboard (I actually have 4 switchboards fed from 4 individual Tx supplies and there's an ADF going on each, so 4 separate systems).

I'd appreciate if you would review the logic behind my cable sizing proposal, which is...maximum current draw capable on each ADF is 360A (based on data sheet); I also applied a de-rating factor of x 1.3 to the full load rating of each ADF to guarantee that the cables are not overheated due to the skin effect caused by the harmonics. This is in accordance with the manufacturer’s recommendations, so I'm selecting my cable based on a max load of 360A x 1.3 = 468A.

I contacted the cable manufacturers and they advised that the single phase figures could be used from table 4F2A, but that a 0.7 de-rating factor should be applied for trefoil formation. Based on this info, if I apply the inverse of the 0.7 de-rating factor (1.4285), to the max load of 468A, I get a nominal current carrying capacity figure of 668.5A which I have used to ref Table 4F2A. This brings the size of single core cables required for this installation method to 240mm.

The 1 second short circuit withstand current rating for the 240mm H07ZZ-F cable is 34.3kA, and I am satisfied that the cables will safely withstand a fault current long enough for the trip to operate.

However, the minimum trip setting on the supply ACB as mentioned above is 500A, as the max load capacity is lower that this setting, the ACB is not intended to provide overload protection for the equipment (as each ADF module has its own incoming fuse), just fault protection for the cables, but with the de-rating factor of 0.7 applied to the 240mm cable ccc value from Table 4F2A (673A x 0.7 = 471A), does this mean that the 240mm cables would be undersized for the minimum ACB trip setting??

My thoughts are that as the connected load is not capable of reaching 471A under normal operating conditions and the capacity for withstanding a fault condition is satisfied and unlike a sub-mains to a switchboard, there is no capacity to add-on load to the ADF, this proposal would be sound. I just have this nagging in my head from my years of sparking that the downstream cable should be rated at or above the breaker setting that feeds it...

Any advice appreciated,


Regards, Seán 

Hi Mike

Thanks for your response and apologies for my delay in responding (it was the last thing I looked at before the Christmas break and just getting back to it now!).

OK so, some more info; yes the ADF is connected in parallel to the load via the dedicated ACB on the main switchboard (I actually have 4 switchboards fed from 4 individual Tx supplies and there's an ADF going on each, so 4 separate systems).

I'd appreciate if you would review the logic behind my cable sizing proposal, which is...maximum current draw capable on each ADF is 360A (based on data sheet); I also applied a de-rating factor of x 1.3 to the full load rating of each ADF to guarantee that the cables are not overheated due to the skin effect caused by the harmonics. This is in accordance with the manufacturer’s recommendations, so I'm selecting my cable based on a max load of 360A x 1.3 = 468A.

I contacted the cable manufacturers and they advised that the single phase figures could be used from table 4F2A, but that a 0.7 de-rating factor should be applied for trefoil formation. Based on this info, if I apply the inverse of the 0.7 de-rating factor (1.4285), to the max load of 468A, I get a nominal current carrying capacity figure of 668.5A which I have used to ref Table 4F2A. This brings the size of single core cables required for this installation method to 240mm.

The 1 second short circuit withstand current rating for the 240mm H07ZZ-F cable is 34.3kA, and I am satisfied that the cables will safely withstand a fault current long enough for the trip to operate.

However, the minimum trip setting on the supply ACB as mentioned above is 500A, as the max load capacity is lower that this setting, the ACB is not intended to provide overload protection for the equipment (as each ADF module has its own incoming fuse), just fault protection for the cables, but with the de-rating factor of 0.7 applied to the 240mm cable ccc value from Table 4F2A (673A x 0.7 = 471A), does this mean that the 240mm cables would be undersized for the minimum ACB trip setting??

My thoughts are that as the connected load is not capable of reaching 471A under normal operating conditions and the capacity for withstanding a fault condition is satisfied and unlike a sub-mains to a switchboard, there is no capacity to add-on load to the ADF, this proposal would be sound. I just have this nagging in my head from my years of sparking that the downstream cable should be rated at or above the breaker setting that feeds it...

Any advice appreciated,


Regards, Seán