Cable sizing Query for Harmonic Filter

Don't worry about the delay - even if you had come back sooner,  something funny with the software meant that I could not login to respond at all over Christmas anyway - just got a 'down for maintenance' message, and I assume by the lack of regular  posters until yesterday, a number of folk who do not login with google or whatever but actually type in a name and password were in the same boat .

I know what you mean about the gut feel that it is better if the downstream cable should be rated at or above the breaker setting that feeds it... however it is not always required, and for big stuff it often is impractical - the cable to the house is protected at the substation by a fuse of many times the steady state rating, but the overload protection comes from the fuse at  the service head  end,  as to bring full sized 300mm2 wavecon or whatever  to the meter cupboard in every house and flat would be impractical in the extreme.

This is a similar case.

I suspect your 240mm cables will run pretty cool actually, you may even be able to drop a size, but I have not seen the route..

(I think open air trefoil 240mmm reaches 90C with about 600A  based on this) and there is no need to worry about the 500A breaker setting, as there is overload protection at the load end.

Even if there was not a load end fuse,  the likely fault condition is likely to be more or less 50Hz, sinewave, if one of the internal switching devices gets shorted or stuck 'on'. As the skin effect thing will only apply to fast changing waveforms, so you do not need the additional de-rating  of 1.3 during faults (!).


Mike.

Don't worry about the delay - even if you had come back sooner,  something funny with the software meant that I could not login to respond at all over Christmas anyway - just got a 'down for maintenance' message, and I assume by the lack of regular  posters until yesterday, a number of folk who do not login with google or whatever but actually type in a name and password were in the same boat .

I know what you mean about the gut feel that it is better if the downstream cable should be rated at or above the breaker setting that feeds it... however it is not always required, and for big stuff it often is impractical - the cable to the house is protected at the substation by a fuse of many times the steady state rating, but the overload protection comes from the fuse at  the service head  end,  as to bring full sized 300mm2 wavecon or whatever  to the meter cupboard in every house and flat would be impractical in the extreme.

This is a similar case.

I suspect your 240mm cables will run pretty cool actually, you may even be able to drop a size, but I have not seen the route..

(I think open air trefoil 240mmm reaches 90C with about 600A  based on this) and there is no need to worry about the 500A breaker setting, as there is overload protection at the load end.

Even if there was not a load end fuse,  the likely fault condition is likely to be more or less 50Hz, sinewave, if one of the internal switching devices gets shorted or stuck 'on'. As the skin effect thing will only apply to fast changing waveforms, so you do not need the additional de-rating  of 1.3 during faults (!).


Mike.