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Topic Title: Design current for Luminaires with more than 15% Total Harmonic Distortion
Topic Summary: Each luminaire > 15% THD but < 33%
Created On: 01 May 2014 06:39 PM
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 01 May 2014 06:39 PM
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Nakhil

Posts: 10
Joined: 11 April 2014

A circuit feeding a string of High Density Sodium Vapour lamps installed on each lighting pole with 30-metre spacing between each (spacing is not important here, though). Each lamp is the same type and has more than 15% THD (but less than 30%).

How should I go about sizing the feeder cable (the cable connecting the source to the first fixture) while taking into account the harmonic currents? Since the THD% of each fixture is between 15 - 33%, should I just divide the total current of the fixtures by 0.86 and the result obtained will be the design current of that circuit?
 01 May 2014 07:32 PM
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OMS

Posts: 19860
Joined: 23 March 2004

Single or three phase ?

Whilst you might want to put a small adjustment factor on the line conductor and neutral on a single phase load (to capture non triplen harmonics), you would only want to derate the supply cabling if it was 3 phase (and the neutral was distributed within the cable) for teh predominant triplen currents plus the potential out of balance neutral current.

If it is 3 phase and reasonably balanced, and the neutral is within the cable, then typically Appendix 4 derating of about 0.86 would be usual.

The simple rule is leave plenty of headroom in your cable selection compared to Ib - running your conductors to not more than say 50C usually gives you all the margin you need

Regards

OMS

-------------------------
Failure is always an option
 01 May 2014 07:57 PM
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Nakhil

Posts: 10
Joined: 11 April 2014

Yes, it is three phase, with each fixture connected between alternate phase and neutral, and with the same number of fixtures on each phase (each fixture has the same load, as I've already said).

I should also mention that the cable size up to the last pole is the same, and yes, the neutral conductor is within the cable.

So I guess I would go for the 0.86 derating. Thank you.
 01 May 2014 08:06 PM
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OMS

Posts: 19860
Joined: 23 March 2004

Sounds about right to me.

Bsically, if you derate a cable to say 50C then you''ll have achieved at least 0.86 derating anyway, plus any factors for buried cables etc

Regards

OMS

-------------------------
Failure is always an option
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